Effects of Acetabular Rim Trimming on Hip Joint Contact Pressures
5-in-5
How Much Is Too Much? Sanjeev Bhatia,*y MD, Simon Lee,z MD, MPH, Elizabeth Shewman,z MS, Richard C. Mather III,§ MD, Michael J. Salata,|| MD, Charles A. Bush-Joseph,z MD, and Shane J. Nho,z MD, MS Investigation performed at Rush University Medical Center, Chicago, Illinois, USA Background: In patients with femoroacetabular impingement (FAI), acetabular rim trimming removes the offending area of the acetabular deformity in patients with pincer-type and mixed-type FAI to improve hip joint kinematics. Although the rationale for arthroscopic acetabular rim trimming in patients with FAI is well established, the amount of rim resection has not been quantified, and the threshold at which excessive rim resection results in abnormal hip contact pressures has not been described. Purpose: To investigate the changes in contact areas, contact pressures, and peak forces within the hip joint with sequential acetabular rim trimming. Study Design: Controlled laboratory study. Methods: Six fresh-frozen, nondysplastic, human cadaveric hemipelvises were analyzed utilizing thin-film piezoresistive load sensors to measure the contact area, contact pressure, and peak force after anterosuperior acetabular rim trimming at depths of 0 mm (intact), 2 mm, 4 mm, 6 mm, and 8 mm. Each specimen was examined at 20° of extension and 60° of flexion. Analysis was performed on 2 regions of interest: the acetabular rim and the acetabular base (deep part of the acetabulum). After each experimental condition, the acetabulum was normalized with respect to the intact state to account for specimen variability. Statistical analysis was conducted through 1-way analysis of variance with post hoc Games-Howell tests. Results: At the acetabular base, there were significant increases in the contact area after 4-mm resection (60°: 169.12% 6 30.64%; P = .0138), contact pressure after 6-mm resection (60°: 292.76% 6 79.07%; P = .009), and peak force after 6-mm resection (60°: 166.00% 6 34.40%; P = .027). At the acetabular rim, there were significant decreases in the contact area after 6-mm resection (60°: 66.32% 6 18.80%; P = .0354) (20°: 65.47% 6 15.87%; P = .0127), contact pressure after 6-mm resection (60°: 50.77% 6 11.49%; P \ .001) (20°: 58.01% 6 23.10%; P = .0335), and peak force after 6-mm resection (60°: 60.67% 6 9.29%; P \ .001) (20°: 74.44% 6 9.84%; P = .007). Conclusion: Resecting more than 4 to 6 mm of the acetabular rim during hip arthroscopic surgery to address a pincer deformity may dramatically increase contact pressures by 3-fold at the acetabular base. The study suggests that excessive rim resection may lead to increased loads in the hip joint and may predispose to premature joint degeneration. Clinical Relevance: Resecting more than 4 to 6 mm of the acetabular rim may significantly alter hip joint biomechanics, increasing joint reactive forces and subsequent chondrolabral degeneration. Keywords: hip; femoroacetabular impingement; pincer deformity; hip arthroscopic surgery; biomechanics; rim trimming
The number of hip arthroscopic procedures has grown dramatically, owing to an improved understanding of femoroacetabular impingement (FAI) as well as other sources of intra-articular and extra-articular hip pain.8,19,25,26 During the treatment of FAI with arthroscopic approaches, acetabular rim trimming may be performed, in conjunction with other procedures, to help alleviate the pathomechanics
associated with pincer-type and mixed pincer-cam–type FAI. First described by Ganz and colleagues,7 pincer FAI occurs when repetitive contact is made between the acetabular rim and the femoral head-neck junction, leading to labral tears, chondral injuries, and potentially osteoarthritis.5,24 Arthroscopic rim trimming is performed to address acetabular overcoverage and oftentimes performed with osteochondroplasty of the proximal femur to allow for hip motion without impingement.19,21,24 Numerous studies have reported on the improvement of hip pain and hip function after arthroscopic treatment of focal acetabular overcoverage, global acetabular overcoverage, and mixed-
The American Journal of Sports Medicine, Vol. 43, No. 9 DOI: 10.1177/0363546515590400 Ó 2015 The Author(s)
2138
Vol. 43, No. 9, 2015
Effects of Acetabular Rim Trimming on Hip Contact Pressures
type FAI.19,20 Additionally, rim trimming allows for a bleeding bed of cancellous bone to facilitate healing and also results in protection of the repaired labrum from repetitive abutment with the proximal femur.21 Although the benefits of acetabular rim trimming have been well described, the ideal amount of rim resection has not yet been established. Previous authors have described resection of anywhere from 1 to 9 mm of bone from the acetabular rim, likely varying depending on the size and location of the acetabular deformity.17,20,21 Excessive resection of the acetabulum can lead to increased load transfer to the chondrolabral surface and even iatrogenic hip instability.10,17 Some authors have recommended that the lateral center-edge angle of Wiberg (CEA) should be greater than 20° to 25° after acetabular rim trimming.17,21 Philippon and colleagues21 correlated the amount of rim resection with a change in the CEA on anteroposterior (AP) radiographs. These investigators determined that 1 mm of bone resection resulted in a 2.4° decrease of the CEA and that each additional millimeter of bone resection results in a 0.6° decrease of the CEA. Although the CEA is a useful guide, there are several limitations with the use of CEA alone to quantify rim resection. The CEA can only measure the superolateral aspect of the acetabulum and cannot detect changes in the remainder of the anterior wall, which is commonly altered with arthroscopic rim trimming. Moreover, the AP pelvis view does not account for pelvic tilt and rotation, which can affect the CEA and the presence of a crossover sign.23 The purpose of this study was to evaluate changes in contact pressures, contact areas, and peak pressures within the hip joint with sequential acetabular rim trimming. Our hypothesis was that a threshold for rim resection exists, after which the benefits of rim resection are outweighed by the harmful increases in the contact pressure profile of the hip joint.
METHODS Six fresh-frozen, unpaired, human cadaveric hemipelvises were used for biomechanical testing. The sample size of this biomechanical cadaveric study was determined based on the availability of specimens and on prior experience with cadaveric models using thin-film piezoelectric pressure sensors. A post hoc analysis of collected data demonstrated a minimal statistical power of 0.989 for statistically significant results, providing evidence of a sufficient sample size in this study. Specimens were included only if they had
2139
a CEA greater than 25° and To¨nnis grade of 0 or 1. Additionally, specimens with damage due to the procurement process, a severe joint deformity, advanced osteoarthritis (To¨nnis grade .1), prior surgery, previous hardware placement, or previous fractures were also excluded. The mean age of the specimens was 57.5 6 5.41 years (range, 46-62 years). Each hemipelvis came from a separate cadaveric specimen to allow for increased variability. There were 3 left and 3 right hemipelvises. The mean CEA as measured on computed tomography was 35.7° 6 3.2° (range, 29.8°38.8°). Each specimen was maintained in a freezer at 220°C until approximately 24 hours before use and then thawed to room temperature for testing.
Specimen Preparation After tissues were appropriately thawed to room temperature, all extracapsular soft tissues were dissected from each hemipelvis. The proximal femur and acetabular portion of the pelvis were each potted separately and stabilized using polymethyl methacrylate (Isocryl; Lang Dental Manufacturing Co). Both the proximal femur and acetabulum were placed in custom fixtures that were rigidly mounted on a test platform of a materials testing system (MTS Insight 5; MTS Systems Corp) with a positional resolution of 60.001 mm, positional accuracy of 60.01 mm, and speed accuracy (% of set speed) of 60.005%. A 5-kN load cell was used (Model 569329-03; MTS Systems Corp), which was calibrated with a nonlinearity of 20.013% full scale and hysteresis of 20.002% full scale (Figure 1). The capsule was then incised and removed, followed by separation of the ligamentum teres from the base of the acetabular floor and the femoral head. Reserved synovial fluid from the joint space was mixed with normal saline for constant lubrication of the joint space during testing. The articular surface was hydrated immediately before and after each test. Given that some cadaveric specimens had occult anterosuperior labral tears present as well as variable turgor of labral tissue, the entire labrum was then excised to minimize variability between specimens and to better analyze the true effect of bony acetabular rim trimming. Ossified labral pathology was not noted in any specimen. To maintain the consistency of placement for each specimen, a set of standardized angles in relation to readily identifiable pelvic landmarks were developed. A vertical acetabular angle of 40° was determined by measuring the angle between a completely vertical line and a line through the 6- and 12-o’clock positions at the labrum’s edge when viewed laterally (Figure 1). This measurement was based
*Address correspondence to Sanjeev Bhatia, MD, Center for Hip Preservation, Cincinnati Sports Medicine and Orthopaedic Center, Mercy Health, 10663 Montgomery Road, Cincinnati, OH 45242, USA (email: [email protected]). y Center for Hip Arthroscopy and Joint Preservation, Cincinnati Sports Medicine and Orthopaedic Center, Mercy Health, Cincinnati, Ohio, USA. z Hip Preservation Center, Division of Sports Medicine, Department of Orthopedic Surgery, Rush University Medical Center, Rush Medical College of Rush University, Chicago, Illinois, USA. § Division of Sports Medicine, Department of Orthopaedic Surgery, Duke University School of Medicine, Durham, North Carolina, USA. || Division of Sports Medicine, Department of Orthopaedic Surgery, Case Western Reserve University, Cleveland, Ohio, USA. Presented at the interim meeting of the AOSSM, Las Vegas, Nevada, March 2015. One or more of the authors has declared the following potential conflict of interest or source of funding: S.J.N. is a paid consultant for Ossur and Stryker and receives research support from Allosource, Arthrex, Athletico, DJ Orthopaedics, Linvatec, Miomed, Smith & Nephew, and Stryker. M.J.S. is a paid consultant for Linvatec and Smith & Nephew. R.C.M. is a paid consultant for KNG Health Consulting, Pivot Medical, Smith & Nephew, and Stryker.
2140 Bhatia et al
The American Journal of Sports Medicine
111.8 3 111.8–mm area matrix with a sensel density of 15.5 sensels/cm2 (total number of sensels = 1936) and was precalibrated on the MTS machine. A 2-point calibration was performed per the manufacturer’s guidelines, applying 2 separate compressive loads across a custom jig. The jig consisted of a piece of dense foam sandwiched between 2 steel plates in the shape of the sensor, and it was manufactured to mimic contact pressures of a human joint with cartilage. The calibration points were obtained by applying loads at 10 N/s across the jig and sensor, resulting in contact pressures of 20% and 80% of the maximum test pressure created from the 700-N load across the hip joint. Each load was held as the calibration data were acquired using I-Scan software (v 5.83; Tekscan), and the sensor was allowed to rest for 5 minutes before applying the second calibration load. After calibration, each sensor was then applied within the femoroacetabular joint, with a load of 700 N (approximately simulating three-fourths the body weight) subsequently placed onto the joint for 30 seconds. This resulted in real-time pressure and area map measurements (Figure 2). Specimens were examined at 20° of extension and 60° of flexion, both in neutral position, to simulate the complete arc of the natural hip range of motion during normal gait on a level surface and during stair climbing. Measurements were repeated 3 times for each condition on each specimen. These values were then averaged for utilization in the final analysis.
Sequential Acetabular Rim Trimming
Figure 1. To maintain the consistency of placement for each specimen, a set of standardized angles in relation to readily identifiable pelvic landmarks were developed based on prior work.12 A vertical acetabular angle of 40° was determined by measuring the angle between a completely vertical line and a line through the 6- and 12-o’clock positions at the labrum’s edge when viewed laterally. A pubic-femoral neck angle of 140° was then set to establish standardized neutral positioning of the hip joint, measuring the angle between the extension of the pubic bone from the acetabulum and the vector in line with the femoral neck. on an anatomic study by Krebs et al12 that demonstrated the average acetabular abduction angle to be 39.8°. A pubic-femoral neck angle of 140° was then established, measuring the angle between the extension of the pubic bone from the acetabulum and the vector in line with the femoral neck. This angle was employed to establish standardized neutral positioning of the hip joint. All specimens were prepared and positioned in a similar fashion.
Contact Area and Pressure Measurement A dynamic contact pressure and area mapping system that utilized thin-film piezoresistive load sensors (Model 5101; Tekscan) was used based on the validated work of previous authors.2,11,13,14,27 Each sensor consisted of a
Five testing conditions were performed on each specimen: intact hip and 2-mm, 4-mm, 6-mm, and 8-mm rim resections. Although acetabular rim trimming is typically performed as an arthroscopic procedure, the current biomechanical model utilized an open technique.6 For each acetabular rim trim, a handheld rotary tool with a bur attachment (Dremel; Bosch North America) was used to perform each rim resection in a region of the acetabulum clinically relevant to a pincer deformity and arthroscopic pincer resection: from the 12- to 3-o’clock position. For each acetabular resection, a digital caliper was utilized to precisely indicate the appropriate rim-trim depth at several points along the region of resection to ensure consistency.
Data Acquisition and Statistical Analysis Contact area (cm2), contact pressure (kg/cm2), and peak force (N) were recorded at each acetabular resection depth. These measures are reported as a percentage of the native acetabular state. The resected states were then normalized with respect to their own intact state to control for specimen variability. To differentiate acetabular base and rim measurements, area of interest polygons were created to allow for the discrete analysis of each area. The Tekscan software contains a function that enables the user to delineate specific regions of the recorded pressure map by drawing polygons, outputting area, pressure, and force results from that region only. A best-fit perfect circle polygon was first created from the pressure map recorded from
Vol. 43, No. 9, 2015
Effects of Acetabular Rim Trimming on Hip Contact Pressures
2141
each specimen’s intact acetabulum. Next, a circle covering 70% of the original polygon’s area was created and positioned in the center of the original polygon to isolate contact areas, contact pressures, and peak pressures from the acetabular base, or deepest area of the acetabulum. The original polygon was then redrawn as a ring to isolate contact areas, contact pressures, and peak pressures representative of the acetabular rim. Once the polygon was created for each intact specimen, it was used for analysis of the subsequent trimmed states of that particular specimen. Statistical analysis was conducted through Pearson correlations and 1-way between-group analysis of variance (ANOVA) on normalized data with subsequent post hoc Games-Howell tests to evaluate the nature of the differences between groups (SPSS Statistics v 21.0; IBM Corp). The Games-Howell test was selected for post hoc analysis based on the Levene test for homogeneity of variances. All reported P values were 2-tailed, with an a level of .05 detecting significant differences.
RESULTS Gross Observations Qualitatively, there appeared to be a marked increase in the ease of subluxation of all hip specimens with greater than 6 to 8 mm of rim resection with manual femoroacetabular manipulation. Fortunately, because of the static loading design of the study, the reduced static constraints of the acetabulum did not affect the stability of the femoroacetabular joint when it was being loaded on the universal testing system. Visually, there was also an increase in the contact pressure in the deeper part of the acetabulum with sequential rim trimming on contact pressure mapping (Figure 2).
Contact Area Measurements Significant differences in the contact area between rim-trim depths were detected after ANOVA for the acetabular rim (20°: P \ .001; 60°: P \ .001) as well as the acetabular base at 60° (P \ .001). There was a significant decrease in the acetabular rim contact area after 6-mm resection at both 60° of flexion (66.32% 6 18.80%; P = .0354) and 20° of extension (65.47% 6 15.87%; P = .0127). With regard to the acetabular base, there was a significant increase in the contact area after 4-mm resection at 60° of flexion (169.12% 6 30.64%; P = .0138) (Figure 3; also see the Appendix, available online at http://ajsm.sagepub.com/supplemental).
Contact Pressure Measurements Figure 2. Qualitative observations of contact area and pressure maps after sequential acetabular rim resection from the 12- to 3-o’clock position. (A) Intact state (0-mm rim resection), (B) 2-mm rim resection, (C) 4-mm rim resection, (D) 6-mm rim resection, and (E) 8-mm rim resection. Note the gradual increase in the contact pressure in the central circle (acetabular base) with sequential rim trimming.
Significant differences in the contact pressure between rimtrim depths were detected after ANOVA for the acetabular rim (20°: P \ .001; 60°: P \ .001) as well as the acetabular base at 60° (P \ .001). There was a significant decrease in the acetabular rim contact pressure after 6-mm resection at both 60° of flexion (50.77% 6 11.49%; P \ .001) and 20° of extension (58.01% 6 23.10%; P = .0335). The acetabular base, however, had a significant increase in the contact
2142 Bhatia et al
The American Journal of Sports Medicine
Figure 3. Normalized changes in the contact area of the acetabular rim and acetabular base with sequential rim trimming. (A) Contact area with 60° of flexion. (B) Contact area with 20° of extension. An asterisk denotes the rim resection level that first led to a statistically significant change as compared with the intact state; all rim resection levels beyond the asterisk were statistically significant in each given plot line. Bars represent SD.
Figure 4. Normalized changes in the contact pressure of the acetabular rim and acetabular base with sequential rim trimming. (A) Contact pressure with 60° of flexion. (B) Contact pressure with 20° of extension. An asterisk denotes the rim resection level that first led to a statistically significant change as compared with the intact state; all rim resection levels beyond the asterisk were statistically significant in each given plot line. Bars represent SD. pressure after 6-mm resection at 60° of flexion (292.76% 6 79.07%; P = .009) (Figure 4).
Peak Force Measurements Significant differences in the peak force between rim-trim depths were detected after ANOVA for the acetabular rim (20°: P \ .001; 60°: P \ .001) as well as the acetabular base at 60° (P \ .001). There was a significant decrease in the acetabular rim peak force after 6-mm resection at 60° of flexion (60.67% 6 9.29%; P \ .001) and 20° of extension (74.44% 6 9.84%; P = .007). As for the acetabular base, there was a significant increase in the peak force after 6-mm resection at 60° of flexion (166.00% 6 34.40%; P = .027) (Figure 5; also see the Appendix).
P \ .001). At 20° of extension, rim-trim depth was significantly correlated in an inverse fashion to the acetabular rim contact area (R = 20.846, P \ .001), rim contact pressure (R = 20.732, P \ .001), and rim peak force (R = 20.838, P \ .001) (Figure 6).
Rim-Trim Depth Versus Acetabular Base Variables At 60° of flexion, rim-trim depth was significantly correlated to the acetabular base contact area (R = 0.805, P \ .001), base contact pressure (R = 0.849, P \ .001), and base peak force (R = 0.834, P \ .001). At 20° of extension, rim-trim depth was significantly correlated to the acetabular base contact area (R = 0.362, P = .049) (Figure 6).
Rim-Trim Depth Versus Acetabular Rim Variables
DISCUSSION
At 60° of flexion, rim-trim depth was significantly correlated in an inverse fashion to the acetabular rim contact area (R = 20.769, P \ .001), rim contact pressure (R = 20.892, P \ .001), and rim peak force (R = 20.874,
Although pincer resection is commonly performed in hip arthroscopic surgery, the effect of excessive pincer resection on contact pressures within the hip joint has never previously been studied. No prior studies have described
Vol. 43, No. 9, 2015
Effects of Acetabular Rim Trimming on Hip Contact Pressures
2143
Figure 5. Normalized changes in the peak force of the acetabular rim and acetabular base with sequential rim trimming. (A) Peak force at 60° of flexion. (B) Peak force at 20° of extension. An asterisk denotes the rim resection level that first led to a statistically significant change as compared with the intact state; all rim resection levels beyond the asterisk were statistically significant in each given plot line. Bars represent SD. a well-defined threshold for rim resection based on biomechanical data. The primary results of this investigation demonstrate that at 60° of flexion, rim-trim depth was inversely correlated to the acetabular rim contact area, rim contact pressure, and rim peak force but positively correlated with the acetabular base contact area, base contact pressure, and base peak force. Additionally, at 60° of flexion and 6-mm resection, there was a statistically significant 300% increase in the acetabular base contact pressure and a statistically significant 165% increase in the base peak force. Based on these findings, it should be noted that excessive rim resection results in a higher contact area, contact pressure, and peak force of the articular surface at the acetabular base in hip flexion, which suggests a transfer of loads from the periphery to the base of the acetabulum. When observing the typical loading pattern of the intact state, the superolateral, anteroinferior, and posteroinferior aspects of the acetabular rim demonstrated an increased contact pressure relative to the rest of the acetabulum. As the rim was progressively resected, the contact pressure map changed from a 3-point rim loading pattern to an increased load at the base of the acetabulum when in flexion. Sequential rim trimming beyond 6 mm resulted in significantly higher contact pressures in the weightbearing portion of the acetabulum. The 3-fold increase in the contact pressure in the acetabulum with over 6 mm of rim resection quantifies the amount of load transfer to the remaining articular surface, and therefore, orthopaedic surgeons should be aware of the biomechanical effect of overresection.4,18 Perhaps the most interesting finding in this study was that an increase in contact pressures within the acetabulum was created with rim resection beyond 6 mm in hips that had a CEA of well over 30°. Unfortunately, as noted by others, CEA measurements after resection on cadaveric hip specimens are too inaccurate to measure radiographically because of proximal subluxation of the femur that occurs after only 3 mm of rim resection (the center point of the femoral head is necessary for measurement).6 However, previous authors of clinical studies have noted that the hip CEA decreases in the following manner with each
millimeter of rim resection (x): Decrease in CEA = 1.8 1 0.64x.21 Thus, based on our preprocedural CEA measurements, no hip specimen in this study had a postprocedural CEA of less than 23° even after 8 mm of rim resection. Our data support that excessive rim resection, more than 4 to 6 mm, should be cautioned against even in hips with preoperative CEAs greater than 25° because a dramatic increase in contact pressures could occur. Relying on preoperative and postoperative CEAs to guide rim trimming may inadvertently lead to increased contact pressures and peak forces if greater than 4 to 6 mm of the rim is removed, except in isolated situations of extreme acetabular protrusio or ossified labral lesions. Iatrogenic dysplasia is a concept that was described by Philippon et al21 as a potential danger with excessive rim resection. Similar to the pathomechanics that occur in congenitally dysplastic hip joints,22 it is theorized that iatrogenic dysplasia may predispose an otherwise nondysplastic person to accelerated chondral degeneration because of increased contact pressures within the femoroacetabular hip joint. In a study involving contact pressure profiles of 12 dysplastic patients who subsequently underwent periacetabular osteotomy, Armiger et al1 found that contact pressure profiles were highly associated with eventual outcomes of patients. Given the popularity of hip arthroscopic surgery, it is presently unclear what percentage of pincer resections performed in recent years has resulted in rim resections beyond 4 to 6 mm in the anterosuperior acetabulum, thus making it very difficult to clinically pinpoint earlier acetabular degeneration in these populations. In the short term, increases in femoroacetabular contact pressures after large rim resection may be clinically offset by various chondroprotective factors such as activity modification, labral repair, and femoral cam decompression. Nonetheless, there are reports of accelerated hip joint degeneration when performing large rim resection in patients who are highly sensitive to the development of iatrogenic dysplasia.16 Similar to how meniscectomy follow-up studies have shown earlier joint degeneration after 5 to 10 years in the knee,3 it is theorized that long-term studies in patients undergoing rim resection beyond 4 to 6 mm are
2144 Bhatia et al
The American Journal of Sports Medicine
Figure 6. Correlation between acetabular rim and acetabular base contact area, contact pressure, and peak force with sequential rim trimming. (A) Acetabular rim correlations with increasing rim-trim depth. (B) Acetabular base correlations with increasing rim-trim depth. At 60° of flexion, rim-trim depth was inversely correlated to the acetabular rim contact area (R = 20.769, P \ .001), rim contact pressure (R = 20.892, P \ .001), and rim peak force (R = 20.874, P \ .001) but positively correlated with the acetabular base contact area (R = 0.805, P \ .001), base contact pressure (R = 0.849, P \ .001), and base peak force (R = 0.834, P \ .001). At 20° of extension, rim-trim depth was inversely correlated to the acetabular rim contact area (R = 20.846, P \ .001), rim contact pressure (R = 20.732, P \ .001), and rim peak force (R = 20.838, P \ .001) but positively correlated with the acetabular base contact area (R = 0.362, P = .049). needed to fully appreciate the radiographic effects of increased contact pressures in the hip. In this study, excessive rim resection beyond 6 mm qualitatively resulted in subluxation of cadaveric hip specimens when they were not being statically loaded on the universal testing system. Colvin et al,6 in a cadaveric open rim resection study, noted that 70% of hip specimens with greater than 5 mm of rim resection had proximal subluxation of the femoral head affecting postresection CEA measurements. Matsuda and Khatod17 described a clinical case report of a 39-year-old woman who had an anterior hip dislocation in the recovery room after a labral debridement and rim resection procedure in which 4 to 7 mm of bone was removed. The patient underwent closed reduction and miniopen capsulorrhaphy immediately after the index procedure. Although hip dislocations after hip arthroscopic surgery are quite rare, likely attributed to the powerful dynamic stabilizers of the hip joint, excessive rim resection could contribute to significant microinstability of the hip joint, a condition marked by persistent pain and tightness of the hip musculature resulting from excessive spasm of the dynamic stabilizers. The current study also demonstrated that overresection of the acetabular rim caused a qualitative increase in the translation of the femoral head relative to the acetabulum. Although hip joint stability was not quantified in the present study, increased rim trimming does appear to affect the containment of the femoral head; therefore, the amount of rim resection does need to monitored. Previous authors have also evaluated the threshold at which iatrogenic hip dysplasia occurs, but unfortunately, many recommendations for overresection have been based solely on radiographic parameters, namely the CEA.6,15,20 Matsuda and Khatod17 described an elaborate method for fluoroscopically templating the desired amount of rim resection based on the
CEA and horizontal AP section of the distracted hip intraoperatively. Although this method helps provide an accurate assessment of the location of rim resection and CEA correction, it likely could lead to overresection if a large correction in the CEA is desired. In a retrospective review of 58 hips treated with acetabular rim trimming, Philippon et al21 recommended the use of preoperative templating based on the CEA for rim resection and stated that hips with high preoperative CEA measurements could require a large amount of rim resection. However, the authors also acknowledged that there are significant shortcomings in solely relying on the CEA measurement when determining the ideal amount of rim resection. The CEA, although a useful measurement for the assessment of hip dysplasia, often underestimates anterior acetabular overcoverage and may lead to errors in bone resection when used perioperatively to guide rim trimming.9,21 For this reason, Gross et al9 described using the anterior wall angle, anterior rim angle, and anterior margin ratio to provide a more comprehensive illustration of the acetabulum and to evaluate rim resection intraoperatively and postoperatively. Although this cadaveric investigation benefitted from an open and accurate testing design with an assessment of contact pressures at various areas of the acetabulum, there certainly are limitations. The specimens were selected based on the absence of dysplasia and osteoarthritis but may not necessarily reflect hips with a pincer deformity. Because of the imprecise definition of the pincer pathologic structure, the study design focused on the relative effect of rim resection of the cadaveric specimens. Only 2 flexion angles were assessed, making it impossible to truly measure the effect of sequential rim resection in other planes of femoroacetabular motion. Additionally, the labrum had to be removed for accurate and precise rim resection and to ensure consistency among specimens.
Vol. 43, No. 9, 2015
Effects of Acetabular Rim Trimming on Hip Contact Pressures
Labral tissue has been noted to be imperative for maintaining the labral suction seal of the hip joint and plays a significant role with regard to hip stability.8 Moreover, the static loading study design employed in this investigation could not include the effects of dynamic stabilizers in the hip, which also play an important role in the loading profile and stability of the femoroacetabular joint. Furthermore, the Tekscan sensors provided reproducible results, although they also have some limitations. First, although they are relatively thin and compliant, the addition of sensors within a joint may affect how contact pressures are distributed. In addition, because the sensors are not custom fit to each hip, some wrinkling did occur. However, little evidence of this was seen in the pressure maps, and because of the nature of the repeated test design, we believe these limitations to have a minimal effect. Lastly, the effects of iatrogenic hip instability were not quantified but would provide additional data to support the effect of hip stability.
CONCLUSION Overresection of the acetabular rim of more than 4 to 6 mm of bone increased hip joint contact pressures by 3-fold and changed the contact pressure profile from a 3-point rim loading pattern to increased loading at the base of the acetabulum. The authors caution that excessive rim resection may cause an increased contact area, contact pressure, and peak force in the acetabulum and may predispose patients to early hip degeneration. Further studies to characterize pincer deformities need to be conducted to provide a more accurate assessment of pathologic structural abnormalities and ultimately guide treatment. REFERENCES 1. Armiger RS, Armand M, Tallroth K, Lepisto¨ J, Mears SC. Threedimensional mechanical evaluation of joint contact pressure in 12 periacetabular osteotomy patients with 10-year follow-up. Acta Orthop. 2009;80(2):155-161. 2. Bachus KN, DeMarco AL, Judd KT, Horwitz DS, Brodke DS. Measuring contact area, force, and pressure for bioengineering applications: using Fuji Film and TekScan systems. Med Eng Phys. 2006;28(5):483-488. 3. Bhatia S, Laprade CM, Ellman MB, Laprade RF. Meniscal root tears: significance, diagnosis, and treatment. Am J Sports Med. 2014; 42(12):3016-3030. 4. Byrd JWT, Jones KS. Hip arthroscopy in the presence of dysplasia. Arthroscopy. 2003;19(10):1055-1060. 5. Clohisy JC, Kim Y-J. Femoroacetabular impingement research symposium. J Am Acad Orthop Surg. 2013;21 Suppl 1:vi-viii. 6. Colvin AC, Koehler SM, Bird J. Can the change in center-edge angle during pincer trimming be reliably predicted? Clin Orthop Relat Res. 2011;469(4):1071-1074. 7. Ganz R, Parvizi J, Beck M, Leunig M, No¨tzli H, Siebenrock KA. Femoroacetabular impingement: a cause for osteoarthritis of the hip. Clin Orthop Relat Res. 2003;417:112-120.
2145
8. Gomberawalla MM, Kelly BT, Bedi A. Interventions for hip pain in the maturing athlete: the role of hip arthroscopy? Sports Health. 2013;6(1):70-77. 9. Gross CE, Salata MJ, Manno K, et al. New radiographic parameters to describe anterior acetabular rim trimming during hip arthroscopy. Arthroscopy. 2012;28(10):1404-1409. 10. Harris MD, Anderson AE, Henak CR, Ellis BJ, Peters CL, Weiss JA. Finite element prediction of cartilage contact stresses in normal human hips. J Orthop Res. 2012;30(7):1133-1139. 11. Konrath GA, Hamel AJ, Sharkey NA, Bay BK, Olson SA. Biomechanical consequences of anterior column fracture of the acetabulum. J Orthop Trauma. 1998;12(8):547-552. 12. Krebs V, Incavo SJ, Shields WH. The anatomy of the acetabulum: what is normal? Clin Orthop Relat Res. 2009;467(4):868-875. 13. Lee S, Wuerz TH, Shewman E, et al. Labral reconstruction with iliotibial band autografts and semitendinosus allografts improves hip joint contact area and contact pressure: an in vitro analysis. Am J Sports Med. 2014;43(1):98-104. 14. Levine RG, Renard R, Behrens FF, Tornetta P. Biomechanical consequences of secondary congruence after both-column acetabular fracture. J Orthop Trauma. 2002;16(2):87-91. 15. Matsuda DK. Acute iatrogenic dislocation following hip impingement arthroscopic surgery. Arthroscopy. 2009;25(4):400-404. 16. Matsuda DK. Fluoroscopic templating technique for precision arthroscopic rim trimming. Arthroscopy. 2009;25(10):1175-1182. 17. Matsuda DK, Khatod M. Rapidly progressive osteoarthritis after arthroscopic labral repair in patients with hip dysplasia. Arthroscopy. 2012;28(11):1738-1743. 18. Noguchi Y, Miura H, Takasugi S-I, Iwamoto Y. Cartilage and labrum degeneration in the dysplastic hip generally originates in the anterosuperior weight-bearing area: an arthroscopic observation. Arthroscopy. 1999;15(5):496-506. 19. Philippon MJ, Briggs KK, Yen Y-M, Kuppersmith DA. Outcomes following hip arthroscopy for femoroacetabular impingement with associated chondrolabral dysfunction: minimum two-year follow-up. J Bone Joint Surg Br. 2009;91(1):16-23. 20. Philippon MJ, Stubbs AJ, Schenker ML, Maxwell RB, Ganz R, Leunig M. Arthroscopic management of femoroacetabular impingement: osteoplasty technique and literature review. Am J Sports Med. 2007;35(9):1571-1580. 21. Philippon MJ, Wolff AB, Briggs KK, Zehms CT, Kuppersmith DA. Acetabular rim reduction for the treatment of femoroacetabular impingement correlates with preoperative and postoperative centeredge angle. Arthroscopy. 2010;26(6):757-761. 22. Reijman M, Hazes J, Pols H. Acetabular dysplasia predicts incident osteoarthritis of the hip: the Rotterdam study. Arthritis Rheum. 2005;52(3):787-793. 23. Richards PJ, Pattison JM, Belcher J, Decann RW, Anderson S, Wynn-Jones C. A new tilt on pelvic radiographs: a pilot study. Skeletal Radiol. 2009;38(2):113-122. 24. Sankar WN, Nevitt M, Parvizi J, Felson DT, Agricola R, Leunig M. Femoroacetabular impingement: defining the condition and its role in the pathophysiology of osteoarthritis. J Am Acad Orthop Surg. 2013;21 Suppl 1:S7-S15. 25. Smart LR, Oetgen M, Noonan B, Medvecky M. Beginning hip arthroscopy: indications, positioning, portals, basic techniques, and complications. Arthroscopy. 2007;23(12):1348-1353. 26. Tibor LM, Sekiya JK. Differential diagnosis of pain around the hip joint. Arthroscopy. 2008;24(12):1407-1421. 27. Wirz D, Becker R, Li SF, Friederich NF, Mu¨ller W. [Validation of the Tekscan system for statistic and dynamic pressure measurements of the human femorotibial joint]. Biomed Tech (Berl). 2002;47(78):195-201.
For reprints and permission queries, please visit SAGE’s Web site at http://www.sagepub.com/journalsPermissions.nav
5-in-5
How Much Is Too Much? Sanjeev Bhatia,*y MD, Simon Lee,z MD, MPH, Elizabeth Shewman,z MS, Richard C. Mather III,§ MD, Michael J. Salata,|| MD, Charles A. Bush-Joseph,z MD, and Shane J. Nho,z MD, MS Investigation performed at Rush University Medical Center, Chicago, Illinois, USA Background: In patients with femoroacetabular impingement (FAI), acetabular rim trimming removes the offending area of the acetabular deformity in patients with pincer-type and mixed-type FAI to improve hip joint kinematics. Although the rationale for arthroscopic acetabular rim trimming in patients with FAI is well established, the amount of rim resection has not been quantified, and the threshold at which excessive rim resection results in abnormal hip contact pressures has not been described. Purpose: To investigate the changes in contact areas, contact pressures, and peak forces within the hip joint with sequential acetabular rim trimming. Study Design: Controlled laboratory study. Methods: Six fresh-frozen, nondysplastic, human cadaveric hemipelvises were analyzed utilizing thin-film piezoresistive load sensors to measure the contact area, contact pressure, and peak force after anterosuperior acetabular rim trimming at depths of 0 mm (intact), 2 mm, 4 mm, 6 mm, and 8 mm. Each specimen was examined at 20° of extension and 60° of flexion. Analysis was performed on 2 regions of interest: the acetabular rim and the acetabular base (deep part of the acetabulum). After each experimental condition, the acetabulum was normalized with respect to the intact state to account for specimen variability. Statistical analysis was conducted through 1-way analysis of variance with post hoc Games-Howell tests. Results: At the acetabular base, there were significant increases in the contact area after 4-mm resection (60°: 169.12% 6 30.64%; P = .0138), contact pressure after 6-mm resection (60°: 292.76% 6 79.07%; P = .009), and peak force after 6-mm resection (60°: 166.00% 6 34.40%; P = .027). At the acetabular rim, there were significant decreases in the contact area after 6-mm resection (60°: 66.32% 6 18.80%; P = .0354) (20°: 65.47% 6 15.87%; P = .0127), contact pressure after 6-mm resection (60°: 50.77% 6 11.49%; P \ .001) (20°: 58.01% 6 23.10%; P = .0335), and peak force after 6-mm resection (60°: 60.67% 6 9.29%; P \ .001) (20°: 74.44% 6 9.84%; P = .007). Conclusion: Resecting more than 4 to 6 mm of the acetabular rim during hip arthroscopic surgery to address a pincer deformity may dramatically increase contact pressures by 3-fold at the acetabular base. The study suggests that excessive rim resection may lead to increased loads in the hip joint and may predispose to premature joint degeneration. Clinical Relevance: Resecting more than 4 to 6 mm of the acetabular rim may significantly alter hip joint biomechanics, increasing joint reactive forces and subsequent chondrolabral degeneration. Keywords: hip; femoroacetabular impingement; pincer deformity; hip arthroscopic surgery; biomechanics; rim trimming
The number of hip arthroscopic procedures has grown dramatically, owing to an improved understanding of femoroacetabular impingement (FAI) as well as other sources of intra-articular and extra-articular hip pain.8,19,25,26 During the treatment of FAI with arthroscopic approaches, acetabular rim trimming may be performed, in conjunction with other procedures, to help alleviate the pathomechanics
associated with pincer-type and mixed pincer-cam–type FAI. First described by Ganz and colleagues,7 pincer FAI occurs when repetitive contact is made between the acetabular rim and the femoral head-neck junction, leading to labral tears, chondral injuries, and potentially osteoarthritis.5,24 Arthroscopic rim trimming is performed to address acetabular overcoverage and oftentimes performed with osteochondroplasty of the proximal femur to allow for hip motion without impingement.19,21,24 Numerous studies have reported on the improvement of hip pain and hip function after arthroscopic treatment of focal acetabular overcoverage, global acetabular overcoverage, and mixed-
The American Journal of Sports Medicine, Vol. 43, No. 9 DOI: 10.1177/0363546515590400 Ó 2015 The Author(s)
2138
Vol. 43, No. 9, 2015
Effects of Acetabular Rim Trimming on Hip Contact Pressures
type FAI.19,20 Additionally, rim trimming allows for a bleeding bed of cancellous bone to facilitate healing and also results in protection of the repaired labrum from repetitive abutment with the proximal femur.21 Although the benefits of acetabular rim trimming have been well described, the ideal amount of rim resection has not yet been established. Previous authors have described resection of anywhere from 1 to 9 mm of bone from the acetabular rim, likely varying depending on the size and location of the acetabular deformity.17,20,21 Excessive resection of the acetabulum can lead to increased load transfer to the chondrolabral surface and even iatrogenic hip instability.10,17 Some authors have recommended that the lateral center-edge angle of Wiberg (CEA) should be greater than 20° to 25° after acetabular rim trimming.17,21 Philippon and colleagues21 correlated the amount of rim resection with a change in the CEA on anteroposterior (AP) radiographs. These investigators determined that 1 mm of bone resection resulted in a 2.4° decrease of the CEA and that each additional millimeter of bone resection results in a 0.6° decrease of the CEA. Although the CEA is a useful guide, there are several limitations with the use of CEA alone to quantify rim resection. The CEA can only measure the superolateral aspect of the acetabulum and cannot detect changes in the remainder of the anterior wall, which is commonly altered with arthroscopic rim trimming. Moreover, the AP pelvis view does not account for pelvic tilt and rotation, which can affect the CEA and the presence of a crossover sign.23 The purpose of this study was to evaluate changes in contact pressures, contact areas, and peak pressures within the hip joint with sequential acetabular rim trimming. Our hypothesis was that a threshold for rim resection exists, after which the benefits of rim resection are outweighed by the harmful increases in the contact pressure profile of the hip joint.
METHODS Six fresh-frozen, unpaired, human cadaveric hemipelvises were used for biomechanical testing. The sample size of this biomechanical cadaveric study was determined based on the availability of specimens and on prior experience with cadaveric models using thin-film piezoelectric pressure sensors. A post hoc analysis of collected data demonstrated a minimal statistical power of 0.989 for statistically significant results, providing evidence of a sufficient sample size in this study. Specimens were included only if they had
2139
a CEA greater than 25° and To¨nnis grade of 0 or 1. Additionally, specimens with damage due to the procurement process, a severe joint deformity, advanced osteoarthritis (To¨nnis grade .1), prior surgery, previous hardware placement, or previous fractures were also excluded. The mean age of the specimens was 57.5 6 5.41 years (range, 46-62 years). Each hemipelvis came from a separate cadaveric specimen to allow for increased variability. There were 3 left and 3 right hemipelvises. The mean CEA as measured on computed tomography was 35.7° 6 3.2° (range, 29.8°38.8°). Each specimen was maintained in a freezer at 220°C until approximately 24 hours before use and then thawed to room temperature for testing.
Specimen Preparation After tissues were appropriately thawed to room temperature, all extracapsular soft tissues were dissected from each hemipelvis. The proximal femur and acetabular portion of the pelvis were each potted separately and stabilized using polymethyl methacrylate (Isocryl; Lang Dental Manufacturing Co). Both the proximal femur and acetabulum were placed in custom fixtures that were rigidly mounted on a test platform of a materials testing system (MTS Insight 5; MTS Systems Corp) with a positional resolution of 60.001 mm, positional accuracy of 60.01 mm, and speed accuracy (% of set speed) of 60.005%. A 5-kN load cell was used (Model 569329-03; MTS Systems Corp), which was calibrated with a nonlinearity of 20.013% full scale and hysteresis of 20.002% full scale (Figure 1). The capsule was then incised and removed, followed by separation of the ligamentum teres from the base of the acetabular floor and the femoral head. Reserved synovial fluid from the joint space was mixed with normal saline for constant lubrication of the joint space during testing. The articular surface was hydrated immediately before and after each test. Given that some cadaveric specimens had occult anterosuperior labral tears present as well as variable turgor of labral tissue, the entire labrum was then excised to minimize variability between specimens and to better analyze the true effect of bony acetabular rim trimming. Ossified labral pathology was not noted in any specimen. To maintain the consistency of placement for each specimen, a set of standardized angles in relation to readily identifiable pelvic landmarks were developed. A vertical acetabular angle of 40° was determined by measuring the angle between a completely vertical line and a line through the 6- and 12-o’clock positions at the labrum’s edge when viewed laterally (Figure 1). This measurement was based
*Address correspondence to Sanjeev Bhatia, MD, Center for Hip Preservation, Cincinnati Sports Medicine and Orthopaedic Center, Mercy Health, 10663 Montgomery Road, Cincinnati, OH 45242, USA (email: [email protected]). y Center for Hip Arthroscopy and Joint Preservation, Cincinnati Sports Medicine and Orthopaedic Center, Mercy Health, Cincinnati, Ohio, USA. z Hip Preservation Center, Division of Sports Medicine, Department of Orthopedic Surgery, Rush University Medical Center, Rush Medical College of Rush University, Chicago, Illinois, USA. § Division of Sports Medicine, Department of Orthopaedic Surgery, Duke University School of Medicine, Durham, North Carolina, USA. || Division of Sports Medicine, Department of Orthopaedic Surgery, Case Western Reserve University, Cleveland, Ohio, USA. Presented at the interim meeting of the AOSSM, Las Vegas, Nevada, March 2015. One or more of the authors has declared the following potential conflict of interest or source of funding: S.J.N. is a paid consultant for Ossur and Stryker and receives research support from Allosource, Arthrex, Athletico, DJ Orthopaedics, Linvatec, Miomed, Smith & Nephew, and Stryker. M.J.S. is a paid consultant for Linvatec and Smith & Nephew. R.C.M. is a paid consultant for KNG Health Consulting, Pivot Medical, Smith & Nephew, and Stryker.
2140 Bhatia et al
The American Journal of Sports Medicine
111.8 3 111.8–mm area matrix with a sensel density of 15.5 sensels/cm2 (total number of sensels = 1936) and was precalibrated on the MTS machine. A 2-point calibration was performed per the manufacturer’s guidelines, applying 2 separate compressive loads across a custom jig. The jig consisted of a piece of dense foam sandwiched between 2 steel plates in the shape of the sensor, and it was manufactured to mimic contact pressures of a human joint with cartilage. The calibration points were obtained by applying loads at 10 N/s across the jig and sensor, resulting in contact pressures of 20% and 80% of the maximum test pressure created from the 700-N load across the hip joint. Each load was held as the calibration data were acquired using I-Scan software (v 5.83; Tekscan), and the sensor was allowed to rest for 5 minutes before applying the second calibration load. After calibration, each sensor was then applied within the femoroacetabular joint, with a load of 700 N (approximately simulating three-fourths the body weight) subsequently placed onto the joint for 30 seconds. This resulted in real-time pressure and area map measurements (Figure 2). Specimens were examined at 20° of extension and 60° of flexion, both in neutral position, to simulate the complete arc of the natural hip range of motion during normal gait on a level surface and during stair climbing. Measurements were repeated 3 times for each condition on each specimen. These values were then averaged for utilization in the final analysis.
Sequential Acetabular Rim Trimming
Figure 1. To maintain the consistency of placement for each specimen, a set of standardized angles in relation to readily identifiable pelvic landmarks were developed based on prior work.12 A vertical acetabular angle of 40° was determined by measuring the angle between a completely vertical line and a line through the 6- and 12-o’clock positions at the labrum’s edge when viewed laterally. A pubic-femoral neck angle of 140° was then set to establish standardized neutral positioning of the hip joint, measuring the angle between the extension of the pubic bone from the acetabulum and the vector in line with the femoral neck. on an anatomic study by Krebs et al12 that demonstrated the average acetabular abduction angle to be 39.8°. A pubic-femoral neck angle of 140° was then established, measuring the angle between the extension of the pubic bone from the acetabulum and the vector in line with the femoral neck. This angle was employed to establish standardized neutral positioning of the hip joint. All specimens were prepared and positioned in a similar fashion.
Contact Area and Pressure Measurement A dynamic contact pressure and area mapping system that utilized thin-film piezoresistive load sensors (Model 5101; Tekscan) was used based on the validated work of previous authors.2,11,13,14,27 Each sensor consisted of a
Five testing conditions were performed on each specimen: intact hip and 2-mm, 4-mm, 6-mm, and 8-mm rim resections. Although acetabular rim trimming is typically performed as an arthroscopic procedure, the current biomechanical model utilized an open technique.6 For each acetabular rim trim, a handheld rotary tool with a bur attachment (Dremel; Bosch North America) was used to perform each rim resection in a region of the acetabulum clinically relevant to a pincer deformity and arthroscopic pincer resection: from the 12- to 3-o’clock position. For each acetabular resection, a digital caliper was utilized to precisely indicate the appropriate rim-trim depth at several points along the region of resection to ensure consistency.
Data Acquisition and Statistical Analysis Contact area (cm2), contact pressure (kg/cm2), and peak force (N) were recorded at each acetabular resection depth. These measures are reported as a percentage of the native acetabular state. The resected states were then normalized with respect to their own intact state to control for specimen variability. To differentiate acetabular base and rim measurements, area of interest polygons were created to allow for the discrete analysis of each area. The Tekscan software contains a function that enables the user to delineate specific regions of the recorded pressure map by drawing polygons, outputting area, pressure, and force results from that region only. A best-fit perfect circle polygon was first created from the pressure map recorded from
Vol. 43, No. 9, 2015
Effects of Acetabular Rim Trimming on Hip Contact Pressures
2141
each specimen’s intact acetabulum. Next, a circle covering 70% of the original polygon’s area was created and positioned in the center of the original polygon to isolate contact areas, contact pressures, and peak pressures from the acetabular base, or deepest area of the acetabulum. The original polygon was then redrawn as a ring to isolate contact areas, contact pressures, and peak pressures representative of the acetabular rim. Once the polygon was created for each intact specimen, it was used for analysis of the subsequent trimmed states of that particular specimen. Statistical analysis was conducted through Pearson correlations and 1-way between-group analysis of variance (ANOVA) on normalized data with subsequent post hoc Games-Howell tests to evaluate the nature of the differences between groups (SPSS Statistics v 21.0; IBM Corp). The Games-Howell test was selected for post hoc analysis based on the Levene test for homogeneity of variances. All reported P values were 2-tailed, with an a level of .05 detecting significant differences.
RESULTS Gross Observations Qualitatively, there appeared to be a marked increase in the ease of subluxation of all hip specimens with greater than 6 to 8 mm of rim resection with manual femoroacetabular manipulation. Fortunately, because of the static loading design of the study, the reduced static constraints of the acetabulum did not affect the stability of the femoroacetabular joint when it was being loaded on the universal testing system. Visually, there was also an increase in the contact pressure in the deeper part of the acetabulum with sequential rim trimming on contact pressure mapping (Figure 2).
Contact Area Measurements Significant differences in the contact area between rim-trim depths were detected after ANOVA for the acetabular rim (20°: P \ .001; 60°: P \ .001) as well as the acetabular base at 60° (P \ .001). There was a significant decrease in the acetabular rim contact area after 6-mm resection at both 60° of flexion (66.32% 6 18.80%; P = .0354) and 20° of extension (65.47% 6 15.87%; P = .0127). With regard to the acetabular base, there was a significant increase in the contact area after 4-mm resection at 60° of flexion (169.12% 6 30.64%; P = .0138) (Figure 3; also see the Appendix, available online at http://ajsm.sagepub.com/supplemental).
Contact Pressure Measurements Figure 2. Qualitative observations of contact area and pressure maps after sequential acetabular rim resection from the 12- to 3-o’clock position. (A) Intact state (0-mm rim resection), (B) 2-mm rim resection, (C) 4-mm rim resection, (D) 6-mm rim resection, and (E) 8-mm rim resection. Note the gradual increase in the contact pressure in the central circle (acetabular base) with sequential rim trimming.
Significant differences in the contact pressure between rimtrim depths were detected after ANOVA for the acetabular rim (20°: P \ .001; 60°: P \ .001) as well as the acetabular base at 60° (P \ .001). There was a significant decrease in the acetabular rim contact pressure after 6-mm resection at both 60° of flexion (50.77% 6 11.49%; P \ .001) and 20° of extension (58.01% 6 23.10%; P = .0335). The acetabular base, however, had a significant increase in the contact
2142 Bhatia et al
The American Journal of Sports Medicine
Figure 3. Normalized changes in the contact area of the acetabular rim and acetabular base with sequential rim trimming. (A) Contact area with 60° of flexion. (B) Contact area with 20° of extension. An asterisk denotes the rim resection level that first led to a statistically significant change as compared with the intact state; all rim resection levels beyond the asterisk were statistically significant in each given plot line. Bars represent SD.
Figure 4. Normalized changes in the contact pressure of the acetabular rim and acetabular base with sequential rim trimming. (A) Contact pressure with 60° of flexion. (B) Contact pressure with 20° of extension. An asterisk denotes the rim resection level that first led to a statistically significant change as compared with the intact state; all rim resection levels beyond the asterisk were statistically significant in each given plot line. Bars represent SD. pressure after 6-mm resection at 60° of flexion (292.76% 6 79.07%; P = .009) (Figure 4).
Peak Force Measurements Significant differences in the peak force between rim-trim depths were detected after ANOVA for the acetabular rim (20°: P \ .001; 60°: P \ .001) as well as the acetabular base at 60° (P \ .001). There was a significant decrease in the acetabular rim peak force after 6-mm resection at 60° of flexion (60.67% 6 9.29%; P \ .001) and 20° of extension (74.44% 6 9.84%; P = .007). As for the acetabular base, there was a significant increase in the peak force after 6-mm resection at 60° of flexion (166.00% 6 34.40%; P = .027) (Figure 5; also see the Appendix).
P \ .001). At 20° of extension, rim-trim depth was significantly correlated in an inverse fashion to the acetabular rim contact area (R = 20.846, P \ .001), rim contact pressure (R = 20.732, P \ .001), and rim peak force (R = 20.838, P \ .001) (Figure 6).
Rim-Trim Depth Versus Acetabular Base Variables At 60° of flexion, rim-trim depth was significantly correlated to the acetabular base contact area (R = 0.805, P \ .001), base contact pressure (R = 0.849, P \ .001), and base peak force (R = 0.834, P \ .001). At 20° of extension, rim-trim depth was significantly correlated to the acetabular base contact area (R = 0.362, P = .049) (Figure 6).
Rim-Trim Depth Versus Acetabular Rim Variables
DISCUSSION
At 60° of flexion, rim-trim depth was significantly correlated in an inverse fashion to the acetabular rim contact area (R = 20.769, P \ .001), rim contact pressure (R = 20.892, P \ .001), and rim peak force (R = 20.874,
Although pincer resection is commonly performed in hip arthroscopic surgery, the effect of excessive pincer resection on contact pressures within the hip joint has never previously been studied. No prior studies have described
Vol. 43, No. 9, 2015
Effects of Acetabular Rim Trimming on Hip Contact Pressures
2143
Figure 5. Normalized changes in the peak force of the acetabular rim and acetabular base with sequential rim trimming. (A) Peak force at 60° of flexion. (B) Peak force at 20° of extension. An asterisk denotes the rim resection level that first led to a statistically significant change as compared with the intact state; all rim resection levels beyond the asterisk were statistically significant in each given plot line. Bars represent SD. a well-defined threshold for rim resection based on biomechanical data. The primary results of this investigation demonstrate that at 60° of flexion, rim-trim depth was inversely correlated to the acetabular rim contact area, rim contact pressure, and rim peak force but positively correlated with the acetabular base contact area, base contact pressure, and base peak force. Additionally, at 60° of flexion and 6-mm resection, there was a statistically significant 300% increase in the acetabular base contact pressure and a statistically significant 165% increase in the base peak force. Based on these findings, it should be noted that excessive rim resection results in a higher contact area, contact pressure, and peak force of the articular surface at the acetabular base in hip flexion, which suggests a transfer of loads from the periphery to the base of the acetabulum. When observing the typical loading pattern of the intact state, the superolateral, anteroinferior, and posteroinferior aspects of the acetabular rim demonstrated an increased contact pressure relative to the rest of the acetabulum. As the rim was progressively resected, the contact pressure map changed from a 3-point rim loading pattern to an increased load at the base of the acetabulum when in flexion. Sequential rim trimming beyond 6 mm resulted in significantly higher contact pressures in the weightbearing portion of the acetabulum. The 3-fold increase in the contact pressure in the acetabulum with over 6 mm of rim resection quantifies the amount of load transfer to the remaining articular surface, and therefore, orthopaedic surgeons should be aware of the biomechanical effect of overresection.4,18 Perhaps the most interesting finding in this study was that an increase in contact pressures within the acetabulum was created with rim resection beyond 6 mm in hips that had a CEA of well over 30°. Unfortunately, as noted by others, CEA measurements after resection on cadaveric hip specimens are too inaccurate to measure radiographically because of proximal subluxation of the femur that occurs after only 3 mm of rim resection (the center point of the femoral head is necessary for measurement).6 However, previous authors of clinical studies have noted that the hip CEA decreases in the following manner with each
millimeter of rim resection (x): Decrease in CEA = 1.8 1 0.64x.21 Thus, based on our preprocedural CEA measurements, no hip specimen in this study had a postprocedural CEA of less than 23° even after 8 mm of rim resection. Our data support that excessive rim resection, more than 4 to 6 mm, should be cautioned against even in hips with preoperative CEAs greater than 25° because a dramatic increase in contact pressures could occur. Relying on preoperative and postoperative CEAs to guide rim trimming may inadvertently lead to increased contact pressures and peak forces if greater than 4 to 6 mm of the rim is removed, except in isolated situations of extreme acetabular protrusio or ossified labral lesions. Iatrogenic dysplasia is a concept that was described by Philippon et al21 as a potential danger with excessive rim resection. Similar to the pathomechanics that occur in congenitally dysplastic hip joints,22 it is theorized that iatrogenic dysplasia may predispose an otherwise nondysplastic person to accelerated chondral degeneration because of increased contact pressures within the femoroacetabular hip joint. In a study involving contact pressure profiles of 12 dysplastic patients who subsequently underwent periacetabular osteotomy, Armiger et al1 found that contact pressure profiles were highly associated with eventual outcomes of patients. Given the popularity of hip arthroscopic surgery, it is presently unclear what percentage of pincer resections performed in recent years has resulted in rim resections beyond 4 to 6 mm in the anterosuperior acetabulum, thus making it very difficult to clinically pinpoint earlier acetabular degeneration in these populations. In the short term, increases in femoroacetabular contact pressures after large rim resection may be clinically offset by various chondroprotective factors such as activity modification, labral repair, and femoral cam decompression. Nonetheless, there are reports of accelerated hip joint degeneration when performing large rim resection in patients who are highly sensitive to the development of iatrogenic dysplasia.16 Similar to how meniscectomy follow-up studies have shown earlier joint degeneration after 5 to 10 years in the knee,3 it is theorized that long-term studies in patients undergoing rim resection beyond 4 to 6 mm are
2144 Bhatia et al
The American Journal of Sports Medicine
Figure 6. Correlation between acetabular rim and acetabular base contact area, contact pressure, and peak force with sequential rim trimming. (A) Acetabular rim correlations with increasing rim-trim depth. (B) Acetabular base correlations with increasing rim-trim depth. At 60° of flexion, rim-trim depth was inversely correlated to the acetabular rim contact area (R = 20.769, P \ .001), rim contact pressure (R = 20.892, P \ .001), and rim peak force (R = 20.874, P \ .001) but positively correlated with the acetabular base contact area (R = 0.805, P \ .001), base contact pressure (R = 0.849, P \ .001), and base peak force (R = 0.834, P \ .001). At 20° of extension, rim-trim depth was inversely correlated to the acetabular rim contact area (R = 20.846, P \ .001), rim contact pressure (R = 20.732, P \ .001), and rim peak force (R = 20.838, P \ .001) but positively correlated with the acetabular base contact area (R = 0.362, P = .049). needed to fully appreciate the radiographic effects of increased contact pressures in the hip. In this study, excessive rim resection beyond 6 mm qualitatively resulted in subluxation of cadaveric hip specimens when they were not being statically loaded on the universal testing system. Colvin et al,6 in a cadaveric open rim resection study, noted that 70% of hip specimens with greater than 5 mm of rim resection had proximal subluxation of the femoral head affecting postresection CEA measurements. Matsuda and Khatod17 described a clinical case report of a 39-year-old woman who had an anterior hip dislocation in the recovery room after a labral debridement and rim resection procedure in which 4 to 7 mm of bone was removed. The patient underwent closed reduction and miniopen capsulorrhaphy immediately after the index procedure. Although hip dislocations after hip arthroscopic surgery are quite rare, likely attributed to the powerful dynamic stabilizers of the hip joint, excessive rim resection could contribute to significant microinstability of the hip joint, a condition marked by persistent pain and tightness of the hip musculature resulting from excessive spasm of the dynamic stabilizers. The current study also demonstrated that overresection of the acetabular rim caused a qualitative increase in the translation of the femoral head relative to the acetabulum. Although hip joint stability was not quantified in the present study, increased rim trimming does appear to affect the containment of the femoral head; therefore, the amount of rim resection does need to monitored. Previous authors have also evaluated the threshold at which iatrogenic hip dysplasia occurs, but unfortunately, many recommendations for overresection have been based solely on radiographic parameters, namely the CEA.6,15,20 Matsuda and Khatod17 described an elaborate method for fluoroscopically templating the desired amount of rim resection based on the
CEA and horizontal AP section of the distracted hip intraoperatively. Although this method helps provide an accurate assessment of the location of rim resection and CEA correction, it likely could lead to overresection if a large correction in the CEA is desired. In a retrospective review of 58 hips treated with acetabular rim trimming, Philippon et al21 recommended the use of preoperative templating based on the CEA for rim resection and stated that hips with high preoperative CEA measurements could require a large amount of rim resection. However, the authors also acknowledged that there are significant shortcomings in solely relying on the CEA measurement when determining the ideal amount of rim resection. The CEA, although a useful measurement for the assessment of hip dysplasia, often underestimates anterior acetabular overcoverage and may lead to errors in bone resection when used perioperatively to guide rim trimming.9,21 For this reason, Gross et al9 described using the anterior wall angle, anterior rim angle, and anterior margin ratio to provide a more comprehensive illustration of the acetabulum and to evaluate rim resection intraoperatively and postoperatively. Although this cadaveric investigation benefitted from an open and accurate testing design with an assessment of contact pressures at various areas of the acetabulum, there certainly are limitations. The specimens were selected based on the absence of dysplasia and osteoarthritis but may not necessarily reflect hips with a pincer deformity. Because of the imprecise definition of the pincer pathologic structure, the study design focused on the relative effect of rim resection of the cadaveric specimens. Only 2 flexion angles were assessed, making it impossible to truly measure the effect of sequential rim resection in other planes of femoroacetabular motion. Additionally, the labrum had to be removed for accurate and precise rim resection and to ensure consistency among specimens.
Vol. 43, No. 9, 2015
Effects of Acetabular Rim Trimming on Hip Contact Pressures
Labral tissue has been noted to be imperative for maintaining the labral suction seal of the hip joint and plays a significant role with regard to hip stability.8 Moreover, the static loading study design employed in this investigation could not include the effects of dynamic stabilizers in the hip, which also play an important role in the loading profile and stability of the femoroacetabular joint. Furthermore, the Tekscan sensors provided reproducible results, although they also have some limitations. First, although they are relatively thin and compliant, the addition of sensors within a joint may affect how contact pressures are distributed. In addition, because the sensors are not custom fit to each hip, some wrinkling did occur. However, little evidence of this was seen in the pressure maps, and because of the nature of the repeated test design, we believe these limitations to have a minimal effect. Lastly, the effects of iatrogenic hip instability were not quantified but would provide additional data to support the effect of hip stability.
CONCLUSION Overresection of the acetabular rim of more than 4 to 6 mm of bone increased hip joint contact pressures by 3-fold and changed the contact pressure profile from a 3-point rim loading pattern to increased loading at the base of the acetabulum. The authors caution that excessive rim resection may cause an increased contact area, contact pressure, and peak force in the acetabulum and may predispose patients to early hip degeneration. Further studies to characterize pincer deformities need to be conducted to provide a more accurate assessment of pathologic structural abnormalities and ultimately guide treatment. REFERENCES 1. Armiger RS, Armand M, Tallroth K, Lepisto¨ J, Mears SC. Threedimensional mechanical evaluation of joint contact pressure in 12 periacetabular osteotomy patients with 10-year follow-up. Acta Orthop. 2009;80(2):155-161. 2. Bachus KN, DeMarco AL, Judd KT, Horwitz DS, Brodke DS. Measuring contact area, force, and pressure for bioengineering applications: using Fuji Film and TekScan systems. Med Eng Phys. 2006;28(5):483-488. 3. Bhatia S, Laprade CM, Ellman MB, Laprade RF. Meniscal root tears: significance, diagnosis, and treatment. Am J Sports Med. 2014; 42(12):3016-3030. 4. Byrd JWT, Jones KS. Hip arthroscopy in the presence of dysplasia. Arthroscopy. 2003;19(10):1055-1060. 5. Clohisy JC, Kim Y-J. Femoroacetabular impingement research symposium. J Am Acad Orthop Surg. 2013;21 Suppl 1:vi-viii. 6. Colvin AC, Koehler SM, Bird J. Can the change in center-edge angle during pincer trimming be reliably predicted? Clin Orthop Relat Res. 2011;469(4):1071-1074. 7. Ganz R, Parvizi J, Beck M, Leunig M, No¨tzli H, Siebenrock KA. Femoroacetabular impingement: a cause for osteoarthritis of the hip. Clin Orthop Relat Res. 2003;417:112-120.
2145
8. Gomberawalla MM, Kelly BT, Bedi A. Interventions for hip pain in the maturing athlete: the role of hip arthroscopy? Sports Health. 2013;6(1):70-77. 9. Gross CE, Salata MJ, Manno K, et al. New radiographic parameters to describe anterior acetabular rim trimming during hip arthroscopy. Arthroscopy. 2012;28(10):1404-1409. 10. Harris MD, Anderson AE, Henak CR, Ellis BJ, Peters CL, Weiss JA. Finite element prediction of cartilage contact stresses in normal human hips. J Orthop Res. 2012;30(7):1133-1139. 11. Konrath GA, Hamel AJ, Sharkey NA, Bay BK, Olson SA. Biomechanical consequences of anterior column fracture of the acetabulum. J Orthop Trauma. 1998;12(8):547-552. 12. Krebs V, Incavo SJ, Shields WH. The anatomy of the acetabulum: what is normal? Clin Orthop Relat Res. 2009;467(4):868-875. 13. Lee S, Wuerz TH, Shewman E, et al. Labral reconstruction with iliotibial band autografts and semitendinosus allografts improves hip joint contact area and contact pressure: an in vitro analysis. Am J Sports Med. 2014;43(1):98-104. 14. Levine RG, Renard R, Behrens FF, Tornetta P. Biomechanical consequences of secondary congruence after both-column acetabular fracture. J Orthop Trauma. 2002;16(2):87-91. 15. Matsuda DK. Acute iatrogenic dislocation following hip impingement arthroscopic surgery. Arthroscopy. 2009;25(4):400-404. 16. Matsuda DK. Fluoroscopic templating technique for precision arthroscopic rim trimming. Arthroscopy. 2009;25(10):1175-1182. 17. Matsuda DK, Khatod M. Rapidly progressive osteoarthritis after arthroscopic labral repair in patients with hip dysplasia. Arthroscopy. 2012;28(11):1738-1743. 18. Noguchi Y, Miura H, Takasugi S-I, Iwamoto Y. Cartilage and labrum degeneration in the dysplastic hip generally originates in the anterosuperior weight-bearing area: an arthroscopic observation. Arthroscopy. 1999;15(5):496-506. 19. Philippon MJ, Briggs KK, Yen Y-M, Kuppersmith DA. Outcomes following hip arthroscopy for femoroacetabular impingement with associated chondrolabral dysfunction: minimum two-year follow-up. J Bone Joint Surg Br. 2009;91(1):16-23. 20. Philippon MJ, Stubbs AJ, Schenker ML, Maxwell RB, Ganz R, Leunig M. Arthroscopic management of femoroacetabular impingement: osteoplasty technique and literature review. Am J Sports Med. 2007;35(9):1571-1580. 21. Philippon MJ, Wolff AB, Briggs KK, Zehms CT, Kuppersmith DA. Acetabular rim reduction for the treatment of femoroacetabular impingement correlates with preoperative and postoperative centeredge angle. Arthroscopy. 2010;26(6):757-761. 22. Reijman M, Hazes J, Pols H. Acetabular dysplasia predicts incident osteoarthritis of the hip: the Rotterdam study. Arthritis Rheum. 2005;52(3):787-793. 23. Richards PJ, Pattison JM, Belcher J, Decann RW, Anderson S, Wynn-Jones C. A new tilt on pelvic radiographs: a pilot study. Skeletal Radiol. 2009;38(2):113-122. 24. Sankar WN, Nevitt M, Parvizi J, Felson DT, Agricola R, Leunig M. Femoroacetabular impingement: defining the condition and its role in the pathophysiology of osteoarthritis. J Am Acad Orthop Surg. 2013;21 Suppl 1:S7-S15. 25. Smart LR, Oetgen M, Noonan B, Medvecky M. Beginning hip arthroscopy: indications, positioning, portals, basic techniques, and complications. Arthroscopy. 2007;23(12):1348-1353. 26. Tibor LM, Sekiya JK. Differential diagnosis of pain around the hip joint. Arthroscopy. 2008;24(12):1407-1421. 27. Wirz D, Becker R, Li SF, Friederich NF, Mu¨ller W. [Validation of the Tekscan system for statistic and dynamic pressure measurements of the human femorotibial joint]. Biomed Tech (Berl). 2002;47(78):195-201.
For reprints and permission queries, please visit SAGE’s Web site at http://www.sagepub.com/journalsPermissions.nav
