The Journal of Arthroplasty xxx (2015) xxx–xxx
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What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? John V. Tiberi III, MD, Valentin Antoci, MD, PhD, Henrik Malchau, MD, PhD, Harry E. Rubash, MD, Andrew A. Freiberg, MD, Young-Min Kwon, MD, PhD Department of Orthopaedic Surgery, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts
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Article history: Received 19 July 2014 Accepted 15 March 2015 Available online xxxx Keywords: total hip arthroplasty acetabular cup position standing safe zone EOS X-ray imaging Acquisition System
a b s t r a c t This retrospective study measured the change of the acetabular component orientation between supine and standing radiographs in 113 THA patients and identified the associated anatomical parameters that may help direct pre-operative planning. The mean change of the acetabular component inclination and version from supine to standing was 4.6° and 5.9° respectively (P b 0.0001), with 49 (43%) hips showing inclination change N 5° and 69 (53%) hips showing version change N5°. Twelve (43%) of 28 ‘malpositioned’ cups became ‘well-positioned’ and 26 (31%) of 85 ‘well-positioned’ cups became ‘malpositioned’ upon standing. Changes in inclination were associated with leg length discrepancy and pelvic obliquity; and changes in version were associated with pelvic tilt and pelvic incidence. Standing position and patient factors should be considered when defining “optimal” acetabular orientation. © 2015 Elsevier Inc. All rights reserved.
Optimal acetabular component positioning represents one of the major goals of total hip arthroplasty (THA). Although there is currently no consensus in the literature, the conventionally defined desired acetabular position of 30°–50° inclination and 5°–25° anteversion is routinely referenced as “safe” acetabular orientation [1]. For bearings of all types, positioning outside of this “safe zone” has been previously associated with adverse outcomes, which include instability, increased wear, decreased range of motion, squeaking, and elevated serum metal ion levels [1–5]. Despite such definitions, no clear safe zone exists as the acetabular inclination remains dynamic, with a high rate of malpositioning observed by all surgeons [6]. Furthermore, despite the correlations between malpositioning and clinical adverse outcomes, questions remain regarding why many so-called “malpositioned” cups are well-functioning and many “well positioned” cups fail [1–5]. Acetabular inclination and version, representing the twodimensional parameters of the three-dimensional acetabular component orientation, are often quoted as a reference the Lewinneck et al standard [7]. Inclination is most commonly measured radiographically using an internal pelvic reference, as originally described by Sharp [8] for the purpose of quantifying acetabular dysplasia. Both internal and external markers have been described for measuring version [9,1,10]. Both abduction and version though provide references to pelvic One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.03.025. Reprint requests: Young-Min Kwon, MD, PhD, Department of Orthopedic Surgery, Massachusetts General Hospital, Harvard Medical School, 55 Fruit Street, Yawkey 3B, Boston, MA 02114.
anatomy only and do not account for overall patient alignment. As such, measurements based on supine radiographs do not recognize the effect of the spatial orientation of the pelvis on acetabular component position in other orientations, seated or standing. Changing of acetabular component orientation with movement of the pelvis is both logical and reported in the literature [11–19]. Although attempts have been made to quantify the corresponding change in acetabular component position with a particular change in the pelvic position [11,15,16], the amount of change among individuals is variable [11–19]. A recently introduced three dimensional x-ray orthopedic imaging system (EOS X-Ray Imaging Acquisition System; EOS Imaging Inc., Paris, France) allows the acquisition of high quality coronal and sagittal images of patients in the standing position using low dose radiation [20]. Previous studies have provided an insight into the pelvis acetabular position in sitting and standing positions, supporting the fact that EOS provides an excellent correlation to conventional x-ray imaging [21]. Currently, there are still limited data quantifying acetabular component position changes with respect to the previously defined target zones or identifying pelvis characteristics that are associated with patients in whom a significant pelvic range of motion is present during that transition. Kanawade et al. examined 85 hips and suggest that acute inclination during sitting results in a more vertical cup, especially in hypermobile pelvises [22]. The current study aims to increase the cohort size and provides information regarding the patient specific characteristics including body mass index (BMI), limb length discrepancy (LLD), and spino-pelvic parameters that may help the surgeon during pre-operative and intraoperative planning. The purpose of this retrospective study was thus to (1) quantify the difference and magnitude in acetabular component orientation between the supine and standing positions, and (2) identify
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Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
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J.V. Tiberi III et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
any patient related factors associated with acetabular orientation changes with patient position. Methods The authors’ hospital database was utilized to identify all total hip arthroplasty patients who had undergone standing radiographic evaluation at 3–6 months follow-up during the study period of June 2012 through December 2012. There were 113 patients who had obtained both supine and standing radiographic imaging on the same day and these patients were included in the study. The imaging was performed as part of the acquisition of an EOS system and initial testing. The selection of patients was thus on all-comers basis with no specific selection or exclusion criteria outside of standard follow-up for a unilateral total hip arthroplasty. Additional patient information obtained included age at surgery, gender, laterality, date of surgery, height, weight, and body mass index (BMI), and femoral component size. The supine images were anteroposterior views of the pelvis obtained with conventional radiography. The standing images were obtained utilizing commercially available EOS X-Ray Imaging Acquisition System (EOS Imaging Inc., Paris, France), a three dimensional x-ray orthopedic imaging system that uses low dose radiation to obtain high quality coronal and sagittal images of patients in the standing position (Figs. 1 and 2). This device is a vertical slot scanner where a patient is positioned standing, and both AP and lateral (orthogonal) images are acquired simultaneously. The maximum scan height is 175 cm; the detector is 1764 pixels wide with a pixel size of 254 μm2. The length of the segment being imaged can be adjusted from a full body scan to any subset of a full scan. The system has been tested and validated in the previous literature by Lazennec et al [21] The radiation dose is reported to be less than that of a conventional CR image [20]. Radiographic Measurements and Definitions Acetabular Component Position Based on EOS films, the acetabular face was delineated by overlying a ring on the film, similar to previous validated work by Journe et al [23]. Digital edge detection software (Martell Hip Analysis Suite, Chicago, IL) was used to measure radiographic component inclination and version (Figs. 1 and 2) on both the supine AP pelvis radiograph and the standing coronal image. The standing sagittal image was utilized to establish ante-version or retro-version. Multiple authors were directly involved with the reading and have been trained on the use of the Martell Hip Analysis Suite and certified by the validation of their reading through a series of standard cases in order to confirm inter-observer and intraobserver reproducibility. Pelvic Obliquity Pelvic obliquity (PO; the angle between a line across the superior most aspect of the iliac crests and a horizontal line) was measured on
Fig. 1. Represents a supine anteroposterior radiograph of the pelvis with an acetabular component with 30.6° inclination and 2.7° anteversion.
Fig. 2. The standing coronal image showing of the same patient showing significantly increased inclination of 40° and anteversion of 16.7°.
the standing coronal image (Fig. 3). A positive value was assigned when the operative hip was ipsilateral to the more inferior lying iliac crest (pelvis tilted toward operative hip), and a negative value was assigned when the operative hip was ipsilateral to the more superior lying iliac crest (pelvis tilted away from operative hip). Leg Length Discrepancy Leg length discrepancy (LLD; the vertical distance between the inter-teardrop line and the maximal prominence of the lesser trochanter) was measured on the standing coronal image. In cases where the inter-teardrop line could not easily be established, the trans-ischial line was used for the pelvic reference (Fig. 3). A positive value was assigned for a longer operative limb; a negative value was assigned for a shorter operative limb. The leg length discrepancy could not be obtained in two cases of distortion of the anatomy of the lesser trochanter; these cases were excluded from analyses pertaining to leg length discrepancy. Spino-Pelvic Parameters (Sacral Slope, Pelvic Tilt, and Pelvic Incidence) Sacral slope (SS; the angle between a horizontal line and a line along the superior endplate of S1) and pelvic tilt (PT; the angle between a vertical line and a line from the midpoint of superior endplate of S1 and the midpoint between the centers of the femoral heads) were measured on the standing, sagittal image (Fig. 4). Pelvic incidence (PI: the angle between a line perpendicular to the superior endplate of S1 and a line from the midpoint of superior endplate of S1 and the midpoint between the centers of the femoral heads) was calculated as the sum of the sacral slope and pelvic tilt (Fig. 4). These parameters were measured to determine whether these anatomic variables correlated with a change in pelvic, and therefore acetabular, orientation from supine to standing. In four patients, these measurements could not be made because S1 was obscured or could not be clearly defined on the image; these cases were excluded from analyses pertaining to these pelvic parameters.
Fig. 3. Demonstrates the measurement technique for pelvic obliquity (PO) and leg length discrepancy (LLD).
Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
J.V. Tiberi III et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
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Table 1 Statistically Significant Difference in Mean Acetabular Component Inclination and Version Were Observed. There Was a Significant Change (Absolute Value) in Both Parameters, and a Significant Number of Hips Changed N5° and N10°.
Number of Hips Supine Position Standing Position PValue Change (Absolute Value) N5° Change N10° Change
Fig. 4. Demonstrates the measurement technique for sacral slope (SS) and pelvic tilt (PT). Pelvic incidence (PI) is calculated as the sum of these two parameters.
“Safe Zone” Definition In accordance with previously published studies, the “malpositioned” acetabular component was defined as those hips outside the so-called “safe zone” of 30°–50° inclination and 5°–25° anteversion [1].
Acetabular Cup Inclination
Acetabular Cup Version
113 40.6° (22.6°–67.6°) 43.3° (19°–71°) b0.0001 4.6° (0.01°–16.2°) 49 (43%) 7 (6%)
113 12.8° (0.9°–33°) 17.6° (4.5°–37.3°) b0.0001 5.9° (0°–17.2°) 69 (53%) 17 (15%)
With respect to inclination, there were statistically significant differences in the mean pelvic obliquity (P b 0.0001), leg length discrepancy (P b 0.0001), and pelvic tilt (P = 0.04) when comparing those hips with inclination that substantially increased (−2.87°, 5.65 mm, and 20.11°, respectively), did not substantially change (0.93°, − 0.97 mm, and 16.86°, respectively), and substantially decreased (4.13°, − 8.68 mm, and 11.57°, respectively). With respect to version, there was a statistically difference in the mean pelvic tilt (P = 0.02) when comparing those hips with version that substantially increased (19.87°), did not substantially change (15.04°), and substantially decreased (22°). When excluding the group of hips that substantially decreased (three hips only), statistically significant differences were observed in both pelvic tilt (P = 0.003) and pelvic incidence (P = 0.04) between hips with version that substantially increased (19.87°) and those that did not substantially change (15.04°). No differences were observed in gender, height, weight, BMI, and sacral slope (Table 2).
Supine vs. Standing Acetabular Component Orientation “Safe Zone” Measurements The “difference” in acetabular component inclination was calculated by subtracting the value measured on the supine AP pelvis radiograph from the value measured on the standing, coronal image. The “difference” in acetabular component version was calculated by subtracting the value measured on the supine AP pelvis radiograph from the value measured on the standing, coronal image. The “change” in acetabular component inclination and version was defined as the absolute value of the difference. A “substantial change” in either parameter was defined as N 5°. Statistical Analysis Paired student t-tests and analyses of variance (ANOVA) were used for continuous variables to compare two and more than two groups, respectively. Chi square tests were used to compare categorical variables. In one instance (comparison among substantial increase, substantial decrease, and no substantial change of version), a secondary analysis was performed after excluding one group (substantial decrease) because the number of hips (three) in this group was deemed too low for valid comparison. Results Supine vs. Standing Acetabular Component Orientation There were statistically significant differences in supine (40.6°; range 22.6°–67.6°) versus standing (43.3°; range 19°–17°) inclination (P b 0.0001) as well as supine (12.8°; range 0.9°–33°) versus standing (17.6°; range 4.5°–37.3°) version (P b 0.0001). The mean change, by absolute value, of the acetabular component inclination and version from supine to standing was 4.6° (0.01°–16.2°) and 5.9° (0–17.2), respectively. With respect to inclination, 49 (43%) hips had a change N5°, and 7 (6%) hips had a change greater than 10°. With respect to version, 69 (53%) hip has a change N 5°, and 17 (15%) hips had a change N 10° (Table 1).
Overall, there were a total of 38 cups (34%) that “moved” either into or out of the safe zone upon standing. More specifically, nine (8%) had inclination and 20 (18%) had version outside of the “safe zone” accounting for a total of 28 (25%; one patient malpositioned on both parameters) acetabular components meeting criteria for “malpositioned” (Table 3). Of these, one (11%) mal-inclined cup, 15 (75%) malversioned cups, and 12 (43%) combined “malpositioned” cups became located within the “safe zone” upon standing, achieving the definition of “standing well-positioned” (Table 3, Fig. 5). Correspondingly, 104 (92%) had inclination, 93 (82%) had version, and 85 (75%) had both parameters inside of the “safe zone,” meeting criteria for “well positioned” (Table 3). Of these, 21 (20%) well-inclined cups, 13 (14%) wellversioned cups, and 26 (31%) combined “well positioned” cups became located outside of the “safe zone” upon standing, achieving the definition of “standing mal-positioned” (Table 3, Fig. 6). With respect to overall component position, there were statistically significant differences in BMI (27.25 vs. 29.35; P = 0.04), sacral slope (41.78 vs. 36.82; P = 0.01), pelvic tilt (20.32 vs. 16.19; P = 0.03), and pelvic incidence (62.11 vs. 53.01; P = 0.0004) when comparing acetabular components that moved into or out of the “safe zone” versus those that did not. For version, there were statistically significant differences pelvic tilt (21.11 vs. 16.38; P = 0.04) and pelvic incidence (62.04 vs. 54.05; P = 0.01) when comparing acetabular components that moved into or out of the “safe zone” versus those that did not. For inclination, there were no differences in demographic or anatomic variables between the groups (Table 4). Discussion Acetabular component orientation has an important impact on clinical outcomes following THA [1–5], yet achieving satisfactory position remains one of the challenges of this overall highly successful procedure [6]. Adverse outcomes including, but not limited to, instability and wear are often associated with malpositioning, but these
Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
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J.V. Tiberi III et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
Table 2 Table Represents a Summary of the Comparison of Demographic and Anatomic Variables among Hips with a Substantial Increase, No Substantial Change, or a Substantial Decreased in Acetabular Inclination or Version. Acetabular Cup Inclination
n (Hips) Male Female Height (in) Weight (lbs) BMI PO LLD (mm) SS PT PI
Acetabular Cup Version
Substantial Increase (N5°)
No Substantial Change
Substantial Decrease (N5°)
P Value
Substantial Increase (N5°)
No Substantial Change
P Value (t-Test)
Substantial Decrease (N5°)
P Value
38 24 14 67.34 181.92 28.49 −2.87° 5.65 39.03° 20.11° 59.14°
67 32 35 66.77 182.52 28.89 0.93° −0.97 37.98° 16.86° 54.85°
8 7 1 69.13 195.08 27.36 4.13° −8.68 40.71° 11.57° 52.29°
NA 0.05
57 32 25 67.42 184.95 28.92 0.25° −0.75 38.75° 19.87° 58.62°
53 30 23 66.83 182.23 28.64 −0.60° 2.17 38.70° 15.04° 53.74°
NA 0.96
3 1 2 64.67 159.33 24.17 0.33° 0.40 33.33° 22.00° 55.33°
NA 0.73
0.22 0.71 0.73 b0.0001* b0.0001* 0.72 0.04* 0.18
0.54 0.79 0.77 0.23 0.08 0.92 0.003* 0.04*
0.37 0.58 0.33 0.43 0.22 0.63 0.02* 0.14
Legend: * — statistically significant, BMI — body mass index, PO — pelvic obliquity, LLD — leg length discrepancy, SS — sacral slope, PT — pelvic tilt, PI — pelvic incidence.
complications are also observed in hips with well-positioned acetabular components. Additionally, good outcomes are not uncommonly observed in hips with malpositioned acetabular components. That may not necessarily just relate to dislocation but also edge loading, limb length discrepancy, and other long term outcomes. The current study is one of the largest studies quantifying the difference in standing and supine acetabular component orientation in patients with THA, demonstrating that acetabular component orientation changes significantly as the patient moves from the supine to the standing position. Furthermore, this is one of the first studies to evaluate the effect of a change in body position (supine to standing) on conventionally defined “malpositioned” and “well-positioned” acetabular components, suggesting the importance of considering functional standing position when defining an “ideal” window for acetabular component orientation in THA. Furthermore, the work correlates patient specific factors to the component position, thus providing insight for pre-operative planning and subsequent intra-operative adjustments. Overall, both inclination and version increased from the supine and standing position. Furthermore, substantial changes (N5°) in both inclination (43%) and version (53%) were common, and N 10° changes (6% for inclination; 15% for version) were not rare. The overall change and incidence of substantial change for version were similar to that of a previously performed study [18]. In contrast to the prior study, however, we found no reason to lower the threshold (3°) for inclination [18]. While the previous study reported the difference in version (mean 7°, range − 2° to 18°) [9], we elected to report the change by absolute value for inclination (mean 4.6°, range 0.01°–16.2°) and version (mean 5.9°, range 0°–17.2°) as negative values would lower the mean and detract from the understanding of the true magnitude of changes that occur (Table 1).
A substantial change in orientation from supine to standing was experienced by many, but not all, acetabular components. Even though ultimately not related to dislocation, a surgeon must understand specific factors that will affect outcome. We investigated demographic and anatomic variables to identify differences in cups with and without substantial changes in position. With respect to inclination, cups that substantially increased had a pelvic obliquity away from the operative hip (mean − 2.87°) and a longer operative limb (mean 5.65 mm) while cups that substantially decreased in inclination had a pelvic obliquity toward the operative hip (mean 4.13°) and a shorter operative limb (mean 8.68 mm). These data suggest that a pelvic obliquity may cause a change in standing inclination compared to anatomic inclination and that limb length discrepancy is likely to be a cause for pelvic obliquity following THA. Pelvic tilt was found to be higher in cups that substantially increased in both inclination and version. Prior reports have demonstrated increasing inclination and version with posterior pelvic tilt [24]. It is possible that patients with a high standing pelvic tilt have an even higher supine pelvic tilt; however, our study included pelvic parameters measured only in the standing position. A higher pelvic incidence was also observed in those cups that substantially increased in
Table 3 Table Represents a Summary of Acetabular Component Position Change in Reference to the “Safe Zone.” Inclination Version Change in ‘Safe Zone’ from Supine to Standing Malpositioned Became Well Positioned 1 (11%) Well positioned Became 21 (20%) Malpositioned Total (Safe Zone Change) 12 (19%) No Change in ‘Safe Zone’ from Supine to Standing Malpositioned Remained 8 (89%) Malpositioned Well Positioned Remained Well-Positioned 83 (80%) Total (No Safe Zone Change) 91 (81%)
Overall Position
15 (75%) 13 (14%)
12 (43%) 26 (31%)
28 (25%)
38 (34%)
5 (25%)
16 (57%)
80 (86%) 85 (75%)
59 (69%) 75 (66%)
Fig. 5. Is a graphical representation of fate of all supine malpositioned acetabular components upon standing.
Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
J.V. Tiberi III et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
Fig. 6. Is a graphical representation of the fate of all supine well-positioned acetabular components upon standing.
version. A high pelvic incidence describes a tilted pelvis with pronounced lumbar lordosis [25] a pelvis with these parameters may be subject to more sagittal range-of-motion, corresponding to an increase in version upon standing (Table 2). A change in acetabular component position may be most relevant when cups transition into and out of the acetabular target zone [1–5]. In this study, there were high rates of change in position with respect to the reference “safe zone” for inclination (12 cups, 19%), version (28 cups, 25%), and overall cup position (38 cups, 34%). Even though we do not attempt to correlate the “safe zone” to any effect on clinical outcomes and complications, changes from “malpositioned” to “well-positioned” and “well-positioned” to “malpositioned” were common for inclination, version, and overall component position. This observation may, in part, explain why many “malpositioned” cups have been reported previously to be well-functioning and many “well positioned” cups are reported as failures in the literature. When the demographic and anatomic factors were evaluated with respect to a change in the acetabular safe zone, there was no difference in the factors (pelvic obliquity, leg length
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discrepancy, and pelvic tilt) previously found to correlate with an increase or decrease in inclination. This observation is likely a result of cups both increasing and decreasing to either move into or out of the safe zone. Pelvic tilt and pelvic incidence were noted to be higher in cups that moved into or out of the safe zone for version, which may be explained by the fact that 25 of 28 cups that moved into or out of the safe zone did so as a result of an increase in version, shown in our data to be associated with high pelvic values of these parameters. There are several limitations to the current study. One limitation is that it included the standing position as a surrogate for the functional position of the pelvis. Although variable, post-operative pelvic sagittal range-of-motion is approximately 5° [17], therefore, standing cup orientation is a good estimate of functional orientation. Furthermore, information regarding static measurements is clinically useful, as most orthopedic surgeons do not have the capability to perform dynamic analyses. The standing position is the general standard of care for making clinical decisions and has significant value of pre-operative planning. Although the recently reported 3D fluoroscopic and kinematic studies in vivo in THA patients during functional activities provide invaluable insights, it is not readily available [26,27]. Another limitation to this study is the definition of a “safe zone” as the limits of what is safe are quite arbitrary. The literature does support, however, fewer adverse outcomes with cups positioned around the “safe zone.” With dislocation being a major concern, other factors including edge wear, limb length discrepancy, and impingement are related to cup position. The current study is also limited in that we were not able to correlate acetabular component positioning in our patient cohort to their respective long-term clinical outcomes. However, we are conducting an ongoing prospective study wherein we are following these total hip arthroplasty patients for 10 years. Lastly, it was rare for hips to substantially decrease in inclination or version, making these subgroups too small for statistical analysis. Information regarding substantial increases in inclination and version, however, are more important as it is much more common. In summary, the current study data suggest that with movement from the supine to the standing body position, substantial changes in both acetabular component inclination and version occur in many (N5° in more than half of cases), but not in all patients. Many of these changes involve position changes into or out of the conventionally defined “safe zone” with nearly half of all conventionally defined “malpositioned” cups were well-positioned in standing position. This finding may, in part, explain why many “malpositioned” cups have been previously reported to be well-functioning and many “well positioned” cups may fail. These data also suggest that the true acetabular component “safe” or “optimal” zone may need to be individualized for patients undergoing THA. Therefore, patient factors such as fixed pelvic obliquity, uncorrectable leg length discrepancy, and patients with high or low standing pelvic tilt need to be considered in determining ‘safe’ functional acetabular component orientation.
Table 4 Table Represents a Summary of the Comparison of Demographic and Anatomic Variables among Hips That Did and Did Not Move Either into or Out of the Safe Zone upon Standing. Acetabular Cup Inclination
n (Hips) Male Female Height (in) Weight (lbs) BMI PO LLD (mm) SS PT PI
Acetabular Cup Version
Overall Acetabular Cup Position
No Safe Zone Change
Safe Zone Change
P Value
No Safe Zone Change
Safe Zone Change
P Value
No Safe Zone Change
Safe Zone Change
P Value
91 47 44 66.94 183.61 28.84 −0.04 0.16 38.10 16.90 55
22 16 6 67.95 181.66 27.80 −0.45 2.48 40.09 20.36 60.45
NA 0.74
85 50 35 67.41 184.21 28.72 0.15 0.00 37.67 16.38 54.05
28 13 15 66.36 180.55 28.43 −0.93 2.38 40.93 21.11 62.04
NA 0.25
75 41 34 67.24 187.21 29.35 0.07 0.33 36.82 16.19 53.01
38 22 16 66.93 175.50 27.25 −0.50 1.19 41.78 20.32 62.11
NA 0.74
0.29 0.84 0.43 0.64 0.35 0.42 0.19 0.09
0.17 0.71 0.82 017 0.22 0.11 0.04* 0.01*
0.69 0.15 0.04* 0.42 0.63 0.01* 0.03* 0.0004*
Legend: * — statistically significant, BMI — body mass index, PO — pelvic obliquity, LLD — leg length discrepancy, SS — sacral slope, PT — pelvic tilt, PI — pelvic incidence.
Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
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Acknowledgement The authors would like to thank Selami Cakmak, MD and Dov Goldvasser, MS for their technical assistance with the radiographic data analysis. References 1. Lewinnek GE, Lewis JL, Tarr R, et al. Dislocations after total hip-replacement arthroplasties. J Bone Joint Surg Am 1978;60(2):217. 2. Barrack RL, Schmalzried TP. Impingement and rim wear associated with early osteolysis after a total hip replacement. J Bone Joint Surg Am 2002;84:1218. 3. Langton DJ, Jameson SS, Joyce TJ, et al. The effect of component size and orientation on the concentrations of metal ions after resurfacing arthroplasty of the hip. J Bone Joint Surg (Br) 2008;90:1143. 4. Walter WL, O’toole GC, Walter WK, et al. Squeaking in ceramic-on-ceramic hips: the importance of acetabular component orientation. J Arthroplasty 2007;22:496. 5. Wan Z, Boutary M, Dorr LD. The influence of acetabular component position on wear in total hip arthroplasty. J Arthroplasty 2008;23:51. 6. Callanan MC, Jarrett B, Bragdon CR, et al. The John Charnley Award: risk factors for cup malpositioning: quality improvement through a joint registry at a tertiary hospital. Clin Orthop Relat Res 2011;469(2):319. 7. Murray DW. The definition and measurement of acetabular orientation. J Bone Joint Surg (Br) 1993;75(2):228. 8. Sharp Ian K. Acetabular dysplasia: the acetabular angle. J Bone Joint Surg (Br) 1961;43-B:268. 9. Tiberi JV, Pulos N, Kertzner M, et al. A more reliable method to assess acetabular component position. Clin Orthop Relat Res 2012;470(2):471. 10. Woo RY, Morrey BF. Dislocations after total hip arthroplasty. J Bone Joint Surg Am 1982;64(9):1295. 11. Ackland MK, Bourne WB, Uhthoff HK. Anteversion of the acetabular cup. Measurement of angle after total hip replacement. J Bone Joint Surg (Br) 1986;68(3):409. 12. Babisch JW, Layher F, Amiot LP. The rationale for tilt-adjusted acetabular cup navigation. J Bone Joint Surg Am 2008;90(2):357. 13. Kalteis TA, Handel M, Herbst B, et al. In vitro investigation of the influence of pelvic tilt on acetabular cup alignment. J Arthroplasty 2009;24(1):152.
14. Lazennec JY, Boyer P, Gorin M, et al. Acetabular anteversion with CT in supine, simulated standing, and sitting positions in a THA patient population. Clin Orthop Relat Res 2011;469(4):1103. 15. Legaye J. Influence of the sagittal balance of the spine on the anterior pelvic plane and on the acetabular orientation. Int Orthop 2009;33(6):1695. 16. Lembeck B, Mueller O, Reize P, et al. Pelvic tilt makes acetabular cup navigation inaccurate. Acta Orthop 2005;76(4):517. 17. Parratte S, Pagnano MW, Coleman-Wood K, et al. The 2008 Frank Stinchfield award: variation in postoperative pelvic tilt may confound the accuracy of hip navigation systems. Clin Orthop Relat Res 2009;467(1):43. 18. Polkowski GG, Nunley RM, Ruh EL, et al. Does standing affect acetabular component inclination and version after THA? Clin Orthop Relat Res 2012; 470(11):2988. 19. Zhu J, Wan Z, Dorr LD. Quantification of pelvic tilt in total hip arthroplasty. Clin Orthop Relat Res 2010;468(2):571. 20. Deschenes S, Charron G, et al. Diagnostic imaging of spinal deformities: reducing patients radiation dose with a new slot-scanning X-ray imager. Spine (Phila Pa 1976) 2010;35(9):989. 21. Lazennec J, Rousseau M, Rangel A, et al. Pelvis and total hip arthroplasty acetabular component orientations in sitting and standing positions: measurements reproductibility with EOS imaging system versus conventional radiographies. Orthop Traumatol Surg Res 2011;97(4):373. 22. Kanawade V, Dorr LD, Wan Z. Predictability of acetabular component angular change with postural shift from standing to sitting position. J Bone Joint Surg 2014;96(12): 978. 23. Journé A, Sadaka J, Bélicourt C, et al. New method for measuring acetabular component positioning with EOS imaging: feasibility study on dry bone. Int Orthop 2012; 36(11):2205. 24. Kyo T, Nakahara I, Miki H. Factors predicting change in pelvic posterior tilt after THA. Orthopedics 2013;36(6):e753. 25. Legaye J, Duval-Beaupère G, Hecquet J, et al. Pelvic incidence: a fundamental pelvic parameter for three-dimensional regulation of spinal sagittal curves. Eur Spine J 1998;7(2):99. 26. Tsai TY, Li JS, Wang S, et al. In-vivo 6 degrees-of-freedom kinematics of metal-on-polyethylene total hip arthroplasty during gait. J Biomech 2014; 47(7):1572. 27. Tsai TY, Li JS, Wang S, et al. A novel dual fluoroscopy imaging for determination of THA kinematics: in-vitro and in-vivo study. J Biomech 2013;46(7):1300.
Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
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What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? John V. Tiberi III, MD, Valentin Antoci, MD, PhD, Henrik Malchau, MD, PhD, Harry E. Rubash, MD, Andrew A. Freiberg, MD, Young-Min Kwon, MD, PhD Department of Orthopaedic Surgery, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts
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Article history: Received 19 July 2014 Accepted 15 March 2015 Available online xxxx Keywords: total hip arthroplasty acetabular cup position standing safe zone EOS X-ray imaging Acquisition System
a b s t r a c t This retrospective study measured the change of the acetabular component orientation between supine and standing radiographs in 113 THA patients and identified the associated anatomical parameters that may help direct pre-operative planning. The mean change of the acetabular component inclination and version from supine to standing was 4.6° and 5.9° respectively (P b 0.0001), with 49 (43%) hips showing inclination change N 5° and 69 (53%) hips showing version change N5°. Twelve (43%) of 28 ‘malpositioned’ cups became ‘well-positioned’ and 26 (31%) of 85 ‘well-positioned’ cups became ‘malpositioned’ upon standing. Changes in inclination were associated with leg length discrepancy and pelvic obliquity; and changes in version were associated with pelvic tilt and pelvic incidence. Standing position and patient factors should be considered when defining “optimal” acetabular orientation. © 2015 Elsevier Inc. All rights reserved.
Optimal acetabular component positioning represents one of the major goals of total hip arthroplasty (THA). Although there is currently no consensus in the literature, the conventionally defined desired acetabular position of 30°–50° inclination and 5°–25° anteversion is routinely referenced as “safe” acetabular orientation [1]. For bearings of all types, positioning outside of this “safe zone” has been previously associated with adverse outcomes, which include instability, increased wear, decreased range of motion, squeaking, and elevated serum metal ion levels [1–5]. Despite such definitions, no clear safe zone exists as the acetabular inclination remains dynamic, with a high rate of malpositioning observed by all surgeons [6]. Furthermore, despite the correlations between malpositioning and clinical adverse outcomes, questions remain regarding why many so-called “malpositioned” cups are well-functioning and many “well positioned” cups fail [1–5]. Acetabular inclination and version, representing the twodimensional parameters of the three-dimensional acetabular component orientation, are often quoted as a reference the Lewinneck et al standard [7]. Inclination is most commonly measured radiographically using an internal pelvic reference, as originally described by Sharp [8] for the purpose of quantifying acetabular dysplasia. Both internal and external markers have been described for measuring version [9,1,10]. Both abduction and version though provide references to pelvic One or more of the authors of this paper have disclosed potential or pertinent conflicts of interest, which may include receipt of payment, either direct or indirect, institutional support, or association with an entity in the biomedical field which may be perceived to have potential conflict of interest with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2015.03.025. Reprint requests: Young-Min Kwon, MD, PhD, Department of Orthopedic Surgery, Massachusetts General Hospital, Harvard Medical School, 55 Fruit Street, Yawkey 3B, Boston, MA 02114.
anatomy only and do not account for overall patient alignment. As such, measurements based on supine radiographs do not recognize the effect of the spatial orientation of the pelvis on acetabular component position in other orientations, seated or standing. Changing of acetabular component orientation with movement of the pelvis is both logical and reported in the literature [11–19]. Although attempts have been made to quantify the corresponding change in acetabular component position with a particular change in the pelvic position [11,15,16], the amount of change among individuals is variable [11–19]. A recently introduced three dimensional x-ray orthopedic imaging system (EOS X-Ray Imaging Acquisition System; EOS Imaging Inc., Paris, France) allows the acquisition of high quality coronal and sagittal images of patients in the standing position using low dose radiation [20]. Previous studies have provided an insight into the pelvis acetabular position in sitting and standing positions, supporting the fact that EOS provides an excellent correlation to conventional x-ray imaging [21]. Currently, there are still limited data quantifying acetabular component position changes with respect to the previously defined target zones or identifying pelvis characteristics that are associated with patients in whom a significant pelvic range of motion is present during that transition. Kanawade et al. examined 85 hips and suggest that acute inclination during sitting results in a more vertical cup, especially in hypermobile pelvises [22]. The current study aims to increase the cohort size and provides information regarding the patient specific characteristics including body mass index (BMI), limb length discrepancy (LLD), and spino-pelvic parameters that may help the surgeon during pre-operative and intraoperative planning. The purpose of this retrospective study was thus to (1) quantify the difference and magnitude in acetabular component orientation between the supine and standing positions, and (2) identify
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Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
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any patient related factors associated with acetabular orientation changes with patient position. Methods The authors’ hospital database was utilized to identify all total hip arthroplasty patients who had undergone standing radiographic evaluation at 3–6 months follow-up during the study period of June 2012 through December 2012. There were 113 patients who had obtained both supine and standing radiographic imaging on the same day and these patients were included in the study. The imaging was performed as part of the acquisition of an EOS system and initial testing. The selection of patients was thus on all-comers basis with no specific selection or exclusion criteria outside of standard follow-up for a unilateral total hip arthroplasty. Additional patient information obtained included age at surgery, gender, laterality, date of surgery, height, weight, and body mass index (BMI), and femoral component size. The supine images were anteroposterior views of the pelvis obtained with conventional radiography. The standing images were obtained utilizing commercially available EOS X-Ray Imaging Acquisition System (EOS Imaging Inc., Paris, France), a three dimensional x-ray orthopedic imaging system that uses low dose radiation to obtain high quality coronal and sagittal images of patients in the standing position (Figs. 1 and 2). This device is a vertical slot scanner where a patient is positioned standing, and both AP and lateral (orthogonal) images are acquired simultaneously. The maximum scan height is 175 cm; the detector is 1764 pixels wide with a pixel size of 254 μm2. The length of the segment being imaged can be adjusted from a full body scan to any subset of a full scan. The system has been tested and validated in the previous literature by Lazennec et al [21] The radiation dose is reported to be less than that of a conventional CR image [20]. Radiographic Measurements and Definitions Acetabular Component Position Based on EOS films, the acetabular face was delineated by overlying a ring on the film, similar to previous validated work by Journe et al [23]. Digital edge detection software (Martell Hip Analysis Suite, Chicago, IL) was used to measure radiographic component inclination and version (Figs. 1 and 2) on both the supine AP pelvis radiograph and the standing coronal image. The standing sagittal image was utilized to establish ante-version or retro-version. Multiple authors were directly involved with the reading and have been trained on the use of the Martell Hip Analysis Suite and certified by the validation of their reading through a series of standard cases in order to confirm inter-observer and intraobserver reproducibility. Pelvic Obliquity Pelvic obliquity (PO; the angle between a line across the superior most aspect of the iliac crests and a horizontal line) was measured on
Fig. 1. Represents a supine anteroposterior radiograph of the pelvis with an acetabular component with 30.6° inclination and 2.7° anteversion.
Fig. 2. The standing coronal image showing of the same patient showing significantly increased inclination of 40° and anteversion of 16.7°.
the standing coronal image (Fig. 3). A positive value was assigned when the operative hip was ipsilateral to the more inferior lying iliac crest (pelvis tilted toward operative hip), and a negative value was assigned when the operative hip was ipsilateral to the more superior lying iliac crest (pelvis tilted away from operative hip). Leg Length Discrepancy Leg length discrepancy (LLD; the vertical distance between the inter-teardrop line and the maximal prominence of the lesser trochanter) was measured on the standing coronal image. In cases where the inter-teardrop line could not easily be established, the trans-ischial line was used for the pelvic reference (Fig. 3). A positive value was assigned for a longer operative limb; a negative value was assigned for a shorter operative limb. The leg length discrepancy could not be obtained in two cases of distortion of the anatomy of the lesser trochanter; these cases were excluded from analyses pertaining to leg length discrepancy. Spino-Pelvic Parameters (Sacral Slope, Pelvic Tilt, and Pelvic Incidence) Sacral slope (SS; the angle between a horizontal line and a line along the superior endplate of S1) and pelvic tilt (PT; the angle between a vertical line and a line from the midpoint of superior endplate of S1 and the midpoint between the centers of the femoral heads) were measured on the standing, sagittal image (Fig. 4). Pelvic incidence (PI: the angle between a line perpendicular to the superior endplate of S1 and a line from the midpoint of superior endplate of S1 and the midpoint between the centers of the femoral heads) was calculated as the sum of the sacral slope and pelvic tilt (Fig. 4). These parameters were measured to determine whether these anatomic variables correlated with a change in pelvic, and therefore acetabular, orientation from supine to standing. In four patients, these measurements could not be made because S1 was obscured or could not be clearly defined on the image; these cases were excluded from analyses pertaining to these pelvic parameters.
Fig. 3. Demonstrates the measurement technique for pelvic obliquity (PO) and leg length discrepancy (LLD).
Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
J.V. Tiberi III et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
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Table 1 Statistically Significant Difference in Mean Acetabular Component Inclination and Version Were Observed. There Was a Significant Change (Absolute Value) in Both Parameters, and a Significant Number of Hips Changed N5° and N10°.
Number of Hips Supine Position Standing Position PValue Change (Absolute Value) N5° Change N10° Change
Fig. 4. Demonstrates the measurement technique for sacral slope (SS) and pelvic tilt (PT). Pelvic incidence (PI) is calculated as the sum of these two parameters.
“Safe Zone” Definition In accordance with previously published studies, the “malpositioned” acetabular component was defined as those hips outside the so-called “safe zone” of 30°–50° inclination and 5°–25° anteversion [1].
Acetabular Cup Inclination
Acetabular Cup Version
113 40.6° (22.6°–67.6°) 43.3° (19°–71°) b0.0001 4.6° (0.01°–16.2°) 49 (43%) 7 (6%)
113 12.8° (0.9°–33°) 17.6° (4.5°–37.3°) b0.0001 5.9° (0°–17.2°) 69 (53%) 17 (15%)
With respect to inclination, there were statistically significant differences in the mean pelvic obliquity (P b 0.0001), leg length discrepancy (P b 0.0001), and pelvic tilt (P = 0.04) when comparing those hips with inclination that substantially increased (−2.87°, 5.65 mm, and 20.11°, respectively), did not substantially change (0.93°, − 0.97 mm, and 16.86°, respectively), and substantially decreased (4.13°, − 8.68 mm, and 11.57°, respectively). With respect to version, there was a statistically difference in the mean pelvic tilt (P = 0.02) when comparing those hips with version that substantially increased (19.87°), did not substantially change (15.04°), and substantially decreased (22°). When excluding the group of hips that substantially decreased (three hips only), statistically significant differences were observed in both pelvic tilt (P = 0.003) and pelvic incidence (P = 0.04) between hips with version that substantially increased (19.87°) and those that did not substantially change (15.04°). No differences were observed in gender, height, weight, BMI, and sacral slope (Table 2).
Supine vs. Standing Acetabular Component Orientation “Safe Zone” Measurements The “difference” in acetabular component inclination was calculated by subtracting the value measured on the supine AP pelvis radiograph from the value measured on the standing, coronal image. The “difference” in acetabular component version was calculated by subtracting the value measured on the supine AP pelvis radiograph from the value measured on the standing, coronal image. The “change” in acetabular component inclination and version was defined as the absolute value of the difference. A “substantial change” in either parameter was defined as N 5°. Statistical Analysis Paired student t-tests and analyses of variance (ANOVA) were used for continuous variables to compare two and more than two groups, respectively. Chi square tests were used to compare categorical variables. In one instance (comparison among substantial increase, substantial decrease, and no substantial change of version), a secondary analysis was performed after excluding one group (substantial decrease) because the number of hips (three) in this group was deemed too low for valid comparison. Results Supine vs. Standing Acetabular Component Orientation There were statistically significant differences in supine (40.6°; range 22.6°–67.6°) versus standing (43.3°; range 19°–17°) inclination (P b 0.0001) as well as supine (12.8°; range 0.9°–33°) versus standing (17.6°; range 4.5°–37.3°) version (P b 0.0001). The mean change, by absolute value, of the acetabular component inclination and version from supine to standing was 4.6° (0.01°–16.2°) and 5.9° (0–17.2), respectively. With respect to inclination, 49 (43%) hips had a change N5°, and 7 (6%) hips had a change greater than 10°. With respect to version, 69 (53%) hip has a change N 5°, and 17 (15%) hips had a change N 10° (Table 1).
Overall, there were a total of 38 cups (34%) that “moved” either into or out of the safe zone upon standing. More specifically, nine (8%) had inclination and 20 (18%) had version outside of the “safe zone” accounting for a total of 28 (25%; one patient malpositioned on both parameters) acetabular components meeting criteria for “malpositioned” (Table 3). Of these, one (11%) mal-inclined cup, 15 (75%) malversioned cups, and 12 (43%) combined “malpositioned” cups became located within the “safe zone” upon standing, achieving the definition of “standing well-positioned” (Table 3, Fig. 5). Correspondingly, 104 (92%) had inclination, 93 (82%) had version, and 85 (75%) had both parameters inside of the “safe zone,” meeting criteria for “well positioned” (Table 3). Of these, 21 (20%) well-inclined cups, 13 (14%) wellversioned cups, and 26 (31%) combined “well positioned” cups became located outside of the “safe zone” upon standing, achieving the definition of “standing mal-positioned” (Table 3, Fig. 6). With respect to overall component position, there were statistically significant differences in BMI (27.25 vs. 29.35; P = 0.04), sacral slope (41.78 vs. 36.82; P = 0.01), pelvic tilt (20.32 vs. 16.19; P = 0.03), and pelvic incidence (62.11 vs. 53.01; P = 0.0004) when comparing acetabular components that moved into or out of the “safe zone” versus those that did not. For version, there were statistically significant differences pelvic tilt (21.11 vs. 16.38; P = 0.04) and pelvic incidence (62.04 vs. 54.05; P = 0.01) when comparing acetabular components that moved into or out of the “safe zone” versus those that did not. For inclination, there were no differences in demographic or anatomic variables between the groups (Table 4). Discussion Acetabular component orientation has an important impact on clinical outcomes following THA [1–5], yet achieving satisfactory position remains one of the challenges of this overall highly successful procedure [6]. Adverse outcomes including, but not limited to, instability and wear are often associated with malpositioning, but these
Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
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Table 2 Table Represents a Summary of the Comparison of Demographic and Anatomic Variables among Hips with a Substantial Increase, No Substantial Change, or a Substantial Decreased in Acetabular Inclination or Version. Acetabular Cup Inclination
n (Hips) Male Female Height (in) Weight (lbs) BMI PO LLD (mm) SS PT PI
Acetabular Cup Version
Substantial Increase (N5°)
No Substantial Change
Substantial Decrease (N5°)
P Value
Substantial Increase (N5°)
No Substantial Change
P Value (t-Test)
Substantial Decrease (N5°)
P Value
38 24 14 67.34 181.92 28.49 −2.87° 5.65 39.03° 20.11° 59.14°
67 32 35 66.77 182.52 28.89 0.93° −0.97 37.98° 16.86° 54.85°
8 7 1 69.13 195.08 27.36 4.13° −8.68 40.71° 11.57° 52.29°
NA 0.05
57 32 25 67.42 184.95 28.92 0.25° −0.75 38.75° 19.87° 58.62°
53 30 23 66.83 182.23 28.64 −0.60° 2.17 38.70° 15.04° 53.74°
NA 0.96
3 1 2 64.67 159.33 24.17 0.33° 0.40 33.33° 22.00° 55.33°
NA 0.73
0.22 0.71 0.73 b0.0001* b0.0001* 0.72 0.04* 0.18
0.54 0.79 0.77 0.23 0.08 0.92 0.003* 0.04*
0.37 0.58 0.33 0.43 0.22 0.63 0.02* 0.14
Legend: * — statistically significant, BMI — body mass index, PO — pelvic obliquity, LLD — leg length discrepancy, SS — sacral slope, PT — pelvic tilt, PI — pelvic incidence.
complications are also observed in hips with well-positioned acetabular components. Additionally, good outcomes are not uncommonly observed in hips with malpositioned acetabular components. That may not necessarily just relate to dislocation but also edge loading, limb length discrepancy, and other long term outcomes. The current study is one of the largest studies quantifying the difference in standing and supine acetabular component orientation in patients with THA, demonstrating that acetabular component orientation changes significantly as the patient moves from the supine to the standing position. Furthermore, this is one of the first studies to evaluate the effect of a change in body position (supine to standing) on conventionally defined “malpositioned” and “well-positioned” acetabular components, suggesting the importance of considering functional standing position when defining an “ideal” window for acetabular component orientation in THA. Furthermore, the work correlates patient specific factors to the component position, thus providing insight for pre-operative planning and subsequent intra-operative adjustments. Overall, both inclination and version increased from the supine and standing position. Furthermore, substantial changes (N5°) in both inclination (43%) and version (53%) were common, and N 10° changes (6% for inclination; 15% for version) were not rare. The overall change and incidence of substantial change for version were similar to that of a previously performed study [18]. In contrast to the prior study, however, we found no reason to lower the threshold (3°) for inclination [18]. While the previous study reported the difference in version (mean 7°, range − 2° to 18°) [9], we elected to report the change by absolute value for inclination (mean 4.6°, range 0.01°–16.2°) and version (mean 5.9°, range 0°–17.2°) as negative values would lower the mean and detract from the understanding of the true magnitude of changes that occur (Table 1).
A substantial change in orientation from supine to standing was experienced by many, but not all, acetabular components. Even though ultimately not related to dislocation, a surgeon must understand specific factors that will affect outcome. We investigated demographic and anatomic variables to identify differences in cups with and without substantial changes in position. With respect to inclination, cups that substantially increased had a pelvic obliquity away from the operative hip (mean − 2.87°) and a longer operative limb (mean 5.65 mm) while cups that substantially decreased in inclination had a pelvic obliquity toward the operative hip (mean 4.13°) and a shorter operative limb (mean 8.68 mm). These data suggest that a pelvic obliquity may cause a change in standing inclination compared to anatomic inclination and that limb length discrepancy is likely to be a cause for pelvic obliquity following THA. Pelvic tilt was found to be higher in cups that substantially increased in both inclination and version. Prior reports have demonstrated increasing inclination and version with posterior pelvic tilt [24]. It is possible that patients with a high standing pelvic tilt have an even higher supine pelvic tilt; however, our study included pelvic parameters measured only in the standing position. A higher pelvic incidence was also observed in those cups that substantially increased in
Table 3 Table Represents a Summary of Acetabular Component Position Change in Reference to the “Safe Zone.” Inclination Version Change in ‘Safe Zone’ from Supine to Standing Malpositioned Became Well Positioned 1 (11%) Well positioned Became 21 (20%) Malpositioned Total (Safe Zone Change) 12 (19%) No Change in ‘Safe Zone’ from Supine to Standing Malpositioned Remained 8 (89%) Malpositioned Well Positioned Remained Well-Positioned 83 (80%) Total (No Safe Zone Change) 91 (81%)
Overall Position
15 (75%) 13 (14%)
12 (43%) 26 (31%)
28 (25%)
38 (34%)
5 (25%)
16 (57%)
80 (86%) 85 (75%)
59 (69%) 75 (66%)
Fig. 5. Is a graphical representation of fate of all supine malpositioned acetabular components upon standing.
Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
J.V. Tiberi III et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
Fig. 6. Is a graphical representation of the fate of all supine well-positioned acetabular components upon standing.
version. A high pelvic incidence describes a tilted pelvis with pronounced lumbar lordosis [25] a pelvis with these parameters may be subject to more sagittal range-of-motion, corresponding to an increase in version upon standing (Table 2). A change in acetabular component position may be most relevant when cups transition into and out of the acetabular target zone [1–5]. In this study, there were high rates of change in position with respect to the reference “safe zone” for inclination (12 cups, 19%), version (28 cups, 25%), and overall cup position (38 cups, 34%). Even though we do not attempt to correlate the “safe zone” to any effect on clinical outcomes and complications, changes from “malpositioned” to “well-positioned” and “well-positioned” to “malpositioned” were common for inclination, version, and overall component position. This observation may, in part, explain why many “malpositioned” cups have been reported previously to be well-functioning and many “well positioned” cups are reported as failures in the literature. When the demographic and anatomic factors were evaluated with respect to a change in the acetabular safe zone, there was no difference in the factors (pelvic obliquity, leg length
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discrepancy, and pelvic tilt) previously found to correlate with an increase or decrease in inclination. This observation is likely a result of cups both increasing and decreasing to either move into or out of the safe zone. Pelvic tilt and pelvic incidence were noted to be higher in cups that moved into or out of the safe zone for version, which may be explained by the fact that 25 of 28 cups that moved into or out of the safe zone did so as a result of an increase in version, shown in our data to be associated with high pelvic values of these parameters. There are several limitations to the current study. One limitation is that it included the standing position as a surrogate for the functional position of the pelvis. Although variable, post-operative pelvic sagittal range-of-motion is approximately 5° [17], therefore, standing cup orientation is a good estimate of functional orientation. Furthermore, information regarding static measurements is clinically useful, as most orthopedic surgeons do not have the capability to perform dynamic analyses. The standing position is the general standard of care for making clinical decisions and has significant value of pre-operative planning. Although the recently reported 3D fluoroscopic and kinematic studies in vivo in THA patients during functional activities provide invaluable insights, it is not readily available [26,27]. Another limitation to this study is the definition of a “safe zone” as the limits of what is safe are quite arbitrary. The literature does support, however, fewer adverse outcomes with cups positioned around the “safe zone.” With dislocation being a major concern, other factors including edge wear, limb length discrepancy, and impingement are related to cup position. The current study is also limited in that we were not able to correlate acetabular component positioning in our patient cohort to their respective long-term clinical outcomes. However, we are conducting an ongoing prospective study wherein we are following these total hip arthroplasty patients for 10 years. Lastly, it was rare for hips to substantially decrease in inclination or version, making these subgroups too small for statistical analysis. Information regarding substantial increases in inclination and version, however, are more important as it is much more common. In summary, the current study data suggest that with movement from the supine to the standing body position, substantial changes in both acetabular component inclination and version occur in many (N5° in more than half of cases), but not in all patients. Many of these changes involve position changes into or out of the conventionally defined “safe zone” with nearly half of all conventionally defined “malpositioned” cups were well-positioned in standing position. This finding may, in part, explain why many “malpositioned” cups have been previously reported to be well-functioning and many “well positioned” cups may fail. These data also suggest that the true acetabular component “safe” or “optimal” zone may need to be individualized for patients undergoing THA. Therefore, patient factors such as fixed pelvic obliquity, uncorrectable leg length discrepancy, and patients with high or low standing pelvic tilt need to be considered in determining ‘safe’ functional acetabular component orientation.
Table 4 Table Represents a Summary of the Comparison of Demographic and Anatomic Variables among Hips That Did and Did Not Move Either into or Out of the Safe Zone upon Standing. Acetabular Cup Inclination
n (Hips) Male Female Height (in) Weight (lbs) BMI PO LLD (mm) SS PT PI
Acetabular Cup Version
Overall Acetabular Cup Position
No Safe Zone Change
Safe Zone Change
P Value
No Safe Zone Change
Safe Zone Change
P Value
No Safe Zone Change
Safe Zone Change
P Value
91 47 44 66.94 183.61 28.84 −0.04 0.16 38.10 16.90 55
22 16 6 67.95 181.66 27.80 −0.45 2.48 40.09 20.36 60.45
NA 0.74
85 50 35 67.41 184.21 28.72 0.15 0.00 37.67 16.38 54.05
28 13 15 66.36 180.55 28.43 −0.93 2.38 40.93 21.11 62.04
NA 0.25
75 41 34 67.24 187.21 29.35 0.07 0.33 36.82 16.19 53.01
38 22 16 66.93 175.50 27.25 −0.50 1.19 41.78 20.32 62.11
NA 0.74
0.29 0.84 0.43 0.64 0.35 0.42 0.19 0.09
0.17 0.71 0.82 017 0.22 0.11 0.04* 0.01*
0.69 0.15 0.04* 0.42 0.63 0.01* 0.03* 0.0004*
Legend: * — statistically significant, BMI — body mass index, PO — pelvic obliquity, LLD — leg length discrepancy, SS — sacral slope, PT — pelvic tilt, PI — pelvic incidence.
Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
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J.V. Tiberi III et al. / The Journal of Arthroplasty xxx (2015) xxx–xxx
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Please cite this article as: Tiberi JV, et al, What Is the Fate of THA Acetabular Component Orientation When Evaluated in the Standing Position? J Arthroplasty (2015), http://dx.doi.org/10.1016/j.arth.2015.03.025
