Proposed Referential Index to Resect Femoroacetabular Cam-Type Impingement During Arthroscopy Using a Cadaveric Hip Model Shinya Yamasaki, M.D., Yusuke Hashimoto, M.D., Ph.D., Shozaburo Terai, M.D., Junsei Takigami, M.D., Shinji Takahashi, M.D., Ph.D., and Hiroaki Nakamura, M.D., Ph.D.

Purpose: To establish a reference index for the simple identification of the optimum resection point for cam-type impingement on arthroscopy. Methods: Twelve cadaveric left hips with a 20 to 40 center-edge angle, without osteoarthritis, were examined (mean age, 85  10.1 years). The pelvis was fixed such that the anterior pelvic plane and femur were parallel to the table. The resection line for impingement was first defined on the femoral head surface 5 mm distal to the acetabular labrum, from the 9-o’clock (anterior) to 12-o’clock (superior) position. Next, we measured the hip flexion angle necessary for the head-neck junction to reach the resection line. After positioning the wire on the femoral head surface along the resection line from the 9- to 12-o’clock area of the femoral head, we measured the target alpha angle on radiographs at 0 , 15 , 30 , 45 , and 60 of hip flexion using the frog-leg 45/45/30 view (45 of flexion, 45 of abduction, and 30 of external rotation) and Dunn 45 view (45 of flexion, 20 of abduction, and neutral rotation). Results: The mean hip flexion angle at which the head-neck junction reached the resection line was 31  4.6 . For 0 , 15 , 30 , 45 , and 60 of hip flexion, the mean target alpha angle was 75.5  5.5 , 65.3  5.6 , 56.3  5.8 , 49.0  6.6 , and 42.6  5.8 , respectively, using the frog-leg 45/45/30 view and 75.0  6.0 , 65.8  6.2 , 57.2  7.3 , 50.7  6.9 , and 44.2  5.8 , respectively, using the Dunn 45 view. There were no significant differences between the 2 radiographic techniques (P ¼ .82, P ¼ .84, P ¼ .76, P ¼ .57, and P ¼ .52, respectively). Conclusions: A description of the degree of hip flexion during cam resection can affect the final alpha angle when using the labrum as a reference for resection. Clinical Relevance: The described index allows systematic navigation of cam lesions during arthroscopy for femoroacetabular impingement patients using the hip flexion angle.

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emoroacetabular impingement (FAI) is a pathologic condition characterized by bone morphologic abnormalities of the femoral head-neck junction and acetabulum. This mechanical impingement induced by motion, particularly flexion and internal rotation, causes articular cartilage delamination, labral damage, and progressive degenerative processes that may eventually lead to From the Department of Orthopaedic Surgery, Osaka City University Graduate School of Medicine, Osaka, Japan. Presented at the 2013 International Society for Hip Arthroscopy (ISHA) meeting (October 2013, Munich, Germany) and nominated for the “Top Ten Posters.” The authors report that they have no conflicts of interest in the authorship and publication of this article. Received April 9, 2014; accepted December 19, 2014. Address correspondence to Yusuke Hashimoto, M.D., Ph.D., Department of Orthopaedic Surgery, Osaka City University Graduate School of Medicine, 14-3, Asahimachi, Abeno-ku, Osaka 545-8585, Japan. E-mail: [email protected] osaka-cu.ac.jp Ó 2015 by the Arthroscopy Association of North America 0749-8063/14295/$36.00 http://dx.doi.org/10.1016/j.arthro.2014.12.024

early osteoarthritis of the hip joint over time.1-3 Specifically, it has been recently pointed out that cam-type impingement is essential for pathology of hip degeneration.2,4,5 Surgical treatment to improve the femoral head-neck offset is achieved by resecting the excessive bony bump on the anterolateral aspect of the femoral head-neck junction. There are 2 procedures to correct cam lesions: an open surgery technique that incorporates surgical dislocation of the hip6 and arthroscopic surgery. Good clinical results using both procedures have been reported.7 The surgical dislocation technique offers advantages such as the ability to view the entire hip joint and direct access. However, the disadvantages of this technique include exposure of the articular cartilage to air, extensive soft-tissue dissection, trochanteric fixation failure, and heterotopic ossification.8,9 Arthroscopic resection, conversely, is thought to be less invasive than open surgery in terms of softtissue dissection, low complication rates, and recovery period.10,11 However, arthroscopic surgery requires technical skill to access and adequately visualize the hip

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joint. Because the femoral head is spherical in shape and there are few landmark in the arthroscopic field, surgeons sometimes have difficulty identifying mild cam lesions. Therefore preoperative image planning is required to determine the amount and position of a cam lesion to be resected. Cam lesions are usually diagnosed as having a greater than 50 to 55 alpha angle radiographically using frog-leg lateral, Dunn, cross-table lateral, and anteroposterior (AP) view projections, in accordance with the measurement described by Notzli et al.12-16 However, some reports have indicated that the value of the alpha angle varies depending on the radiographic projection. Such findings mean that each projection targets a different area of the femoral headneck junction. That is, it is desirable to evaluate the alpha angle before and after surgery using the same projection. A residual cam lesion because of insufficient resection is one of the common causes of revision surgery,17 whereas excessive resection can often result in a femoral neck fracture, with a negative effect on the labral seal of the hip joint.18 Moreover, trying to be too accurate can increase the duration of fluoroscopy. Therefore it is important to navigate the resection point and predict the postoperative alpha angle accurately through an easier and more useful assistive index, in addition to intraoperative fluoroscopy. The objective of this study was to establish a reference index for the simple identification of the optimum resection point for cam-type impingement on arthroscopy. Our hypothesis was that a referential hip flexion angle, which indicates the head-neck junction on the arthroscopic view, and valid hip flexion angle corresponding to the individual cam size, could be established through this study.

Methods This study was performed after we obtained approval from the ethics committee of our institute. We examined 12 cadaveric hip joints (8 male and 4 female specimens) in this study. Pelves and femurs were provided by the department of anatomy of our institute. The mean age was 85  10.1 years. We excluded osteoarthritic hips and hips in which the center-edge angle was less than 20 or more than 40 . The mean femoral head diameter was 49.6  4.0 mm, and the mean neck shaft angle was 136.3  4.2 . In the acetabulum the mean center-edge angle was 32.3  2.6 , and the mean acetabular inclination angle was 37.5  2.3 . Soft tissue, including musculature and the capsule around the pelvis and the femur, was removed, leaving only the acetabular labrum and ligamentum teres to freely move the hip joint. The pelvis was fixed such that the anterior pelvic plane was parallel to the table, and the femur was also placed parallel to the table. There were 2 key settings in this study. First, the labrum was regarded as a reference to start the resection. We defined the position of the cam resection line on the femoral head surface as 5 mm distal to the free edge of the acetabular labrum from the 9- (anterior) to 12-o’clock (superior) position, where cam-type impingement is commonly found (Fig 1A).12 In addition, the 6-o’clock position was defined as the middle of the transverse acetabular ligament and the 12-o’clock position as directly opposite the transverse ligament, in the position of the stellate crease.19 Left hip joints were used in this study, and the 9-o’clock position was on the anterior side. The second key setting assumed that the aforementioned resection line shifts in the proximaldistal direction with changes in the hip joint flexion angle: When the hip joint is in the extended position, the resection line is positioned in proximity to the

Fig 1. (A) The cam-type impingement resection line was defined on the left femoral head surface at a point 5 mm distal to the free edge of the acetabular left labrum from the 9-o’clock position (anterior in a left hip) to 12-o’clock position (superior). (B, C) The resection line shifts in the proximal-distal direction with changes in the hip joint flexion angle: When the hip joint is in extension, the resection line is positioned proximal to the femoral head (solid circle); this position shifts toward the distal femoral neck when the hip joint is in flexion (dotted circle).

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femoral head and it shifts toward the distal femoral neck with flexion (Figs 1 B and C). Hip Joint Flexion Angle When Resection Line Contacts Head-Neck Junction The hip joint was placed in the intraoperative position: 0 of flexion/extension, 10 of abduction, and neutral position for internal and external rotations. Rotation of the femur was determined by the angle between a line parallel to the posterior aspect of the femoral condyles and the table.20 Then, the hip joint was gradually flexed, and the hip flexion angle necessary for the head-neck junction of the femoral head to reach the resection linedmatching the 9- to 12-o’clock position along the acetabular labrumdwas measured using a goniometer with 1 increments (Fig 2). The pelvis was fixed on the table by manual holding by 1 author (J.T.). Then, another author (S.Y.) examined the angle, and the third author (S.T.) corrected the position if misalignment occurred during measurement. Target Alpha Angle Matching Resection Line at Each Flexion Angle and Distance Between Lines at Flexion Angles The hip joint was gradually flexed from 0 to 15 , 30 , 45 , and 60 . At each flexion angle, a line was drawn on the part of the femoral head surface matching the 9- to 12-o’clock region of the acetabular rim to a position 5 mm distal to the acetabular labrum as the resection line. Soft wire was pasted on each line (Fig 3), and imaging by radiography was obtained to visualize the planned resection lines to find the alpha angle of the cam-type impingement for each resection line. Radiographs including an image of the 10- to 11-o’clock region (the 1- to 2-o’clock region toward the right) of the femoral head were obtained: frog-leg 45/45/30 view, representing the 10-o’clock position (2-o’clock position on right), and Dunn 45 view,

Fig 2. The left hip joint was gradually flexed, and the hip flexion angle necessary for the head-neck junction (arrow) to reach the resection line at a point 5 mm distal to the acetabular labrum was measured.

Fig 3. Lines were drawn on the part of the left femoral head surface corresponding to the 9- to 12-o’clock positions (12- to 3-o’clock positions on right) at a point 5 mm distal to the left acetabular labrum with the hip in 0 , 15 , 30 , 45 , and 60 of flexion (Flex), and soft wires were pasted on the lines. The distance between each line at the 9- to 12-o’clock position was measured.

representing the 11-o’clock position (1-o’clock position on right).21 During acquisition, the frog-leg 45/45/30 view was imaged at 45 of flexion, 45 of abduction, and 30 of external rotation by irradiation from the front, as previously proposed by Cavaignac et al.22 (Fig 4). The Dunn 45 view was also imaged by irradiation from the front, at 20 of abduction, 45 of flexion, and neutral rotation. On the acquired images, a line was drawn from the inflection point on each soft wire on the anterior femoral head contour to the center of the femoral head, and the angle formed by this line and the femoral neck axis was measured. This angle was defined as the “target alpha angle” indexing the size of the cam lesion for resection at each flexion angle at the 10- to 11-o’clock positions (Fig 5). In addition, the distances between the lines drawn on the femoral head surface at the 9-, 10-, 11-, and 12-o’clock positions of the acetabulum were measured. Statistical analysis was performed using SAS software (version 9.3 2; SAS Institute, Cary, NC). Differences in the target alpha angle values between the groups were analyzed using the t test. Trends in the degree of the hip flexion angle and the target alpha angle value were determined using the Cochran-Armitage trend test, and P < .05 was regarded as significant. Intraobserver and interobserver (S.Y. and J.T.) reliabilities for the 12 cadavers, including all 5 parameters in each projection, were assessed using intraclass correlation coefficients (ICCs). The strength of agreement was interpreted as follows: ICC greater than 0.80 indicated almost perfect agreement; ICC of 0.61 to 0.80, substantial agreement; ICC of 0.41 to 0.60, moderate

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Fig 4. Plain radiography using (A) frog-leg 45/45/30 view (45 of flexion, 45 of abduction, and 30 of external rotation) and (B) Dunn 45 view (45 of flexion, 20 of abduction, and neutral rotation) for measuring alpha angle.

agreement; ICC of 0.21 to 0.40, fair agreement; and ICC of 0.20 or less, slight agreement. The sample size required to meet the principal objective was calculated by post hoc power analysis to observe a minimum difference of 5 between the alpha angle measured on the Dunn view and that measured on the frog-leg 45/45/30 view (t test). Assuming that the SD of the alpha angle will be 6 on both views (in accordance with Meyer et al.23) and that the angle measured on our view will be correlated with the angle measured on the Dunn view with a correlation coefficient of 0.7 or higher, we determined that the number of subjects required was 12 for an a risk of 5% and power of 80%.

respectively) (Fig 6). The Cochran-Armitage trend test showed that the more the hip joint was flexed, the significantly smaller the target alpha angle value became (P < .01). The intraobserver and interobserver reliabilities of target alpha angle measurements were 0.97 and 0.97, respectively. The distances between each line at the 9- to 12-o’clock position are shown in Table 1. The distance between 0 and 15 at any clock position was the longest, with the distances shortening as the line lowered. In addition, the distance at the 9-o’clock position was the longest, and the distances shortened as the line shifted more posteriorly.

Discussion Results Hip Joint Flexion Angle Necessary for Head-Neck Junction to Reach Resection Line The mean hip flexion angle at which the head-neck junction reached the resection line was 31  4.6 , showing that the head-neck junction was present at 5 mm distal to the acetabular labrum when the hip joint was flexed around 30 . Target Alpha Angle Matching Resection Line at Each Flexion Angle and Distance Between Lines at Flexion Angles At 0 of flexion, the target alpha angle was large, and it gradually decreased as the hip flexion angle increased. For 0 , 15 , 30 , 45 , and 60 of flexion, the mean target alpha angles were 75.5  5.5 , 65.3  5.6 , 56.3  5.8 , 49.0  6.6 , and 42.6  5.8 , respectively, using the frog-leg 45/45/30 view and 75.0  6.0 , 65.8  6.2 , 57.2  7.3 , 50.7  6.9 , and 44.2  5.8 , respectively, using the Dunn 45 view. Regarding the acquisition method, there were no significant differences in the target alpha angle values between the Dunn 45 and frog-leg 45/45/30 techniques (P ¼ .82, P ¼ .84, P ¼ .76, P ¼ .57, and P ¼ .52,

This is the first report to establish an index to navigate the appropriate orientation of cam-type impingement for systematic resection during arthroscopic surgery by

Fig 5. Measurement method for target alpha angle. A line was drawn from the inflection point of the soft wire on the anterior femoral head contour to the center of the left femoral head, and the angle formed by this line and the femoral neck axis was measured. (Flex, flexion.)

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Fig 6. Target alpha angle matching resection line at each flexion angle. At 0 , 15 , 30 , 45 , and 60 of flexion, the mean target alpha angle was 75.5  5.5 , 65.3  5.6 , 56.3  5.8 , 49.0  6.6 , and 42.6  5.8 , respectively, on the frog-leg 45/45/30 view (45 of flexion, 45 of abduction, and 30 of external rotation) and 75.0  6.0 , 65.8  6.2 , 57.2  7.3 , 50.7  6.9 , and 44.2  5.8 , respectively, on the Dunn 45 view (45 of flexion, 20 of abduction, and neutral rotation). There was no significant difference between the values of the target alpha angle using the Dunn 45 (white bars) and frog-leg 45/45/30 (black bars) views.

adjusting the angle of hip flexion without fluoroscopy. In general, surgeons determine the intended position of the femoral head using hip flexion and radiographic visualization. However, there have been no reports concerning the correlation between the degree of hip flexion and the size of the cam lesion. Therefore we sought to determine a correlation between these 2 parameters. According to the first experiment, the hip joint flexion angle was approximately 30 when the resection line at 5 mm distal to the labrum was present at the head-neck junction. Previous reports indicated that cam lesions generally occur near the head-neck junction.1,24,25 On the basis of these findings, most pathologic cam lesions should be observed 5 mm distal to the labrum under arthroscopy with the hip joint in 30 of flexion, 0 of internal/external rotation, and 10 of abduction. Moreover, the second experiment showed that the

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target alpha angle at the resection line with the hip in 30 of flexion was 56.3  5.8 with frog-leg 45/45/30 measurements and 57.2  7.3 with Dunn 45 measurements on radiographs. Given these results, the value of the alpha angle for a bony bump occurring at the head-neck junction met the diagnostic criterion of a cam lesion (around 55 ), as previous reports described.13 Therefore the design of this study seemed to be reasonable. Although many reports detail arthroscopic techniques in femoroacetabular impingement cases, Philippon et al.26 developed a unique formula to predict the change in the center-edge angle matching the amount of resection at the acetabular rim intraoperatively in pincer-type impingement cases. There is no such index enabling us to predict the postoperative alpha angle matching the intraoperative resection point in cam-type impingement cases. Ejnisman et al.18 indicated that with the hip placed in 45 of flexion, femoral osteoplasty can be performed proximally at 1 cm from the peripheral edge of the labrum, with resection tapering distally along the femoral neck for 1.5 to 2.0 cm. Similarly, Vaughn and Safran27 reported that when the hip is placed in 20 to 45 of flexion, the resection should be less than 1 cm deep, 8 mm from proximal to distal, and 15 mm medial to lateral, beginning 1 cm from the labral margin. In our study the correlation between the hip flexion angle and the alpha angle of the point 5 mm distal to the labrum was confirmed as evidence for the recommendation of the hip position and range in cam resection. When this index is actually applied in surgery, resection will commence from a site 5 mm distal to the labrum at 0 to 30 of hip joint flexion (depending on the size of the cam lesion) and, when the resection reaches a point distal to this position at 60 of flexion, the final alpha angle can be reduced to about 43 ; this result will thus allow cam lesions to be resolved systematically (Fig 7). Previous basic studies of cam resection were mainly concerned with the critical threshold on the depth of resection and its association with the risk of neck fracture.28,29 However, another study reported that not only the depth but also the width and length are important factors associated with the neck fracture incidence.30 Arthroscopic surgery has a learning curve, and technical training is needed. Moreover,

Table 1. Distance Between Lines Drawn at Different Flexion Angles Distance Between Each Line, mm Flexion Angle 0 and 15 15 and 30 30 and 45 45 and 60

9-O’Clock Position (3-O’Clock Position on Right) 5.3 4.9 4.1 3.1

10-O’Clock Position (2-O’Clock Position on Right) 4.7 4.5 3.8 2.8

11-O’Clock Position (1-O’Clock Position on Right) 4.4 3.9 3.4 2.4

12-O’Clock Position 3.8 3.6 2.8 2.1

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Fig 7. Simulation of cam resection based on study findings. For a preoperative (pre ope) alpha angle of 65 , a target alpha angle larger than 65 can be obtained at 15 of hip joint flexion. Thus the upper end of the cam lesion can be removed by commencing resection at a point 5 mm distal to the labrum. To normalize the alpha angle of a cam lesion measuring 50 or smaller, the joint is further flexed to 60 and resection is advanced to 5 mm distal to the labrum, which ensures reduction of the postoperative (post ope) alpha angle to 43 . The same result can be obtained by resecting a 8.5-mm (superior) to 12.1-mm (anterior) width distally, which is the total distance between 2 lines from 15 to 60 of hip flexion, keeping the position at 15 of flexion.

Mardones et al.8 showed that the resection area tended to be positioned more posteriorly when using arthroscopic surgery versus an open surgery technique, thus introducing a potential risk of insufficient resection by arthroscopy. Our reference index is useful to confirm the preoperative planning during arthroscopic resection. Regarding the radiographic measurements of the alpha angle, the most commonly projections are the cross-table lateral, AP, Dunn, and frog-leg lateral views. In one study assessing the usefulness of various views for diagnosing cam lesions, Meyer et al.23 imaged an identical femoral head using 6 radiographic methods to assess femoral head-neck asphericity and reported that the Dunn 45 view was superior to the cross-table lateral and AP views in both sensitivity and specificity. Clohisy et al.31 compared differences between normal hip joints and hip joints with cam lesions using various projections and found that the most significant difference was detected on the frog-leg lateral view compared with differences observed on the AP and cross-table lateral views. The reason for these descriptions is that cam lesions mainly occur at the 1- and 2-o’clock positions in a right hip.32,33 Furthermore, Nepple et al.21 reported that the clock position presented by each acquisition method was the 12-o’clock position on the AP view, 3-o’clock position on the cross-table lateral view, 2-o’clock position on the frog-leg lateral view, and 1-o’clock position on the Dunn 45 view in a right hip. Domayer et al.25 also reported that the Dunn 45 view represents the 1-o’clock position in a right hip. In addition, the cross-table lateral view is not suitable to

diagnose the size of a cam-type lesion because viewing a cam lesion in the cross-table lateral setting is difficult on hip arthroscopy. Therefore the Dunn 45 and frog-leg lateral views adopted in this study are practical and applicable for the evaluation of cam lesions before and during surgery. Limitations There are some limitations in this study. First, removal of the soft tissue, such as the capsule, could yield different kinematics from that of the native hip. Myers et al.34 showed that removing the soft tissue including the muscle and capsule increased the rotation angle and anterior translation of the hip joint compared with the native hip. Second, some variations in the morphologic parameters in the hip joint might affect the results of our study. The index values of the target alpha angle at each hip flexion angle might change depending on the degree of coverage of the acetabulum over the femoral head, as well as pelvic tilt. When the amount of anterior coverage is small or the pelvis is tilted posteriorly, it may be necessary to increase the hip flexion angle compared with the normal hip setting to identify the same target alpha angle as would be seen under normal hip morphology conditions. Overcoverage will also necessitate less flexion. In addition, the necessary width of resection may increase to some extent when the diameter of the femoral head is larger. Finally, although the post hoc power analysis showed that the current sample number was sufficient to evaluate the differences in the target alpha angle between the 2 projection methods, more samples may

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be needed. Nevertheless, the principle described in this study could be sufficiently applied to individual patients. Indeed, examination of the individual morphology of the hip joint using computed tomography or magnetic resonance imaging could allow surgeons to determine the positional relation between the acetabulum and the femoral head 3-dimensionally and then use the presented index for the patient’s assessment. Thus this study may serve as a tool by which to understand the correlation between the alpha angle and hip flexion angle. Careful preoperative planning using this index might facilitate accurate cam lesion resection and shorten the radiation exposure time. The described index was validated using 4 cadaveric hips. The mean target alpha angles after resection at 15 and 60 of hip flexion were 62.4 and 41.8 , respectively, on the frog-leg 45/45/30 view and 63.5 and 40.8 , respectively, on the Dunn 45 view. These results almost matched the current index. However, this index has not been validated in the clinical setting. As a further study, validation of this index in the clinical setting is needed.

Conclusions A description of the degree of hip flexion during cam resection can affect the final alpha angle when using the labrum as a reference for resection.

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Acknowledgment The authors express their appreciation to the Department of Anatomy, Graduate School of Medicine, Osaka City University, for the cadaveric samples.

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Proposed Referential Index to Resect Femoroacetabular Cam-Type Impingement During Arthroscopy Using a Cadaveric Hip Model.

To establish a reference index for the simple identification of the optimum resection point for cam-type impingement on arthroscopy...
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