Cam and Pincer Femoroacetabular Impingement: CT Findings of Features Resembling Femoroacetabular Impingement in a Young Population Without Symptoms

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1 Musculoskeletal Imaging Original Research Chakraverty et al. CT of Femoroacetabular Impingement Musculoskeletal Imaging Original Research Julian K. Chakraverty 1 Carl Sullivan Chee Gan Srikanth Narayanaswamy Sridhar Kamath Chakraverty JK, Sullivan C, Gan C, Narayanaswamy S, Kamath S Keywords: femoroacetabular impingement, hip, osteoarthritis DOI: /JR Received January 9, 2012; accepted after revision June 10, ll authors: Department of Radiology, University Hospital of Wales, Heath Park, Cardiff, Wales CF14 4XW, UK. ddress correspondence to J. K. Chakraverty (jchakraverty@doctors.org.uk). JR 2013; 200: X/13/ merican Roentgen Ray Society Cam and Pincer Femoroacetabular Impingement: CT Findings of Features Resembling Femoroacetabular Impingement in a Young Population Without Symptoms OJECTIVE. The purpose of this study was to use CT to determine the presence of radiologic parameters associated with cam and pincer femoroacetabular impingement (FI) in a young population without symptoms. MTERILS ND METHODS. retrospective review of 50 patients (age range, years) with no current or previous hip disorder who underwent CT of the abdomen and pelvis was conducted. Multiplanar images were reformatted with a soft-tissue and bone algorithm and assessed for the presence of parameters associated with FI; alpha angle greater than 55, femoral head-neck offset less than 8 mm, angle of acetabular version less than 15, lateral center edge angle greater than 40, acetabular index less than 0, pistol-grip deformity, acetabular crossover, and prominent posterior wall signs. RESULTS. t least one abnormal parameter was present in 66% of joints, and two or more abnormal parameters were present in 29% of joints. In seven patients the findings were bilateral. Parameters of mixed morphologic characteristics (cam and pincer) were found in 22% of joints. In side-by-side comparison, high alpha angles were seen in 36 joints measured in the radial plane compared with only three joints measured in the axial oblique plane. CONCLUSION. The CT finding of FI-like features was made with high frequency in a young symptom-free population. Cutoff values for defining morphologic abnormalities associated with FI may have been set too low in the current literature. lpha angle measurements in the radial plane may be a more accurate quantitative assessment of asphericity of the femoral head-neck junction than are measurements in the axial oblique plane. F emoroacetabular impingement (FI) is recognized as an important cause of premature osteoarthritis among young people [1 5]. number of established imaging findings support a diagnosis of FI [1]. The natural course of the disease is largely unknown, but early diagnosis and appropriate treatment have been found to reduce symptoms and improve function, at least in the short term [6, 7]. Early surgical intervention is also believed to reduce the risk of degenerative changes in later life [4]. Two distinct pathoanatomic types of FI exist, although mixed types have been described [8]. In the cam lesion (femoral component of FI) there is a decrease in the normal waist and hence asphericity at the femoral head-neck junction. The pincer lesion (acetabular component of FI) results from focal or generalized overcoverage of the acetabulum. These variances are thought to predispose a person to early pathologic contact between the fem- oral head and acetabular articular and labral surfaces during movement of the hip joint. The prevalence of FI among adults is estimated to be 10 15% [9]. number of studies reported in the literature examined the prevalence of asymptomatic FI [10, 11]. Only a few studies, however, have evaluated the presence of morphologic parameters associated with FI in people without symptoms [12 19]. The objective of our study was to use CT to further evaluate the presence of the morphologic factors associated with cam- and pincer-type FI in a young symptom-free population and hence further our understanding of FI. Materials and Methods The study was approved by our institutional research review board, and the requirement for consent was waived. retrospective review was undertaken to identify individuals years old who presented to our tertiary center between December 2009 and February 2010 and underwent a JR:200, February

2 Chakraverty et al. CT of the abdomen and pelvis. The CT scans were obtained for conditions unrelated to disorders of the hip. ll scans were obtained with a 64-MDCT scanner (Optima CT660, GE Healthcare). Images with a mm section thickness were obtained at a tube voltage 120 kvp with dose modulation and a pitch of Of 120 patients, a continuous set of 50 eligible patients were selected for review. Our aim was to include only individuals without preexisting or current symptoms pertaining to the hip joint. This goal was achieved in a 10-year retrospective review of our radiology database and computerized hospital records. We identified three patients whose circumstances were unsuitable for the study: Two patients had undergone previous hip and pelvic imaging at our institution, and one patient had previously consulted an orthopedic surgeon for hip pain. Other patients excluded from the study had images of inadequate quality or images on which the area of interest was deemed inadequate. One of these patients had a femoral intramedullary nail, and seven patients had CT scout images that did not include the lesser trochanters. Once 50 eligible subjects had been selected, the other 59 patients were also excluded. The raw helical CT data were loaded into a workstation (dvantage version 4.5, GE Healthcare), and multiplanar reconstructions were generated with a bone and soft tissue algorithm. One hundred hip joints were analyzed for the presence of parameters predisposing to FI. Cutoff measurements for various parameters and specific radiologic signs associated with FI according to the current standard literature were chosen (see later). To prevent measurement errors generated by patient position, the reformatted CT images were corrected with the oblique tool of the Fig. 1 Measurements recorded in axial oblique plane., CT scan shows alpha angle formed between line drawn from center of femoral head through central axis of femoral neck and second line drawn from center of femoral head to anterior point where distance from center of head exceeds radius of femoral head. Large alpha angle (> 55 ) implies asphericity of anterior femoral head-neck junction., CT scan shows femoral head-neck offset. nterior offset is distance d, which lies between lines b and c, which lie parallel to anterior aspect of femoral neck cortex and anterior cortex of femoral head. Lines b and c lie parallel to midpoint of femoral neck at its narrowest dimension. Reduced offset was defined as less than 8 mm. workstation. Image interpretation, including all measurements, was conducted by an experienced fellowship-trained consultant musculoskeletal radiologist and a senior fellow specializing in musculoskeletal radiology. The study sample was divided into two groups so that each radiologist individually reviewed 50 hips. Image Interpretation and nalysis lpha angle and femoral head-neck offset were measured in the oblique axial plane. n alpha angle greater than 55 [13] and a femoral head-neck offset less than 8 mm [1] were considered to represent threshold values consistent with a cam lesion (Fig. 1). lpha angles were measured in four planes at intervals of 30 in the 12- to 3-o clock positions of the anterosuperior quadrant of the femoral head and neck on radial reformatted images (Fig. 2). The femoral neck axis was used as Fig. 2 lpha angles., Drawing shows alpha angles recorded in four positions around anterosuperior quadrant of femoral head (12- to 3-o clock positions)., CT scan shows alpha angle recorded in 1-o clock position. the center of rotation. We determined the femoral neck axis by aligning the rotation axis along two cardinal points: the center of the femoral head and the center of the femoral neck at the narrowest portion of the neck. For the purpose of this study, an abnormal alpha angle was again considered greater than 55 [13]. On an orthogonal axial reformatted image (Fig. 3), we measured the angle of acetabular version between a line joining the anterior and posterior rims of the acetabulum and a line perpendicular to a tangent running through both posterior corners of the acetabula. The angle was measured at a level where the acetabular cup was at its deepest, determined by cross-referencing to the images obtained in the coronal plane [17]. n angle less than 15 was chosen to indicate the presence of acetabular retroversion [17]. From a reconstructed pelvis model (tip of coccyx pointing toward the 390 JR:200, February 2013

3 CT of Femoroacetabular Impingement Fig. 3 CT scan shows angle of acetabular version. Line a is drawn between both posterior corners of acetabula. Line b is tangential to both anterior and posterior corners of acetabulum. Line c is perpendicular to line a. ngle of acetabular version is angle between lines b and c. cetabular retroversion was recorded at angle less than 15. Fig. 4 Simulated anteroposterior radiograph. symphysis pubis) and a simulated standard anteroposterior radiograph of a pelvis (Fig. 4) the following factors were assessed. cetabular index negative acetabular index (< 0 ) was chosen to represent overcoverage of the femoral head by the acetabulum, consistent with pincer morphologic characteristics [1] (Fig. 5). Lateral center-edge angle n angle greater than 40 was chosen to represent overcoverage of the femoral head by the acetabulum, consistent with pincer morphologic features [19] (Fig. 5). cetabular crossover sign The acetabular crossover sign is a qualitative sign of acetabular retroversion consistent with pincer-type morphologic features [1] (Fig. 6). crossover sign was defined as being positive when the posterior wall of the acetabulum was more medial to the anterior wall of the acetabulum in relation to the superior aspect of the acetabular rim. Pistol-grip deformity Pistol-grip deformity is an indicator of asphericity at the femoral head-neck junction (Fig. 6). It is consistent with cam-type morphologic features [20]. Prominent posterior wall sign Prominent posterior wall sign is an indicator of focal overcoverage of the femoral head by the acetabulum in the posterior aspect (Fig. 7). It is associated with posterior pincer impingement [1]. Statistical nalysis ecause the measurements were performed by two radiologists, interrater reliability was tested. The two readers repeated the measurements on a random sample of 15 hips. For the quantitative assessments (alpha angle, femoral head-neck offset, angle of acetabular version, lateral center edge angle, and acetabular index), intraclass correlation coefficient (ICC) was used to evaluate interrater reliability. ICC was interpreted as indicating poor (ICC, 0 0.2), fair (ICC, ), moderate (ICC, ), strong (ICC, ), or almost perfect (ICC, > 0.8) agreement. For the qualitative assessments (acetabular crossover sign, pistol-grip deformity, and posterior wall sign), kappa analysis was used to evaluate interrater validity. Level of agreement was interpreted as follows: less than chance agreement (k 0), slight agreement (k = ), fair agreement (k = ), moderate agreement, (k = ), substantial agreement, (k = ), almost perfect agreement (κ = ), or perfect agreement (κ = 1). SPSS for Microsoft Windows software (version 18.0, SPSS) was used for the statistical data analysis. Results One hundred hip joints from 50 patients (20 women, 30 men; age range, years; mean age, 30.8 [SD, 6.1] years) were assessed for the presence of parameters associated with FI. t least one abnormal parameter was present in 66% of joints. Twenty-three of 40 (57.5%) joints in women and 43 of 60 (71.7%) joints in men had at least one predisposing factor. Two or more abnormal parameters were present in 29% of joints, and in seven patients these abnormalities were bilateral. Ten of 40 (25%) joints in women and 19 of 60 (31.7%) joints in men had two or more abnormalities associated with FI. Parameters of mixed morphologic features (cam and pincer types) were found in 22% of joints: 8 of 40 (20%) in joints in women and 14 of 60 (40%) in joints in men. Parameters ssociated With Cam Femoroacetabular Impingement The total number of abnormal cam-type parameters measured in the sample population was 81. Thirty-six of 60 (60%) hips in men displayed at least one abnormality associated with cam FI, and two or more were seen in 16 of 60 (26.7%) hips. Conversely, 14 of 40 (35%) women had at least one cam abnormality, and 4 of 40 (10%) had at least two. Table 1 shows the number and distribution of cam-type abnormalities according to sex. lpha angles ranged from 32 to 57 (mean, 45 ; median, 45 ). lpha angles greater than 55 were found in three joints. Forty-nine abnormal alpha angles (> 55 ) in total were found in 36 joints. Table 2 shows the distribution of alpha angles and the number of alpha angles greater than 55 with respect to clock position of the femoral head-neck junction. nterior femoral head-neck offset ranged from 5 to 13.7 mm (mean, 9 mm; median, 9 mm). Twenty-one of 100 joints (21%) had a reduced anterior head-neck offset distance (< 8 mm). Eight of 100 (8%) joints on a reconstructed pelvic model had pistol-grip deformity. Parameters ssociated With Pincer Femoroacetabular Impingement There were 40 pincer-type abnormalities in the sample population. Twenty-two of 60 (36.7%) hips in men and 17 of 40 (42.5%) hips in women had at least one pincer-type characteristic. Only one hip in the men had two or more characteristics associated with pincer FI (a negative acetabular index and a lateral center edge angle of 45 ), but these were not observed at all in the women. Table 3 shows the number and distribution of pincer-type abnormalities according to sex. The acetabular version angle ranged from 11 to 36 (mean, 21.1 ; median, 20 ). Twelve of 100 joints had a low version angle (< 15 ). The acetabular crossover sign was found in one hip only. This hip also had an acetabular ver- JR:200, February

4 Chakraverty et al. sion angle less than 15 (12 ). ccording to our figures, the presence of an acetabular crossover sign in detecting acetabular retroversion, with an acetabular version angle less than 15 as the reference standard, revealed a sensitivity of 7% and a specificity of 100%. The acetabular index ranged from 9.9 to 14.2 (mean, 4.4 ; median, 4 ). Nine of 100 joints (9%) had a negative acetabular index (< 0 ). The lateral center edge angle ranged from 17 to 45 (mean, 31.5 ; median, 32.5 ). Two of 100 (2%) joints had a lateral center edge angle greater than 40. Sixteen of 100 (16%) joints had a prominent posterior wall sign on the reconstructed pelvic model. Interrater nalysis ICC was used to evaluate agreement between the two observers for each quantitative measurement, which were repeated on 15 hips. The ICCs ranged from 0.9 to 1 (Table 4). Kappa analysis was performed on the same 15 hips for the qualitative parameters (Table 5). kappa score of 1 for each parameter was recorded. Discussion FI is a distinct entity in which morphologic and mechanical influences on the hip joint predispose an individual to premature development of osteoarthritis. The diagnosis depends on a defined constellation of painful hip symptoms, restricted motion, and a positive result of an impingement test at clinical orthopedic examination that occur in the presence of certain morphologic abnormalities that can be detected radiologically. The etiologic mechanism of the morphologic features associated with FI is debated. With regard to the cam deformity, similar morphologic features are seen in slipped Fig. 5 cetabular index and lateral center-edge angle., Drawing shows acetabular index formed between horizontal line joining medial end of sclerotic weightbearing surface of acetabulum and line drawn between two ends of sclerotic arched weightbearing area of acetabular roof. Overcoverage was defined as angle less than 0., Drawing shows lateral center edge angle between line drawn from center of femoral head parallel to longitudinal body axis and another line drawn between center of femoral head and most external edge of acetabular roof. Overcoverage was defined as angle greater than 40. Fig year-old man with acetabular crossover sign. and, Schematic () and reformatted CT image () show acetabular crossover sign. nterior wall of acetabulum overlaps posterior wall of acetabulum in superior aspect. Pistol-grip deformity and herniation pits are evident in. upper femoral epiphysis and Perthes disease, suggesting that a genetic influence or a subclinical event may occur before the onset of skeletal maturity [21]. lthough the importance of genetic factors as causes of acetabular protrusion is well established [22, 23], the cause of pincer FI is unknown. Research results have shown, however, that patients with cam FI may acquire pincer deformities secondary to repeated microtrauma to the labrum, and these acquired deformities result in acetabular remodelling and overcoverage [24, 25]. number of reports in the literature have described the radiologic parameters associated with FI, but these have largely focused on patients with symptoms. The prevalence of radiographic parameters associated with FI in patients reporting hip pain is high. Ochoa et al. [10] found at least one case of FI in 135 of 155 patients with symptoms. Seymour et al. [11] found that 74% of 27 primary care patients with hip pain had two or more abnormal parameters associated with FI. With respect to the prevalence of radiologic parameters associated with FI in symptomfree populations, most studies have focused on the measurement of alpha angles [12 15] and cam-type morphologic features [12 17]. To our knowledge, there have been only two previous studies of symptom-free individuals in whom both cam and pincer radiologic parameters were analyzed. In a study of 100 hips in 50 patients without symptoms, Kang et al. [18] found a high prevalence of radiographic 392 JR:200, February 2013

5 CT of Femoroacetabular Impingement parameters associated with FI; 39% of joints had at least one parameter and 11% had more than one parameter predisposing to FI. In that study, however, only six parameters (three cam and three pincer) were measured, and interrater and intrarater validity analyses were not performed. Laborie et al. [19] in a populationbased survey also found a high prevalence of both cam and pincer morphologic features in a radiographic analysis of 2081 healthy young adults. In that study six parameters (three cam and three pincer) were assessed in a large cohort of subjects, but the analysis was purely qualitative and restricted to morphologic analysis in only two planes. We evaluated nine parameters (four cam and five pincer) associated with FI and to our knowledge conducted the most comprehensive evaluation to date of Fig year-old woman with posterior wall sign. and, Schematic () and reformatted CT image () show posterior wall sign. Posterior wall of acetabulum is lateral to point that marks center of femoral head. TLE 1: Number and Distribution of Cam bnormalities ccording to Sex lpha ngle > 55 Femoral Sex xial Oblique Plane Radial Plane Head-Neck Offset < 8 mm Pistol-Grip Deformity Men Women TLE 2: lpha ngle ccording to Clock Position of the Femoral Head- Neck Junction Clock Position Value Range Mean Median No. of hips with alpha angle > imaging parameters associated with FI in a symptom-free population. We performed both quantitative and qualitative analyses and did so in multiple planes. Our study showed a higher than expected occurrence of morphologic parameters associated with FI in a symptomfree population. t least one abnormal parameter associated with FI was present in 66% of joints, and two or more abnormal parameters were present in 29% of joints. Sixty percent of hips in men had at least one abnormality associated with cam FI, and two or more abnormalities were seen in 26.7% of hips. Conversely, in women 35% of hips had at least one abnormality, and 10% had two or more. Our findings are supportive evidence that cam abnormalities are more common in men and that the overall prevalence is higher than previously reported. In a populationbased study with results reported in 2011 [19], the prevalence of cam-type abnormalities in men and women was as follows: pistol-grip deformity, 21.5% in men and 3.3% in women; focal femoral neck prominence, 10.3% in men and 2.6% in women; and flattening of the lateral femoral head, 14.4% in men and 6.2% in women. Similarly, in a study by Reichenbach et al. [16], nearly one fourth of symptom-free young men had cam-type morphologic findings at hip MRI. To our knowledge, our study is the first of its kind to use reformatted CT images to calculate alpha angles along the anterosuperior quadrant of the femoral head junction in a noncadaveric population. We found that calculation of abnormal alpha angles in the radial reformatted imaging planes resulted in better representation of head-neck surface contour than measurement in the oblique axial plane alone (36 joints versus three joints). We considered an alpha angle greater than 55 abnormal in any plane, though we accept that this cutoff has not yet been firmly established in the literature. We found that more than one half of the 49 abnormal alpha angles measured in the radial plane were measured at the 1 o clock position. The alpha angle is a parameter frequently used to assess for cam impingement caused by femoral head asphericity and diminished femoral head-neck offset. It is conventionally measured in the oblique axial plane on crosssectional images. Previous studies [26 28] have shown that the reduced offset is maximal in the anterosuperior portion of the femoral head-neck junction but that only the anterior aspect of the offset can be accurately measured in the oblique axial plane on crosssectional images. Rakhra et al. [26] used MRI and multiplanar reformation of hip images in the radial plane to assess the offset along its entirety. Calculation of abnormal alpha angles with this technique also was reported to be more accurate than calculation of angles in the conventional axial oblique plane alone. Similar findings were made by udenaert et al. [28] in a CT study of cadaveric femurs. lthough radial imaging of the hip with MRI is well established, only preliminary data exist on CT. CT cannot be used to assess chondral and labral injury. CT, however, may have an advantage over MRI because contrast agents are not used, and the bony contour of the femoroacetabular articulation can be better assessed owing to the relative transparency of the soft-tissue envelope [12]. The use of CT JR:200, February

6 Chakraverty et al. TLE 3: Number and Distribution of Pincer bnormalities ccording to Sex Sex cetabular Version ngle > 15 Lateral Center Edge ngle > 40 Negative cetabular Index Posterior Wall Sign cetabular Crossover Sign Men Women as a surrogate for conventional radiography may also have advantages. lpha angles can be evaluated and 3D surface-rendered images assessed but at the expense of an extra radiation dose to the patient. With regard to pincer morphologic features, 36.7% of hips in men and 42.5% of hips in women had at least one pincer-type characteristic. Only one hip (a male patient) had two or more characteristics associated with pincer FI (a negative acetabular index and a lateral center edge angle of 45 ). The findings indicate that the prevalence of pincer-type morphologic features is similar in the sexes, with a slight female predominance. Established cases of pincer FI are also reported to be more prevalent among women [1]. Our study showed poor correlation between an abnormal acetabular version angle and a positive crossover sign (sensitivity, 7%; specificity, 100%). In contrast, Kang et al. [18] found that in the detection of acetabular retroversion, the crossover sign had 71% sensitivity and 88% specificity. The reason for this difference is not clear and was beyond the scope of our study. ecause the analysis of the hips was conducted by two radiologists, an interrater validity analysis was performed on 15 hips. ICC was used for the quantitative parameters measured, and agreement ranged from almost perfect to perfect. For the qualitative parameters, kappa analysis revealed perfect agreement. Weaknesses of our study include the small sample size and the retrospective nature of the investigation. We worked on the assumption that the individuals selected had no symptoms with regard to the hip joint after a careful review of our radiology database and hospital records. However, the patients were not contacted to verify their asymptomatic status. The retrospective study design also did not allow us to perform range of motion testing or impingement provocation maneuvers on the hip joints. Furthermore, occupational and activity levels were not evaluated or recorded. With regard to the simulated anteroposterior radiograph, care was taken to perform specific analyses on a well-centered and neutral pelvis with the tip of the coccyx pointing toward the symphysis pubis. Calibration to correct for pelvic tilt in the sagittal plane was TLE 4: Interrater Reliability Estimates for Measurements of Nine Parameters With Intraclass Correlation Coefficient Parameter Intraclass Correlation Coefficient lpha angle by clock position lpha angle, axial oblique plane 0.98 Femoral head-neck offset 0.97 cetabular version angle 0.92 Lateral center edge angle 0.98 cetabular index 1.0 TLE 5: Interrater Reliability Estimates With the Kappa Statistic Parameter κ cetabular crossover sign 1 Pistol-grip deformity 1 Posterior wall sign 1 not addressed, as has been previously reported for conventional radiography [29]. Therefore, although we considered the risk small, there was the potential for underestimation or overestimation of an acetabular crossover or prominent posterior wall sign. With regard to alpha angle measurements in the radial plane, consensus has yet to be achieved about the angle that defines the presence of asphericity in the anterosuperior quadrant of the femoral head-neck junction. lthough there is emerging evidence [14] that the use of the 55 cutoff at this site may lead to overestimates of contour abnormality, we chose an angle greater than 55 as the threshold to define abnormality, which is in accordance with previous work by others [26, 28, 30]. In essence, the frequency of radiologic parameters associated with cam and pincer FI is high even in a symptom-free population. With our current level of understanding, what this actually means to patients remains uncertain. We do not know what proportion of these people will have symptomatic FI in the future. It is also not clear how many parameters need to be present for development of FI. Other variables, such as activity level and previous injury, may be important in determining whether this supposedly abnormal morphologic characteristic results in the development of symptoms. The findings in this study and others make us wonder whether the measurement threshold for diagnosing abnormal morphologic parameters predisposing to FI has been set too low in the current literature and that the figures we have presented may simply reflect anatomic variation. There is therefore a risk that radiologists overreport FI. longitudinal study to assess the proportion of symptom-free individuals with morphologic parameters believed to predispose to FI who actually eventually have FI would be of great interest and go a long way in answering these questions. Until we know the significance of these findings, the emphasis to place on these findings will be debated. Palmer [31] questioned the role of imaging of FI and suggested that caution be exercised in approaching the radiologic findings of FI. We agree with Palmer s view that imaging abnormalities alone are not sufficient. Clinical symptoms and signs are essential to the diagnosis of FI. 394 JR:200, February 2013

7 CT of Femoroacetabular Impingement Conclusion The diagnosis of FI is based on a constellation of clinical features, including hip pain, restricted motion, a positive result of a clinical impingement test, and a number of predictable imaging features. We found in this study that the frequency of features resembling those of FI (both cam and pincer) on CT images is high even in a population without symptoms. Cutoff values for defining a morphologic abnormality predisposing to FI may have been set too low in the literature, and thus the prevalence of FI in symptom-free populations may have been overestimated. Consensus has yet to be achieved with regard to what constitutes an abnormal alpha angle from radial measurements, but if the cutoff is greater than 55, our results strengthen the view that alpha angle measurements in the radial plane are a more accurate quantitative assessment of asphericity of the femoral head-neck junction than measurements in the traditional axial oblique plane. Further research is needed to establish the range of morphologic findings in healthy people without symptoms and to assess the benefit of surgery in stalling the progress to osteoarthritis. References 1. Tannast M, Siebenrock K, nderson SE. Femoroacetabular impingement: radiographic diagnosis what the radiologist should know. JR 2007; 188: Ganz R, Parvizi J, eck M, Leunig M, Notzli H, Siebenrock K. Femoroacetabular impingement: a cause for osteoarthritis of the hip. Clin Orthop Relat Res 2003; 417: Murphy S, Tannast M, Kim YJ, uly R, Millis M. 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