Prevalence of Radiographic Parameters Predisposing to Femoroacetabular Impingement in Young Asymptomatic Chinese and White Subjects

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1 310 COPYRIGHT Ó 2015 BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED Prevalence of Raiographic Parameters Preisposing to Femoroacetabular Impingement in Young Asymptomatic Chinese an White Subjects Jan Van Houcke, MD, Wan Pan Yau, MBBS, FRCSE, FHKAM, FHKCOS, Chun Hoi Yan, MBBS, FRCSE, FHKAM, FHKCOS, Wouter Huysse, MD, Hannes Dechamps, Wing Hang Lau, MBBS, FHKAM, Chun Sing Wong, MBChB, FRCR, FHKCR, FHKAM, Christophe Pattyn, MD, PhD, an Emmanuel Albert Auenaert, MD, PhD Investigation performe at Ghent University Hospital, Ghent, Belgium, an Queen Mary Hospital, Hong Kong Backgroun: Osteoarthritis of the hip is five to ten times more common in white people than in Chinese people. Little is known about the true prevalence of femoroacetabular impingement or its role in the evelopment of osteoarthritis in the Chinese population. A cross-sectional stuy of both white an Chinese asymptomatic iniviuals was conucte to compare the prevalences of raiographic features posing a risk for femoroacetabular impingement in the two groups. It was hypothesize that that there woul be proportional ifferences in hip anatomy between the white an Asian populations. Methos: Pelvic compute tomography scans of 201 subjects (ninety-nine white Belgians an 102 Chinese; 105 men an ninety-six women) without hip pain who were eighteen to forty years of age were assesse. The original axial images were reformatte to three-imensional pelvic moels simulating stanarize raiographic views. Ten raiographic parameters preisposing to femoroacetabular impingement were measure: alpha angle, anterior offset ratio, an caput-collumiaphyseal angle on the femoral sie an crossover sign, ischial spine projection, acetabular anteversion angle, centerege angle, acetabular angle of Sharp, Tönnis angle, an anterior acetabular hea inex on the acetabular sie. Results: The white subjects ha a less spherical femoral hea than the Chinese subjects (average alpha angle, 56 compare with 50 ; p < 0.001). The Chinese subjects ha less lateral acetabular coverage than the white subjects, with average center-ege angles of 35 an 39 (p < 0.001) an acetabular angles of Sharp of 38 an 36 (p < 0.001), respectively. A shallower acetabular configuration was preominantly present in Chinese women. Conclusions: Significant ifferences in hip anatomy were emonstrate between young asymptomatic Chinese an white subjects. However, the absolute size of the observeifferences appears to contrast with the reporte low prevalence of femoroacetabular impingement in Chinese iniviuals compare with the high prevalence in white populations. Peer Review: This article was reviewe by the Eitor-in-Chief an one Deputy Eitor, an it unerwent bline review by two or more outsie experts. It was also reviewe by an expert in methoology an statistics. The Deputy Eitor reviewe each revision of the article, an it unerwent a final review by the Eitor-in-Chief prior to publication. Final corrections an clarifications occurreuring one or more exchanges between the author(s) an copyeitors. The concept of femoroacetabular impingement was first suggeste by Ganz et al. 1 in Originally, the conition was ientifie as a long-term complication following femoral neck fractures, although this cause of impingement is rarer than iiopathic femoroacetabular impingement. Femoroacetabular impingement is efine as premature abutment of the waist of the femoral neck against the anterior acetabular rim uring repetitive high flexion movement leaing to early labral an cartilage lesions of the hip 2,3. The premature impingement can be attribute to a ecrease in femoral hea sphericity (cam-type impingement) an/or increase overhang by the acetabular rim (pincer-type impingement) 4-7. Although Disclosure: One or more of the authors receive payments or services, either irectly or inirectly (i.e., via his or her institution), from a thir party in support of an aspect of this work. None of the authors, or their institution(s), have ha any financial relationship, in the thirty-six months prior to submission of this work, with any entity in the biomeical arena that coul be perceive to influence or have the potential to influence what is written in this work. Also, no author has ha any other relationships, or has engage in any other activities, that coul be perceive to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitte by authors are always provie with the online version of the article. J Bone Joint Surg Am. 2015;97:

2 311 TABLE I Definition of Raiographic Parameters Use to Describe Variation in Hip Joint Morphology Value Parameter Definition Normal Risk of Femoroacetabular Impingement Stuy Proximal part of femur CCD angle Alpha angle Anterior offset ratio Acetabulum Central acetabular anteversion Crossover sign Ischial spine projection Center-ege angle Acetabular angle of Sharp Tönnis angle Angle forme by the axis of the femoral neck an the proximal femoral iaphyseal axis (on anteroposterior view) Angle forme by the femoral neck axis an a line connecting the center of the femoral hea with the point of beginning asphericity (on anteroposterior, cross-table lateral, or Dunn view) Ratio between the anterior offset an the maximal iameter of the femoral hea. The anterior offset is the ifference in raius between the anterior aspect of the femoral hea an the anterior aspect of the femoral neck (on cross-table lateral view) Angle between a line rawn between the anterior an posterior acetabular riges an a reference line rawn perpenicular to a line between the posterior pelvic margins at the level of the sciatic notch at the mipart of the femoral hea (on axial CT slice) Present if the anterior rim runs more laterally in the most proximal part of the acetabulum an crosses the posterior rim istally (on anteroposterior view) Projection of the ischial spine into the pelvic cavity (on anteroposterior view) Angle forme by a line parallel to the longituinal pelvic axis an by the line connecting the center of the femoral hea with the lateral ege of the acetabulum (on anteroposterior view) Angle forme between a horizontallineanalinefromthetearrop to the lateral acetabulum margin (on anteroposterior view) Angle forme between a horizontal line an a line extening from the meial to the lateral ege of the sourcil (on anteroposterior view) <125 Tönnis an Heinecke 30 (1999) <55 >55 Nötzli et al ± 0.03 in normal subjects; 0.13 ± 0.05 in patients with cam-type impingement (2002) <0.13 Eijer et al. 29 (2001) <15 Stem et al. 31 Anterior rim line projects meially to the posterior wall line Anterior rim crosses the posterior rim (2006) Reynols et al. 32 (1999) No projection Projection Kalberer et al. 33 (2008) >45 Wiberg 34 (1939) <33 Sharp 35 (1961) 5-15 <5 Tönnis an Brunken 36 (1968) continue

3 312 TABLE I (continue) Value Parameter Definition Normal Risk of Femoroacetabular Impingement Stuy Anterior acetabular hea inex Ratio between a horizontal line rawn from the most posterior aspect of the femoral hea to the anterior aspect of the acetabulum an a horizontal line rawn from the most posterior aspect of the femoral hea to the most anterior aspect of the hea (on false-profile view) ± >0.9 Chosa an Tajima 37 (2003) osseous abutment at the en of the hip range of motion can be normal an asymptomatic 8, it may occur earlier an to a greater egree in patients with femoroacetabular impingement morphology, with possible progression to a symptomatic femoroacetabular impingement synrome. This conition is hypothesize to be a likely cause of hip osteoarthritis in white young aults 4,9,10. Hip osteoarthritis has a heterogeneous geographic istribution, with the prevalence in white populations approximately five to ten times higher than that in Chinese people of the same age an sex 11,12. This racial variation in osteoarthritis prevalence is emonstrate by the much higher rate of total hip replacements in white iniviuals. Stuies have reveale that the rate of total hip replacement for primary osteoarthritis in white patients is as much as twenty times the rate for Chinese 13,14. Whereas most cases of hip osteoarthritis in white populations are consiere primary an possibly relate to femoroacetabular impingement 4,9,10,15-17, in Chinese patients hip osteoarthritis is nearly always seconary to congenital hip isease, in particular evelopmental ysplasia of the hip 18,19. Little is known about the true prevalence of femoroacetabular impingement in the Chinese population, an even the value of current evience on the contribution of femoroacetabular impingement to the evelopment of hip osteoarthritis in general has been challenge by some 20. For example, compare with a rather moerate prevalence of hip osteoarthritis of 5% to 10% in white people, the reporte prevalence of raiographic characteristics posing a risk of femoroacetabular impingement in stuies of white subjects seems to be substantially larger, ranging from 17% to 48% in men an 4% to 31% in women We are aware of only one stuy comparing the prevalences of raiographic features relate to femoroacetabular impingement between Chinese an white patients (women) 27. Therefore, we conucte a cross-sectional stuy of asymptomatic iniviuals of both races with use of multietector compute tomography (CT). We hypothesize that there woul be significant ifferences in the anatomy of the proximal part of the femur an the acetabulum between the two populations with regar to the well-known raiographic parameters of femoroacetabular impingement. It was further hypothesize that there woul be significant ifferences in the number of raiographic signs preisposing to femoroacetabular impingement in asymptomatic iniviuals in the two groups. Materials an Methos Across-sectional stuy was esigne to prospectively collect pelvic multietector CT scans of subjects at the raiology epartments of Ghent University Hospital an the University of Hong Kong from September 1, 2011, to January 31, The selecte CT scans were of subjects with abominal trauma or abominal pain requiring further investigation for iagnosis. The subjects were not expose to aitional raiation for the stuy, which was approve by the local ethics committee of both universities. First, the minimal sample size was etermine on the basis of an estimate ifference in alpha angle of 5 between the two racial groups. An average anterolateral alpha angle of 50 (stanareviation [SD] = 8 ) ha previously been reporte in an observational stuy of 200 asymptomatic white subjects by Hack et al. 26. The sample size calculation was performe with the assumption of a type-i error of 0.05, a type-ii error of 0.2, an estimateifference of 5, an an SD of 8. Theminimalnumberofhipstobeinclueineachracialgroupwas calculate to be forty-one hips. Subjects unergoing a pelvic CT scan for nonorthopaeic reasons were inclue in the stuy if they reporte an unremarkable hip history an if they were between eighteen an forty years of age. A total of 202 subjects met these initial criteria. One patient in whom osteonecrosis with collapse of the femoral hea was note on CTwas exclue, leaving 201 subjects (402 hips), ninety-nine (fifty-eight men an forty-one women) of whom were white an 102 (fortyseven men an fifty-five women) of whom were Chinese. The average ages of the racial an sex subgroups range from thirty to thirty-three years (see Appenix). The absolute exclusion criteria were (1) any history of hip pain mentione by the patient; (2) evience of hip surgery on imaging; or (3) a history or raiographic evience of osteoarthritis of the hip, previous hip fracture, Legg-Calvé-Perthes isease, slippe capital femoral epiphysis, congenital hip ysplasia, or arthropathies capable of causing seconary alterations to the hip joint. CT scanning was performe in a stanarize fashion at 120 kvp with use of ose moulation with a sixty-four-slice multietector CT scanner. The ata sets consiste of slices of 1 mm in thickness at 0.7-mm increments epicting the pelvis an proximal parts of the femora, incluing the lesser trochanters. Ten raiographic parameters preisposing to cam or pincer-type femoroacetabular impingement were measure. These inclue three parameters of proximal femoral morphology (alpha angle 28,29, anterior offset ratio 29, an caput-collum-iaphyseal [CCD] angle 30 ), three parameters of acetabular orientation (central acetabular anteversion angle 31, crossover sign 32, an ischial spine projection 33 ), an four parameters of acetabular coverage (center-ege angle of Wiberg 34, acetabular angle of Sharp 35,Tönnis angle 36, an anterior acetabular hea inex 37 ). The efinition an normal values of these raiographic

4 313 TABLE II Differences in Continuous Raiographic Parameters Preisposing Chinese an White Subjects, Classifie by Sex, to Femoroacetabular Impingement* Raiographic Parameter Chinese Men (N = 94 Hips) White Men (N = 116 Hips) P Value Chinese Women (N = 110 Hips) White Women (N = 82 Hips) P Value Femur (eg) Alpha angle: 1:30 o clock 52 (50-54) 56 (54-58) (48-50) 56 (53-59) <0.001 Anterior offset ratio 0.19 ( ) 0.19 ( ) NS 0.21 ( ) 0.19 ( ) CCD angle 131 ( ) 127 ( ) < ( ) 132 ( ) NS Acetabulum Central acetabular 18 (17-19) 19 (18-20) NS 21 (20-22) 21 (20-22) NS anteversion (eg) Center-ege angle (eg) 37 (36-38) 40 (38-42) (32-34) 38 (36-40) <0.001 Acetabular angle of 37 (37-38) 35 (34-36) < (38-39) 36 (35-37) <0.001 Sharp (eg) Tönnis angle (eg) 9.6 ( ) 10.7 ( ) < ( ) 10.9 ( ) NS Acetabular hea inex 0.90 ( ) 0.88 ( ) NS 0.90 ( ) 0.89 ( ) NS *The crossover sign an ischial spine projection are shown in Table III. The values are given as the mean an 95% confience interval. NS = not significant. parameters are summarize in Table I. The techniques for measurement of each parameter are illustrate in figures in the Appenix. All of these parameters were originally measure on raiographs, except for the acetabular anteversion angle, for which the original axial CT images were reformatte with use of the Mimics software package (Materialise, Leuven, Belgium).Thefollowingreconstructionsweremaeforeachsubject(seeAppenix): a transparent three-imensional rotational moel simulating the anteroposterior pelvic view with pelvic tilt stanarize as escribe by Siebenrock et al. 38 i.e., with a pubic symphysis-to-sacrococcygeal joint istance of approximately 32 mm in men an 47 mm in women; oblique, coronal, an axial views along the axis of the femoral neck for each hip; an orthogonal axial view of the pelvis at the level of the sciatic notch where the acetabulum is the eepest; an a lateral view of each hip joint with the pelvis at an angle of 65, simulating the false profile. The ata were analyze with use of the SPSS software package (version 20; IBM, Armonk, New York). The morphometric parameters were analyze as epenent variables, while the inepenent variable was the racial group subivie accoring to sex. Continuous ata (alpha angle, anterior offset ratio, CCD angle, acetabular version, center-ege angle, acetabular angle of Sharp, Tönnis angle, an anterior acetabular hea inex) of men an women separately were compare between Chinese an white subjects with use of the inepenent t test. The stanar assumptions for normality of istribution were met by means of the Shapiro-Wilk test an evaluation of the quantilequantile (Q-Q) plot. The homogeneity of variances was assesse with use of the Levene test. The remaining two variables, the crossover sign an ischial spine projection, were categorical an were evaluate by means of the chi-square test. TABLE III Prevalence of Raiographic Parameters Preisposing Chinese an White Subjects, Classifie by Sex, to Femoroacetabular Impingement % of Hips with Parameter* % of Hips with Parameter* Raiographic Parameter* Chinese Men (N = 94 Hips) White Men (N = 116 Hips) P Value Chinese Women (N = 110 Hips) White Women (N = 82 Hips) P Value Femur Alpha angle: 1:30 o clock 22 (14-31) 34 (25-42) NS 15 (8-21) 32 (21-42) Anterior offset ratio 1 (0-3) 4 (1-8) NS 2 (0-4) 2 (0-6) NS CCD angle 15 (8-22) 28 (20-37) (5-17) 4 (0-8) NS Acetabulum Central acetabular anteversion 32 (22-42) 16 (9-22) (6-19) 11 (4-18) NS Crossover sign 12 (5-19) 41 (32-50) < (0-5) 16 (8-24) <0.001 Ischial spine projection 33 (23-43) 29 (21-37) NS 6 (2-10) 12 (5-19) NS Center-ege angle 15 (18-22) 19 (12-26) NS 3 (0-6) 13 (6-21) Acetabular angle of Sharp 4 (0-8) 16 (10-23) (0-4) 7 (2-13) NS Tönnis angle 7 (2-13) 2 (0-4) NS 1 (0-3) 0 NS Acetabular hea inex 52 (42-62) 38 (29-47) (41-60) 28 (18-36) *The eight continuous variables were converte to binary ones, with hips classifie as at risk for femoroacetabular impingement when the value exceee the normal range inicate in the literature. The crossover sign an ischial spine projection are binary variables. The values are given as the prevalence an 95% confience interval. NS = not significant.

5 314 Fig. 1 Box-an-whisker plot of the measure alpha angle in the hips of Chinese an white subjects classifie by sex. The top an bottom of the box represent the interquartile range, the horizontal line in each box represents the meian, an the whiskers represent the minimum an maximum. The eight continuous raiographic parameters were also converte to ichotomous variables on the basis of cutoff values that imply the risk of femoroacetabular impingement. The prevalence of these raiographic signs in asymptomatic Chinese an white men an women was evaluate by using the chi-square test. Two observers (J.V.H. an H.D.) performe all of the raiographic measurements on anonymize imaging files. Both observers each repeate the measurements on forty-eight hips one week later to obtain test-retest ata. The interobserver an intraobserver reliability was evaluate by using the intraclass correlation coefficient (ICC). The level of significance for all tests was set at a = Source of Funing Two authors (E.A.A. an J.V.H.) were supporte by the Research Founation- Flaners (FWO). Results The ICC for interobserver an intraobserver reliability showe an agreement of more than 80% for most parameters, inicating strong reliability. The Tönnis angle, anterior offset ratio, anterolateral alpha angle, an acetabular angle of Sharp showe moerate agreement for interobserver reliability (ICC, 72% to 78%) (see Appenix). Proximal Part of the Femur The average alpha angle over the anterolateral aspect of the femoral neck (at the 1:30 o clock position) was significantly larger in white subjects (56 in both white men an white women compare with 52 in Chinese men an 49 in Chinese women; p = 0.01 an p < 0.001) (Fig. 1 an Table II). The femoral neck ha a more pronounce varus configuration in white men, with an average CCD angle of 127 compare with 131 in Chinese men (p < 0.001). The anterior offset ratio was significantly higher in Chinese women (0.21 compare with 0.19 in white women; p = 0.038). Twenty-four percent of the hip joints in the Chinese subjects (31% of those in the Chinese men an 17% of those in the Chinese women) compare with 40% of the hip joints in the white subjects (41% of those in the white men an 39% of those in the white women) ha an aspherical femoral hea at the hea-neck junction (p < 0.001) (see Appenix). Asphericity of the femoral hea was efine as an anterior, anterolateral, or lateral alpha angle that exceee 55 in at least one of these three planes. Thirty-four percent of the Chinese subjects an 56% of the white subjects ha an aspherical femoral hea in one or both hips (p < 0.001). Elevate alpha angles were most common at the 1:30 o clock (anterolateral) position in both racial groups an were present in 33% of the hips in the white subjects (34% of those in the white men an 32% of those in the white women) compare with just 18% of the hips in the Chinese subjects (22% of those in the Chinese men an 15% of those in the Chinese women) (p = 0.001) (Table III). Acetabulum The acetabula of the Chinese subjects appeare more shallow in the coronal plane, with a mean center-ege angle of 37 in Chinese men compare with 40 in white men (p = 0.006) an 33 in Chinese women compare with 38 in white women (p < 0.00) (Fig. 2). The acetabular angle of Sharp average 37 in Chinese men an 35 in white men (p < 0.001) an 38 in Chinese women an 36 in white women (p < 0.001). The Tönnis angle was significantly lower in Chinese men than in white men (p < 0.001) (Table II). Increase center-ege angles, inicative of acetabular overcoverage, were significantly more prevalent in white women than in Chinese women (13% of the hips compare with 3%; p = 0.005). The prevalence of a ecrease acetabular angle of Sharp, also inicative of acetabular overcoverage, was significantly higher in white men than in Chinese men (16% of the hips compare with 4%; p = 0.005). The central acetabular version angle inicate a tenency towar retroversion in 32% of the hips of Chinese men compare with 16% of those of the white men (p = 0.005). However, the crossover sign was more prevalent in both white men an white women (41% an 16% of the hips, respectively, compare with 12% an 2% in the Chinese group; p < 0.001). Overcoverage at the anterior acetabular rim, base on the anterior acetabular hea inex, was present in 52% of the hips of Chinese men compare with 38% of those of white men (p = 0.039) an in 51% of the hips of Chinese women compare with 28%ofthoseofwhitewomen(p= 0.00) (Table III). Fig. 2 Box-an-whisker plot of the measure center-ege angle in the hips of Chinese an white subjects classifie by sex. The box represents the interquartile range, the horizontal line in each box represents the meian, an the whiskers represent the minimum an maximum.

6 315 Fig. 3 Percentages of hips in Chinese subjects compare with those in white subjects as a function of the number of femoroacetabular impingement (FAI) at-risk signs. All ten morphometric variables were taken into account: two categorical raiographic parameters (the crossover an ischial spine signs) an eight continuous variables that were converte into ichotomous variables by selecting the subjects who ha a value exceeing the normal range. Signs Preisposing to Femoroacetabular Impingement There were at least two or more raiographic signs of a risk of femoroacetabular impingement in 41% of the hips of the Chinese subjects compare with 53% of the hips of the white subjects (p = 0.017) (Fig. 3). Discussion The current stuy emonstrates that the proximal part of the femur tens towar more varus in white men an generally has a less spherical femoral hea in white subjects. On the other han,chinesepeople,anespeciallychinesewomen,clearly possess a shallower acetabulum. The prevalence of raiographic signs inicating a risk of femoroacetabular impingement was significantly higher in white subjects than in Chinese subjects. However, the absolute size of the observeifferences appears to contrast with the reporte low prevalence of femoroacetabular impingement in Chinese people compare with the high number of impingement cases iagnose in white populations as well as with the significantly lower prevalence of hip osteoarthritis in Chinese people 13,14,20.Thisfining suggests that some of the current raiographic concepts irectly linking femoroacetabular impingement morphology to actual impingement synrome an the evelopment of hip osteoarthritis may nee to be revise. We believe that this stuy is the first to comprehensively compare, with use of multietector CT, the morphology of the proximal part of the femur an the acetabulum between asymptomatic young Chinese an white subjects in a large sample. To our knowlege, the only stuy comparing hip joint morphology between Chinese an white subjects (women) was performe by Dua et al. 27, who use anteroposterior raiographs. They reporte that the white female population ha significant increases in lateral overcoverage (average centerege angle, 30.4 compare with 25.5 ) an femoral hea asphericity (average impingement angle, 83.6 compare with 87 ), finings that agree with our results. We inclue three parameters for the evaluation of the occurrence of acetabular retroversion in the racial groups: a ecrease central acetabular anteversion angle (<15 ), the crossover sign, an ischial spine projection 31-33,39.Wefoun that the prevalence of hips with a ecrease acetabular anteversion anglewassignificantly higher in Chinese subjects (22% compare with 13% in white subjects), whereas the crossover sign was more prevalent in white subjects (30% compare with 6%). The ischial spine projection showe no significant ifference in racial prevalence. The acetabular anteversion angle in this stuy was measure accoring to the methoescribe by Stem et al. 31, at the mifemoral hea level on the axial CT slices, whereas Reynols et al. 32 claime that measurement of acetabular version at the superior part of the acetabulum was of greater value. With acetabular retroversion, the version angle is ecrease at both levels, albeit less so at the mifemoral level 32. The measurement of central instea of superior acetabular anteversion can be consiere a limitation; however, it oes not sufficiently explain the observe inverse relationship between the crossover sign an the acetabular anteversion angle. This contraiction might be ue to a low reliability of the crossover sign in the iagnosis of acetabular retroversion, as ebate in recent stuies 40,41. The prevalence of raiographic evience of hip osteoarthritis is 5% in white people compare with barely 1% in Chinese people (sixty to seventy-four years of age) 11,42. Symptomatic hip osteoarthritis in the Chinese population is as low as

7 % (in patients fifty-five years of age or oler 12 ). However, the prevalences of han 12 an knee 43 osteoarthritis are rather similar between the two racial populations. Knee arthrosis is even moreprevalentinchinesewomenthaninwhitewomen. These contraictory finings in ifferent joint types make it less probable that genetic mutations of general articular cartilage metabolism are responsible for the observeifferences in the prevalence of hip osteoarthritis between races. Nevertheless, genetic factors may help provie an explanation, since hip osteoarthritis has a reporte os ratio of 6.4 in siblings 44,45.Itis possible that inheritance of anatomic variants that preispose to hip osteoarthritis contributes to such familial clustering 11. Recent stuies suggest that acetabular overcoverage an femoral hea asphericity contribute to the evelopment of hip osteoarthritis 4,9,14-16.Ourfinings confirme a higher prevalence of lateral acetabular overcoverage an osseous bumps in white subjects compare with Chinese subjects, a fining that agrees with those of previous stuies 27,46.However, the absolute ifference in these values is not in proportion to the ifference in the prevalence of hip osteoarthritis. Thus, our fining provies little aitional evience that femoroacetabular impingement plays a role in the evelopment of hip osteoarthritis in white populations. Clearly, femoroacetabular impingement is a ynamic conition, with the raiographic features merely risk factors preisposing to the actual evelopment of a true impingement synrome. The present stuy was not esigne to escribe the epiemiology of impingement synrome. We can conclue only that aitional work seems to be require to establish the variables that ifferentiate iniviuals with raiographic variation from those who will evelop clinical symptoms or even articular cartilage efects. The current stuy ha limitations. First, the selecte sample consiste of young iniviuals requiring pelvic CT scans for nonorthopaeic reasons, which is probably not a true representation of the general population. However, it woul be ifficult an probably unethical to recruit healthy volunteers to unergo pelvic CT scans for purely research purposes. Furthermore, the bias towar younger subjects is actually a strength rather than a limitation of our stuy. If femoroacetabular impingement morphology is actually a risk factor for later hip osteoarthritis, the risk factor shoul be present before the onset of osteoarthritis. In oler people (commonly the subjects of stuies in the literature), early osteophytes are often misinterprete as representing proximal femoral asphericity or acetabular overcoverage. A secon limitation of this stuy is that, although the hips were asymptomatic, they i not unergo an actual physical examination. It is not actually known whether the recruite subjects were truly free of impingement signs, espite the fact that none of them reporte previous hip symptoms. Thir, femoral anteversion was not measure. It has been ocumente that ecrease femoral anteversion as to the risk of femoroacetabular impingement 47,48. However, measuring femoral torsion requires scans mae through the femoral conyles, which was impractical for the stuy protocol. Fourth, some of the cutoff values for efining the femoroacetabular impingement risk factors aopte in this stuy (summarize in Table I) might be consiere arbitrary. Although the current literature suggests that these risk factors for femoroacetabular impingement might contribute to the pathogenesis of femoroacetabular impingement synrome, the exact values associate with the evelopment of femoroacetabular impingement have not yet been establishe. We believe, however, that it is the overall morphology of the hip in terms of specific combinations of anatomical abnormalities, combine with an increase activity level, that gives rise to femoroacetabular impingement. In conclusion, this stuy showe that Chinese an white subjects iffer significantly with regar to hip anatomy: the proximal part of the femur tens more towar varus in white men an has a less spherical femoral hea in white subjects, whereas the acetabulum is more shallow Chinese subjects. The prevalence of raiographic signs associate with a risk of the evelopment of femoroacetabular impingement was significantly higher in white subjects than in Chinese subjects. The observe absolute ifferences between the two racial groups, however, appeare too small to explain the reporteifferences in the prevalence of hip osteoarthritis. This fining seems to suggest that femoroacetabular impingement morphology is probably at most a seconary risk factor for the evelopment of hip osteoarthritis, at least in the Chinese population. Future work shoul focus on further investigating the cause for this contraictory fining. Appenix Tables showing the ages of the subgroups as well as the interobserver an intraobserver repeatability of measurements of the raiographic parameters an figures emonstrating techniques for measuring the raiographic parameters are available with the online version of this article as a ata supplement at jbjs.org. n Jan Van Houcke, MD Wouter Huysse, MD Hannes Dechamps Christophe Pattyn, MD, PhD Emmanuel Albert Auenaert, MD, PhD Departments of Orthopaeic Surgery an Traumatology (J.V.H., H.D., C.P., an E.A.A.) an Raiology (W.H.), Ghent University Hospital, De Pintelaan 185, B-9000 Ghent, Belgium. aress for E.A. Auenaert: emmanuel.auenaert@ugent.be Wan Pan Yau, MBBS, FRCSE, FHKAM, FHKCOS Chun Hoi Yan, MBBS, FRCSE, FHKAM, FHKCOS Wing Hang Lau, MBBS, FHKAM Chun Sing Wong, MBChB, FRCR, FHKCR, FHKAM Departments of Orthopaeic Surgery an Traumatology (W.P.Y. an C.H.Y.) an Raiology (W.H.L. an C.S.W.), Queen Mary Hospital, University of Hong Kong, 102 Pokfulam Roa, Hong Kong, China

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