Acetabular retroversion was found in thirty-one (53%) of fifty-nine hips (five [50%] of ten Stulberg class-I hips; nine [53%] of seventeen class-II hips; nine [56%] of sixteen class-III hips; three [33%] of nine class-IV hips; and five [71%] of seven class-V hips). Acetabular retroversion was not found in the seven hips treated by varus osteotomy, but was found in 32% (six) of nineteen hips treated conservatively, 40% (two) of five hips treated by arthrodiastasis, and 90% (nineteen) of twenty-one hips treated by Salter or triple osteotomy.
Abnormalities of the acetabular labrum and cartilage were found on MRI scans of 75% (forty-four) of fifty-nine hips and 47% (twenty-four) of fifty-one hips, respectively. The overall frequency of labral and cartilage abnormalities identified on MRI scans was 75% (forty-four hips) and 45% (twenty-three hips), respectively, for reader 1 and 76% (forty-five hips) and 47% (twenty-four hips) for reader 2. Interobserver and intraobserver agreement was considered at least substantial for labrum and cartilage evaluation (interobserver weighted kappa, 0.82 and 0.81, respectively; intraobserver weighted kappa varied from 0.78 to 0.83)46.
The frequency of labral abnormalities included type I in two hips (3%), type II in twelve (20%), type III in nineteen (32%), and type IV in eleven (19%). Of the forty-four labral abnormalities, most were smaller than one quadrant (type A in four hips [9%] and type B in nineteen hips [43%]). The remaining hips had abnormalities extending greater than one quadrant (type C in twelve hips [27%] and type D in nine hips [20%]). Articular cartilage delamination, which was detected in fifteen (29%) of fifty-one hips, was the most frequent MRI abnormality of the acetabular cartilage. Most labral and acetabular cartilage abnormalities were located at the anterolateral aspect of the acetabulum (3h to 12h; Fig. 6). There was strong correlation between labral and acetabular cartilage abnormalities (p = 0.002; OR = 16). The MRI abnormalities of the acetabular labrum and cartilage did not change significantly between different durations of disease (four to fifteen years; t test, p = 0.21).
Estimating a normal range of alpha angle in patients with Legg-Calvé-Perthes disease, the receiver operating characteristic curve analyses showed a graphic of true-positive rates compared with false-positive rates, with an area under the curve of 0.74. The cutoff of 55° represented the best correlated value (sensitivity of 71% and specificity of 93%). The mean MRI alpha angle value was 71° ± 33° (range, 30° to 130°; median, 78°; 95% confidence interval [CI], 63° to 80°) for all samples, 45° ± 20° (range, 30° to 113°; median, 42°; 95% CI, 33° to 56°) for hips with a normal labrum, and 80° ± 32° (range, 31° to 130°; median, 92°; 95% CI, 70° to 90°) for hips with a labral abnormality on MRI. There was significant correlation between increased alpha angle and the presence of acetabular labral (p < 0.001) and articular cartilage (p < 0.01) MRI abnormalities (see Appendix).
Absolute values of the radiographic and MRI alpha angle showed no significant agreement (paired t test, p < 0.05; mean of differences, 5°; 95% CI, 0.5° to 10°), but there was effective correlation of variability (coefficient, 0.85). The alpha angle was not accurately measurable in oblique axial MRI plane or lateral hip radiographs in 32% (nineteen) of the fifty-nine hips because of a major loss of sphericity. This feature was not observed in Stulberg class-I or II hips, but it was present in 31% of Stulberg class-III hips and 85% to 89% of Stulberg class-IV or V hips.
The frequency distribution of MRI findings, according to the Stulberg classification and the presence or absence of acetabular retroversion, is shown in a table in the Appendix. A normal labrum is significantly correlated with the Stulberg class-I group without acetabular retroversion (p < 0.001). Labral abnormalities were associated with Stulberg class I with acetabular retroversion and with Stulberg class-II, III, IV, and V groups. Among these groups with hip deformities, no significant differences were detected for labral (p > 0.05) or cartilage abnormalities (p = 0.15).
When all radiographic deformities were considered independently (Table II), the increased alpha angle was the factor most significantly associated with both acetabular labral and cartilage abnormalities, followed by coxa brevis. Coxa magna and a higher greater trochanter also showed significant association with labral abnormalities only. However, as deformities coexist and are considered dependent variables, a logistic regression for multivariate analysis was used. In the presence of other deformities, the alpha angle showed the strongest correlation and the highest relative risk for MRI abnormalities of the acetabular labrum (p = 0.01; OR = 17) and articular cartilage (p = 0.02; OR = 5). Coxa brevis showed significant correlation and increased relative risk only for labral abnormalities (p = 0.02; OR = 6).
For hips with a normal alpha angle (Table II), coxa brevis and a higher greater trochanter showed significant association with MRI labral abnormalities only. In addition, acetabular retroversion and coxa magna also showed increased relative risk for MRI labral abnormalities, but this difference was not significant on the basis of our sample size.
The radiographic data for our ninety-nine patients (108 hips) with healed Legg-Calvé-Perthes disease showed that only nine patients (9%) had no deformity associated with labral and/or chondral abnormalities (Stulberg class I and no acetabular retroversion). Most of the patients (91%) had at least one deformity that could increase the risk for acetabular labral abnormalities. Coxa brevis was found in 74% (eighty) of all 108 hips; coxa magna, in 69% (seventy-four); an abnormal alpha angle, in 53% (fifty-seven); a higher greater trochanter, in 50% (fifty-four); and acetabular retroversion, in 50% (fifty-four).
The cases of two patients with Legg-Calvé-Perthes disease are illustrated in figures in the Appendix.
A complex variety of deformities may occur in patients with healed Legg-Calvé-Perthes disease, which might result in later hip osteoarthritis7,12. The classical mechanical concepts of coverage, containment, concentricity, and congruence have been considered the primary prognostic factors associated with hip osteoarthritis in Legg-Calvé-Perthes disease4,29,48. However, the development of preceding hip problems has recently been associated with the femoroacetabular impingement phenomenon, which is more related to repetitive motion than to axial mechanical overloading10,11. On the basis of clinical and imaging findings20, the proper diagnosis and treatment of femoroacetabular impingement may hypothetically relieve the progression of hip degeneration12,49,50.
As the hip is a ball-and-socket joint whose range of motion is limited by the osseous geometry and soft-tissue components51,52, deformities secondary to Legg-Calvé-Perthes disease may affect the joint clearance.
Femoroacetabular impingement and hinge flexion are thought to be an important source of pain in healed Legg-Calvé-Perthes disease4,12. However, patients affected by Legg-Calvé-Perthes disease can show high grades of radiographic deformity but remain almost asymptomatic with daily activities5,7,8. As the study purpose was to evaluate imaging findings, we did not take into account symptoms as a criterion of inclusion, but aimed at prospectively identifying radiographic conditions possibly related to femoroacetabular impingement, such as labral and cartilage MRI abnormalities, and abnormal head-neck morphology25. When the imaging diagnostic methods are considered, MRI is the most reasonable noninvasive method to evaluate intra-articular structures of the hip24,28 (see Appendix).
The loss of offset was the main variable related to MRI abnormalities of the acetabular labrum and cartilage. On the basis of previous studies43,44,53,54, alpha angles of <55° are considered normal for healthy individuals. Siebenrock et al.55 reported a mean alpha angle of 37° at the three o’clock position for healthy nonathlete volunteers with a mean age of seventeen years. However, a cutoff value has not been established for femoroacetabular impingement in children or for children with Legg-Calvé-Perthes disease. Despite the lack of clinical correlation, our results suggested that 55° might also be an applicable cutoff value for MRI labral abnormalities in Legg-Calvé-Perthes disease.
Nevertheless, as it was originally described, the alpha angle may not be completely feasible for all hips. For deformities like coxa plana, the center of the femoral head may be posteriorly dislocated and not coincident to the femoral neck axis in the MRI oblique axial plane or lateral radiographic view. A best-fit circle may not be perfectly achieved, and the aspherical point may be located over the articular surface and not over the cervicocapital junction. Absolute values of alpha angles with aspherical femoral heads showed low quantitative reliability, but there was qualitative agreement to assert whether the head-neck contour was normal or not.
For hips with an increased alpha angle, we found a remarkably high frequency of labral abnormalities (97%), predominantly of type III, located at the anterolateral region of the acetabular rim. However, we also found labral abnormalities on MRI scans in hips with a normal alpha angle. In these cases, our results showed that coxa brevis is the deformity most significantly associated, but an overriding greater trochanter and acetabular retroversion also trended toward an increased relative risk.
Some limitations of this study must be noted. As both observers were aware of the possibility of the diagnosis of femoroacetabular impingement, an expectative bias might have occurred. To decrease the chance of false-positive interpretations, we evaluated very carefully images suggestive of sublabral recess (see Appendix), which are considered normal variations41,42, and only abnormalities found in two or more MRI slices or planes were considered. Some images of acetabular cartilage in peripheral MRI sagittal slices of the acetabulum may actually correspond to the chondrolabral junction, and not to the articular cartilage. Therefore, a triplanar evaluation is mandatory. Nevertheless, we do not have any surgical data as a gold standard to validate our MRI findings and our comparative group is small. Although our results strongly suggest morphological and degenerative findings, several patients in our study had functional and asymptomatic hips that did not require surgical treatment. Another limitation is the lack of correlation with specific symptoms and signs of femoroacetabular impingement.
The classification system of Stulberg et al.29 could presumably identify hips prone to labral and cartilage abnormalities. However, the acetabular side has to be evaluated for the presence of acetabular retroversion. Our data showed that the Stulberg class-I group without acetabular retroversion had significant correlation with the presence of a normal labrum on MRI, whereas the class-I group with acetabular retroversion and the groups with class-II through V deformities showed no significant correlation.
In many patients with Legg-Calvé-Perthes disease, the acetabulum remodels congruently with the femoral head deformity. Lateral subluxation or extrusion of the lateral pillar due to coxa magna may also occur and, as a result, the labrum may be intermittently submitted to abnormal compression and shearing loads. Perilabral cysts may occur56. Retroversion may also occur after acetabular remodeling4,15, increasing the chance of pincer femoroacetabular impingement development. Our results did not show an increased risk for abnormalities of the Wiberg angle, but there is a fourfold risk for acetabular retroversion when the alpha angle was normal.
The high frequency of lesions located at the anterolateral portion of the acetabular rim (2h to 12h) in our study is in agreement with the data reported for patients not diagnosed with Legg-Calvé-Perthes disease35-37,41,42,57-59. A higher frequency of lesions situated at the superior portion (12h) could be explained by the enlarged cervicocapital region typically found in Legg-Calvé-Perthes disease, which is larger and wider than in patients with idiopathic femoroacetabular impingement.
In conclusion, we identified an elevated frequency of MRI abnormalities of the acetabular labrum and articular cartilage in hips with healed Legg-Calvé-Perthes disease. The loss of sphericity of the femoral head associated with a reduced offset (an abnormal alpha angle) was the most significantly associated predisposing factor for abnormalities of the acetabular labrum and cartilage, but coxa brevis with an overriding greater trochanter or coxa magna also showed an increased risk for labral abnormalities. Acetabular retroversion may predispose to labral abnormalities even when the alpha angle is normal. Hip deformities possibly related to labral and cartilage abnormalities were found in 91% of our patients.
A description of MRI evaluation of the hip (including references 60 through 73, which are cited only in the Appendix) as well as tables showing the distribution of MRI abnormalities of the acetabular labrum and articular cartilage with regard to hips with alpha angles of <55° or ≥55°; the distribution of such abnormalities with regard to the Stulberg groups and the presence or absence of acetabular retroversion; and the sensitivity, specificity, and accuracy of MRI with regard to the evaluation of acetabular labral and cartilage abnormalities; figures demonstrating the classification systems used for MRI grading of labral findings and the extent of labral abnormalities according to clock-face nomenclature, MRI scans of two patients with Legg-Calvé-Perthes disease, and an oblique axial MRI scan showing a sublabral recess are available with the online version of this article as a data supplement at jbjs.org.
Investigation performed at the Division of Pediatric Orthopaedics of the Departments of Biomechanics, Medicine, and Rehabilitation of the Locomotor System, and Division of Imaging Sciences, School of Medicine of Ribeirão Preto, University of São Paulo, Ribeirão Preto, São Paulo, Brazil
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Disclosure: None of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of any aspect of this work. None of the authors, or their institution(s), have had any financial relationship, in the thirty-six months prior to submission of this work, with any entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. Also, no author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.Copyright 2013 by The Journal of Bone and Joint Surgery, Incorporated