Anterior Hip Clearance in Residual Legg-Calvé-Perthes Disease.

Background: The mechanical environment in the hip in people with residual Legg-Calvé-Perthes disease (LCPD) deformity is still poorly understood. Anterior impingement is thought to contribute to poor long-term outcomes such as early-onset osteoarthritis, but it has not been measured directly using imaging in high flexion. Our objective in this study was to determine the association between radiographic morphologic scores of LCPD deformity and reduced anterior hip clearance.

Methods: We measured the anterior clearance (β-angle) of 20 LCPD-affected hips (17 patients) scanned in 4 functional postures using an upright open MRI scanner. Mixed effects models were used to describe the relationship between β, posture, and morphologic measures of deformity (Stulberg classification and Sphericity Deviation Score).

Results: Hip β-angle was significantly associated with posture (P<0.001). Aspherical (Stulberg III to V) hips had lower β across all postures compared with spherical (Stulberg I to II) hips (difference in β = -39.1 degrees; 95% CI: -71.9 to -6.2 degrees; P=0.020). An increased SDS was strongly associated with reduced β in neutral hip postures (P=0.002, 0.005, respectively), but not with elevated adduction and internal rotation. Due to morphologic heterogeneity, 6 hips (out of 20 overall) did not fit the overall trend.

Conclusions: Our results show an association between more severe radiographic deformity and a greater potential for anterior impingement, a known contributor to cartilage degradation. However, the large proportion of cases where morphologic deformity does not align with functional clearance suggests current radiographic measures of deformity may not be sufficient to predict long-term outcomes in every LCPD patient.

Clinical relevance: (1) Hip joint asphericity in LCPD is associated with less anterior clearance and a greater potential for anterior impingement in high flexion postures. (2) Considering both morphologic and functional parameters may improve our understanding of the causes of pain and early-onset osteoarthritisin LCPD, as opposed to morphology alone.