Theoretical study of subluxation in early Legg-Calvé-Perthes disease

The relationship between mechanical subluxation and femoral head necrosis geometry in Legg-Calvé-Perthes disease (LCP) was investigated with a three-dimensional rigid body-spring method hip model. Femoral head models with progressively larger regions of necrosis, corresponding to the four Catterall grades, were placed in a spherical acetabular model and studied in static single-limb stance configuration, with variable mechanical rigidity of the necrotic segment. The degree of subluxation was dependent on the geometric region of involvement, mechanical properties of the segment, and direction of loading force. In general, femoral head subluxation was always in the direction of the necrosis, modified by the anatomic and force environment. In the neutral position, the Catterall I models exhibited minimal subluxation. The Catterall II model subluxated anteriorly as collapse occurred, and the Catterall III model subluxated anteriorly and superiorly. Lateral subluxation could be produced by changing the loading force to a more vertical orientation. The Catterall IV model collapsed directly along the line of force application rather than subluxating. Reorientation of models with minor necrosis could improve stability, but reorientation of models with extensive necrosis had minimal effect on subluxation behavior. Higher-grade LCP involvement may lead to early subluxation, particularly anteriorly, which is difficult to visualize radiographically. The appearance of lateral subluxation may signal a change in the mechanical environment of the hip from clinical progression of the disease. Femoral head reorientation (osteotomy) may improve femoral head stability when necrosis is limited but is unlikely to reduce subluxation or collapse when extensive necrosis is present.