Poor long-time results in Legg-Calvé-Perthes disease (LCP) have in previous reports been attributed to the presence of degenerative joint disease due to deformities of the femoral head. Reports have stated that hip dysplasia (HD) pre-dispose to hip osteoarthritis (OA). The increased risk of OA in LCP disease might be caused by an increased risk of secondary HD. We have evaluated the prevalence of secondary HD by applying Wiberg's CE angle, acetabular index angle, femoral head extrusion index, the acetabular depth ratio, and the ACM angle in both the affected and non-affected hip.

From 1941 to 1962, 167 patients with LCP presented to The Community of Disabled in Kolding, Denmark. All hips included were treated conservatively by a Thomas splint. At follow-up weight-bearing AP pelvis radiographs were obtained. As control group radiographs of sex- and age-matched persons were obtained from The Copenhagen City Heart Study.

The following exclusion criteria were applied: bilateral involvement, emigrated persons, persons lost to follow-up, diseased persons, patients who refused to participate, surgery at pelvis or the lower limb before follow-up, foramen obturator index 1.8 and missing or destroyed radiographs. Forty-nine patients were included.

The prevalence of secondary HD evaluated from pre-defined radiographic parameters was overall significant increased regardless of Stulberg class in the affected hip compared to a sex- and age-matched control group. The prevalence of secondary HD in the non-affected hip was significantly increased evaluated from the CE angle and AA angle in Stulberg class III/IV/V.

The strengths of our study were that the same conservative treatment regime was applied to all the children regardless of gender, age at debut and degree of femoral head involvement. 118 patients were excluded, however we do not have any reason to believe that they were excluded systematically.

The reviewed Stulberg classification was used. The advantages of the classification included the simplicity of application and reliability with good inter- and intraobserver agreement. Previous report has also shown good inter- and intraobserver agreement for the CE angle, AA angle and FHEI, while ADR and the ACM angle suffers from poor inter- and intraobserver reliability

Pelvis rotation and inclination/reclination have an effect on the CE angle and ADR. We assessed the pelvis rotation using Tönnis foramen obturator index and included patients within 0.7-1.8.

In conclusion the study showed that LCP patients have a significantly increased risk of secondary HD in the affected hip compared to a sex- and age-matched control group, as well as an increased risk in the non-affected hip in patients in Stulberg class III/IV/V. It may be that what we considered as a non-affected hip is affected anyway.

Fishers exact test was used. A significance level of p