Radiographic classifications in Perthes disease

Abstract

Background and purpose - Different radiographic classifications have been proposed for prediction of outcome in Perthes disease. We assessed whether the modified lateral pillar classification would provide more reliable interobserver agreement and prognostic value compared with the original lateral pillar classification and the Catterall classification. Patients and methods - 42 patients (38 boys) with Perthes disease were included in the interobserver study. Their mean age at diagnosis was 6.5 (3-11) years. 5 observers classified the radiographs in 2 separate sessions according to the Catterall classification, the original and the modified lateral pillar classifications. Interobserver agreement was analysed using weighted kappa statistics. We assessed the associations between the classifications and femoral head sphericity at 5-year follow-up in 37 non-operatively treated patients in a crosstable analysis (Gamma statistics for ordinal variables, γ). Results - The original lateral pillar and Catterall classifications showed moderate interobserver agreement (kappa 0.49 and 0.43, respectively) while the modified lateral pillar classification had fair agreement (kappa 0.40). The original lateral pillar classification was strongly associated with the 5-year radiographic outcome, with a mean γ correlation coefficient of 0.75 (95% CI: 0.61-0.95) among the 5 observers. The modified lateral pillar and Catterall classifications showed moderate associations (mean γ correlation coefficient 0.55 [95% CI: 0.38-0.66] and 0.64 [95% CI: 0.57-0.72], respectively). Interpretation - The Catterall classification and the original lateral pillar classification had sufficient interobserver agreement and association to late radiographic outcome to be suitable for clinical use. Adding the borderline B/C group did not increase the interobserver agreement or prognostic value of the original lateral pillar classification.

Radiographs of a boy with Perthes disease of the left hip. A and B. AP and lateral projections at diagnosis (age 8 years) of Perthes disease. C and D. Radiographs taken 8 months after diagnosis show fragmentation of the femoral head. The observers classified the radiographs with the following categories: Catterall group 3 (4 observers), Catterall group 2 (1 observer); original lateral pillar type B (4 observers), type C (1 observer); modified lateral pillar type B/C (3 observers), type B (2 observers). E and F. AP and frog-leg radiographs taken 4 years and 7 months after diagnosis, at an age of 13 years. Both projections show healing and were classified according to the modified Stulberg classification as follows: round femoral head (2 observers) and ovoid femoral head (2 observers). 2 observers reassessed the radiographs. These observers agreed upon round femoral head as “true” modified Stulberg classification.