The sphericity deviation score: a quantitative radiologic outcome measure of Legg-Calvé Perthes disease applicable at the stage of healing and at skeletal maturity.

Aim: This study aimed to determine if a new measure of sphericity of the femoral head, the Sphericity Deviation Score (SDS), could be estimated reliably at the stage of healing of Legg-Calvé-Perthes disease (LCPD) and to determine if the SDS at the healing stage reflects the outcome at skeletal maturity.

Methods: The SDS was measured with image analysis software on radiographs of 120 children with unilateral LCPD at healing of the disease and at skeletal maturity. The reproducibility of measurement was assessed. SDS values for hips in different Stulberg classes were calculated. On the basis of SDS values at healing of 82 children, a logistic regression model was developed to determine the probability of a good outcome at skeletal maturity. The validity of the model was tested on another 38 children.

Results: The SDS values at the stage of healing and at skeletal maturity were comparable. The SDS values were lowest for Stulberg Class I hips and highest for Class IV and V hips. The validated regression model showed a very high probability of a good outcome (Stulberg Class I or II) at skeletal maturity if the SDS at the stage of healing was below 10.

Conclusions: A reliable estimate of the outcome of LCPD at skeletal maturity can be made by computing the SDS as soon as the disease heals.