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The deformity index as a predictor of final radiological outcome in Perthes' disease

D. Nelson,
 M. Zenios,
 K. Ward,
 M. Ramachandran, D.G. Little
Children’s Hospital at Westmead, Westmead, Australia
JBJS (British), Vol. 89-B, No. 10, October 2007

Abstract Introduction: The long-term outcome in Perthes’ disease is related to the deformity of the femoral head and its congruency with the acetabulum. There is no satisfactory system, which allows assessment of outcome before skeletal maturity. The deformity index is a new radiological measurement of the degree of deformity of the femoral head in unilateral Perthes’ disease. Its values represent a continuous outcome measure of deformity incorporating changes in femoral epiphyseal height and width compared with the unaffected side. Methods: The sphericity of the femoral head in 30 radiographs (ten normal and 20 from patients with Perthes’ disease) were rated blindly as normal, mild, moderate or severe by three observers. Further blinded measurements of the deformity index were made on two further occasions with intervals of one month. We also reviewed retrospectively 96 radiographs of children with Perthes’ disease, who were part of a multicentre trial, which followed them to skeletal maturity. We found that the deformity index at two years correlated well with the Stulberg grading at skeletal maturity. A deformity index value above 0.3 was associated with the development of an aspherical femoral head. Using a deformity index value of 0.3 to divide groups for risk gives a sensitivity of 80% and specificity of 81% for predicting a Stulberg grade of III or IV. Results: There was good agreement between the deformity index score and the subjective grading of deformity. Intra and interobserver agreement for the deformity index was high. The intraobserver intraclass correlation coefficient for each observer was 0.98, 0.99 and 0.97, respectively, while the interobserver intraclass correlation coefficient was 0.98 for the first and 0.97 for the second set of calculations. Conclusions: Deformity index at two years is a valid and reliable radiological outcome measure in unilateral Perthes’ disease.

Comment by Shalin Maheshwari

The ultimate goal when managing a child with Perthes’ disease is to maximize congruity of femoral head and acetabulum. The premise is that by limiting the degree of deformity of the femoral head, the likelihood of premature osteoarthritis and leg length discrepancy is reduced.

Deformity Index (DI) as defined in this paper is the maximal orthogonal differences in height and width between the affected and normal epiphyses summed and divided by the width of the normal growth plate.

Various radiographic classification systems have been developed to assess femoral head deformity. These include the Catterall, Salter-Thompson, lateral pillar of Herring in fragmentation stage and Stulberg's classification as an outcome measure at skeletal maturity. Numerous studies exist demonstrating inter and intra-observer reliability of these systems, however it is controversial as to which of these has the best prediction potential. Nelson et al, in their paper, cite various authors who have shown that Herring's method is the best predictor of outcome and the most reliable. Clearly the reliability of the different systems depends not only on factors intrinsic to the systems, but also on observer factors such as experience and interpretation of, and familiarity with, a given classification. So, is there room for yet another classification system? The answer to this is probably: Yes, as long as it fulfills the following requirement equal (or improved) prediction of long-term outcome with improved inter and intra-observer reliability when compared to existing classifications.

Deformity Index was designed as a grading system in the healing phase, which could reliably predict radiological outcome, thus giving a continuous outcome measure of deformity of femoral head. Newer treatment options for Perthes’ disease are emerging, including pharmacological treatment, acetabular augmentation and distraction techniques. The authors have chosen a time period of two years after presentation at which to assess the DI and predict the final Stulberg score, this is roughly the time that remodelling begins. Nelson et al have suggested that an objective measurement of deformity of the femoral head by the deformity index at that time predicts the outcome at skeletal maturity. This contrasts with the lateral pillar classification which, although an early marker of prognosis has not been shown to be altered by intervention. The authors hope that the DI at two years will serve as a baseline research tool to monitor the effects of treatment protocols on outcome at maturity. This would call for large randomised, controlled trials, the results of which would have significant impact.

Perhaps the most obvious current pitfall of the DI as presented is its limitation to unilateral disease. The authors rise to its defence by stating that patients with bilateral Perthes’ disease may have an underlying disorder and therefore may not truly have Perthes’ disease and should be excluded from research trials. While it is true that some patients with bilateral involvement will not have Perthes’ disease, this is not universal. Bilateral Perthes’ disease is recognized in 8 to 24% of cases; in one series being synchronous in 52% and metachronous in 48% of 50 patients. Does bilateral disease have a worse prognosis than unilateral? With emerging drug therapies will the incidence and outcome of bilateral disease be reduced or improved? It would be interesting to include patients with bilateral disease in any prospective research protocols. Their results could be analyzed separately.

The authors shun the use of current measures of sphericity such as Mose circles. It may be possible from a large database of normal hips of children at varying age to develop standards for use in assessing the DI in children with bilateral disease. This could have the additional benefit of limiting radiographs to one side in children with unilateral disease. In addition to sphericity of the femoral head, the age of the child at the onset of the disease is an important predictor of long-term outcome. Also a two dimensional analysis and rotation of hip during X-rays can alter the measurements. The authors have not addressed this in their current paper, but perhaps plan to do so in future.

In summary, there is no satisfactory system, which allows assessment of outcome before skeletal maturity. Since follow-up of at least 15 years is required to assess outcome, it is not feasible to use the Stulberg grading to evaluate new forms of treatment for Perthes’ disease. Nelson et al introduced us to Deformity Index as a novel measure that allows prediction of Stulberg classification from radiographs obtained in children with unilateral Perthes’ disease at two years following the onset of the disease. The use of a continuous outcome measure enables trials of a feasible size to be adequately powered to test the hypothesis that new treatments may reduce the progression of deformity of the femoral head. Trials with promising results at two years based on the deformity index could be expanded and extended to meet the Stulberg outcome. If reliable, the DI could be an important research tool for randomized, controlled trials of interventions in Perthes’ disease. Certainly inter and intra-observer reliability in the hands of the authors is high; further evaluation by many observers is required.