Multiplier method for predicting limb-length discrepancy

Background: In patients with a congenital or developmental limb-length disc repancy, the short limb grows at a rate proportional to that of the normal, long limb. This is the basis of predicting limb-length discrepancy with ex isting methods, which are complicated and require multiple data points. The purpose of our study was to derive a simple arithmetic formula that can ea sily and accurately predict limb-length discrepancy at skeletal maturity. Methods: Using available databases, we divided the femoral and tibial lengt hs at skeletal maturity by the femoral and tibial lengths at each age for e ach percentile group. The resultant number was called the multiplier, Using the multiplier, we derived formulae to predict the limb-length discrepancy and the amount of growth remaining. We verified the accuracy of these form ulae by evaluating two groups of patients with congenital shortening who we re managed with epiphysiodesis or limb-lengthening. We also calculated and compared the multipliers for other databases according to radiographic, cli nical, and anthropological lower-limb measurements. Results: The multipliers for the femur and tibia were equivalent in all per centile groups, varying only by age and gender. Because congenital limb-len gth discrepancy increases at a rate proportional to growth, the discrepancy at maturity can be calculated as the current discrepancy times the multipl ier for the current age and the gender. This calculation can be performed,v ith use of a single measurement of limb-length discrepancy. For progressive developmental (noncongenital) discrepancies, the discrepancy at skeletal m aturity can be calculated as the current discrepancy plus the growth inhibi tion times the amount of growth remaining. The tinting of the epiphysiodesi s can also be calculated with the multiplier. The predictions made,vith use of the multiplier method correlated well with those made,vith use of the M oseley method as well as with the actual limb-length discrepancy in both th e limb-lengthening and epiphgsiodesis groups. The multipliers derived from the radiographic, clinical, and anthropological measurements of femora and tibiae were all similar to each other despite differences in race, ethnicit y, and generation. Conclusions: The multiplier method allows for a quick calculation of the pr edicted limb-length discrepancy at skeletal maturity, without the need to p lot graphs, and is based on as few as one or two measurements. This method is independent of percentile groups and is the same for the prediction of f emoral, tibial, and total-limb lengths. The multiplier values are also inde pendent of generation, height, socioeconomic class, ethnicity and race. We verified the accuracy of this method clinically by evaluating patients who had been managed with limb-lengthening or epiphysiodesis. The method was al so comparable with or more accurate than the Moseley method of limb-length prediction.