A formula for diaphyseal limb lengthening.

Calculation of the expected limb length discrepancy at maturity plays an important part in the timing of operations designed to equalise length. Present methods assume similar pre-operative and postoperative rates of bone growth in limbs undergoing diaphysea! lengthening procedures. Moseley (1977) conceived a straight line graph to predict the optimal timing ofsurgery to lengthen the shorter limb, or to slow growth in the longer one. Recently Shapiro (1987) has calculated more precise growth rates after diaphysea! lengthening ; he defined different pre-operative and postoperative longitudinal growth rates in femora and tibiae. Before operation the rate of growth of congenitally short femora was, on average, 82% of that on the normal side ; after operation the figure was 90%. An average of 88% pre-operatively and 64% postoperatively was found in congenitally short tibiae and in tibiae which were short secondary to O!!ier’s disease, poliomyelitis and haemangioma. These rates of growth may vary according to the disease process. Method. A simple formula for predicting the length of a normal bone at maturity is proposed. It combines the differential growth rates found by Shapiro, with the standard charts produced by Anderson and Green (1948), Anderson, Messner and Green (1964) and by Maresh (1955). This formula should allow the amount of lengthening required for equalisation to be predicted with greater accuracy. If SAP is the length of the shorter bone at presentation, LAP is the length of the longer bone at presentation, LAM is the length of the longer bone at maturity, and PLN is the predicted lengthening necessary, then the formula is: