The growth inhibition can sometimes be estimated accurately on the basis of the pathogenesis of the discrepancy.

There are usually two components of the discrepancy of which the orthopaedic surgeon should be aware, the absolute discrepancy and the growth inhibition. The absolute discrepancy describes the patient at one point in time and can be measured objectively. The growth inhibition provides a view of the patient’s growth over time and a means for extrapolating growth and discrepancy into the future.It cannot be measured directly and must be calculated from measurements of discrepancy over time.

There are several clinical and radiological ways to measure leg length discrepancy, but only the radiological methods provide sufficient accuracy for the analysis of growth and the prediction of the future. These methods include the teleoroentgenogram, orthoroentgenogram, scanogram and computerized tomography.

Consider, for example, the effect on the apparent leg length discrepancy of an adduction contracture of the hip.

The orthopaedic surgeon should not be blinded by radiologic measurements, however, since they usually represent only that part of the discrepancy that is located between the ankle and the femoral head. There may be other factors, both anatomical and functional, which are involved in the asymmetry of the patient’s gait and these must be taken into account.

The clinical examination should include placing blocks under the heel of the short leg to produce the optimum posture. If the height of the blocks is not equal to the measured discrepancy then it behooves the surgeon to understand why not. In addition, Menelaus’s method for timing epiphyseodeis was intended to be used only with clinical measurement of discrepancy by blocks, and not with rodiologic measurements of leg length.

This term is used, somewhat inappropriately perhaps, even when the discrepancy is caused by one leg growing abnormally rapidly. Growth inhibition relates the growth rate of the short leg to that of the long leg and is constant throughout growth once the active phase of the disturbance causing the discrepancy is past. This quantity is determined by comparing the amounts of growth of the two legs over the same period of time. The assumption that this relative rate of growth will be continued into the future allows the prediction of future growth.