Analysis and Prediction

Introduction

Patients with leg length discrepancy who are still in their growing years have a discrepancy that is measurable and perhaps also an abnormal growth rate that is not. Arriving at a treatment decision in such cases involves an analysis of past growth, a prediction of future growth, and a prediction of the effects of the surgical alternatives. Several methods are available to help the orthopaedic surgeon in this task.
There are at least three methods useful in the management of patients with leg length discrepancy. Only the Straight Line Graph method is a general tool for the description, analysis and prediction of growth. The others are specific tools designed for the timing of epiphysiodesis. The methods also differ in their accuracy and ease of use.

The Arithmetic Method

This method is used in the timing of epiphysiodesis and has been presented, in one form or another, by several authors. Pedipod/LLD applies the method as described by Westh and Menelaus.
Their method is based on several assumptions:
  1. The distal femoral growth plate grows at 3/8 inch per year.
  2. The proximal tibial growth plate grows at 1/4 inch per year.
  3. Boys stop growing at 16 years of age.
  4. Girls stop growing at 14 years of age.
  5. If one of the growth plates is completely arrested, the discrepancy will increase at a rate corresponding to the normal rate of growth for that plate. Otherwise, it will increase at 1/8 inch per year.
  6. Calendar age is adequate in assessing growth and skeletal age need not be assessed.
Menelaus intends that his method only be used with clinical measurements of the discrepancy by blocks, and not by calculation from radiological measurements of leg length.
Menelaus makes the point that the calculations can be done easily by mental arithmetic if English measure is used.
The six assumptions, applied to the patientís age and measured discrepancy, allow the prediction of future growth and the determination of the timing of epiphysiodesis by relatively simple arithmetic calculations.
Menelaus advises that the method should not be used for children under the age of 8 years, presumably because the above assumptions do not approximate the true growth pattern closely enough. He also advises that it should not be used if chronological age and calendar age differ by more than one year.

Advantages

This method is the most convenient of the three described here. It does not require special resources or tools and the arithmetic calculations are simple.

Disadvantages

The normal pattern of growth is not as simple as expressed in these assumptions. The growth rate is not constant and growth does not stop abruptly at a specific age. Nevertheless, this approximation is fairly accurate and allows the timing of epiphysiodesis to be made with acceptable accuracy. The accuracy of this method diminishes for children not yet in the final years of growth. It should therefore not be used for children younger than eight.
The use of calendar age can give rise to significant errors in children who are significantly advanced or delayed in maturation. Children of advanced skeletal age will be undercorrected by epiphysiodesis, and those of delayed skeletal age will be overcorrected. To avoid this type of error, this method is not used for children whose skeletal and chronological ages differ by more than one year. Menelaus has not specifically recommended the use of skeletal age in such cases.
The assumptions used to arrive at the rate of increase of the discrepancy may not always apply. In the case of children with discrepancies that began at birth the assumption that it is increasing at the rate of 1/8 inch per year is acceptably accurate. Such children, in order to have attained discrepancies within the range considered suitable for epiphysiodesis, must have developed their discrepancies at about that rate. Also, in children whose discrepancies are the result of events later in their lives that have completely obliterated one growth plate, it is accurate to assume that the rate of increase is equal to the normal growth rate of that plate. However, in children whose discrepancies were caused by a childhood event or illness which cannot be determined to have completely arrested growth Menelaus does not provide us with a method for determining the rate of increase.

The Growth Remaining Method

This method is based on data from the Growth Study of the Childrenís Hospital Medical Center in Boston as published by Anderson, Messner and Green. They published, in two different papers, the absolute lengths of the femur and tibia, and the growth remaining in the epiphyseal plates about the knee, for boys and girls of ages from 1 to maturity.
Pedipod/LLD shows the patientís leg lengths superimposed on their graph, but does not implement their method for timing epiphysiodesis.

Advantages

This method is based not on approximations of growth but on scientific data describing patterns of growth. Skeletal age is usually used in the determinations. Two possible sources of error in the arithmetic method are thus avoided. In addition, the growth inhibition is calculated, not assumed.

Disadvantages

This method is less convenient than the arithmetic method since users must have available the published graphs. That showing growth remaining is used to predict the amount of correction to be gained by epiphysiodesis. The graphs of absolute bone length are used to compare the patient with the population and predict future growth and the discrepancy at maturity. The method uses only the most recent skeletal age estimate and is therefore subject to the errors inherent in single skeletal age estimation. The methodology is not obvious and the original articles presenting the growth data do not describe it.

The Straight Line Graph Method

This method is based on the leg length data of the Boston Growth Study but, instead of using arithmetic calculations, uses the plotting of points and the drawing of lines on a special graph.

Advantages

The method, for use with the paper graph, has been described explicitly in step by step fashion by the original author.
Like the Growth Remaining Method, this one is based on scientific data describing patterns of growth. It avoids errors due to arithmetic mistakes, and automatically takes into account the growth percentile of the child and the growth inhibition and relative growth rates of the legs. It minimizes, by using all skeletal ages, errors inherent in single estimates. It is a general method for analyzing growth and not just a method for timing epiphysiodesis.
The paper version of this method, applied by drawing points and lines, is not designed for use once the patient has undergone corrective surgery. On the other hand, Pedipod/LLD is able to take into account the effects of such surgery, is able to continue to plot the patientís progress, and to predict the final outcome at maturity.

Disadvantages

This method is also less convenient than the arithmetic method since it requires either a special graph and some skill in plotting points and drawing lines, or this computer program and an accessible computer. The methodology is straightforward but not immediately obvious.
 
 
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