Analysis and Prediction IntroductionPatients 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 MethodThis 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:
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 patient’s 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.
AdvantagesThis method is the most
convenient of the three described here. It does not require special
resources or tools and the arithmetic calculations are simple.
DisadvantagesThe 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 MethodThis method is based on
data from the Growth Study of the Children’s 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
patient’s leg lengths superimposed on their graph, but does not implement
their method for timing epiphysiodesis.
AdvantagesThis 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.
DisadvantagesThis 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 MethodThis 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.
AdvantagesThe 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 patient’s progress, and to predict
the final outcome at maturity.
DisadvantagesThis 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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