Apparent length is measured from the umbilicus to the medial malleolus, usually with the patient in the supine position. It is affected by pelvic obliquity and abduction or adduction contractures of the hips.
Real length is measured from the anterior superior iliac spine to the medial malleolus. It is less affected by the positional factors than is the apparent length.
Pelvic obliquity is measured by comparing the relative heights of the two iliac crests with the patient standing with the legs together and the knees fully extended. This assessment takes into account differences in the heights of the feet.
The term ‘pelvic obliquity’, confusingly, may have a different meaning and sometimes relates the transverse axis of the pelvis to the longitudinal axis of the spine. This is more in line with the way the term is used by spinal surgeons and is a concept important in the management of leg length discrepancy.
The meaning of the term is usually easily inferred from the context.
Blocks Under the Heels
Placing blocks under the heels is a direct method of assessing the discrepancy in leg length. It allows the orthopaedic surgeon to predict the effect of corrective surgery, particularly lengthening or shortening procedures. It permits one to take into account factors above the hips and below the ankles. This is the method of assessment required by the Menelaus method.
It must be remembered that this examination only provides the present discrepancy and not the discrepancy that will be present at maturity. The present discrepancy therefore cannot be used as the goal in determining the amount of lengthening or shortening to be obtained in a growing child.
There are a number of factors that are of importance in patients with leg length discrepancy, either because they affect the functional leg length or the posture of the patient.
Some patients have a small foot on the same side as the short leg. This affects the functional leg length and must be considered when determining the goal of corrective surgery. These patients will require more correction than is immediately apparent from their measurements.
Knee Flexion Deformity
Flexion of the knee shortens the leg and affects measurements of leg length. Correction of the deformity lengthens the leg and should be considered a lengthening procedure.
Hip Abduction or Adduction Contracture
Patients with leg length discrepancy compensate by a number of mechanisms, one of which is pelvic obliquity with abduction of the hip on the short side and adduction of the hip on the long side. Patients with contractures cannot compensate in this way.
In much the same way a hip contracture can cause a functional leg length discrepancy in the absence of any true discrepancy.
Spinal deformity or an abnormality of the lumbo-sacral takeoff may cause the long axis of the spine to deviate from its normal posture perpendicular to the transverse axis of the pelvis. Such patients, in order to stand with their trunks erect, must accept pelvic obliquity. Equal leg lengths may not be an appropriate goal of treatment in such patients, and they must be evaluated carefully by placing blocks under the heel or providing temporary shoe lifts.
In order to facilitate clearing of the foot or toes in the swing phase of gait patients who wear braces or who have weakness of ankle dorsiflexion may benefit from having the weak leg somewhat short. Equality may not be a suitable goal for such patients.
Most children with spasticity have an asymmetrical gait. Parents often mistakenly assume that it is due to leg length discrepancy.
The techniques available for the radiological measurement of leg length differ with respect to accuracy and convenience.
This technique involves a single exposure of both legs in the standing position with a tube to film distance of 72 inches. Measurement accuracy is compromised by the magnification error due to parallax, but it allows the visualization of the entire limb and the evaluation of angular and other deformities.
This technique involves placing a radio-opaque ruler beneath the limb and moving the x-ray tube to obtain separate exposures of the hip, knee and ankle, each with the central x-ray beam passing through the joint perpendicular to the film. It avoids errors due to magnification and provides accurate measurements. Because it requires three separate exposures its accuracy is compromised by any movement of the patient between exposures.
In principle, this technique is the same as the orthoroentgenogram except the film is also moved between exposures so that all three exposures fit on a 14 by 17 inch film that is easier to handle.
The software that is part of all CT scan units allows certain measurements to be made. Leg lengths can be measured in this way on just the scout film, without actually performing the CT examination. This technique can be performed at less cost and less radiation than those using plain radiography.
Many different techniques have been described for the assessment of skeletal age using various parts of the growing skeleton. Techniques in current use are based on x-rays of the hand and wrist, and involve comparing x-rays of the patient with standards published in an Atlas.
The assessment of skeletal age is the weakest link and the greatest source of error in predicting future growth.
Greulich and Pyle Technique
Greulich and Pyle published an atlas, based on that of ????, showing x-rays of the hand and wrist typical of boys and girls of various ages. Comparing a patient’s x-rays with these standards can be difficult and there is significant inter- and intra-observer error with this technique. Accuracy is also compromised by large intervals between standards in some parts of the atlas.
Tanner and Whitehouse Technique
Tanner, Whitehouse, et al., with the aid of computers, developed a more mathematical approach to the estimation of skeletal age. Their technique involves the assessment of maturation of 20 different bony landmarks in the hand and wrist, and the assigning of a letter score to each landmark by comparison with standards in their atlas. These letter scores can then be converted to skeletal age accurate to tenths of a year. Unfortunately this method gives different results than the Greulich and Pyle method and has not been independently correlated with leg length so that it is of uncertain value in the context of leg length discrepancy.