Limb Length Inequality

Ashok Johari
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Radiographic Measurement of Limb Length Discrepancy
Because clinical measurement of limb lengths has some fallacies and inter-observer errors, x-rays are utilized for limb length measurements. Two commonly employed methods of x - ray measurement are the teleoroentgenogram and the scanogram. Both techniques use a radio-opaque ruler kept alongside the extremity during the exposure.

The teleoroentgenogram is used for small children below the age of 5 to 6 years. In this a single exposure is made from the hip to the ankles and the length read on the ruler markings. The scanogram is used for larger children and it utilizes sequential exposures of the hips, knees and ankles and reads their level on the ruler. This technique requires that the child be still so that lower limb position in relation to the ruler is not altered during different exposures. These techniques provide a reliable method of storing data for future reference and for calculating the final growth discrepancy.

Limb lengths can also be measured utilizing CT scans but at present this is costly. Ultrasound has also been employed for this but is subject to inter-observer errors.

Projection of limb length discrepancy

It is important to know what the final discrepancy of limb length at skeletal maturity will be. A rough idea can be gained by repeated measurements and calculation of the percentage inhibition in relation to the normal limb. The growth of the normal limb in terms of percentile is charted and on this basis the length of the normal limb at maturity is calculated. Applying the percentage inhibition the length of the affected limb can be calculated.

There are a number of fallacies involved in these methods. Firstly growth does not proceed in a linear fashion. It is age dependent and in the case of the affected lower limb it is dependent on the etiology of limb shortening. Secondly, normal limb length data is not available for Indian population. The available charts are those of Green - Anderson growth prediction chart and the Moseley straight line graph. Thirdly, the skeletal age according to the Greulich and Pyle's atlas itself is an approximation.

Menelaus predicts growth by a simple method of calculating growth of the distal femur at 3/8 inch per year and that of the proximal tibia at 1/4 inch per year in adolescents over 9 years of old age. Growth ceases at age 14 years in girls and in age 16 years in boys.


References
Limb Length Inequality Limb Length Inequality 02/20/2001
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