The midsagittal and interpedicular diameters and the trefoil shape of lumbar vertebrae of known age at death were measured in skeletons from a population aged between 1 and 70 years. All the trefoil configurations were at L5 with the exception of one at L4. The overall prevalence was 25%, but this shape was not generally apparent until adulthood. The midsagittal diameter in the trefoil canals was found to be significantly smaller than that in the unaffected canals. This did not change significantly after six years of age indicating that the cause of the trefoil configuration is probably present early in life. The trefoil shape was no more common in the spines of the elderly subjects. Our findings indicate that the trefoil configuration of the lumbar vertebral canal has a developmental origin and is not a consequence of degenerative processes.

We report the case of a child with cerebral palsy and spastic diplegia treated for bilateral fixed flexion of the knee by bilateral hamstring lengthening. An attempt to straighten the legs from 90 degrees to 20 degrees flexion damaged the sciatic nerve. There are no objective means of estimating how much deformity can be reduced safely. We present a method of calculating the extra strain in the sciatic nerve produced by reducing a flexion deformity. The result, combined with clinical judgement, provides guidelines for safe corrective surgery.