A case report.

A Jamaican RAF corporal aged 24 years complained of intermittent low back pain for two years. His first attack occurred when he twisted whilst in a crouching position playing cricket, then in March 1 977 he kicked a football with his left foot and he felt a severe pain in the back of the left thigh. This pain persisted and was accompanied by numbness of the outer side of the left calf and foot, with foot drop and weakness of eversion of the left foot. Two months later he presented as a very fit, well-built man and gave no history of direct injuries to his thighs. Examination of the lumbar spine showed no abnormalities apart from slight limitation of forward flexion. Straight leg raising was full and painless on both sides. However, there was complete paralysis of the left tibialis anterior, toe extensors and peronei, with weakness of the left calf muscles, absence of the left ankle jerk and complete loss of sensibility on the outer side of the left calf and the dorsum of the left foot. Examination of the left thigh revealed a large hard mass in the posterior thigh muscles, extending for I 0 centimetres distally from the fold of the buttock. A similar but smaller mass was felt in the same position in the posterior muscles ofthe right thigh. Radiographs (Figs 1 and 2) showed these masses to consist of ossification in the posterior thigh muscles. Some exostosis formation at the right anterior superior and anterior inferior iliac spines was also shown. Electrophysiological studies showed normal conduction in the left sciatic nerve as far distally as the upper margin of the ossified mass, but not below it; positive waves and fibrillation potentials were elicited from the anterior crural muscles on electromyography. The extensor digitorum brevis muscle however did not show any evidence of denervation and there was no neurological deficit in the right lower limb. Radiographs of the upper limbs showed no evidence of ectopic calcification. The biochemical levels in the blood were normal. The left buttock and upper half of the left thigh were explored through a posterior approach. An irregular mass of bone was found in the substance of the biceps femoris muscle, from which it was excised with difficulty in two pieces by sharp dissection (Fig. 3). The upper end of this mass lay in the sciatic notch, and the sciatic nerve which was attached to muscle at this point was stretched tightly over it. The lower end of the mass consisted of a number of fingers of bone which blended with the tendinous fibres in the distal part of the muscle. The blending