Response of the growth plate to distraction close to skeletal maturity: is fracture necessary?

Axial force applied during epiphyseal distraction has been measured close to skeletal maturity in patients having leg lengthening, in a rabbit model, and in vitro from an amputation specimen. In the patient study, both slow distraction rates and low constant distraction loads were applied. For all the distraction regimens, it was not possible to lengthen the limb significantly without evidence of fracture as demonstrated by a sudden decrease in distraction force. Growth plate failure was observed from 600 to 800 N, these levels being lower than those recorded from the in vitro tests. In the animal study, three distraction regimens (0.13, 0.26, and 0.53 mm/day) were applied across the upper tibial growth plate of New Zealand white rabbits close to skeletal maturity. Distraction was applied and force measured using a strain-gauge dual-frame external fixator. The force-time results revealed two distinct patterns. One pattern, in which the forces rapidly increased to maximum values of approximately 25 N and then suddenly decreased, indicated fracture of the growth plate, which was confirmed histologically. In the other pattern, forces increased steadily throughout distraction, reaching maximum values at the end of distraction of approximately 16 N. Histologic observations indicated hyperplasia of the growth plate without fracture, however, only a small increase in limb length was detectable. Hence, if a significant increase in leg length is required close to skeletal maturity, then fracture of the growth plate must occur.