Longitudinal growth rate following slow physeal distraction - The proximal tibial growth plate studied in rabbits

Studies in animals by de Bastiani et al. (1986) on leg lengthening by physeal (growth plate) distraction have shown that the integrity of the growth plate can be preserved intact if slow rates of distraction are employed. Clinically, however, this technique has been restricted to the period shortly before skeletal maturity, due to uncertainty about the behavior of the growth plate following distraction. We conducted 2 studies. 11 immature rabbits used in a study on the normal growth at the proximal tibial physis established that the growth rate was unchanged with transfixing K-wires in the epiphysis. The normal growth rate of the proximal physis of the tibia decreased with age and was expressed as a quadratic function, G(n) (mm/day) = 0.44-0.002 age (days). At 6 weeks of age, the growth rate was 0.33 mm/day, slowly decelerating to a rate of 0.15 mm/ day by the 16th week. In the lengthening study, to determine whether the growth plate would maintain a normal rate of growth following slow distraction, a custom-made bilateral distraction device was applied to the proximal tibial epiphysis of 32 immature rabbits aged 6 weeks and weighing approximately 500 gm. The growth behavior of the growth plate following 2, 3 and 4 weeks of distraction was studied. The rate of distraction was set at 0.5 mm/ day. The mean amount of distraction achieved was 8.5 mm, 11.3 mm and 14.6 mm resulting in a mean 'net' increase in length as compared to the experimental control after the distracter was removed amounting to 3.0 mm (55% of the control growth), 3.6 mm (47%) and 4.2 mm (40%), respectively. Subsequent serial measurements, up to 13 weeks post-distraction, showed no significant change in the discrepancy between the length of the tibia and the growth rate at the proximal tibial epiphysis and between the distracted and the contralateral controls in all 3 groups. Our findings suggest that the proximal tibial growth plate in the rabbit would maintain a normal growth rate after slow physeal distraction for periods up to 4 weeks.