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 th e growth plate can be preserved intact if slow rates of distraction ar e 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 uncha nged with transfixing K-wires in the epiphysis. The normal growth rate of the proximal physis of the tibia decreased with age and was expres sed 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 ra bbits aged 6 weeks and weighing approximately 500 gm. The growth behav ior 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 amou nt of distraction achieved was 8.5 mm, 11.3 mm and 14.6 mm resulting i n a mean 'net' increase in length as compared to the experimental cont rol after the distracter was removed amounting to 3.0 mm (55% of the c ontrol growth), 3.6 mm (47%) and 4.2 mm (40%), respectively. Subsequen t serial measurements, up to 13 weeks post-distraction, showed no sign ificant change in the discrepancy between the length of the tibia and the growth rate at the proximal tibial epiphysis and between the distr acted and the contralateral controls in all 3 groups. Our findings sug gest that the proximal tibial growth plate in the rabbit would maintai n a normal growth rate after slow physeal distraction for periods up t o 4 weeks.