Limb lengthening by low rate epiphyseal distraction. An experimental study in the caprine tibia.

Low-rate epiphyseal distraction of the left proximal tibia using a modified bilateral Hoffmann external fixation device, was accomplished in 14 goats (age 6-7 months). Daily distraction at a rate of 0.5 mm was continued for 5.5 weeks. Epiphysiolysis occurred after 5-9 days. After distraction had been discontinued, the external device was left in situ for fixation for 4 weeks. The animals in group 1 (n = 6), group 2 (n = 3), and group 3 (n = 5) were killed at 4, 16, and 32 weeks, respectively, from the end of the distraction period. The proximal growth plate of the elongated tibia appeared radiolucent at the conclusion of the study but was less distinct and reduced in height as compared with the control leg. The gained length at the end of the distraction period was on the average 2.2 cm (11.9%). This gain in length was reduced with further growth. In group 3, the final gain in length was reduced to half. A reduced cortical thickness of the bone within the area subjected to distractional force was observed. This finding is attributable to stress protection. An interesting observation comparing control and operated tibia was an increase of the cross-sectional diameter of the elongated bone. Both tibiae of all animals were tested mechanically. The torsional strength of the elongated tibiae as compared with controls was on the average 29% in group 1, 79% in group 2, and 95% in group 3. We conclude that leg lengthening by low-rate epiphyseal distraction in the proximal tibia is a justifiable alternative to other lengthening procedures in individuals approaching adolescence. In juveniles, lengthening by epiphyseal distraction may result in reduction in the longitudinal growth.