Fetal tibial bone healing in utero: the effects of miniplate fixation.

Although clinical and experimental findings have demonstrated that fetal soft-tissue wounds heal without scarring, very little is known about the process of fetal bone healing. This study examined fetal long bone healing in utero, both histologically and biochemically, with and without fracture fixation in a fetal sheep model. Our study group consisted of 25 live fetuses (from 16 ewes). There were 50 fetal tibias in this group; 12 were control, 17 were fixed (miniplate fixation), and 21 were nonfixed. A midshaft osteotomy of the tibia, either fixed or non-fixed, was performed on fetal sheep at 95 days' gestation (term = 145 days) in utero. The sheep were then killed at one of five postoperative time intervals (weeks 1, 2, 3, 4, and 7), and fetal bone healing was examined. The variables reviewed included gross morphology, histology, radiology, and collagen analysis (proportions of types II to I and III to I collagen). Fetal bone healing without fixation was accompanied by a large callus with rapid and abundant cartilage and collagen deposition. Bone healing was characterized by malunion or nonunion at 7 weeks. However, with miniplate and screw fixation, callus formation was minimal; primary bone healing occurred by 3 weeks and did not adversely affect long bone growth. Analysis of callus samples revealed a minimal amount of type III collagen, whereas the proportion of type II collagen was variable and proportional to the content of callus cartilage.