Fetal bone gap healing in utero.

The healing of fetal tibial bone after osteotomy with and without stable fixation has been reported previously. The present study was designed to evaluate fetal bone gap healing using a tibial ostectomy model in fetal sheep. Eighteen time-dated pregnant ewes (20 fetuses, 34 experimental hind limbs) underwent intrauterine surgery at 95 days gestation (term, 145 days). A titanium miniplate was applied to the anterior aspect of the tibia and a longitudinal length of bone approximately 1.5 times its diameter was removed and the incision closed. The pregnancies were then allowed to progress until the ewe was killed at postoperative weeks 1, 2, 4, or 7. Assessments at that time included evaluation of gross morphology, histologic and radiologic appearance, and collagen analysis and hydroxyproline determination of the tissue within and at the borders of the gap. At 7 weeks, seven of nine bone gap specimens exhibited radiographic and histologic evidence of union with woven and lamellar bone. Hydroxyproline concentrations gradually increased within the bone gap over the period of the study. At all intervals, type I collagen composed over 90% of the collagen within the healing bony gap. Histologically and biochemically, the process appears to be similar to postnatal bone healing, albeit occurring at an accelerated rate.