To evaluate and compare the spontaneous regeneration repair process in femoral bone defects in 6-week-old rabbits and to compare the healing time periods between different rabbit groups.

Bone defects were created at the femur mid-shaft and an external fixator was applied in 50 rabbits. The periosteum was resected in 10 rabbits (defect size: 15%, 25%) and left untouched in the remaining rabbits. Forty rabbits were divided into four groups according to the percentage of bone defects (15%, 20%, 25%, 30%). Radiographs were taken weekly to evaluate the bone regeneration and union. The bone union time was measured between the osteotomy and the cortico-medullary differentiation by examining radiographs. The healing index was defined as the union time (week) per amount (cm) of bone defect. Eight rabbits, 2 from each groups with the bone defects, were investigated by histopathologic examination.

The mean union time was approximately 7.0 to 7.3 weeks. The healing index in groups that had a large percentage of bone defects was less than in groups that had a small percentage of bone defects. The periosteum-resected group did not show bone regeneration. Histopathologic examinations showed intramembranous and atypical endochondral ossifications along the periosteum and typical endochondral ossification at the center of the bone defects.

Spontaneous bone regeneration may be used in children to fill the bone defect instead of performing an internal bone transport. Spontaneous bone regeneration is useful in cases of mid-shaft bone defects or when the remaining bone fragments are large enough for an external fixation application.