Repair and remodeling in Millipore-isolated defects in cortical bone

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Abstract
The pattern of osseous repair in Millipore-isolated defects (chambers) of cortical bone (canine radii) was studied histologically during the interval from the 1st through the 320th day after operation. Within 24 hr., extensive necrosis was noted of most of the cellular elements in lacunae and small vascular channels in the cortex, bordering the operative defect. Cells within and near larger vascular channels remained viable. These and regenerating endosteal vasculature provided the major source of cells entering the Millipore-isolated chambers, through transected Haversian canals at either end of the defects. The chambers were bridged, centripetally, within 30 days by fibrous tissue which appeared to be transformed directly in fiber bone. Longitudinally-oriented, collagen bundles bridged the interface between fiber bone and fibrous tissue. Along the bundles, active osteoblasts were observed. Fiber bone was reorganized into lamellar bone without significant numbers of multinucleated osteoclasts, so that, after 320 days, the histologic pattern (lamellar) in the center of the operative region resembled that in the unoperated cortical bone proximal and distal to the defect. Cortical bone, ensheathed in Millipore and surrounding the defects, was reorganized by creeping substitution. The 3 cm long bars of cortex, medial and lateral to the defects were invaded from their proximal and distal ends, so that revitalization was occurring last in the middle of the bars, 320 days after operation.