Characterization of rat calvarial nonunion defects

Abstract

This study examined the healing of nonunions by describing the histology and ultrastructural appearance of craniotomy defects as a model. Bone defects (3.4 and 8 mm) were created in thecal varia of adult rats. Central and peripheral specimens of 8-mm defects were retrieved at 1,3,7,10.14,21,28 and 42 days and examined using both light and transmission electron microscopy. Specimens from the 3- and 4-mm defects were retrieved at 28 days and examined using light microscopy. In all sizes of defects, bony repair was consistently localized to the durai side of the defect. The 3- and 4 mm defects demonstrated the greatest degree of osseous bridging and evidence of normal osseous repair throughout the defect. The 8-mm defects repaired in general with the formation of nonunions which contained asmall amount of bone at the periphery and fibrous connective tissue. Bone formation was evident at 10 days in the peripheral regions of the 8-mm defects and exhibited bony peninsulas with normal primary calcification fronts. Matrix vesicles containing hydroxyapatite-like crystals were present. In contrast, the central regions of the 8-mm defects were characterized by several islands of cartilage-like cells which stained metachromatically with toluidine blue. Transmission electron microscopy of this region at 14 days demonstrated a dense, collagenous extracellular matrix with matrix vesicles infiltrating the collagen bundles. There was no evidence of crystal formation in the matrix vesicles nor of calcification in the collagenous matrix. At 21 days, both the central and peripheral regions of the 8-mm calvarial nonunions were characterized by dense fibrous connective tissue repair and inactive fibroblasts. The collagen became more densely packed throughout the region of the defect by 42 days. Inactive fibroblasts with long cytoplasmic processes were observed throughout the defect. These data confirm that defect size is important in the development of bone fibrous union defects. One characteristic of nonunion development may be the failure of cells to calcify this matrix, perhaps due to the lack of appropriate bone-derived growth and differentiation factors. © 1990 S. Karger AG, Basel.