Histomorphometric description of allograft bone remodeling and union in a canine segmental femoral defect model: a comparison of rhBMP-2, cancellous bone graft, and absorbable collagen sponge

The purpose of this study was to determine the effect of recombinant human bone morphogenetic protein type 2 (rhBMP-2) on the histomorphometry of femo ral allograft-host bone union and allograft remodeling. A 6 cm mid-diaphyse al femoral defect was created and filled with an allograft stabilized with an interlocking nail in 21 dogs. Dogs were randomly divided into three equa l groups and the allograft-host bone junctions and the mid-diaphyses of the allografts were treated with either an absorbable collagen sponge (ACS) lo aded with rhBMP-2 (BMP group), an autogenous cancellous bone graft (CBG gro up), or ACS loaded with buffer solution (ACS group). All dogs received dail y tetracycline until sacrifice at 24 weeks to label new bone formation. His tomorphometric analyses on sections of proximal and distal allograft-host b one junctions and the mid-diaphyseal portion of allografts were performed u sing fluorescent and regular light microscopy. Analyses of the host bone an d junctions between allograft and host bone revealed significantly greater new bone formation and larger osteon radii in the BMP group compared to CBG and ACS groups and contralateral intact bone. Porosity in CBG and ACS grou ps was significantly higher than in the BMP group, which had similar values to intact bone. In transverse sections of allografts. the largest pore dia meters were present in the CBG group. Based on all parameters measured, sig nificantly higher bone turnover occurred in the outer cortical area of the allograft in all groups as compared to the inner cortical and mid-cortical al eas. New bone formation and osteon radius/osteon width in allografts wer e similar for all three groups. Higher porosity and larger pore diameters i n the CBG and ACS groups suggested higher bone resorption vel sus formation in these groups compared to the BMP group. The results of this study revea l more balanced allograft bone resorption and bone formation in the BMP gro up, with greater resorptive activity in the CBG and ACS groups. However, ne ither rbBMP-2 nor autogenous bone graft increased allograft incorporation w hen compared to the negative control (ACS group). (C) Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.