10. AN IN-VIVO EVALUATION OF THE EFFECT OF A HYDROXYAPATITE COATING WITH AND WITHOUT THE USE OF BMP-7 ON EXTRACORTICAL BONE BRIDGING USING A CANINE SEGMENTAL DEFECT MODEL

Abstract

Purpose: Extracortical bone bridging and ingrowth have been shown to reduce stresses on the stem and cement mantle of tumor endoprostheses. The purpose of this study was to assess the effect of bone morphogenetic protein 7 (BMP-7) delivered by Peri-Apatiteâ (PA, Stryker Orthopaedics) hydroxyapatite coating on porous segmental replacement prostheses.

Method: Eighteen mature mongrel canines were implanted with unilateral segmental replacement prostheses made of a cobalt-chromium (Co-Cr) alloy and coated with two layers of sintered Co-Cr alloy beads (diameter 600 to 800mm). The control group consisted of a plain porous coated segmental prosthesis without any PA coating. Group 2 consisted of a PA-coated segmental prosthesis coated with buffer solution. Group three consisted of a PA-coated segmental prosthesis loaded with rhBMP-7 (Stryker Biotech) in a buffer solution carrier. Group 1 had the implant only. Group 2 had the buffer solution evenly applied to the porous coat and group 3 had 2.9 mg of BMP-7 in liquid buffer solution evenly applied. The canines were allowed to fully bear weight without restrictions. The femurs were retrieved at twelve weeks for radiographic and histologic analysis.

Results: Gross and radiographic data of the retrieved specimens showed that all six PA-coated implants augmented with BMP-7 had complete bone bridging; only one of the PA-coated implants and only two of the plain porous implants were completely bridged. There was a greater percentage of bone apposition for the BMP-7 augmented PA-coated group compared to both the plain (p=0.0026) and the PA-coated (p=0.0001). There was no difference in bone formation or bone apposition between the plain and PA-coated groups. Histology revealed greater depth of bone ingrowth in the BMP-7 augmented PA-coated group as compared to the plain (p< 0.0001) and the PA-coated (p< 0.0001) groups. There was also significantly greater bone apposition in the BMP-7 augmented PA coated groups as compared to the plain (p=0.0014) and PA-coated (p=0.0067) groups. There was no significant difference in depth of bone ingrowth or bone apposition between the plain and PA-coated groups.

Conclusion: BMP-7 when used to augment PA-coated prostheses in a canine segmental defect model can significantly improve extracortical bone bridging and bone ingrowth. PA-coated implants may be considered to deliver the exogenous biological growth factors.