Biomechanical evaluation of hydroxyapatite and autogenous bone intervertebral grafts in an instability model of the porcine cervical spine

Hydroxyapatite ceramics are currently used for anterior cervical fusion following discectomy and osteophytectomy. However, there are few reports describing the biomechanical properties of hydroxyapatite intervertebral grafts such as structural capacity and mechanical resistance. The purpose of this study is to evaluate the biomechanical properties of a porous hydroxyapatite intervertebral graft. We used cadaveric porcine cervical spine that had undergone C3/4 discectomy and dissection of the posterior longitudinal ligament as an instability model. The experimental groups included the intact (n - 11), instability (n - 11), autogenous bone implanted (n - 6) and hydroxyapatite graft implanted (n - 5) groups. Porous hydroxyapatite with 40% porosity and scapular bone were used as the hydroxyapatite and autogenous bone graft, respectively. The displacement rates of the cervical spine by compressive forces to flexural, extensional, and lateral bending directions were evaluated using a video recording followed by computer assisted analysis. The stiffness at 80 N of compressive load was calculated from the load displacement curve. Further, linear and nonlinear coefficients, possibly representing the biomechanical characteristics of the grafts, were calculated based on Fung's theory. Compared with the intact group, the instability group showed significant decreases in the stiffness in all directions except flexural compression. Both the hydroxyapatite and the autogenous bone graft had significantly increased the stiffness in all directions except extensional load. There were no statistical differences in the stiffness between the hydroxyapatite and the autogenous bone graft in all directions except right lateral bending load. The nonlinear coefficient value in Fung's equation was far larger in the hydroxyapatite than in those observed in the autogenous graft. The present study indicates that the porous hydroxyapatite graft exhibits compressive strength equal to the autogenous graft in vitro. A larger nonlinear coefficient value may represent the brittleness of the hydroxyapatite graft.