Biomechanical evaluation of hydroxyapatite intervertebral graft and anterior cervical plating in a porcine cadaveric model

The biomechanical properties of a porous hydroxyapatite (HA) intervertebral graft with or without anterior cervical plating were evaluated in cadaveric porcine cervical spine model using C3-4 discectomy and dissection of the posterior longitudinal ligament to cause instability. The experimental groups were intact (n = 11), instability (n = 11), autogenous bone implant (n = 6), HA graft implant (n = 5), autogenous bone with plating (n = 6), and HA graft with plating (n = 5). Porous HA with 40% porosity and scapular bone were used as grafts. The displacement rates of the cervical spine by compressive forces in the flexural, extensional, and lateral bending directions were evaluated using video-recording followed by computer-assisted analysis. The stiffness to compressive load was calculated from the load-displacement rate curve. The linear and non-linear coefficients of the Fung's equation were obtained based on the plot of Young's modulus against load. There were no statistical differences in the stiffness between the HA and autogenous bone graft in all directions. The two plating groups showed significantly increased stiffness in all directions. The non-linear coefficient value in Fung's equation was far larger in both HA and HA graft with plating groups than in the other groups in flexural compression. Porous HA graft has a compressive strength similar to autogenous graft in vitro, and anterior plating provides additional stiffness to the cervical spine. The larger non-linear coefficient value of the HA groups may represent the characteristic biomechanical brittleness of HA graft, but this is manifest only in flexural compression.