Effect of Endplate Conditions and Bone Mineral Density on the Compressive Strength of the Graft–Endplate Interface in Anterior Cervical Spine Fusion

Abstract

Destructive compression tests and finite element analyses were conducted to investigate the biomechanical strength at the graft–endplate interface in anterior cervical fusion. To investigate the effect of endplate thickness, endplate holes, and bone mineral density of the vertebral body on the biomechanical strength of the endplate–graft interface in an anterior interbody fusion of the cervical spine. Subsidence of the graft into the vertebral body is a well-known complication in anterior cervical fusion. However, there is no information in the literature regarding the compressive strength of the graft–endplate interface in relation to the endplate thickness, holes in the endplate, and bone mineral density of the vertebral body. Biomechanical destructive compression tests and finite element analyses were performed in this study. Cervical vertebral bodies (C3–C7) isolated from seven cadaveric cervical spines (age at death 69–86 years, mean 79 years) were used for compression tests. Bone mineral density of each vertebral body was measured using a dual energy radiograph absorptiometry unit. Endplate thickness was measured using three coronal computed tomography images of the middle portion of the vertebral body obtained using a computer-assisted imaging analysis. Then each vertebral body was cut into halves through the horizontal plane. A total of 54 specimens, consisting of one endplate and half of the vertebral body, were obtained after excluding eight vertebrae with gross pathology on plain radiograph. Specimens were assigned to one of three groups with different endplate conditions (Group I, intact; Group II, partial removal; and Group III, complete removal) so that group mean bone mineral density became similar. Each endplate was slowly compressed until failure using an 8-mm-diameter metal indenter, and the load to failure was determined as a maximum force on a recorded force–displacement curve. The effect on the strength of the graft–endplate interface of various hole patterns in the endplate was studied using a finite element technique. The simulatedhole patterns included the following: one large central hole, two lateral holes, two holes in the anterior and posterior portion of the endplate, and four holes evenly distributed from the center of the endplate. Stress distribution in the endplate was predicted in response to an axial compressive force of 110 N, and the elements with von Mises stress greater than 4.0 MPa were determined as failed. The endplate thickness and bone mineral density were similar at all cervical levels, and the superior and inferior endplates had similar thickness at all cervical levels. There was no significant association between bone mineral density and endplate thickness. Load to failure was found to have a significant association with bone mineral density but not with endplate thickness. However, load to failure tends to decrease with incremental removal of the endplate, and load to failure of the specimens with an intact endplate was significantly greater than that of the specimens with no endplate. Finite element model predictions showed significant influence of the hole pattern on the fraction of the upper endplate exposed to fracture stress. A large hole was predicted to be more effective than the other patterns at distributing a compressive load across the remaining area and thus minimizing the potential fracture area. Results of this study suggest that it is important to preserve the endplate as much as possible to prevent graft subsidence into the vertebral body, particularly in patients with poor bone quality. It is preferable to make one central hole rather than multiple smaller holes in the endplate for vascularity of the bone graft because it reduces the surface area exposed to fracture stresses.