Influences of disc degeneration and bone mineral density on the structural properties of lumbar end plates

BACKGROUND CONTEXT: Implants subsidence is a frequent complication of interbody fusion, which can result in pain, deformity, nerve damage, and even failure of surgery. The end plates as the interface between implants and the vertebral bodies play a very important role in sharing the compression on the vertebral bodies. The information on the structural property distribution of the end plate and its relationship with bone mineral density (BMD) and disc degeneration will be of great significance for the reduction in implants subsidence and improvement in related operative procedures to increase the success rate of interbody fusion. PURPOSE: To investigate the structural property distribution of the lumbar end plate; the effects of disc degeneration on the biomechanical properties of the lumbar end plate; and the relationship between the biomechanical properties of the lumbar end plate and BMD. STUDY DESIGN: A biomechanical study was conducted in human cadaveric lumbar spine models. METHODS: Indentation tests were performed at 24 standardized test sites in 120 bony end plates of intact human vertebrae (L1-L5) using a 1.5-mm-diameter, hemispherical indenter at a speed of 0.2 mm/s. The failure load at each test site was determined using the load-displacement curve. Disc condition was evaluated using a four-point grading scale and bone density was measured using the lateral dual-energy radiograph absorptiometry scans. All end plates were divided into different disc degeneration groups based on the states of the adjacent degenerative discs and BMD groups according to BMD values of the corresponding vertebral bodies. The experimental results were statistically analyzed using the SPSS 15.0 with the disc degeneration and BMD being considered as independent factor, and the failure loads of the superior and inferior end plates were also compared. RESULTS: The peripheral regions of lumbar end plates were stronger than the central regions (p<.05), with the posterolateral sites in front of vertebral pedicles being the strongest regions. The inferior lumbar end plates were found to be stronger than the superior lumbar end plates (p<.05). The disc degeneration was negatively correlated with the failure loads of the lumbar end plates (r(s)=-0.563; p<.01). With increasing disc degeneration, the decreases of failure loads were nonuniform across the lumbar end plate, and the central region became weak with little strength change on the end plate periphery. The BMD was positively correlated with the failure loads of the lumbar end plates (r(s)=0.812; p<.01). The failure loads decreased uniformly across the end plate surfaces as the BMD dropped, and the BMD decrease did not change the structural property distributions of lumbar end plates. CONCLUSIONS: Preoperative evaluation of the states of intervertebral discs and BMD of patients is necessary for predicting risks of implants subsidence after interbody fusion. For patients with or without disc degeneration or osteoporosis, the implants should be placed at the peripheral regions, especially the posterolateral sites, to acquire higher mechanical strength to reduce subsidence as much as possible. (C) 2012 Elsevier Inc. All rights reserved.