Biomechanics of Intervertebral Spine Implants
Problem: Developing Intervertebral Spine Implant Sizing Protocol
Vertebral endplate support is pivotal for successful lumbar disc replacement. Factors related to failure or subsidence of an interbody device at the endplate include regional strength of the endplate, amount of cartilaginous endplate removal during surgery, and implant size and shape. Undersized spinal arthroplasty devices may subject the central portion of the vertebral body to higher stresses and therefore increase the risk for implant subsidence. Many studies have cited implant undersizing as the reason for subsidence and migration. Thus, placement of the largest feasible implant is recommended, yet a larger implant requires extensive posterolateral dissection and may be more difficult to routinely implant. Guidelines for the proper sizing of spinal arthroplasty devices have yet to be established.
Solution: Experimentally Exploring the Effect of Implant Size on Compressive Load Bearing
The goal of this research was to examine the compressive biomechanical behavior of the vertebral endplate with varying sizes of disc replacement implants. Using three different sizes of vertebral implants provided by Synthes Spine, and an Instron electromechanical testing platform, we applied varying levels of force to cadaver spine endplates (various ages and sizes) to determine the average pressure distribution and maximum force applied before endplate failure. Pressure distribution and maximum force information is used to determine which implant size is best suited to a given patient and will best mitigate the incidence of destructive loading.
This study demonstrates that under relatively low loads (400N), implant sizing can have significant biomechanical effects on lumbar vertebrae. Significant differences were found under loads which would correspond roughly to lying in the supine position. It could be expected that under loads corresponding to normal daily activities, which could be as much as fivefold greater, even more dramatic differences would be detected.
Our results support the use of the largest footplate possible to avoid large endplate displacements, and reduce the potential for device subsidence and catastrophic endplate failure. Likewise, device undersizing may significantly compromise the integrity of the endplate structural support and predispose to early subsidence and failure.
- J. Auerbach, C. Ballester, F. Hammond III, E. Carine, R. Balderston, D. Elliott. “The effect of implant size and device keel on vertebral compression properties in lumbar total disc replacement, The Spine Journal, Volume 10, Issue 4, pp. 333-340, 2010.
- J. Auerbach, C. Ballester, F. Hammond III, E. Carine, R. Balderston, D. Elliott (2010). “The effect of implant size and device keel on vertebral compression properties in lumbar total disc replacement, Transactions of the 53rd Annual Meeting of the Orthopaedic Research Society, San Diego, CA, February 11-14, 2006.
- J. Auerbach, C. Ballester, E. Carine, F. Hammond III, R. Balderston, D. Elliott. “Lumbar Vertebral End Plate Biomechanics Are Affected by Disc Replacement Size, The Spine Journal, Volume 6, Issue 5, pp. 111S-112S, 2006.