Optimization of spinal instrumentation stiffness and its effect on interbody fusion

Abstract

We developed spinal fusion implants to investigate these goals. Extradiscal implants of various bending stiffnesses were designed, fabricated and validated. We also designed, fabricated, and validated custom interbody implants which measure interbody forces during normal spinal motions. Each of these implants was developed for both the goat and the human cervical spine. In vitro testing using synthetic models, excised goat motion segments, and excised human motion segments demonstrated that increased plate stiffness decreased interbody loading, increased loading on the posterior elements, and shifted the axis of rotation of the spine anteriorly.

Overall, we have demonstrated that fusion outcomes can be modulated and optimized through varying extradiscal implant stiffness and that interbody force magnitude correlates to the progress of fusion. This work can lead to the development of a new generation of implants which have been designed to induce more robust fusion and improve patient outcomes and quality of life.

We further tested the effect of spinal instrumentation stiffness in vivo in the goat cervical spine following anterior cervical discectomy and fusion. In vivo results showed that instrumentation with plates of two different stiffnesses resulted in two distinct magnitudes of load-sharing during repeatable activities. In animals instrumented with stiff fusion plates, force maxima were observed when animals were in extension and minima were found when animals were in flexion; the opposite trend was observed in animals instrumented with compliant fusion plates. During the course of fusion, we measured a decrease in interbody load for five weeks following surgery, after which interbody loading remained similar over the remaining weeks of the study. After explanting tissues, histological examination of instrumented motion segments demonstrated that levels instrumented with more stiff plates tended to exhibit new bone formation but lower rates of bridging fusion. Animals instrumented with a more compliant fusion plate were also found to exhibit new bone formation with increased rates of fusion compared to the animals treated with the stiff plates.

Spinal fusion is the gold standard surgical treatment for low back and neck pain. A successful spinal fusion restores intervertebral disc height and eliminates pathologic motion between adjacent vertebrae through formation of mature bridging bone. Bone growth between vertebrae is engendered by placement of bone graft within an interbody implant in the disc space during surgery. However, bridging bony fusion can be impeded by excessive motion. Thus, supplemental extradiscal fixation (i.e. spinal plate) is often placed surgically to provide immediate stability to the spine. This reduces the likelihood of a pseudarthrosis by minimizing interbody motion. However, while supplemental fixation helps to improve stability, it may isolate the bone graft from beneficial mechanical stimuli, thus stress-shielding the graft. In this way, the stiffness of the spinal instrumentation affects the mechanical environment in the interbody space. Ideally, there is a balance of stability and beneficial mechanical loading. However, the optimal properties of spinal instrumentation have not previously been determined to achieve this balance. The overarching goal of the present work is to correlate mechanical properties of spinal fusion instrumentation to load-sharing, stabilization, and fusion.