Understanding the kinematics of the lumbosacral spine and the individual functional spinal units (FSU) is essential in assessing spine mechanics and implant performance. The lumbosacral spine and the FSU are comprised of bones and complex soft tissues such as intervertebral discs (IVD) and ligaments. Prior studies have focused on the behavior of isolated structures, but the contribution of each structure to the overall kinematics of the spine needs to be further understood. In this study, the behavior of various structural conditions was determined by experimentally dissecting each ligament in a stepwise fasion until only the IVD remained, and applying loading conditions to the FSU. The FE model was validated through optimization to match the in vitro load-deflection characteristics and contact mechanics for the various structural configurations.

Objective: Long term clinical data showed that lumbar fusion for Lumbar spinal stenosis (LSS) and lumbar disc degeneration (LDD) therapy could change the loads of disc and articular facet and increase the motion of adjacent segments which lead to facet arthropathy and adjacent level degeneration. This study is to design and analyze an interspinous process device (IPD) that could prevent adjacent level degeneration in the LSS and LDD therapy. Method: The IPD was designed based on anatomical parameters measured from 3D CT images directly. The IPD was inserted at the validated finite element model of the mono-segmental L3/L4. The biomechanical performance of a pair of interbody fusion cages and a paired pedicel screws were studied to compare with the IPD. The model was loaded with the upper body weight and muscle forces to simulate five loading cases including standing, compression, flexion, extension, lateral bending and axial rotation. Results: The interbody fusion cage induced serious stress concentration on the surface of vertebral body, has the worst biomechanical performance among the three systems. Pedicle screws and interbody fusion cage could induce stress concentration within vertebral body which leads to vertebral compression fracture or screw loosening. Regarding to disc protection, the IPD had higher percentage to share the load of posterior lumbar structure than the pedicel screws and interbody fusion cage. Conclusion: IPD has the same loads as pedicle screw-rod which suggests it has a good function in the posterior stability. While the IPD had much less influence on vertebral body. Furthermore, IPD could share the load of intervertebral discs and facet joints to maintain the stability of lumbar spine.

One of the popular methods of treating lumbar spine pathologies involves a posterior lumbar interbody fusion (PLIF) using bilateral interbody non-expandable cages. Due to the geometry of these cages, they can require extensive bony removal and nerve root retraction. Some resultant risks of the procedure include dural lacerations and post-operative neuropraxia. Expandable interbody cages may address some of these concerns and possibly decrease the risks associated with PLIF procedures. This is the first study to our knowledge evaluating the biomechanical characteristics of an expandable lumbar interbody device in a cadaveric human spine model. The objective of this study was to evaluate the biomechanical characteristics of a new expandable interbody cage in single segment posterior lumbar interbody fusion using cadaveric lumbar spines.