DESIGN AND DEVELOPMENT OF IN VITRO TOOLS TO ASSESS FIXATION AND MOTION IN THE SPINE

In vitro biomechanical testing of the spine is an important method for evaluating new surgical methods and components, prior to in vivo implementation. This relies upon special laboratory tools and techniques to create spinal motion and loading similar to those experienced in the body. In this thesis, two different studies were performed to evaluate the effects of spinal fixation and motion. The first study compared the fixation of a novel hollow screw and a conventional solid screw in an in vitro sacral model. Screws were tested in seven cadaveric sacra and subjected to stair-cased cyclic flexion- extension loading to simulate the clinical loading scenario. The hollow screw was less resistant to loosening compared to the solid screw in this model. In the second part of this thesis, a spinal loading simulator was developed as a modification to an existing Instron® materials testing machine to produce motion in a multi-segment spine using applied pure bending moments (i.e. flexibility protocol). A custom-designed 2D optical tracking system was used to record the planar motion achieved. An experimental validation study was performed using the developed apparatus, and showed the device was capable of independently producing repeatable and reproducible spine motions (i.e. flexion-extension, lateral bending, and axial rotation) in a single cadaveric specimen. Future work will focus on the continued development of the simulator for use in the assessment of spinal orthopaedic interventions

Influence of graft size on spinal instability with anterior cervical plate fixation following in vitro flexion-distraction injuries

© 2015 Elsevier Inc. Background Context Anterior cervical discectomy and fusion with plating (ACDFP) is commonly used for the treatment of distractive-flexion cervical spine injuries. Despite the prevalence of ACDFP, there is little biomechanical evidence for graft height selection in the unstable trauma scenario. Purpose This study aimed to investigate whether changes in graft height affect the kinematics of instrumented ACDFP C5–C6 motion segments in the context of varying degrees of simulated facet injuries. Study Design In vitro cadaveric biomechanical study was used as study design. Methods Seven C5–C6 motion segments were mounted in a custom spine simulator and taken through flexibility testing in axial rotation, lateral flexion, and flexion-extension. Specimens were first tested intact, followed by a standardized injury model (SIM) for a unilateral facet perch at C5–C6. The stability of the ACDFP approach was then examined with three graft heights (computed tomography-measured disc space height, disc space height undersized by 2.5 mm, and disc space height oversized by 2.5 mm) within three increasing unstable injuries (SIM, an added unilateral facet fracture, and a simulated bilateral facet dislocation injury). Results In all motions, regardless of graft size, ACDFP reduced range of motion (ROM) from the SIM state. For flexion-extension, the oversized graft had a larger decrease in ROM compared with the other graft sizes (p\u3c.05). Between graft sizes and injury states, there were a number of interactions in axial rotation and lateral flexion, where specifically in the most severe injury, the undersized graft had a larger decrease in ROM than the other two sizes (p\u3c.05). Conclusions This study found that graft size did affect the kinematic stability of ACDFP in a series of distractive-flexion injuries; the undersized graft resulted in both facet overlap and locking of the uncovertebral joints leading to decreased ROM in lateral bending and axial rotation, whereas an oversized graft provided larger ROM decreases in flexion-extension. As such, a graft that engages the uncovertebral joint may be more advantageous in providing a rigid environment for fusion with ACDFP