Kinematics of a Novel Canine Cervical Fusion System

Objective The aim of this study was to determine the biomechanical behaviour of a novel distraction–fusion system, consisting of an intervertebral distraction screw, pedicle locking screws and connecting rods, in the canine caudal cervical spine. Study Design Biomechanical study in cadaveric canine cervicothoracic (C3–T3) spines (n = 6). Cadaveric spines were harvested, stripped of musculature, mounted on a four-point bending jig, and tested using non-destructive four-point bending loads in extension (0–100 N), flexion (0–60 N) and lateral bending (0–40 N). Angular displacement was recorded from reflective optical trackers rigidly secured to C5, C6 and C7. Data for primary and coupled motions were collected from intact spines and following surgical stabilization (after ventral annulotomy and nucleotomy) with the new implant system. Results As compared with the intact spine, instrumentation significantly reduced motion at the operated level (C5-C6) with a concomitant non-significant increase at the adjacent level (C6-C7). Conclusion The combination of a locking pedicle screw-rod system and intervertebral spacer provides an alternative solution for surgical distraction–stabilization in the canine caudal cervical spine and supports the feasibility of using this new implant system in the management of disc-associated cervical spondylomyelopathy in dogs. The increase in motion at C6-C7 may suggest the potential for adjacent level effects and clinical trials should be designed to address this.

In VitroComparison of Dynesys, PEEK, and Titanium Constructs in the Lumbar Spine

Introduction. Pedicle based posterior dynamic stabilization systems aim to stabilize the pathologic spine while also allowing sufficient motion to mitigate adjacent level effects. Two flexible constructs that have been proposed to act in such a manner, the Dynesys Dynamic Stabilization System and PEEK rod, have yet to be directly comparedin vitroto a rigid Titanium rod.Methods. Human lumbar specimens were tested in flexion extension, lateral bending, and axial torsion to evaluate the following conditions at L4-L5: Intact, Dynesys, PEEK rod, Titanium rod, and Destabilized. Intervertebral range of motion, interpedicular travel, and interpedicular displacement metrics were evaluated from 3rd-cycle data using an optoelectric tracking system.Results. Statistically significant decreases in ROM compared to Intact and Destabilized conditions were detected for the instrumented conditions during flexion extension and lateral bending. AT ROM was significantly less than Destabilized but not the Intact condition. Similar trends were found for interpedicular displacement in all modes of loading; however, interpedicular travel trends were less consistent. More importantly, no metrics under any mode of loading revealed significant differences between Dynesys, PEEK, and Titanium.Conclusion. The results of this study support previous findings that Dynesys and PEEK constructs behave similarly to a Titanium rodin vitro.

A Pseudo-Rigid-Body Model of the Human Spine to Predict Implant-Induced Changes on Motion

Injury, instrumentation, or surgery may change the functional biomechanics of the spine. Adverse changes at one level may affect the adjacent levels. Modeling these changes can increase the understanding of adjacent-level effects and may help in the creation of devices that minimize adverse outcomes. The current modeling techniques (e.g., animal models, in vitro testing, and finite element analysis) used to analyze these effects are costly and are not readily accessible to the clinician. It is proposed that the pseudo-rigid-body model(PRBM) may be used to accurately predict adjacent level effects in a quick and cost effective manner that may lend itself to a clinically relevant tool for identifying the adjacent-level effects of various treatment options for patients with complex surgical indications. A PRBM of the lumbar spine (lower back) was developed using a compliant mechanism analysis approach. The global moment-rotation response, relative motion, and local moment-rotation response of a cadaveric specimen were determined through experimental testing under three conditions: intact, fused, and implanted with a prototype total disc replacement. The spine was modeled using the PRBM and compared with the values obtained through in-vitro testing for the three cases. The PRBM accurately predicted the moment-rotation response of the entire specimen. Additionally, the PRBM predicted changes in relative motion patterns of the specimen. The resulting models show particular promise in evaluating various procedures and implants in a clinical setting and in the early stage design process.