UCLA

With advancements in deformity correction surgical strategies, such as the use of thoracic pedicle screws, invasive surgical resections, and correction maneuvers involving high forces, surgeons are pushing the limits of deformity correction in children and adolescents. Unfortunately, due to the lack of an adequate in-vitro model, many clinical questions surrounding these treatments remain unanswered, leading to clinical uncertainty and surgical risk. For the past three decades, in-vitro models have almost universally included the application of pure bending moments in cadaveric spines to produce kinematic responses. Pure moments are intended to produce a constant bending moment along the length of the spine, and offer the advantage of reproducible testing regardless of spine length or stiffness. With this model, the resulting kinematic spine responses have been compared for the evaluation of simulated destabilized and implanted conditions, either with dynamic stabilization or rigid fusion devices. However, the pure moment model was not selected to simulate intraoperative conditions. Moreover, alternative loading models have not been adequately explored.

The purpose of the proposed study was to evaluate three preclinical spine testing models: 1) the traditional pure moment preclinical testing method in spine biomechanics; 2) a novel simultaneous multi-planar loading protocol to represent the three-dimensionality of scoliosis deformities; and 3) a novel torsional loading protocol using a custom-built simulator intended to mimic a representative surgical correction maneuver employing high-magnitude in-vitro torque application. Each of the three models was applied to cadaveric thoracic spines to evaluate the safety and efficacy of representative intraoperative surgical techniques during deformity correction surgery. Specifically, safety and efficacy was measured by quantifying differences in thoracic spine range of motion and strength as a function of surgical resection and loading type. In addition to expanding the loading schematics for preclinical scoliosis testing, the proposed study evaluated and improved upon the validity of using elderly cadaveric specimens for preclinical testing of pediatric spine disorders and treatment, a major criticism of pediatric spine biomechanics. Unlike previous studies where intervertebral disc health has been ignored, the proposed study classified changes in the structural response of the spine as a function of disc health, thereby producing more specific conclusions towards the applicability of the results in the pediatric and adolescent communities.

Under single plane Pure Moments, wide posterior releases provided significant increases in motion beyond that provided by routinely performed releases, with thoracic spine range of motion increases of as much as 12-17° following the clinical releases. This result substantiates the use of wide posterior releases as a supplemental tool in increasing the flexibility of the thoracic spine. Moreover, in specimens with healthy intervertebral discs, multi-level releases provided more pronounced releases, nearly doubling the increase in motion. Furthermore, under the multi-planar loading protocol, the releases were effective in providing simultaneous three-dimensional increases in motion, suggesting their potential use in cases of three-dimensional deformity, such as adolescent idiopathic scoliosis.

The results were further developed under the simulated intraoperative correction technique, direct vertebral rotation. Thoracic spine strength was established, with an average failure moment of 33.3Nm, significantly less that the torques purportedly applied intraoperatively. Additionally, following the posterior release, single level ROM increases of as much as 19° were observed at failure, the most clinically relevant magnitude of motion increase measured to date using an in-vitro model. Using strength predictions based on the relationship between BMD and thoracic spine strength, safe limits of loading can be applied to produce significant increases in flexibility. Even applying as little as 25% of the failure load, the achievable increase in range of motion more than doubled compared to that predicted using 4Nm pure moments, the typical pure moment magnitude. In addition to providing safety and efficacy data towards predictions of deformity correction, the novel model highlighted the limitations of traditional in-vitro models, and may promote the use of novel experimental design in evaluating many spine problems.

In its entirety, the study introduced novel spine biomechanics testing methods which challenged the traditionally used pure moment model, and improved upon its limitations. In turn, the clinical applicability of preclinical scoliosis biomechanics testing results improved as well. Using these novel testing methods, the study helped to provide clinical guidelines for the efficacy and safety of posterior surgical releases and correction techniques. The improved clinical applicability of the results may also serve to stimulate development in other areas of spine testing, such as evaluations of disc degeneration, where the pure moment model has become increasingly unsuitable, as treatment has trended away from solid fusion and towards motion-preserving implantations. With improved models and experimental design, preclinical spine testing will begin to make a greater impact on the clinical outcome for children and adolescents. Moreover, the clinical decision making and outcome for all spine patients, young or old, will improve.