Development and Validation of an Analogue Lumbar Spine Model and its Integral Components
Domann, John P.
University of Kansas
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There is a large need for an anatomically and mechanically correct model of the human lumbar spine. Such a model could have widespread use in the development of new implants and surgical procedures designed to remediate low back pain. Previous work has already been completed on such a model, and the purpose of this study is to approach release of the first generation model to the public. In order to reach this milestone significant work went into the development of a synthetic vertebral cancellous bone model, as well as analysis and development of the overall spine model itself. This work is being completed with Pacific Research Laboratories (Sawbones) who will ultimately manufacture and sell the product. Foam theory was utilized to analyze solid materials that could serve as effective cancellous bone models. These materials were readily available to PRL, and their supply secured for the indefinite future. Following four point bend tests, one material was deemed acceptable for use in a cancellous bone model. Analysis showed that a synthetic model manufactured from this material would require 85% of human bone's relative density to obtain similar stiffness, and 111% of human bone's relative density to obtain adequate strength. Synthetic foams were prepared, mechanically characterized, and compared to the literature. Overall, the model behaved quite similarly to human bone, with mechanical properties slightly higher than the reported literature. The model had stiffness of 375 MPa, strength of 4.33 MPa, and post yield ductility of .51%.Future work will serve to further refine this model, and incorporate it into the vertebral body of the Analogue Spine Model. At the beginning of this study, the Analogue Spine Model was behaving too stiffly, and aberrant behaviors were noticeable in axial rotation. Stiffness was approximately 1 Nm/° higher than the literature in all modes of bending. Aberrant behavior was most notable in axial rotation. In this mode of bending, a "stair step" behavior was observed in what should have been a smooth sigmoid curve. Work was conducted on locating the source of these limitations, and altering them to improve model performance. This was conducted with the use of a systematic dissection, allowing identification of the interactions and elements responsible for the model's behavior and stiffness. After identifications, alterations were made to the manufacturing process, and to specific soft tissues. Once completed, the model now has appropriate neutral zone stiffness in all modes of bending (flexion - 1.82 Nm/°, extension - 2.01 Nm/°, lateral bending - .85 Nm/°, axial rotation - 2.47 Nm/°), extension zone stiffness in three of four modes of bending (flexion - 3.09 Nm/°, lateral bending - 5.30 Nm/°, axial rotation - 11.25 Nm/°), but high neutral zone range of motion in all modes of bending and high extension zone stiffness in extension (9.15 Nm/°). Future work is centered on reduction of neutral zone range of motion, and extension zone stiffness in extension. This model's performance will be compared to cadaveric specimens tested using the experimental setup utilized in this study. Following validation, the model will be released into the market, and become accessible to researchers and companies alike.
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