Abstract

This dissertation contributed three main areas to flexibility testing and biomechanical characterization of human spinal segments. The first was a literature review of existing testing methodology, the second examined common spine fixation procedures, and the third developed an improved fixation method. Spine biomechanical characterization is the primary technique for assessing the healthy, diseased, and surgically treated response of spinal tissues. However, despite decades of use, no standard testing protocol or reporting methodology for flexibility testing conditions has been established. As part of the present work, a comprehensive, systematic literature review was performed, and the methods sections of 242 relevant journal articles provided key information regarding preparation processes, fixation methods, testing temperatures, loading rates, loading magnitudes, and pre-conditioning procedures. The collated information was utilized to recommend best practices for testing and reporting flexibility testing methodology. Exothermic cementing processes were the dominant spinal fixation method from the reviewed articles. In these cases, fixation occurred when the most superior and inferior vertebrae of a functional spinal unit (FSU) were embedded into a cementing material that bound tightly to the cortical surface of the vertebrae, yielding a strong and geometrically favorable attachment point. The four most common fixation materials are highly exothermic and were tested to quantify the temperature rise in the adjacent intervertebral disc (IVD), as well as any associated thermal iatrogenic damage. Three thermocouples, inserted into three distinct locations of the IVD, measured temperatures throughout the cementing process. Polymethyl methacrylate (PMMA), the most frequently used cementing material, resulted in the greatest temperature increases, with a mean temperature rise up to 57°C above room temperature. Visible macro- and microscopic changes occurred in each of the cemented FSU IVDs. Changes included morphological changes, tissue desiccation, cracks, a breakdown in striations in the annulus fibrosus (AF), and denaturation and cell migration in the nucleus pulposus (NP). Based on these results, alternative vertebral attachment methods were considered and investigated. A structured design process was followed, and a mechanical ex vivo spinal fixation device which comprised a compliant vertebral clamp was designed. The geometry of the vertebral clamp was optimized based on pseudo-rigid body and finite element analyses. The final design was fabricated in titanium alloy (Ti6Al4V) using a 3D laser sintering process. Three clamps were manufactured and functionally validated. The opening/deployment and physiological loads of ± 7.5 N•m were cyclically tested on each clamp. The clamps showed no signs of failure and remained securely attached to the vertebrae. The resultant device/vertebrae interface stiffnesses were 4 to 10x greater than their corresponding FSU stiffnesses. The clamp design eliminated thermal iatrogenic damage, had minimal structural iatrogenic damage, was reusable, adjustable, and resulted in less than 10% of the previous preparation time.

Degree

PhD

College and Department

Ira A. Fulton College of Engineering; Mechanical Engineering

Rights

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