The effect of anterior plate stiffness on load-sharing and adjacent level biomechanics following arthrodesis of the cervical spine
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Authors
Levy, Rebecca
Issue Date
2024-12
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Biomedical engineering
Alternative Title
Abstract
Low back and neck pain are estimated to affect approximately 80% of individuals at least once in their lifetime. One common cause underlying both back and neck pain is degenerative disc disease (DDD). This disease is characterized by an overall loss in intervertebral disc height causing subsequent pain. When pain becomes chronic the current gold standard for pain relief in the cervical spine is an anterior discectomy and fusion (ACDF). This surgical intervention involves restoring lost disc height by inserting an anterior cervical cage in the intervertebral disc space and using an anterior cervical plate for stability during healing. Successful fusion occurs when a bony bridge forms between two adjacent vertebrae, relieving pain. While this surgery has a high success rate in one-level fusions, a common pathology that can occur is adjacent-level degeneration following the ACDF procedure. This degeneration occurs at the super and sub adjacent levels to the index level. There is no consensus whether this accelerated degeneration is caused by biomechanical changes or if an individual is pre-disposed to degenerative changes as a natural progression of DDD, independent of surgical intervention.Prior research suggests that anterior cervical plate stiffness could affect the biomechanical environment following instrumentation in an ACDF by altering load sharing through the intervertebral disc space and a shift in the instantaneous axis of rotation (IAR) location. Therefore, to study biomechanical changes at the adjacent levels in vitro we developed a range of novel low stiffness anterior cervical plates and a low stiffness cage using CAD and finite element analysis (FEA). The anterior cervical cage was instrumented with strain gauges to develop a “smart” implant to monitor interbody loads in real-time. A novel mechanical testing system was also designed to apply physiologically relevant loading conditions in vitro to more accurately study the changes to the biomechanical environment at adjacent levels.
We hypothesized that there would be a correlation between anterior cervical plate stiffness and load-sharing: as implant stiffness decreases load sharing between the plate and cage increases. We also hypothesized that there would be a correlation between implant stiffness and intersegmental motion, facet joint loads, and the location of the instantaneous axis of rotation: as stiffness decreases there will be (a) decreased intersegmental motion at the super-adjacent and sub-adjacent motion segments, (b) decreased loading on the facet joints of the index level and super- and sub-adjacent levels, and (c) a more physiologic instantaneous axis of rotation when compared to stiffer cervical plates.
Results demonstrated that anterior cervical plate stiffness does dictate the biomechanical environment by altering load-sharing and the location of the IAR. We further demonstrated in vitro that anterior cervical plate stiffness was one factor that could affect adjacent level biomechanics leading to degenerative changes and should be considered when studying adjacent level disease. However, underlying pathological changes also appeared to play a role in the biomechanical environment both at the index and adjacent levels.
Description
December 2024
School of Engineering
School of Engineering
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY