Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Science

Program

Surgery

Supervisor

Rodrigues Fernandes, Renan

2nd Supervisor

Rasoulinejad, Parham

Co-Supervisor

3rd Supervisor

Bailey, Chris

Co-Supervisor

Abstract

Spinal fusion to correct spinal deformity is typically performed with a 2-rod construct spanning the targeted area of fusion. More evidence is starting to emerge around the utility of multiple-rod constructs (typically 3 or more rods) to increase the stiffness and stability of a spinal fusion construct. Much of this work has focused on the lumbar spine, and little has been published around how these constructs behave in a long construct spanning the thoracic spine. The purpose of this thesis is to compare the stability of a two-rod (dual-rod) construct (DRC) to a four-rod (multiple-rod) construct (MRC) in cadaveric thoracic spines. Nine intact human cadaveric thoracic spines (T1-T12) were instrumented with either a DRC or MRC, and biomechanical testing was then carried out to compare range of motion (ROM) and stiffness between these two constructs. Results demonstrated comparable absolute total ROM and stiffness between DRCs and MRCs, and this was consistent across all measured vertebral levels. However, after undergoing a 1-hour bodyweight simulation fatigue test, DRCs exhibited an increase in flexion/extension ROM and decrease in stiffness while MRCs did not. Overall, these findings support previous clinical and biomechanical results in the lumbar spine and adult spinal deformity literature that MRCs can potentially be used to increase the stability of thoracic spine constructs.

Summary for Lay Audience

Spinal fusion, where two or more segments of the spine are fused together, is a common method to treat various types of spinal pathology. In order to accomplish a successful spinal fusion, the amount of motion at the fusion site must be decreased to a certain level to allow adequate bone formation across the segment. To decrease the amount of motion at a fusion site, the spine is instrumented at each level with screws and rods. Typically, two rods are used in a construct (one on each side), with two screws at each vertebral level. However, sometimes this two-rod construct is not rigid enough and excessive motion at the fusion site persists.

When too much motion occurs at the attempted fusion site, instead of bone forming across the fusion site, the area is filled with stable scar or fibrous tissue—this is called a nonunion or pseudarthrosis. Nonunion can lead to ongoing pain and increased motion that leads to continued stress on implants. As the implants are subjected to ongoing stress, complications can occur, such as the screws pulling out from the bone, or the rods themselves breaking. When instrumentation fails, this can then lead to further progression of deformity, pain, and even neurologic deficits, ultimately resulting in the need for additional surgery.

In an attempt to increase stability at the surgery site and to prevent nonunion, many deformity surgeons have started to use three or more rods (multiple-rod constructs) to augment instrumentation. This has had promising results in the biomechanical and clinical literature within the lumbar spine, but biomechanical research is lacking on how these constructs behave in the thoracic spine. In this thesis, two different types of instrumentation methods—a dual-rod construct and multiple-rod construct—in the thoracic spines of cadavers were compared.

Overall, dual-rod constructs and multiple-rod constructs exhibited similar absolute stability. But when each construct was subjected to 1 hour of simulated day-to-day wear and tear, multiple-rod constructs appeared more resistant to change in stiffness and range of motion. This supports the idea that multiple-rod constructs provide additional stability in the initial phase immediately after surgery, when bone is attempting to fuse. This has important consequences as multiple-rod constructs can be used to increase construct stiffness, which can potentially result in less nonunion and reduce the incidence of revision surgery after spinal fusion, ultimately leading to improved patient satisfaction and outcomes.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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