The effect of construct configuration on the mechanical stiffness and strength of spinal implants for kyphotic spinal deformity

Tsang, Kai Sun

Abstract

Spinal deformity involves a wide spectrum of disease, including congenital, developmental, degenerative and traumatic; affecting infant, adolescence and the elderly. Of these, Sheuermann’s kyphosis is a form of developmental long spinal deformity, characterised by an excess forward bending of thoracic spine. It is not lethal but can lead to chronic problems such as pain and psychological stress. Common late presentation leaves patient with the choice of symptomatic controls or surgical correction. It is an uncommon disease but has posed huge surgical challenges to the surgeons due to its technical difficulties over the years. This has since improved with the advent of the polyaxial pedicle screw instrumentation system, which is using titanium screws to hold long titanium supporting rods posterior to the spine. While the surgical outcome has improved, many questions remain. One vexing problem is the huge cost and relative cost per screw insertion in surgery. This is in the background of classic teaching of using as many screws as possible. Clinical outcomes, however, seems to suggest differently.
However, there is a lack of mechanical study on such a long spinal instrumentation to date. This study picked up the challenge and attempts to provide some clues on this topic. Tests were conducted on a simulated spinal model, which is designed to act as a supporting frame, mimicking the state fresh after surgery. Three metal density constructs are tested: 1) full density (100% screw usage), 2) half density (50%) and 3) cantilever density (60%), under vertical stress on a mechanical testing machine. Mean stiffness of the construct in the same order are: 1) 7.8Nmm, 2) 8.9Nmm and 3) 7.3Nmm. There is no statistical difference between groups and the observed difference is small too.
This indicates the metal density does not matter if the load is mainly transmitted through the rods.

Citation

Tsang, K. S. The effect of construct configuration on the mechanical stiffness and strength of spinal implants for kyphotic spinal deformity. (Thesis). Keele University. https://keele-repository.worktribe.com/output/518875

• Thesis Type: Thesis
• Deposit Date: Jul 20, 2023
• Public URL: https://keele-repository.worktribe.com/output/518875
• Award Date: 2023-06