BS in Biomedical Engineering
Biomedical and General Engineering Department
The study of lateral interbody lumbar spinal surgery and designing an optimal intervetebral device is a complex endeavor. Interbody spinal fusion is performed for a variety of clinical situations, including back pain, vertebral fractures, and tumor conditions that require stabilization of the vertebral segments. The fundamental goal of spinal fusion is to attain a solid fixation at the implant-vertebral interface.
Replacement of a degenerated vertebral disc with an artificial intervertebral disc (AID) is currently possible, but poses problems mainly in the force distribution through the vertebral column. Data on the intervertebral disc space geometry will provide a better fit of the prosthesis to the vertebrae, but current literature on vertebral disc geometry is very scarce or not suitable . During the design phase there are few concerns that must be considered. Subsidence of the interbody cage into the vertebral body may cause collapse of disc space, recurrence of spinal deformity and preoperative symptoms, or failure of the fusion. The core of the vertebral body is made of a softer less dense cancellous bone towards the center of the disk space and a much harder ring of cortical bone called the apophyseal ring on the outer perimeter. Through maximizing the footprints contact area (better stress distribution), and focusing greater percentage of that stress on the harder apophyseal ring we will see improved fusion results in accordance to Wolff’s law. Also improved endplate matching reduces the risk of implant migration or protrusion into the psoas muscle. The psoas houses many sensory nerves, and if compacted could cause pain in the patient (commonly chronic leg pain).
In this study, statistical method and clinical axial x-rays of lumbar vertebra endplates were used to formulate a ratio to size the anterolateral radius of a patients lumbar vertebrae, based on the vertebras medial-lateral length. This technique, which was developed into a repeatable protocol, may be useful to consider in the design of any lumbar intervetebral device (particularly for one that is meant to span the disk space laterally).