Effect of Microdiscectomy on the Biomechanics of the Lumbar Facet Joints

Abstract

A microdiscectomy is the surgical standard of care for a lumbar disc herniation and is the most common lumbar spine surgery performed in the United States. The procedure can be done in “partial” (PD) and “subtotal” (SD) fashions, with the latter being the more aggressive of the two techniques. Currently, there is limited information regarding the effects of microdiscectomy on the biomechanics of the lumbar spine. As a first step in addressing this issue, this research aimed to understand how PD and SD affect the mechanics of the lumbar facet joints at the level of surgery. The response of the facet joints was described by (1) the three-dimensional kinematics (3D) of the facets and (2) the strains in the capsular ligament surrounding the joint. These metrics were explored in two separate in vitro investigations using human cadaveric spinal specimens. In both studies, the specimens were tested with a spine simulator that applied pure moments to the superior vertebra while allowing unrestricted motion of the specimen. The specimens were tested during physiological motions of: flexion-extension (FE), lateral bending (LB), axial rotation, and combined FE with LB. The studies employed a repeated measures approach whereby each specimen was tested with its disc intact and in post-PD and -SD states. The kinematics of the facets were determined via a rigid-body, point-based registration technique; and the capsular strains were measured with a custom 3D digital image correlation system. Of the two procedures, only SD was shown to significantly increase the motion of the facets and the strains in the capsular ligament. Increased motion could lead to mechanical overload of the facets, potentially resulting in degenerative changes at the joint and subsequent pain. Additionally, neuroanatomical studies have shown that the capsular ligament is innervated with mechanosensitive nociceptors. Thus, SD patients may also be at risk of experiencing pain that originates from the capsule. These potential outcomes should be weighed by clinicians when deciding on the best course of treatment for their patients.