Patterns of Height Changes in Anterior and Posterior Cervical Disc Regions Affects the Contact Loading at Posterior Facets During Moderate and Severe Disc Degeneration

Abstract

Biomechanical roles of anterior and posterior portions of the disc (AD and PD, respectively) in governing posterior facets (PF) behavior of a C5-C6 motion segment. To understand how height patterns (loss and gain) at AD and PD affects the PF contact loading during moderate and severe grades of cervical disc degeneration (DD). PF overloading and degeneration after degenerative disc height loss is a clinical concern. This may occur because of a decrease in facet joints space, causing elevated PF contact forces. During a physiologic motion, axial disc height patterns at AD and PD affects the spacing and overlapping of articulating PF surfaces. The question arises as to what percentage of deformation and/or elongation at AD and PD is contributing to a corresponding increase and/or decrease in PF contact loading. A poroelastic, three-dimensional finite element model of a C5-C6 segment with a normal (grade I) disc was used after validation. Two degenerated disc models were developed from the normal disc model: moderate (combined effect of Thompson disc grades II and III) and severe (combined effect of Thompson disc grades IV and V). The models were analyzed under compression, flexion, and extension. Height patterns (loss and gain) at AD and PD, and PF loading were calculated for each model. PF loading increased with PD height loss, and further increased with AD height gain. In moderate DD, PF loading was more affected by PD height loss than AD height gain, whereas in severe DD, it was more affected by AD height gain than PD height loss. The current study conclusions suggest a possible mechanism for PF loading through loss and gain in the AD and PD heights during moderate and severe grades of DD. Further cervical spine based biomechanical investigations are suggested to verify our findings.