Initiation and Progression of Lumbar Disc Degeneration under Cyclic Loading: A Finite Element Study

Low back pain is a major health condition that effects every population worldwide. It can lead to diminished physical function, decreased quality of life, and psychological distress. Although low back pain is a multifactorial condition, disc degeneration disease has been indicated to be a strong etiologic factor. The purpose of the study was to investigate the effects of disc degeneration process on the biomechanical behavior of the lumbar motion segment using computational method of finite element analyses. An already validated poroelastic finite element model of L4/L5 lumbar motion segment developed by our research team, that included biological parameters like porosity, fixed charged density, osmotic pressure and strain dependent permeability was employed. Morphological and biochemical changes observed at different stages of disc degeneration were introduced. Finite element models representing Thompson grade I/II, grade III, grade IV and grade V disc degeneration were developed. The finite element models predicted an increase in the flexibility of the lumbar motion segment with progressive disc degeneration. Annulus fibrosus and endplates were exposed to higher levels of stresses with disc degeneration. Facet joints contact forces exhibit an increasing trend while the intradiscal nucleus pressure decreased with disc degeneration. These results suggested changes in the load bearing pattern in the lumbar motion segment from nucleus pulposus in the healthy disc to annulus fibrosus and facet joints in the degenerated discs. The structural failures in the disc are considered to be the result of damage accumulated over a long period of time rather than the traumatic injury. It is difficult to track the fatigue failure in the disc using experimental techniques. Finite element models discussed above were incorporated with continuum damage mechanics methodology to investigate the initiation and progression of the disc failure under cyclic loading. The analyses showed that the damage initiated and accumulated preferentially in the posterior annulus under all loading conditions. With disc degeneration the location of the failure shifted from the inner annulus layers in the healthy disc to the outer periphery of the annulus in grade IV disc degeneration model. The models predicted that the disc failure is unlikely to happen with repetitive bending in the absence of compressive load. The number of load cycles to disc failure decreased as the disc was subjected to complex loading rather than single axis compression or bending. The finite element models results were consistent with the experimental and clinical observations in terms of the region of failure, magnitude of applied loads and the number of load cycles survived. The finite element models presented in this work can be extended to the whole lumbar spine. This work will increase the understanding of the spine biomechanics and will help in designing better techniques to prevent and treat low back disorders.