INTERLAMINAR SHEAR STRESSES AND LAMINAE SEPARATION IN A DISC - FINITE-ELEMENT ANALYSIS OF THE L3-L4 MOTION SEGMENT SUBJECTED TO AXIAL COMPRESSIVE LOADS

Study Design. This study analyzed interlaminar shear stresses across the laminae of a ligamentous L3-L4 motion segment. A three-dimensional finite element model of the motion segment was developed and its response in axial compression mode was predicted. Objectives. The contributions of ''mechanical'' factors in producing laminae separation in a disc are not well understood, especially when the nucleus is still gel-like in appearance (stage 1 of disc degeneration). All types of stresses are likely to contribute to laminae separation. The authors believe it is partially due to the interlaminar shear stresses at the laminae interfaces in specific regions of an intact disc because the disc is a composite structure. The effects of anular tears on the interlaminar shear stresses were also investigated. These tears can be circumferential or radial in nature, and commonly occur in the aged, degenerated disc. Summary of Background Data. A large number of biomechanical studies dealing with the role of the disc vis-a-vis other spinal components have been reported in the literature. The role of mechanical factors, however, in producing laminae separation, especially when the nucleus is still gel-like in appearance (stage 1 of disc degeneration), is not entirely clear. Methods. A three-dimensional nonlinear finite element model of an intact L3-L4 lumbar motion segment, based on the use of a special type of element for the disc anulus, was created to investigate the interlaminar shear stresses in the anulus. The effects of radial ''out-in,'' radial ''in-out,'' and ''circumferential'' injuries were analyzed. Injury was modeled as element removal in the posterolateral portion of the disc. Models subjected to axial compressive loads, ranging from 200 N to 2000 N, were analyzed. In addition to the interlaminar shear stresses, disc bulge, and displacements including coupled motions were predicted. Results. The theoretical disc bulge predictions for the radial in-out injury case were in agreement with the disc bulge data obtained experimentally. Displacements, disc bulge, and coupled motions increased with injury, as expected. The interlaminar shear stresses were highest in the posterolateral portions of the intact disc model. Interlaminar shear stresses, in general, increased with injury. Also, a slight increase in circumferential injury was sufficient to see a substantial increase in interlaminar shear stresses. Conclusions. The interlaminar shear stresses being higher in the posterolateral regions of the intact disc reinforces that, from clinical studies, tears originate in the posterolateral portion of the disc. The large interlaminar shear stresses, caused by asymmetry in the disc structure due to injury, along with chemical and structural changes in the disc with age, may be an important cause of further degeneration through laminae separation. this is the case for traditional composite laminates. This study points out the importance of interlaminar shear stresses to gain further understanding of the role of mechanical factors in producing disc degeneration, especially delamination of the anulus. Clinical relevance of the findings and possible relationship to the aging process are explored.