ANALYSIS OF STRESS DISTRIBUTION IN THE VERTEBRAL BODIES USING FINITE ELEMENT ANALYSIS

Abstract

Introduction: Despite a very high fusion rate (90%) achievable by present techniques, the clinical success rate for curing back pain is in the range of 50%. We hypothesise that disc degeneration gives rise to abnormal stress patterns in the bone. Although the cages integrate fully, load is taken by the cage producing abnormal stress patterns in the vertebrae. Unless a near normal stress pattern in the vertebrae is established, pain may continue.

Method: A simple finite element model of a disc and its adjacent vertebral bodies was developed using ANSYSS software. The dimensions of the model were based on the human lumbar disc. The normal disc was modelled as a fluid with a bulk modulus of 1720 MPa. The degenerate disc was modelled as having the same material properties for the nucleus and the annulus. Fusion of the disc was modelled by replacing the nucleus with commonly used cages. In all the models, the material properties of the cancellous bone (E=100 MPa; v=0.3) and the cortical bone (E=12000 MPa; v=0.3) remained the same. The model was loaded axially with 1.5 kN.

Results: The vertical and horizontal stress patterns around a loaded degenerate disc showed areas of increased loading in the endplate and the cancellous bone confirming the authors’ previous work using load transducers. The introduction of the cages in the model changed the stress distribution – they caused an increase in the compressive stresses in the cancellous bone, and a high concentration of tensile and compressive stresses at the point of contact with the cages.

Conclusion: This study has shown that fusion cages alter the pattern of stress distribution in the adjacent vertebral bodies similar to that of a degenerate disc. It supports the concept that abnormal weight transfer is a more significant cause of back pain as compared to abnormal mobility.