Removal of the nucleus pulposus from the intervertebral disc : effect on the response of the annulus fibrosus to compression
Intervertebral disc herniation can be a source of pain and sometimes requires surgery. This involves removing a portion of the nucleus pulposus from the disc (denucleation). Little research has looked at how this changes the internal response of the disc to loading, but indications are that it may lead to further degeneration. The aims of this study were to characterise the internal behaviour of the disc under load, investigate the changes that occur after denucleation, and evaluate the replacement of the nucleus with synthetic materials. Three techniques were used: mechanical testing with video recording, magnetic resonance imaging (MRI) and finite element (FE) analysis. Mechanical tests involved cutting discs in the mid-sagittal plane, and sealing them with Perspex. This allowed their internal structure to be observed and recorded onto video tape as compression was applied. Movement of dye markers made on the cut face was then measured from the video images. A highly significant change in the deformation of the annulus was observed after total denucleation in 34 sheep discs. Twelve human discs showed similar changes after partial denucleation. MRI was used to see if the results could be reproduced in whole discs. Observations from the images corroborated the results. A simple FE model of the disc was developed which matched the experimental results. The model was used to investigate the effects of replacing the nucleus with a synthetic material, and to predict the ideal properties of such an implant. Experimental validation of the model predictions was obtained by repeating the mechanical testing experiments with various polymers inserted into the sheep discs.