Replacing the nucleus pulposus of the intervertebral disc

OBJECTIVE: To determine whether replacement of the nucleus with a synthetic material would prevent the effects of nucleus removal. DESIGN: Laboratory experiments on excised tissues and a finite element model. BACKGROUND: Removal of the nucleus from the intervertebral disc causes the inner margins of the annulus to bulge inwards, instead of outwards, during compression. This may cause the annulus to degenerate further. METHODS: Video recordings of sheep discs, sectioned in the sagittal plane, were obtained during compression in a materials testing machine; the cut face of the disc was sealed with a Perspex window. Experiments were repeated with the nucleus removed and then replaced by a synthetic implant. A finite element model of an intact disc was also used to investigate the effect of nucleus replacement. RESULTS: When the nucleus of sectioned discs was replaced with the polymer materials, the inward bulging of the annulus was not observed. The predictions from the finite element model of the intact disc were consistent with this result. CONCLUSIONS: Replacement of the nucleus with a synthetic material can prevent the changes in annulus behaviour that result from removal of the nucleus. RELEVANCE: A suitable implant to replace the nucleus after surgical removal may help prevent inward bulging of the inner layers of the annulus.