Abstract
INTRODUCTION: The elastomeric, monobloc disc prosthesis (Cadisc™-L, Ranier Technology, Cambridge, UK) aims to preserve biomechanics of an implanted spinal motion segment. STUDY DESIGN: This study presents the findings of an in vitro investigation on the effect of implantation of Cadisc™-L. Compressive stiffness, flexion stiffness at 10, 20, 30 and 40 Nm and the instant-axis-of-rotation (IAR) loci are compared before and after implantation of a MC-10 mm-6° Cadisc™-L. METHODS: Fresh frozen human monosegmental lumbar spines (n = 8) were prepared, potted and tested in an environmentally controlled chamber to simulate in vivo conditions. Specimens were preconditioned by loading to 500 N for 30 min. Compressive stiffness of the specimen was determined by applying pure compression of 1 kN at 250 N/s via a loading roller positioned at the central loading axis (CLA). The roller was then offset 12.5 mm anterior of the CLA and the loading regime repeated to test specimens in flexion. Bending moments were calculated from the applied load and corresponding flexion angle. The IAR locus was tracked by a motion-tracking camera. RESULTS: Compressive stiffness was reduced by 50 % (p = 0.0005), flexion stiffness was not statistically significantly reduced (40 % reduction, p > 0.05). IAR locus maintained a 'horizontal figure of eight' characteristic. Change in the locus width in the AP plane of 6.4 mm (p = 0.06) and height in the SI plane of 1.3 mm (p = 0.44) were not significant. The centroid was displaced 4.44 mm (p = 0.0019) and 5.44 mm (p = 0.025) at 3° and 6° flexion, respectively. CONCLUSIONS: Implantation of Cadisc™-L caused a reduction in axial stiffness, but maintained disc height and flexion stiffness. IAR loci remained mobile without large displacement of the centroid from the intact spine position.