Analysis of Porous Ingrowth in Intervertebral Disc Prosthesis: A Non-Human Primate Model

· (a - Depuy-Acromed, Inc., Link Spine Group, Inc.)

†Scoliosis and Spine Center, Orthopaedic Biomechanics Laboratory and *Department of Pathology, Union Memorial Hospital, Baltimore, Maryland, USA; **Depuy-AcroMed Inc., Raynham, Massachusetts, USA; ‡Research Institute International, Phoenix, Arizona, USA

PURPOSE: Using a non-human primate model, the current study was undertaken to investigate the efficacy of the AcroFlex® Lumbar Disc as an intervertebral

METHODS: Twenty mature male baboons were equally randomized into two groups based on post-operative time periods of six (n=10) and twelve months (n=10). Each animal underwent an anterior transperitoneal surgical approach to the lumbar spine, with intervertebral reconstructions performed at L3-L4 and L5-L6 using the following techniques: (1) tricortical iliac autograft and 2) AcroFlex® Lumbar Disc (Depuy-AcroMed, Inc.). The two treatments were equally randomized between the non-contiguous operative lumbar levels. Post-mortem analysis included histopathological assessment of systemic reticuloendothelial tissues, non-destructive biomechanical testing of the operative functional spinal units and quantitative histological analysis of trabecular bone coverage at the prosthesis endplates.

RESULTS: All animals survived the operative procedure and post-operative interval without significant complication. Histopathologic analysis of the systemic reticuloendothelial tissues indicated no significant pathologic changes at the six or twelve month intervals. Plain film radiographic analysis showed no lucencies or loosening of any prosthetic vertebral endplate. Biomechanical testing of the six-month autograft, reconstructions with Acroflex lumbar disc and non-operative control (n=7) intact motion segments indicated no significance in peak range of motion (ROM) in axial compression (p › 0.05). However, axial rotation, flexion-extension and lateral bending produced significantly lower ROM for the autograft treatment compared to the remaining two groups (p ‹ 0 .05). Histomorphometric analysis demonstrated excellent ingrowth at the level of prosthesis-bone interface (bone contact area equals total endplate 54.59%±13.24) without fibrous adhesions.

CONCLUSIONS: Based on the bench-top biomechanics, motion was preserved in axial rotation for the prosthesis, but slightly diminished in the other bending modalities. Histomorphometric analysis of porous ingrowth coverage at the vertebral bone-metal interface was excellent, despite the biomechanically challenging model. The application of this technology in deformity conditions would include treatment of lumbar disc pathology at the uninstrumented distal adjacent segment(s) following thoracolumbar curve correction. This project serves as the first comprehensive in-vivo investigation of the AcroFlex Disc Prosthesis and supports the use of this technology for the surgical management of adjacent level discogenic pathology secondary to thoracolumbar spinal deformities.