Bryan W. Cunningham, MSc, Baltimore, MD, USA; Gary L. Lowery, MD, PhD, Phoenix, AZ, USA; Vera Gonzales, PhD, Galveston, TX, USA; Carols M. Orbegoso, MD, Baltimore, MD, USAIntroduction: The development and use of artificial disc replacements serve as the next frontier in the surgical management of intervertebral disc pathology. Using a nonhuman primate model, the current study was undertaken to investigate the efficacy of the AcroFlex Lumbar Disc (Depuy-AcroMed, Inc.) as intervertebral disc prosthesis, based on radiological, biomechanical, histopathologic and histomorphometric analyses.Materials and methods: A total of 20 mature male baboons (Papio cynocephalus, n=20, average 30 kg) were equally randomized into two groups based on postoperative time periods of 6 (n=10) and 12 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. After anterior annulus resection, complete discectomy and end plate decortication, two tricortical wedges of autologous iliac crest (10×10×20 mm) were impacted into the discectomy site and served as a fusion control. For the disc replacement, the AcroFlex Lumbar Disc (20 mm diameter × 10 mm height) was implanted according to the manufacturer's specifications. The two treatments were equally randomized between the noncontiguous operative lumbar levels. After the respective postoperative time periods, the animals were humanely euthanized. Postmortem analysis included histopathological assessment of the systemic reticuloendothelial tissues, nondestructive biomechanical testing of the operative functional spinal units and quantitative histological analysis of trabecular bone coverage at the prosthesis end plates. Data were statistically compared using a one-way analysis of variance with Student-Newman-Keuls test and Student's t test. The institutional animal care committee granted approval for this project.Results: All animals survived the operative procedure and postoperative interval without significant intra- or perioperative complication. Histopathologic analysis of the paraffin-embedded systemic reticuloendothelial tissues indicated no significant pathologic changes at the 6- or 12-month intervals. Plain film radiographic analysis showed no lucencies or loosening of any prosthetic vertebral end plate. Biomechanical testing of the 6-month autograft, reconstructions with Acroflex Lumbar Disc and nonoperative control (n=7) intact motion segments indicated no significance in peak range of motion (ROM) in axial compression (autograft, .42.46 mm; AcroFlex Lumbar Disc, .36.27 mm; intact, .67.45 mm; p>.05). However, axial rotation produced significantly lower ROM for the autograft treatment compared with the remaining two groups (autograft, .74.34 degrees; AcroFlex Lumbar Disc, 1.67.69 degrees; intact, 1.60.68 degrees; p<.05). The most significant differences in peak ROM were noted between all treatment groups under flexion/extension (autograft, 1.691.20 degrees; AcroFlex Lumbar Disc, 3.851.50 degrees; intact, 5.992.95 degrees; p<.05) and lateral bending loading modalities (autograft, 1.581.54 degrees; AcroFlex Lumbar Disc, 3.791.06 degrees; intact, 13.922.44 degrees; p<.05). Gross histopathologic analysis of the AcroFlex Disc Prosthesis demonstrated excellent ingrowth at the level of the implant–bone interface, without evidence of fibrous tissue or synovium. BioQuant histomorphometric analysis at the metal-bone interface (bone contact area/total end plate area) indicated the mean ingrowth = 54.59%, 13.24 SD (range, 36.18% to 80.12%).Discussion: The current study establishes a excellent research model for the evaluation of artificial disc prostheses. Radiographic analysis showed no lucencies or loosening of the AcroFlex vertebral end plate. Based on the bench-top biomechanics, motion was preserved in axial rotation but was slightly diminished in the other bending modalities. This effect may be the result of the limited surface area of device–vertebral end plate contact. Histomorphometric analysis of porous ingrowth coverage at the vertebral bone–metal interface was more favorable for total disc replacement compared with that reported for cementless femoral components = 28.1%, 5.31 [1], cementless tibial plateaus = 27.1%, 16.1 [2] and cementless acetabular components = 12.1%, 8.2 [3] (t tests = p<.05). This project serves as the first comprehensive in vivo investigation into the AcroFlex Disc Prosthesis and supports the use of this technology for surgical management of discogenic pathology as an alternative to interbody spinal arthrodesis.