Study design: To investigate the biomechanical, histochemical, and biologic ingrowth characteristics of two different lumbar disc prostheses-AcroFlex (DePuy-AcroMed) and the SB Charitè (DePuy-AcroMed)-for total disc replacement arthroplasty.
Methods: A total of 17 mature baboons (n = 17, Papio cynocephalus) underwent L5-L6 total disc replacement procedures. The AcroFlex device (n = 10 levels) consisted of sintered titanium beaded ingrowth surfaces, bound together by a hexene-based polyolefin rubber core. The SB Charitè (n = 7 levels) device prosthetic vertebral end plates were cobalt chrome, covered by two layers of thin titanium with an electrochemically bonded hydroxyapatite coating and an ultra-high molecular weight polyethylene core.
Results: Following 6-month survival periods, the range of motion of the SB Charitè and intact nonoperative controls under axial compression, flexion-extension, and lateral bending showed no statistical difference (P > 0.05). However, both exhibited greater range of motion compared to the AcroFlex treatments (P < 0.05). Plain film radiographic analysis showed no lucencies or loosening of any metallic prosthetic vertebral endplate. Gross histopathologic analysis of the AcroFlex and SB Charité prosthesis demonstrated excellent ingrowth at the level of the implant-bone interface, without evidence of fibrous tissue or synovium. Histochemical assays showed no local or systemic accumulation of particulate wear debris (titanium, ultra-high molecular weight polyethylene, or cobalt chrome) nor cytokines (TNF-alpha, PGE2, IL-1, IL-2, or IL-6). Porous ingrowth calculations showed the mean ingrowth (linear apposition) ranging from 47.9% +/- 9.12 for the SB Charitè device and 54.59% +/- 13.24 for the AcroFlex device.
Conclusions: The porous ingrowth, percentage pore ingrowth coverage at the bone-metal interface was more favorable for total disc replacement compared to that reported for cementless total joint components in the appendicular skeleton (range 10-30%). The reason for the improved degree of porous ingrowth in total disc replacement prostheses is probably due to ligamentotaxis causing sustained compression across the metal-bone interface. This project serves as the first comprehensive in vivo investigation comparing two different types of unconstrained disc prostheses with alternate in-growth surfaces and establishes an excellent research model in the evaluation of lumbar total disc replacement arthroplasty.