Analysis of porous ingrowth in intervertebral disc prostheses - A nonhuman primate model

Study Design. A study was conducted to investigate the biomechanical, histochemical, and biologic ingrowth characteristics of the most widely used total disc prosthesis, the hydroxyapatite-coated SB Charite prosthesis. Objective. To compare the porous ingrowth, linear apposition, or bony ingrowth in total disc replacement with published reports of porous ingrowth prostheses in the appendicular skeleton. Methods. Seven mature baboons (Papio cynocephalus) underwent L5-L6 total disc replacement through an anterior transperitoneal approach. The SB Charite prosthetic vertebral endplates (n = 14) were cobalt-chrome covered by two layers of thin titanium with a hydroxyapatite coating, which was electrochemically bonded to the implant surface. Results. At 6 months after surgery, the range of motion exhibited by the SB Charite and the nonoperative control subjects under axial compression, flexion-extension, and lateral bending showed no statistical difference (P > 0.05). Plain film radiographic analysis showed no lucencies or loosening of any prosthetic vertebral endplate. Gross histopathologic analysis of the hydroxyapatite-coated SB Charite prosthesis demonstrated excellent ingrowth at the level of the implant-bone interface, without evidence of fibrous tissue or synovium. Histochemical assays showed no accumulation of particulate wear debris (no titanium, ultrahigh molecular weight polyethylene, or cobalt-chrome) nor cytokines (tumor necrosis factor-alpha, prostaglandin E-2, interleukin-1, -2, or -6). Total endplate area showed a mean ingrowth (volume fraction) of 47.9% +/- 9.12% and a total ingrowth range of 35.5% to 58.8%. Conclusions. The porous ingrowth (percentage of pore ingrowth coverage at the bone-metal interface) was more favorable for total disc replacement than 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 probably is that ligamentotaxis causes sustained compression across the metal-bone interface.