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 Charité 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 Charité 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 Charité 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 Charité 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 E2, 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.