Paul C. McAfee, MD, Towson, MD, USA; Bryan W. Cunningham, MSc, Baltimore, MD, USA; Norimichi Shimamoto, MD, Sapporo, Japan; John C. Sefter, DO, Towson, MD, USA; Anton E. Dmitriev, BS, Baltimore, MD, USA; Ira L. Fedder, MD, Towson, MD, USAIntroduction: To investigate the biomechanical and biologic ingrowth characteristics of two total disc prostheses: the hydroxyapatite (HA)–coated SB Charit III (Link Spine, Inc.) and the AcroFlex prosthesis using a sintered titanium beaded coating.Methods: Sixteen mature baboons (n=16, Papio cynocephalus) underwent L5–L6 total disc replacement through an anterior transperitoneal approach. In Group 1 (n=6) the SB Charit III prosthetic vertebral end plates were cobalt chrome, covered by two layers of thin titanium with an HA coating, which was electrochemically bonded to the implant surface. In Group 2 (n=10) the AcroFlex prosthesis was used comprised by prosthetic end plates of Ti6Al4V alloy, and the ingrowth surface used sintered titanium beads.Results: Six months after surgery the range of motion of the SB Charit III, the AcroFlex prosthesis and nonoperative controls (n=5) under axial compression, flexion/extension and lateral bending showed no statistical difference (p>.05). Plain film radiographic analysis showed no lucencies or loosening of any prosthetic vertebral end plate. Gross histopathologic analysis of the HA-coated SB Charit III and the AcroFlex prostheses 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, UHMWPE, cobalt chrome) nor cytokines (TGF-β, TNF-α, PGE-2, IL-1, IL-2, IL-6). Bioquant histomorphometric analysis at the metal–bone interface (bone contact area/total end plate area) showed the mean ingrowth was 47.9%, standard deviation was ±8.12 and the total range from 35.5% to 58.8% ingrowth.Discussion: The porous ingrowth, percentage pore ingrowth coverage at the bone–metal interface, was more favorable for total disc replacement compared with that reported for cementless femoral components (28.1%±5.31%, t test, p<.001 [1]), cementless tibial plateaus (27.1%±16.1%, t test, p<.001 [2]) and cementless acetabular components (12.1%±8.2%, t test, p<.001 [3]). The absence of reports of periprosthetic osteolysis in over 2,000 clinical cases of total disc replacement prostheses in Europe is probably the result of an absence of synovium in the anterior column of the spine, which fails to activate chemotactic cytokines implicated in wear debris analysis of diarthrodial total joint arthroplasty. The reason for the improved degree of porous ingrowth in total disc replacement prostheses is probably ligamentotaxis causing sustained compression across the metal–bone interface compared with arthroplasties in the appendicular skeleton.