Study Design: Comparative evaluation of in vitro and in vivo biomechanics, resulting fusion and histomorphometric aspects of Polyetheretherketone (PEEK) versus Titanium (Ti) interbody fusion devices in an animal model with similar volumes of bone graft.
Objective: Identify differences in the characteristics of fusion and biomechanics immediately following implantation (time zero) and at 26 weeks with each interbody implant.
Summary of Background Data: PEEK has been well accepted in spinal surgery, it provides a closer match to the mechanical properties of bone than metallic implants such as titanium. This is thought to reduce graft stress shielding and subsidence of interbody fusion devices. There remains controversy as to the overall influence of this as a factor influencing resultant fusion and initial stability. While material modulus is one factor of importance, other design factors are likely to play a large role determining overall performance of an interbody implant.
Methods: A Ti and PEEK device of similar size with a central void to accommodate graft material were compared. The PEEK device had a ridged surface on the caudal and cephalad surfaces, while Ti device allowed axial compliance and had bone ingrowth endplates and polished internal surfaces. A two level ALIF was performed in 9 sheep and fusion, biomechanics, and bone apposition were evaluated at 26 weeks. Time zero in vitro biomechanical tests were performed to establish initial stability immediately following implantation.
Results: No differences were detected in the biomechanical measures of each of the devices in in vitro time zero tests. All levels were fused by 26 weeks with considerably lower Range of Motion (ROM) when compared to in vitro tests. ROM in all modes of bending was reduced by over 70% when compared to intact values for Axial Rotation (Ti-74%, PEEK-71%), Lateral Bending (Ti-90%, PEEK-88%) and Flexion/Extension (Ti-92%, PEEK-91%). Mechanical properties of fusions formed with each implant did not differ, however bone apposition was variable with polished internal Ti surfaces being lower than PEEK and treated Ti endplates showing the greatest levels. Graft material displayed axial trabecular alignment with both implants.
Conclusions: Although material properties and surface characteristics resulted in differing amounts of biological integration from the host, both implants were capable of producing excellent fusion results using similar volumes of bone graft.
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