Analysis of clinically relevant mechanical and thermal characteristics of titanium foam spinal implants during drilling.

Although high biocompatibility promotes the use of titanium (Ti) alloy in spinal implants, this material shows high stiffness, which is an issue for removal by drilling. The recently developed, porous Ti foam implants, which have shown enhanced osteoformation, may overcome this flaw. Thus, this study aimed to compare the mechanical and thermal characteristics of Ti-foam (80 % porosity) and conventional Ti alloy (0 % porosity) implants drilled in clinically relevant conditions. Mechanical properties were analyzed by measuring axial and torque forces using a pressure sensor with a drill of 2.5-mm diameter at a rotation frequency of 20 Hz. Thermography was used to evaluate the heat generated by a diamond burr attached to a high-speed (80,000 rpm) drill. The torque and axial strengths of Ti foam (13.63 ± 1.43 and 82.60 ± 7.78 N, respectively) were significantly lower (P = 0.001) than those of Ti alloy (73.58 ± 13.60 and 850.72 ± 146.99 N, respectively). Furthermore, irrigation reduced the area of local heating for Ti foam to 56-82 % of that for Ti alloy, indicating lower thermal conductivity. These data suggest that the use of Ti foam implants may be advantageous in cases with a probability of implant drilling in the future.