Thermal optimization of robotic piezoelectric osteotomy motion in porcine trabecular bone

Background: Piezoelectric osteotomy represents a potentially viable alternative to conventional techniques for preparing pilot holes for bone screws. However, piezoelectric osteotomes have demonstrated the potential to heat bone beyond 47 °C, which induces necrosis that could contribute to screw loosening, resulting in osteosynthetic material failure and pseudarthrosis. Thus, we sought to determine the working movements of a piezoelectric drill that resulted in the lowest maximum bone temperatures while ensuring that this optimal drilling motion resulted in temperatures below 47 °C.

Methods: Vertebrae samples were obtained fresh from a local abattoir. A robot was used to drill 15 mm holes through the vertebrae bodies parallel to a sectioned viewing surface, observed with a thermal camera. We tested three significant aspects of the osteotome's movement: the feed rate, retraction speed and working cycle to see which combination resulted in the lowest maximum temperatures. Each factor combination was tested three times for a total of 24 trials.

Findings: A feed rate of 2 mm/s, a retraction speed of 2 mm/s and a novel working cycle termed "multi-pass" were found to generate the lowest maximum temperatures (41.5 °C, σ = 1.91 °C). The factors of retraction speed and working cycle demonstrated statistical significance through analysis of variance, with ordinary least squares regression indicating their optimal settings resulted in reductions in maximum temperatures of 4.95 °C and 5.63 °C, respectively.

Interpretation: Robotic piezoelectric osteotomy appears to represent a safe method of pedicle screw pilot hole preparation from a thermal perspective when using the optimal methods found in this study.