Study and improvement of surgical drill bit geometry for implant site preparation

During surgical interventions where bone machining operations are involved (drilling, sawing, etc.), bone tissue may be damaged by the mechanical and thermal loads, and thus, working conditions should be selected in order to reduce or avoid this problem. In this study, with the purpose of reducing the damage induced when drilling, an analysis and improvement of the geometry of a surgical drill bit had been done. As baseline, a commercially available stainless steel surgical drill indicated for implant site preparation had been considered. The influence of the rake angle, margin width, and body thinning over the workpiece temperatures, cutting forces, and surface roughness has been determined by a sensitivity study. First, eight drill bits had been fabricated on hard metal and drilling tests on bovine femoral cortical bone were done. These tests showed that the rake angle is the most influential factor among the analyzed, both for temperatures and cutting efforts. The two drill designs which achieved the lowest workpiece temperatures, cutting forces, and surface roughness had been fabricated also in surgical stainless steel and tested at wider cutting condition range of cutting speed and feed rates. These two drill bits reduced workpiece temperatures about 50 %, feed force about 60 %, and cutting torque up to 50 %, while the surface roughness was improved significantly when compared with the commercial surgical drill bit at the same cutting conditions. Comparing stainless steel and hard metal drills, no appreciable effects of the tool material have been detected over the measured output variables. This study shows how remarkable improvements on the design of surgical drills could be achieved by defining the influence of the rake angle, margin width, and body thinning when drilling bone.