In Vitro and Ex Vivo Evaluation of a Novel Guided Drill System for Bone-Anchored Hearing Implants

Purpose: The aim of this study was to evaluate a new guided drill system (MIPS) for placement of boneanchored hearing systems (BAHS) with respect to cutting performance, generation of heat, and distortion of the bone during drilling and compare it with a conventional drill system (Ponto). Further, the role of irrigation and drilling procedure with respect to the degree of heat generation was evaluated. Materials and Methods: Compact artificial bone was subjected to each drill in the two systems (conventional vs guided) while measuring thrust force, torque, and temperature. The temperature changes, induced during different drilling procedures, were measured by thermocouples positioned 0.5 mm from the periphery of the drill tract. The degree of bone damage was evaluated histologically after drilling in bovine, compact, tibial bone. Results: The mean thrust energy was significantly lower for the drills of the guided system compared with the corresponding conventional drills. In contrast, the mean torque energy was higher using the guided system but only for the initial guide drilling step. Whereas twist drills in combination with a guided drilling approach (MIPS) generated relatively more heat, especially during a prolonged drilling sequence, it was more forgiving in the case of impaired irrigation. The histologic evaluation showed relatively more even cut surfaces and fewer microcracks in the osteotomy wall using the guided system compared with the conventional system. Conclusion: Provided the clinically recommended drilling procedure was adhered to, the absolute temperatures using either a conventional drill system or a guided drill system were below the threshold for thermally induced tissue damage. This study suggests that the guided MIPS system conveys a promising design for an efficient and still-safe osteotomy site preparation for BAHS placement. Keywords: BAHS, bone drilling, bone-anchored hearing system, bone-anchored prosthesis, drill mechanics, flapless procedure, guide, heat generation, osteotomy, surgical guide, surgical procedure