Comparative experimental study on a new drilling system for minimally invasive implantation of bone-anchored hearing aid systems

摘要

Introduction: For the successful installation of bone anchored hearing aid systems (BAHS), an optimized drilling procedure is essential, both in terms of the quality of the osteotomy and for the prevention of heat-induced tissue trauma. A new minimally invasive technique has been developed. The osteotomy is performed via a cannula inserted in a punch-biopsy incision through the skin, as opposed to the conventional procedure where a skin flap is raised. The purpose of this study was to evaluate a new drilling protocol with respect to drilling force and torque, heat generation and the degree of damage in the drilling site. Materials and methods; The conventional drilling protocol (Control) consists of an initial penetration of the bone with a round burr and subsequent countersinking (Fig. 1 A). The new drilling protocol (Test) uses a guide drill of a twist drill design and subsequent enlargement with a widening drill, which also has a twist drill design (Fig. 1B). All drills were manufactured from stainless steel. In addition, the test drills were coated with diamond like carbon. For the mechanical evaluation, compact artificial bone (50 pcf, Sawbone, USA) was subjected to each drill with a constant feed rate of 1 mm/sec and with a constant rotational speed of 2000 rpm while measuring force and torque using a specially designed test rig (n=10). The test drilling protocol was evaluated with respect to heat generation and compared with the temperature generated by the conventional drilling protocol. For the control protocol, cool saline (20oC) was able to flow directly onto the artificial bone. For the test protocol, a layer of artificial skin was added to the artificial bone, and cooling was applied through the cannula. The temperature changes for the two drilling protocols were measured during the final drilling step by thermocouples positioned 0.5 mm from the periphery of the drill tract of the final drill hole (Fig. 1 A, B). Ten drilling procedures of each drilling protocol were performed. The quality and degree of bone damage of the drill tract was evaluated by drilling in bovine, compact tibial bone. Two osteotomies were created with each of the systems and subjected to histological evaluation. Results and Discussion: The mechanical evaluation demonstrated that less force was required to drill in artificial bone with the new drill system (Fig. 2A). The average maximum temperature increase for the test procedure was 3.0°C (SD 0.8), whereas for the conventional procedure it was 3.5°C (SD 0.8) (Fig. 2B). There was no statistically significant difference between the groups. For comparison, drilling was also performed without any cooling resulting in a temperature increase of 17.9°C (SD 4.0) and 217°C (SD 8.4) for the test system and conventional drilling protocol, respectively. Light microscopy of drill sites from bovine compact bone using the conventional drilling protocol revealed an uneven edge with micro-fractures (Fig. 3A). In comparison, the test drilling protocol provided a clean cut edge of the bone (Fig 3B). Conclusion: The present results show that the temperatures generated using both conventional and new drilling protocols are well below the threshold for thermal induced tissue damage when drilling in artificial bone. The new drill system is more efficient compared to the conventional drilling system reflected in a lower drill force. It is concluded that under bench conditions, the new drill system is compatible with a flapless minimally invasive approach for installation of BAHS.