Bone surgery on the head requires the use of machining techniques for treatment. During machining, structure-borne noise and heat are generated, which are also proportionally introduced into the tissue close to the surgical site and thus transmitted to the auditory organ primarily through structure-borne noise. Drilling procedures at the cortical bone can generate equivalent sound pressure levels of up to 120 dB (SPL) [1]. The project “Interactive user-centred assistance system for bone surgery in the head region” is developing a self-learning AI-based assistance system for ear surgery. Previous studies have shown that the individual drilling method of the surgeon plays an important role in noise generation when milling the skull. First experiments of the project investigate the influence of contact force and alignment of the bur on the generation of noise. The milling process was further examined with regard to its efficiency by evaluating the volume of material removed.

Bone substitute material (Sawbones, Seattle, USA) with mechanical properties based on the bone density of the cortical bone of the skull was milled using a Bien Air drill (Bien Air surgery, Le Noirmont, Suisse) with rose (7mm) and diamond (6mm) milling tool. All experiments were performed by the same surgeon, varying the drill, the speed (7.000-20.000 rpm), the orientation of the cutter (5°-90°) and the force applied (subjectively in three steps). The guidance of the cutter was linear in the opposite direction of the drill's rotation.
The applied forces were recorded using a dynamometer (3-axis force sensor ) mounted below the sample holder. The vibrations induced into the sample (as equivalent for the structure-borne noise) were measured with a three-axis acceleration sensor PCB 356A01 (PCB Piezotronics, Hückelhoven, Germany), which was attached to the clamping jaw of the sample holder by means of wax. The airborne noise generated during milling was recorded with microphone (model PCB-378C20, PCB Piezotronics), preamplifier (model 441A101, PCB Piezotronics). All signals were recorded with a sample rate of 25600 Hz.

The tests show that at high speed and steep alignment of the milling cutter, the airborne noise increases sharply while hardly any material is removed. On the other hand, if the tool is optimal aligned, volume removal can be made much more efficient and quieter. Furthermore, the experiments show that a high force input results in a high noise level.
To improve the validity of the experiments, the tests will be repeated on temporal bone (TB) specimens to ensure consistent material behavior.

Overall, careful balancing of these factors is important to minimize noise exposure during skull milling. Optimal positioning of the drill by fine-tuning the angle and contact force, as well as reducing the speed and selecting the appropriate cutter, can help to control the noise level and reduce the stress for the patient and the surgical team.