Experimental Investigation of Neurosurgical Rotary Ultrasonic Bone Grinding

Abstract

Neurosurgical bone grinding is a procedure in which a part of the bone is removed using mechanical grinding process and then the passage is created to reach the tumor’s locations within the brain and spine; thus, treating abnormal accumulation of the mass causing cancer. A miniature grinding burr is inserted in the nostril along with the endoscope to remove the skull bone with abrasion action. Now, when grinding burr comes in contact with the surface of bone then the bone is removed in the form of microchips and leads to the increment in the temperature at the grinding site. This rise in temperature has adverse effects on soft and hard tissues since a lot of critical nerve branches surrounds the skull bone. These nerve branches include temporal branch, zygomatic branch, cervical branch, facial nerves, posterior auricular branch, greater petrosal nerves etc. These nerves are responsible for the important functioning of human body organs. Blood coagulation, optic nerve damage, thermogenesis and osteonecrosis are other associated concerns of temperature rise. Therefore, it becomes vital to limit the temperature to safe levels to avoid any king of thermal trauma during osteotomy. Hence, the research endeavor of the present work is paying attention and introduces some strategies to minimize the temperature produced during the bone grinding. In present research work, a new strategy of using rotary ultrasonic bone grinding has been introduced and further explored for different response characteristics. A spherical diamond burr has been used for the experimentation and porcine bone is used as the workpiece material. Different input parameters rotational speed, feed rate, and frequency has been investigated at three states in terms of change in temperature and thermal biological damage. The systematic investigation has been carried out to determine the effect of varying process parameters on the osteonecrosis at the cut surface. During the investigation, it has been observed that selected process parameters has significant effects on the temperature rise during bone grinding. It has been revealed that temperature increases as the rotational speed, feed rate, and ultrasonic frequency is increased. Statistical analysis revealed that feed rate (45.43%) has the highest contribution towards temperature rise during grinding followed by ultrasonic frequency (23.87%), and rotational speed (12.85%). For comparative analysis, conventional bone grinding experiments have also been carried out at the parametric sets causing maximum and minimum temperature during bone grinding. Furthermore, viable lacunas (filled osteocytes), non-viable lacunas (empty lacunas), necrosed tissues, and haversian canal was found during histological examination. The histograms revealed that rotary ultrasonic bone grinding possessed greater viability of cells and reduced temperature v compared with conventional bone grinding. Optimized set of machining parameters to avoid osteonecrosis and thermal trauma found to be rotational speed = 35,000 rpm, feed rate = 20 mm/min, and ultrasonic frequency = 20 kHz. Since the neurosurgeons uses conventional bone grinding, therefore, the cutting forces developed during the CBG for different sets of parametric combinations have been measured. It has been observed that the increased forces caused crack initiation and propagation on the surface of bone which affects the bone’s regeneration ability and post-operatively healing time. Later, different shapes of the grinding burrs have also been investigated based upon the consultation with neurosurgeons. Since the saline irrigation is continuously used during the bone grinding therefore, burr loading and burr wear is a concern for the neurosurgeons. It is expected that burr loading and burr wear affects the cutting ability of the abrasives and friction at burr-bone interface which eventually affects the temperature produced during bone grinding. To address this concern, quantification analysis has been made for different shape of the grinding burrs. It is revealed that convex shape burr caused minimum burr loading and wear during bone grinding. The important concern of the neurosurgeons for determining the thermal tissue injury and depth of thermogenesis and osteonecrosis has been addressed by using the hybrid thermal dose model and simulation. The maximum depth of thermogenesis (4.26 mm, beneath the grinding slot) and osteonecrosis (1.28 mm) found for rotational speed of 55000 rpm, feed rate of 60 mm/min, and ultrasonic frequency of 40 kHz. Subsequently, the optimum parameters for bone grinding have been suggested using hybrid thermal dose model which will result in no thermal tissue injury considering the exposure temperature and exposure time simultaneously for bone grinding.