Optical performance and drilling forces of an orthopaedic DRS drill with a stagnant optical probe

Abstract

Introduction. In spinal surgery, the misplacement of spinal screws is a problem that causes (severe) pain, bleedings or even paralysis [2]. Screw misplacements are common as navigating in the spine is difficult due to the small vertebral dimensions and a lack of anatomical landmarks [3] [4]. In order to improve the navigational support of spine surgeons, this research focuses on the development of an optical sensing diffuse reflectance spectroscopy (DRS) surgical drill that identifies bone tissue boundaries. The developed drill concept introduces a stagnant optical fiber-equipped probe into a cannulated orthopaedic drill. To verify the clinical applicability of the developed system, the accuracy of the optical tissue boundary detection has been analysed under different tissue penetration speeds, as well as the axial drilling force increases due to the introduction of a stagnant probe into a drill.
Results. When increasing the drilling feed rate in the optical phantom, the drill overshoot (the difference between the DRS-derived tissue boundary location identification and the actual location of the phantom boundary) shows a larger spread. The maximum feed rate at which no overshoot takes place is 0,5mm/s. Increasing the sampling frequency –especially decreasing the inactive period between the measurements– can improve this.
None of the K-wire equipped drills can penetrate the used Sawbones® cortical bone phantom. The axial peak feed forces occurring in the Sawbones® cancellous bone phantom while using a regular 2,7mmØ orthopaedic drill is 38,2N. When using the 2,7mmØ orthopaedic drill with a 1,6mmØ K-wire, a peak of 57,6N is observed. Because the data from drilling in the Sawbones® cancellous bone is not normally distributed, a benchmark experiment on cheese is analysed. On average, the introduction of a K-wire increases the required drilling forces by a 296% (roughly a factor 3). Among the different feed rates and drill types, the force increase of introducing a stagnant K-wire varies between 16% and 575%.
Conclusion. To prevent orthopaedic screws from breaching the bone surface (an overshoot of 0mm) in spinal surgery, the established feed rate speed limit of 0,5mm/s is too low. To meet the observed feed rates applied by surgeons of up to 5mm/s, it is of interest to reduce the DRS sampling time – the inactive period between two measurements in particular. The feed force increase of approximately a factor 3 can be regarded as a challenge for surgeons, who indicated that they preferred feed forces to be kept low. Further testing on real (cadaveric) vertebrae can more give information about the DRS drill’s optical performance in pedicles, and whether the identified feed force increase proves to be problematic for clinical applications.