Mechanistic models to predict thrust force and torque in bone drilling: An in-vitro study validated with robot-assisted surgical drilling parameters

Abstract

Compression plates are widely used in orthopaedic surgeries for internal fixation of fractured femurs. To fix the plate and thus to provide compression to a fracture, the self-tapping bone screws are tightened through predrilled pilot holes of smaller diameter. Preliminary investigation showed that the holes drilled with the inappropriate cutting parameters cause mechanical and thermal damages to the local host bone, which further lead to loosening of internal fixations. In this paper, the mechanistic models to predict the thrust forces and torques during bone drilling were developed, using a 3.20 mm diameter drill bit. As a procedure, the cutting action was investigated at three different regions of the drill point, namely cutting lips, secondary cutting edges and indentation zone. The models employed the analytical approach to account for the drill-bit geometry and cutting parameters, and an empirical approach to account for the material and friction properties. To complete the procedure, calibration experiments were conducted on bovine cortical femurs with two different spindle speeds (1000 and 3000 r/min) and feeds (0.03 and 0.06 mm/rev), and then the specific normal and friction coefficients were determined. The developed mechanistic models were validated with different ranges of parameters (500–3500 r/min speeds, and 0.02–0.07 mm/rev feeds) those commonly involved in manual and robot-assisted surgery. The validation study revealed that the thrust forces predicted using the mechanistic models showed a maximum error of only 5.80%. However, the torques predicted from the mechanistic model found with more error than the thrust forces. The predominant reasons for this under-prediction might because of the extrapolation used to determine the specific cutting pressures, slip-line field applied to the indentation zone instead of compressive fracture, and chip clogging involved during the bone drilling as demonstrated in earlier studies. Despite the deviations, the developed mechanistic models satisfactorily follow the trends of the thrust forces and torques experienced during bone drilling. The outcomes can be used to practice the bone drilling procedure and monitor the effect of process parameters on thrust forces and torques in the in-silico environment before performing actual surgery.