Abstract
Introduction
Surgical drill-bits are used in a raft of procedures, from trauma, joint reconstruction to Arthroplasty. Drilling of bone is associated with the conversion of mechanical work energy into shear failure of bone and heat generation, causing a transient rise in temperature of hard and soft tissues. Thermal insults above 47°C sustained for one minute or more may cause osteonecrosis, reduced osteogenic potential, compromise fixation and influence tolerances with cutting blocks. Drill design parameters and operational variables have marked effects on cutting performance and heat generation during drilling. Dulling and wear of the cutting surfaces sustained through repeated usage can significantly reduce drill bit performance. Deterioration of cutting performance substantially increases the axial thrust force required to propel the cutting face through bone, compromising surgeon control during drilling and increasing the likelihood of uncontrolled plunging, cortical breakthrough and improper placement of holes as well as other jigs.
Methods
The drilling accuracy and skiving of 2.8 mm 3-fluted SurgiBit (Orthopedic Innovation (OI), Sydney, Australia) (Figure 1) was compared with a standard 2-fluted drill (Synthes) at 15, 30 and 45 degrees using a 4th generation Sawbone as well as bovine cortical bone. A surgical handpiece was mounted in a servo-hydraulic testing machine and the motion of the drill-bit confined to 2 degrees of freedom. The lateral force and skiving distance was measured (n=6 per drill per angle per testing medium). A new drill was used for each test. Wear performance over multiple drilling episodes (1, 10 and 100) was performed in bovine cortical bone. The surface characteristics of the cutting faces of the drills were assessed optically at 10x magnification and at higher magnifications (50, 100 and 500x) using an environmental electron microscope.
Results
The OI 3 fluted design outperformed the 2-fluted design in terms of lateral forces beyond 15 degrees (P<0.05). Skiving occurred with the 2-fluted design at all angles while this did not occur with the OI 3-fluted design. The stiffness (lateral force/skiving distance) for the OI 3-fluted design was more than twice that of the 2-fluted design. Wear performance of the OI 3-fluted design was superior to the 2-fluted design based on optical and SEM analysis.
Discussion and Conclusions
Both 2 and 3-fluted surgical drill-bits are in clinical use. The OI 3 fluted drill is unique in its negatively-raked pyramidal tip geometry which induces a ductility change in the cutting medium, thereby improving cutting efficiency for the ensuing trio of positively raked cutting surfaces. This reduces wear and decreases heat generation compared to their diameter-matched 2 and 3-fluted counterparts. Three fluted drills are inherently more efficient due to the inclusion of an additional cutting face, which can potentially remove 50% more material per rotation. The acute tip angle and geometry in the 3-fluted OI design also improves accuracy and targeting ability for the surgeon. Mechanically, 3-fluted drills have higher bending resistance, even during operation, which decreases the likelihood of intra-operative breakage. The engineering advantages of the OI 3-fluted design are beginning to be realized at the clinical level.
