Drilling of Bone: Practicality, Limitations and Complications Associated with Surgical Drill-Bits

The drilling of bone is ubiquitous in many fields of surgery including orthopaedics, neurosurgery, plastics and reconstructive, craniomaxillofacial and ear nose and throat (ENT). A cylindrical tunnel is typically prepared in bone using a surgical drill-bit to accommodate a screw or other threaded device for rigid fixation which is provided by the integration of bone (cancellous and/or cortical) with the screw threads. In this configuration bone screws are resistant to axial and shear forces as well as bending moments and therefore suited to the load-bearing function of the skeleton during locomotion. Drill-bits are also used in the preparation of bony tunnels, such as in anterior cruciate ligament reconstruction. Drilling is associated with the conversion of mechanical work energy into thermal energy causing a transient rise in temperature of adjacent bone and soft tissues to above normal physiological levels (Matthews and Hirsch 1972; Lavelle and Wedgwood 1980; Eriksson and Albrektsson 1984; Eriksson, Albrektsson et al. 1984b; Abouzgia and Symington 1996; Natali, Ingle et al. 1996; Toews, Bailey et al. 1999; Bachus, Rondina et al. 2000; Davidson and James 2003; Augustin, Davila et al. 2008; Franssen, van Diest et al. 2008; Bertollo, Milne et al. 2010). Primary sources of this thermal energy are plastic deformation and shear failure of bone and friction at the machining face. The magnitude of this temperature rise is determined by a number of factors, including drill geometry and diameter, rotational speed (rpm), feed-rate (mm.s-1), axial thrust force (N), initial drill-bit temperature and internal or external cooling. The negative effect of elevated temperature on the viability of bone is well-acknowledged and measures to reduce them during surgery are frequently employed, such as manual irrigation with sterile saline (Matthews and Hirsch 1972; Jacob and Berry 1976; Lavelle and Wedgwood 1980; Camargo, Faria et al. 2007; Augustin, Davila et al. 2008). Viability (Eriksson, Albrektsson et al. 1984b) as well as the structure and mechanical properties (Bonfield and Li 1968) of bone are indeed compromised through exposure to elevated temperatures. Another important variable that can influence the biological response for drilled bone is the time which a temperature above a threshold value is maintained. Excessive temperatures and durations at these elevated levels can result in the necrosis (death) of bone, a phenomenon termed osteonecrosis, or the impairment of osteogenic