3-Fluted orthopaedic drills exhibit superior bending stiffness to their 2-fluted rivals: Clinical implications for targeting ability and the incidence of drill-bit failure
Introduction
Drilling of bone is routine during fracture fixation and reconstructive surgery in a variety of anatomical sites. Surgical drills are the instruments used to remove a controlled amount of bone prior to the placement of a screw or graft material. They are available in either 2- or 3-fluted configurations, where the flutes are that portion of the drill used to channel debris away from the cutting surface. In reality, a surgeon is often required to drill at non-perpendicular orientations and on a highly curved and irregular surface to achieve the surgical goals. This can be difficult and result in skiving of the drill-tip along (or off) the bone and may result in adjacent soft tissue damage, improper placement of the drill hole or a hole that does not meet the original requirements in geometric terms. Unnecessary removal of bone stock can decrease screw pullout strength and have a negative effect on defect healing.5 Little has been reported on the effects which drill-bit geometry and mechanical properties have on the ability of a surgeon to accurately place a drill hole in bone.
Breakage of orthopaedic drill-bits due to excessive bending rates highly amongst the reported incidences of drill-bit failure.[2], [3], [4] Bending failure of surgical drills is most commonly encountered with 2-fluted drills during bi-cortical drilling of long bones, such as in the placement of a lag screw across a fracture site.1 This can occur due to skiving – or wandering – of the drill-tip along the far cortex prior to purchase which deforms the rotating drill-bit and induces a concentrated moment load causing failure. Where rotational bending failure has previously been reported the broken portion of the drill has been left in situ due to complications associated with removal of the broken portion from the medullary cavity.[2], [3], [4]
This study examined differences in tip geometry (point-angle) and flexural rigidity (EIX) of diameter-matched 2- and 3-fluted surgical drills. We hypothesised that the acute-tip geometry and increased flexural rigidity of 3-fluted drills can – in certain clinical scenarios – equate not only to improved targeting ability for the surgeon, but also account for the clinical prevalence of rotational bending failure amongst 2-fluted drills.
Section snippets
Methods
2.8 mm diameter 2-fluted (Acumed, CA) and 3-fluted (Orthopaedic Innovation, Sydney, Australia) surgical drills were used in this study (Fig. 1). This study consisted of 3 components: (1) examination of accuracy; (2) examination of mechanical properties; (3) mathematical and computer modeling.
Results
Results from the accuracy experiment, shown in Table 1, revealed that the 3-fluted drill outperformed the 2-fluted drill not only in terms of accuracy, but also in the range of allowable approach angles. Our surgeon was unable to drill holes at approach angles beyond 30° using the 2-fluted drill but was able to drill at all angles up to and including 45° using the 3-fluted drill. Mean skiving was contained to within one diameter of the drill (2.8 mm) for both drills at all approach angles.
Discussion
Flexural rigidity (EIX) of a member rotating through a stationary bending moment (such as a surgical drill subjected to a net bending moment) is a function of the principal second (area) moments of inertia (Imax, Imin), Young's modulus (E) of the material, rotation (θ) of the principal axes of the given cross-sectional area relative to the plane of bending according to the following relation:For surgical drills – and twist drills in general –
Conflict of interest statement
None.
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