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

SummaryNon-perpendicular drilling of bone is commonplace in orthopaedic surgery. In the absence of drill-jigs and guides, the drill-tip is prone to skiving along the bone. Skiving can alter the position of the intended hole or result in damage to surrounding tissues. We hypothesised that the acute point-angle and increased flexural rigidity of 3-fluted drills – in certain clinical scenarios – can increase a surgeons’ ability to accurately position a hole. This study examined differences in drill-tip geometry (point-angle) and mechanical properties (flexural rigidity) between 2.8 mm diameter 2-fluted and 3-fluted surgical drills. Our results show that the 3-fluted design offers a significant improvement over the 2-fluted design not only in terms of accuracy; at 15° and 30° approach angles the 3-fluted drill skived significantly less than the 2-fluted drill in the hands of our surgeon, but also in the range of permissible approach angles; the 3-fluted drill was able to drill at a 45° approach angle with skiving equivalent to that experienced by the 2-fluted drill at 15°. Mechanical testing showed that bending stiffness (N/mm) of the 3-fluted drill (9.5 ± 2.1 N/mm) is more than double that of the 2-fluted drill (3.5 ± 0.6 N/mm) during operation. Computer modeling of the drills supported this finding and demonstrated that bending stiffness (IX) for the 2-fluted drill varies dynamically during operation whilst remaining constant for the 3-fluted drill. Our study confirms a correlation between mechanical properties, point-geometry and targeting capability for surgical drills. Increased IX of 3-fluted drills may account for the clinical prevalence of rotational bending failure amongst 2-fluted drills.