Bone drilling is an essential technique in orthopedic surgery for skeletal disorders [[1], [2], [3], [4], [5]]. When osteosynthesis is desired, a pilot hole is made prior to a screw's insertion into the bone. Since bone is a fragile material, the use of a drill bit made of a hard material such as metal is likely to cause damage to the wall of the drill hole. Such hole-wall damage contributes to subsequent screw loosening. Investigations of this damage have focused on the use of solid drills [[4], [5], [6]], although a cannulated drill has been more commonly used for medical purposes, such as for drilling a pilot hole for a cannulated cancellous screw (CCS). CCSs are used for a variety of applications, but one of the main uses is screw-only osteosynthesis at the metaphysis and epiphysis [7]. We have searched for but found no published study concerning the hole-wall damage that occurs with a cannulated drill. Although cannulated drills are controlled by guide pins, it is still possible to damage the hole wall, depending on the bone quality. For example, surgeons have often observed that in patients with osteoporosis, sufficient screw fixation strength cannot be obtained when bone fragments at metaphysis and epiphysis are fixed with a CCS, making treatment difficult.

The numbers of individuals with osteoporosis are increasing globally, particularly in rapidly aging societies such as Japan [8]. Osteoporosis is characterized by bone loss and microarchitectural deterioration, and osteoporotic bones become fragile and easily fractured [9,10]. The pull-out strength of screw fixation in osteoporotic bones is reduced, and the screws are easily loosened [[11], [12], [13], [14]]. In addition, it has been reported that the pull-out strength of a screw in osteoporotic cancellous bone is significantly lower than that of the same screw inserted into healthy cancellous bone [15]. The main reason for this screw-loosening phenomenon is thought to be the fragility of the bone itself. However, an additional potential factor should be considered: the pilot hole that is made for the screw may be larger than expected in osteoporotic bone, contributing to the screw-loosening phenomenon. It has been demonstrated that the pull-out strength in screw fixation in osteoporotic bone can be improved by reducing the pilot-hole diameter [16,17]. In other words, there may be some relationship between osteoporosis and the pilot-hole diameter, but this relationship is unclear.

With the increasing number of osteoporotic patients, the issue of bone fragility and screw fixation strength is expected to become an even more important clinical issue in the future. Knowledge of the precise relationship in cancellous bone between bone fragility and pilot-hole diameters made with a cannulated drill will provide important information for the development of methods that can be used to resolve the problem of screw-loosening in osteoporosis, such as newly designed screws and drills. We hypothesized that the drill-hole diameters in cancellous bone would be enlarged in osteoporotic bone compared to healthy bone, even with the use of hollow drills. The aim of this study was to clarify the relationship in cancellous bone between bone density and drill-hole diameters made by cannulated drills.