The surgical treatment of femoral neck fracture can be classified into internal fixation and arthroplasty. In order to determine the surgical method, various factors such as the patient's age, bone density, fracture dislocation, and type of fracture must be considered . Among them, internal fixation is preferred for undisplaced fracture or for relatively young patients, and either closed or open reduction can be performed [2–4]. Implants for internal fixation in femoral neck fracture include cannulated screws, dynamic hip screw (DHS) with or without antirotation screw, DHS with blade instead of screw or similar implants [5–7]. Parallel multiple cannulated screws (MCS) are commonly used in relatively young patients with femoral neck fractures, but it has lower mean axial stiffness and the number of cyclic loadings compared with DHS in biomechanical tests [8–10]. Although DHS provides more stability to femoral neck fractures than MCS, it requires larger skin incision and more extensive soft tissue dissection .
The recently introduced implant, the Femoral neck system (FNS, DePuy Synthes, Oberdorf, Switzerland) (Fig. 1.), has both advantages of above two implants. It requires small incision like MCS and provides angular stability like DHS .
The FNS is composed of (1) a plate with a barrel and threaded screw holes which accommodates 1 or 2 locking screws, (2) an antirotation screw, and (3) a bolt that supports the head fragment. The proximal fragment with femoral head is held tight by the bolt and antirotation screw, thus it can slide through the axis of barrel to obtain dynamic compression of the fracture site (Fig. 1).
The depth of implant insertion of DHS and multiple cannulated screws are easy to adjust as both use screw mechanism. As the pitch of lag screw of DHS is 3.5 mm, half turn which is the minimum unit of adjustment the operator can make is 1.75 mm. Although the cannulated screws are generally manufactured in the unit of 5mm, surgeon can control the depth of insertion in analogue scale and has more option with washers to adjust the depth. However, the depth of bolt in FNS is difficult to finely control as the length of the bolt is in units of 5 mm.
The manufacturer recommended the subtraction of 5 mm from the measured depth read on the direct measuring device and choose the next shorter bolt size . For example, if you placed the tip of central guide wire into the subchondral bone and the measured depth was 102 mm in measuring device, it is recommended to choose 95 mm bolt.
It is a concern whether the method will ensure the insertion of bolt into sufficient depth and stable fixation of femoral neck fractures. Thus, this study introduces a method to control the depth of FNS bolt in millimeter unit in patients with femoral neck fracture.