Femoral neck fracture (FNF) is a commonly encountered injury in the orthopedic practice, in which the vertically oriented femoral neck fracture (VOFNF) is the most unstable one [1]. The postoperative complications corresponding nonunion of the fracture and osteonecrosis of the femoral head are hard to be overcome, which increases the difficulty of the treatment [2]. Despite the improvement of surgical techniques, the failure rate of the VOFNFs is still relatively high, with a nonunion rate of 20–27 % and an incidence of osteonecrosis of 10–11 % [3].

The cornerstone of managing FNFs in the non-elderly population is anatomical reduction and stable internal fixation of the femoral neck to salvage the femoral head, even in grossly displaced fractures. Due to the instability characteristic of the VOFNFs, Dynamic hip screw (DHS), as the representative of the sliding nail, is applied for internal fixation and the persistent dynamic and static pressure function of which plays an important role in the FNF treatment [4]. Recent studies demonstrated that the DHS is advantageous in biomechanical stability to the VOFNFs [5]. It is the dynamic and static pressure function that could provide adequate pressure stress at the fracture site, whether patients are lying or walking, can meet the requirements of sufficient mechanical stress postoperatively. However, as DHS is a nail-plate structure that can cause great pressure on the lateral cortex of the femur, which can eventually lead to fixation failure due to main nail cutting. Moreover, the stress concentration and coxa vara deformity are also the shortcomings of it [6]. All the shortcomings finally result in its limited clinical application. One conceivable explanation is that DHS is a single nail fixation system, and its anti-rotation performance is poor.

The use of an anti-rotation screw (AS) enables it to overcome the disadvantages above. Many clinical studies have confirmed that this innovation effectively prevents the occurrence of coxa vara deformity, fixation fracture and main nail cutting caused by single DHS fixation, and DHS plus AS system has been proven more effective treatment for VOFNFs [7], [8], [9]. This combination plays a dual role of anti-rotation and further compression at the fracture site, which greatly makes up for the deficiencies of the single DHS system. However, literature review of the previous studies showed little detailed procedure description of how this AS was screwed into the fracture site or the relationship between the DHS and AS. Different degrees of bone resorption or femoral neck shortening may occur during the union process after FNF followed by retrogression of both DHS and AS [10]. In addition, whether the use of an AS might have adverse effects, such as dynamic pressing at the fracture site or stiffness of fixation, on the primary fixation function of DHS or not still needs to be illuminated. Therefore, the relationship of the three-dimensional structure between the DHS and the AS is one of the important factors for the treatment of the VOFNFs [11]. It is worth discussing that how to screw in the AS, so as not to affect the dynamic and static pressure effect of the DHS, but also achieve the goal of sufficient anti-rotation stress. Thus, a biomechanical analysis was conducted to find the most suitable structure between the DHS and the AS, and the clinical study was completed to verify the findings of the biomechanical analysis.