Incidence and Risk Factors of Short Axial Length of the Proximal Femur: A Caution in the Use of Femoral Neck System in Patients with Garden Type I/II Femoral Neck Fractures

In 2018, Femoral Neck System (FNS), a dedicated fixator for femoral neck fractures, was introduced. This device has been in increasing use because it provides excellent rotational and angular stability. However, the shortest bolt of FNS is 75 mm long. Thus, it is not usable when the axial length of the proximal femur (ALPF), the distance between the innominate tubercle and the surface of the femoral head, is less than 80 mm. In this study, we investigated the incidence and associated factors of small ALPF (< 80 mm) in femoral neck fracture patients. We measured the ALPF on preoperative computed tomography (CT) scans of 261 patients (166 women and 55 men), who were operated due to nondisplaced or impacted femoral neck fractures. The ALPF was measured on reconstructed oblique coronal images along the femoral neck. We evaluated the distribution of ALPF, calculated the incidence of small ALPF (< 80 mm), and correlated it with patient's height, weight, body mass index, age, bone mineral density (T-score), and caput-column-diaphysis angle. The ALPF ranged from 67.4 mm to 107.1 mm (mean, 88.4 mm; standard deviation, 7.2 mm). In 19 patients (8.6%, 19 / 221), the length was < 80 mm. The ALPF was strongly correlated with height (correlation coefficient = 0.707, = 0.500, < 0.001) and moderately correlated with weight (correlation coefficient = 0.551, = 0.304, < 0.001). The T-score was moderately correlated with the ALPF (correlation coefficient = 0.433, = 0.187, < 0.001). The age was moderately correlated with the ALPF (correlation coefficient =-0.353, = 0.123, < 0.001). A considerable percentage of femoral neck fracture patients (8.6%) had small proximal femurs (ALPF < 80 mm), which cannot be operated with FNS. We recommend measuring the ALPF using reconstructed oblique coronal CT images or scaled hip radiographs: view of the femoral neck prior to surgery in patients with short stature and/or low body weight. If the ALPF is < 80 mm, the surgeon should prepare other fixation devices.