Clinical outcome of femoral neck system versus cannulated compression screws for fixation of femoral neck fracture in younger patients

We retrospectively analyzed young patients with FNFs in our hospital between January 2017 and January 2020 (all included patients were less than 60 years old). The demographic and radiological data of these patients were retrospectively collected from the institutional database. The study protocol was reviewed and approved by the institutional review board of the hospital. All patients provided informed consent for participation in the study.

After ruling out blood disorders and or bleeding tendency preoperatively, low molecular weight heparin sodium (1 mg/kg body weight, once a day) was routinely used for anticoagulation. Antibiotics were administered 0.5 h before the operation. After anesthesia and awakening, the patient was be instructed to actively exercise isometric contraction of the lower extremity muscles, active ankle pump exercises, and active/assisted active hip and knee flexion exercises. Patients with osteoporosis were treated with calcium tablets and diphosphate. Partial weight-bearing training was performed according to the recovery of the affected limb. Approximately 3 months after the operation, walking with a load was permitted according to bone healing. X-ray examination was performed within 3 days after the operation. X-ray follow-up was performed once a month in the first 6 months after surgery and every 6 months thereafter. Hip function assessment was performed 6 and 12 months after the surgery. If the patient had hip pain on the surgical side during follow-up, computed tomography (CT) or magnetic resonance imaging (MRI) of the hip joint was performed to confirm the presence of fracture nonunion or femoral head necrosis.

The patients were retrospectively identified from the hospital database. Baseline and follow-up data were acquired from the electronic medical records. Patient records were reviewed and the following data were collected: height, weight, body mass index, time from injury to operation, operation time, blood loss, type of fracture internal fixation, types of fractures (Garden typing and Pauwels classification), and length of clinical follow-up. We used the Mercuriali et al. [7] method to calculate the volume of blood loss. All pre- and postoperative hip radiographs of the study cases were evaluated by the authors who reached a consensual decision for each case regarding the type of FNF (according to Garden and Pauwels classification).

The quality of postoperative fracture reduction was evaluated based on standard anteroposterior and lateral radiographs of the femoral neck of the affected side using the Garden alignment index [8]. Assessment of postoperative fracture healing: There was no obvious percussion pain in the hip joint or lower limbs on the operative side. X-ray or CT showed that the fracture line was blurred, and the original fracture end had continuous cancellous bone trabeculae passing through. Assessment for AVN of the femoral head mainly refers to the standard of Slobogean et al. [9]; that is, if the postoperative X-ray film showed partial collapse of the femoral head or subchondral translucent area. In addition, if the patient had local pain in the hip joint, AVN of the femoral head was suspected, and MRI of the hip joint was performed when indicated. The method of Zlowodzki et al. [10] was used to identify femoral neck shortening.

We used the HHS to evaluate hip joint function preoperatively, and at 6 and 12 months after surgery. At the last follow-up, the Harris Hip scoring system was used to score the function of the hip joint: a full score of 100 points, ≥90 points as excellent, 80–89 points as good, 70–79 points as medium, and <70 points as poor.

The statistical software used for all analyses was SPSS 25.0 (SPSS Inc., Chicago, IL, USA). Continuous variables were reported as mean ± standard deviation (with range). Discrete variables were reported as numbers (percentage of total). Chi-squared tests or Fisher’s exact probability method were used to compare binary variables (demographic data and complication rates).

The recently introduced implant FNS (DePuy Synthes, Zuchwil, Switzerland) (Fig. 1) was developed for the dynamic fixation of FNFs. The FNS includes three parts: an ARScrew, a bolt, and a plate. The plate provides angular stability (a fixed angle between the bolt and the ARScrew). The cylindrical bolt design was intended to maintain reduction during insertion. The bolt also provided angular stability. The integrated bolt and ARScrew provided rotational stability. Stoffel et al. [5] evaluated the biomechanical performance of FNS in comparison with established methods for fixation of FNFs in a cadaveric model. They concluded that the FNS showed significantly higher overall construct stability compared to CCS in an unstable FNF model, and no significant difference between the FNS and the DHS systems was observed with regard to the most clinically relevant parameters. Schopper et al. [6] evaluated the biomechanical performance of the FNS versus Hansson Pin System (Hansson Pins). The study showed that the FNS can be considered as a valid alternative to the Hansson Pin System for the treatment of Pauwels II FNFs by providing superior resistance against varus deformation and performing in a less sensitive way to variations in implant placement. According to previous studies, we may conclude that the FNS can provide similar effects as DHS, achieve strong and stable fixation, and prevent postoperative hip varus. Based on our experience with intraoperative FNS, FNS can provide a more strong compression fixation of the fracture site (Fig. 5). FNS combines the advantages of different existing constructs, such as the minimally invasive insertion technique and retention of more viable bone known for CCS with the increased fracture fixation properties of the DHS system.

In CCS, three cannulated screws apply pressure to the fracture and promote fracture healing. In addition, they occupy a relatively small area in the femoral neck and interfere less with femoral head and neck blood flow. Triangular distribution can form a three-dimensional skeleton and bone tissue, which can decrease the stress of femoral head rotation. It can enhance the intraoperative and postoperative compressive stress between fracture ends, promote close contact between fracture ends, and facilitate fracture healing. However, there is no correlation between the three cannulated screws, and the screw position is easily affected by the subjective and objective factors of the surgeon. Therefore, its ability to resist vertical shear and torsion is poor, which may lead to fracture end loosening and displacement, femoral head necrosis and nonunion, and femoral neck shortening [11, 12]. A biomechanical study showed that FNS and DHS have shown similar results in fracture fixation for parameter cycles to failure and femoral neck shortening [5]. These devices allow for a controlled collapse of the fracture site, leading to an increased stimulus for remodeling. For displaced or unstable fracture patterns, a DHS or FNS offers greater mechanical stability to resist the increased shear forces generated [5, 13]. In our study, there was no statistical difference in the incidence of FNF nonunion (12.5% vs. 10.0%, p = 0.795) and femoral head necrosis (12.5% vs. 5.0%, p = 0.389) between the CCS and FNS groups. However, the incidence of femoral neck shortening and screw cut-out was significantly higher in the CCS group than in the FNS group (37.5% vs 10.0%, p = 0.036). Previous studies have shown that femoral neck shortening after CCS treatment in patients with FNFs may even cause hip dysfunction [14, 15]. Weil et al. [16] showed that the quality of reduction of FNFs had a direct effect on the occurrence of postoperative femoral neck shortening. Osteoporosis can lead to a decrease in fixation grip and resistance to stress at the fracture site, resulting in a decrease in stability and a greater likelihood of femoral neck shortening [17, 18]. In our study, the incidence of femoral neck shortening in the FNS group was significantly lower than that in the CCS group, which may be related to the better mechanical stability and shear resistance of FNS. A previous study also reported that cut-out was a common complication and occurred in 14.5% of patients [19]. The study also implied that a nonparallel and widely spread screw trajectory might interfere with shortening of the osteoporotic femoral neck during fracture healing, leading to the screws possibly cutting out from the femoral head [19]. However, due to the locking mechanism of the plate and screw, there were no patients with screw cut-out in the FNS group. In this study, both the CCS and FNS groups achieved relatively satisfactory functions, and there was no statistically significant difference in postoperative HHSs between the two groups. Our meta-analysis was conducted to analyze the clinical outcomes of two implants (CCS and slide DHS) and concluded that two different types of internal fixation could achieve similar clinical outcomes in terms of the HHS [20]. Factors that affect the clinical outcome after fixation of FNFs primarily depend on the condition of the patients, the degree of fracture displacement, adequacy of internal fixations, and quality of surgical reduction. In our study, the operation time in the FNS group was longer than that in the CCS group, which may be related to the surgical instruments and proficiency. Therefore, it is very important to use the FNS skilfully to shorten the operation time. According to the surgeon’s experience, FNS is significantly better than CCS in applying pressure to the fracture site (Fig. 5).

There are also several limitations in our study: (1) Due to the short clinical application time of FNS, a small number of cases were included in this study; (2) this study only compared the FNS and CCS groups, and the results might be more convincing if the DHS group and the FNS group were compared at the same time.