From the perspective of biomechanical studies compared with the CS, the FNS has a higher angle stability in the unstable femoral neck fracture model, and has a strong ability to resist varus deformity[11, 24]. However, due to the limited clinical application time, only a few studies have compared the therapeutic effects of the two types of implants, and the sample size in different studies was small. Therefore, we have formulated comprehensive and rigorous inclusion and exclusion criteria based on published studies to evaluate the safety and efficacy of FNS and CS in the treatment of FNFs. The results showed that FNS is more effective than CS in decreasing the number of X-ray exposures, fracture healing time, length of femoral neck shortening, femoral head necrosis, implant failure/cutout and postoperative VAS Score. It can also significantly improve the postoperative Harris Score.
Intraoperative indicators
The CS group could significantly reduce blood loss compared with the FNS group in terms of intraoperative indicators. Because when using CS to treat FNFs, only a small incision is needed to implant the screws. When FNS is used, a longitudinal incision is required to implant the FNS device due to its structural characteristics. CS should be more advantageous for soft tissue injuries and intraoperative blood loss. Regardless of the CCS, ICCS, TS, or ITCS, to maximize the stability of the structure and accelerate fracture healing, they all need a triangular distribution, and the screws should be implanted as parallel as possible in anteroposterior X-rays[7, 25]. But there is no correlation between the screws, and the position of the screws in the femoral neck needs to be adjusted multiple times. Therefore, various fluoroscopies cannot be avoided during the operation to determine the position of screws. The design of the FNS device simplifies the surgical procedure. It only needs to insert a 130° guide and a central positioning guide pin to complete the implantation of the internal fixation, which can effectively reduce the number of intraoperative fluoroscopies[18]. It is generally believed that repeated intraoperative fluoroscopies will prolong the operation time when CS is used, but we found no difference in the operation time between the two types of implants. The possible reason is the insufficient time of FNS devices for clinical application, and orthopedic surgeons have not fully mastered the surgical skills, which leads to the prolonged operation time. However, most studies did not report the seniority of surgeons. Only one indicated that the operation was performed by four residents under the supervision of a consultant and by seven surgeons[19].To explore the potential sources of heterogeneity, we performed a sensitivity analysis. The results were consistent with previous results.
Postoperative clinical indicators
In the postoperative clinical indicators, the FNS group was significantly better than the CS group in terms of femoral neck shortening and fracture healing time, and there was no difference in hospitalization time between the implants. One of the characteristics of FNS is dynamic compression[11]. The pre-collapsed insertion allows the anti-rotation screw and bolt slide in the maximum 20 mm packaging to meet femoral neck shortening during fracture healing. Because within a certain range, the impaction of the fracture gap can accelerate the healing of the fracture[26]. However, it is generally considered that shortening > 10mm is severe femoral neck shortening, which is detrimental to fracture healing and postoperative function[27]. The biomechanical properties of multiple screws cannot fully resist the high shear force around the hip, and severe shortening is prone to occur after surgery[28]. Zlowodzki et al.[29] found that the shortening rates after fracture fixation with multiple cancellous screws of non-displaced and displaced femoral neck fractures were 31% and 27%, respectively. Angular stable devices, including dynamic hip screw and FNS, have advantages in resisting high shear forces and femoral neck shortening[11]. Our systematic review showed that Compared with CS, FNS can effectively prevent femoral neck shortening and accelerate fracture healing time, which is consistent with the results of previous in vitro studies[11]. The high heterogeneity of the results may be due to the different types of fractures included in the studies, but the limitation of the number of existing studies, we cannot perform subgroup analysis. More research should be conducted in the future.
Postoperative complications
In terms of postoperative complications, we observed a significant reduction in femoral head necrosis and internal fixation failure/cut-out in the FNS group. Although the nonunion/delayed union of the fracture of the two types of implants was not significantly different, the incidence in the FNS group was lower. This is a meaningful discovery for clinical treatment. The most commonly used internal fixation device for treating FNFs in clinics is the CS, which has the advantage of minimal intraoperative soft tissue damage and compression fixation of the fracture site. However, its resistance to shear and rotational stresses is insufficient, and even with good intraoperative repositioning, postoperative complications such as nonunion of the fracture, screw excision, and femoral head necrosis are prone to occur, especially in unstable FNFs[30]. These complications are also the main cause of post-operative reoperation, bringing huge risks to the patient's quality of life and financial burden[31]. There are also adjuvant therapies combined with surgery that can decrease the rate of femoral head necrosis, such as platelet-rich plasma or stem cells[32–34]. However, there are no reports of FNS in combination with other adjuvant therapies. Our systematic review results showed that FNS effectively reduces the incidence of postoperative complications, which is beneficial for clinical application and demonstrates the safety and efficacy of this new internal fixation device.
Postoperative scores
The VAS Score is an important indicator to assess the degree of pain in patients. A lower score is associated with lower postoperative pain. Harris Score is the most frequently used scale to evaluate the postoperative function of the hip joint. It mainly evaluates four aspects: pain, daily activities, deformity, and range of motion. The higher the score, the better the individual's postoperative recovery[35]. The results of our systematic review showed that the FNS group could significantly improve the Harris Score compared to the CS group. We believe that the FNS group had a better score because it can effectively prevent postoperative femoral neck shortening. Many studies have found that shortening of the femoral neck leads to inferior hip function. The more severe the shortening, the worse the function [27, 29].
Limitations
This study had the following limitations: (1) all the included studies were retrospective and observational, there was a risk of selection bias, and systemic and random errors were prone to occur; (2) the follow-up duration of the included studies is relatively short, and some postoperative complications may not occur; and (3) the number of included studies is small, and the level of evidence is not high due to the lack of RCTs, more high-quality researches are needed in the future to improve the reliability of the results.