According to the latest information released by the World Health Organization, young people are defined as those who are 18-65 years old18. Clinically, femoral neck fracture occurring in people aged 18-65 years old is usually referred to as femoral neck fracture in young people. Due to high femoral neck bone mineral density and hard bone substance, the fracture in young people is mostly caused by high-energy injuries, namely violent injuries, resulting in fracture displacement and seriously impaired local blood supply, along with bony defect in the posterior neck of the femur, which is generally called Garden III and IV fractures in clinical practice. It is well known that the internal fixation of femoral neck fracture has been a major difficulty faced by orthopedists due to the high incidence rate of post-operative complications and secondary operation rate. Femoral neck fracture in young patients less than 65 years old is more challenging because of high-energy traumatic mechanism and fracture displacement, and after internal fixation, femoral neck fracture is prone to serious complications, such as bone unhealing, femoral neck shortening, internal fixation failure, coxa vara and coxa valga deformities and ONFH19,20, so it is called "unsolved fracture"21, which has greatly driven scholars to continuously explore the treatment of femoral neck fractures. The studies have mainly focused on the following three aspects: fracture reduction, selection of internal fixation methods, and blood supply of femoral head. The anatomical reduction of fracture and secure and stable internal fixation are the key factors reducing the abovementioned complications. Based on favorable fracture reduction, the selection of internal fixation methods and implants, without any doubt, is the most important influencing factor. Recent studies have also shown that the therapeutic effect can be affected by the selection of implants, over which controversies still exist22. With the advancement of studies, ideal implants conforming to the internal fixation of femoral neck fracture should not only accord with the concept of minimally invasive implantation but also have good biochemical properties, which is a general consensus gradually reached by scholars.
The advantages of FNS and the reasons for their formation are analyzed as follows. First, FNS has biomechanical advantages. FNS, which is mainly composed of four components: power control rod, anti-rotation screw (ARS), bone plate and locking screw, integrates the merits of multiple internal fixation devices. The cylindrical power control rod designed to maintain the reduction in the implantation process presents a 130° angle with the bone plate, which ensures angular stability. Besides, the cross-over design of the power control rod presenting a 7.5° angle with ARS guarantees rotational stability. Meanwhile, the "Z" effect of femoral head truncation is avoided, and the overall stability is enhanced. In a biomechanical study, Stoffel K et al1. verified that the mechanical stability of FNS was equivalent to that of DHS, and its rotational stability and support strength were 1.5 times and twice those of MCCS, respectively. By comparing FNS with Hansson pins in a study, Schopper C et al15. also proved that FNS showed a higher stability than Hansson pins in the treatment of femoral neck fracture and FNS was an effective alternative method for treating displaced and unstable femoral neck fractures, and capable of reducing mechanical instability-induced revision. The above viewpoints were also verified in this study as follows: A) The post-operative partial and complete weight-bearing time in FNS group was 4 weeks earlier than that in MCCS group, manifesting the superior mechanical stability of FNS. B) The initial fracture morphology of one patient in MCCS group was Garden IV type. This patient experienced position loss, internal fixation failure and Varus deformity of hip in the follow-up visit which was considered as a mechanical failure. FNS fixation was performed during revision surgery, and satisfying therapeutic effect was achieved, powerfully proving the biomechanical advantages of FNS. C) Related studies 23–25have revealed that the occurrence of post-operative femoral neck shortening is directly influenced by the reduction quality of femoral neck fracture and the biomechanical stability of internal fixation. In this study, the incidence rate of femoral neck shortening in FNS group was evidently lower than that in MCCS group, which may be associated with the better biomechanical properties in FNS group. Previous biomechanical studies1 have also proved that the FNS structure plays a critical role in the resistance against femoral neck shortening. D) In comparison with MCCS group, the loss of femoral neck-shaft angle in FNS group was reduced, which may be ascribed to the insufficient shear force used by MCCS to withstand femoral neck fracture. Some recent biomechanical studies have also displayed that FNS is of favorable vara resistance by virtue of its angular stability, so the loss of neck-shaft angle can be reduced.
Secondly, while providing sliding compression, FNS can effectively prevent implant protrusion and mitigate the irritation caused by the implant protrusion to the soft tissues of thighs. The power control rod and ARS of FNS are slidable, where the maximum sliding distance of the former is 20 mm, and the backward sliding within the first 15 mm can ensure that the power control rod does not protrude outward at all. This design of FNS avoids the irritation generated by implant protrusion to the soft tissues of thighs and reduces the incidence rate of pain in thighs, which accords with the results in this study. Moreover, it was believed in this study that it might also be one of the reasons why the Harris hip score in FNS group was higher than that in MCCS group.
Thirdly, the implant characterized by small volume and safety design can better prevent the loss of bone mass in the femoral neck. The diameters of power control rod and ARS in FNS are 10 and 6.4 mm, respectively, so the implant volume in FNS is obviously smaller than that of 3 and even 4 cannulated compression screws (the diameter is generally 7.5 mm). Moreover, the cylinder design of power control rod and the round blunt design at the top of ARS effectively avoid the cutting phenomenon and prevent the screws from protruding out of femoral head. In addition, the compact combination and small size of son-mother screws are capable of effectively reducing the injury of femoral head and reserving the bone mass of femoral neck.
In addition, the operation is relatively simple and convenient, along with short learning curve and reduced operation time. The repeated fluoroscopy and adjustment can be effectively avoided during operation, thus reducing the radiant quantity of X rays, which has been reported by Yang et al26. and also well embodied in our study. Moreover, it has been realized in clinical practice that the operation time and frequency of fluoroscopy will be relatively further reduced with the increased operation cases and enhanced technical proficiency.
Finally, the trauma is small thanks to the minimally invasive implantation. The whole operation can generally be completed just by a small incision with the length of about 4-5 cm. It is only necessary to cut partial vastus lateralis muscle open, which will not result in the tendon injury of gluteus medius or post-operative irritation of gluteus medius27. Besides, the small outer bone plate of FNS can shrink the footprint surface of implant while ensuring the secure fixation with the femoral shaft, which can, to some extent, alleviate the irritation induced by the implantation to the surrounding soft tissues and the post-operative foreign body sensation generated by the implant.
It should be pointed out that in terms of length of operative incision, the open incision (4-5 cm) in FNS group was longer than that in MCCS group, with a statistically significant difference. Nevertheless, the difference between the two groups in concrete numerical value was small, and the amounts of intraoperative bleeding were equivalent, so it was reflected that both internal fixation methods embodied the concept of minimally invasive treatment. Meanwhile, an inevitable problem was that the current cost of one set of FNS components is significantly higher than that of three cannulated screws,so the study results revealed that the hospitalization costs in FNS group were higher than those in MCCS group, which was one of the very few disadvantages of FNS in comparison with MCCS.
This study was a multicenter retrospective study, but there were some limitations. First, the cases were not randomly selected in the retrospective study, which may cause selection bias.Second, The clinical results may also be affected by the relatively small sample size and short follow-up period. In the end, three main complications, namely ONFH, femoral neck shortening and bone nonunion, should be observed after the internal fixation of femoral neck fracture. Due to the short follow-up period in this study, only femoral neck shortening and fracture healing can be reported through an early study, while the further follow-up observation of ONFH remains to be done. In addition, the long-term function of internal fixation for femoral neck fracture should be followed up and further evaluated.
To sum up, by comparing FNS with MCCS in treating Garden III and IV femoral neck fractures, it is reflected that FNS is featured by simple and convenient operation, shorter operation time and earlier post-operative weight bearing. Meanwhile, FNS displays superior biomechanical properties and clinical effects, providing a new treatment choice for femoral neck fractures in young people, especially the complete fracture accompanied by displacement.