The operation time, length of incision, surgical blood loss, perioperative complications and incidence of bone healing complications were not statistically different between DCLS group and MCCS group, suggesting that the surgical incision in both groups was small with the small trauma and less blood loss during the surgery, so both surgeries could be minimally invasive with less complications. DCLS group had fewer healing complications than did the MCCS group, but there was no statistical difference. The Harris score, fracture healing time, femoral neck shortening, partial weight-bearing time and complete weight-bearing time at the last follow-up were significantly better in the DCLS group compared with the MCCS group. The fracture healing rate and mobility were better in the DCLS group than in the MCCS group. These suggested that DCLS, compared with MCCS, could increase the fracture healing rate, improve the patient's mobility and hip function, accelerate earlier bone healing and prevent excessive shortening of the femoral neck. So patients in the DCLS group can carry out weight-bearing activities earlier, and have the better quality of life.
Femoral neck fractures are one of the problems in orthopedic treatment. The prognosis is uncertain. Nonunion and femoral head necrosis are recognized as serious complications after internal fixation. The type of femoral neck fracture and improper treatment are considered to be the main factors leading to nonunion and femoral head necrosis. Garden classification is the mainstream classification system for femoral neck fractures and guides clinical treatment. Decisions on the treatment of displaced (unstable) fractures (Garden III and IV) in young patients are still controversial. Surgical methods include closed/open reduction internal fixation, hemi-hip replacement, and total hip replacement . Stable fractures(Garden I and II) tend to be treated with internal fixation.
Femoral neck fractures, no matter what treatment method is selected, have a significant impact on the living quality of patients and bring a large economic burden to society . Compared with hip replacement, internal fixation has become the main treatment method due to its advantages such as less trauma, shorter operation time, less bleeding, lower incidence of perioperative complications, lower early mortality and lower cost. For Garden I and II femoral neck fractures, internal fixation is currently preferred . However, there is still no consensus on which internal fixation method can better maintain the stability of fractured ends, promote fracture healing, and avoid and reduce complications such as femoral head necrosis, nonunion, and internal fixation failure.
Three cannulated screws can exert pressure on the fracture end and promote fracture healing. In addition, they occupy a relatively small area of the femoral neck, and have less interference with the blood flow for femoral head and neck. The triangular distribution can form a three-dimensional frame with bone tissue, which can improve the stress against the rotation of the femoral head. It can enhance the compressive stress intraoperatively and postoperatively between fracture ends, which could promote closer contact between fracture ends and be conducive to fracture healing. However, because the three cannulated screws are not related to each other; the position of the screws are easily affected by the subjective and objective factors of the operator. So its resistance to vertical shear and torsion is relatively poor, which can lead to loosening and re-displacement of fracture end, femoral head necrosis, nonunion, and femoral neck shortening [23, 24]. And in the process of healing, the lack of sustained and effective solid support affects the rehabilitation training of the affected limb and the growth of fractures .
Although the dynamic hip screw can provide better angular stability and sliding compression, its anti-rotational stability is poor, especially when the hip screw is screwed, which can easily cause poor alignment of the femoral head and neck. Furthermore,dynamic hip screw require large soft tissue exposure, and hip screw insertion damages the cancellous bone of the femoral head and neck and destroy its blood supply, which affects the healing of femoral neck fractures.
Therefore, DCLS combines the advantages of the three cannulated screws and dynamic hip screw, which can not only improve the strong, uniform and accurate compression of the fracture section intraoperatively, but also have the stable frame structure to stabilize the broken end of the fracture and the controlled dynamic compression to prevent excessive shortening of the femoral neck. So it can obtain good initial and continuous stability to prevent displacement of the fracture ends and help fracture healing. Early biomechanical experiment of human corpses showed that DCLS, compared with MCCS, has better biomechanical stability, stronger compressive and torsional resistance .
DCLS is a new method for femoral neck fracture fixation. The main features are as follows. ①The positions of the three parallel compression screws are distributed on the triangular protuberance of the safety cross section of femoral neck with high bone density, which conforms to the "cortical support" principle and has the characteristics of screw dispersion and maximum holding force. ② Controlled axial uniform dynamic compression can effectively control femoral neck shortening. ③The three compression screws press the fracture end parallelly to the axial direction of the femoral neck through the lateral locking plate postoperatively can provide axial and uniform positive pressure. ④The frame configuration is stable and has good resistance to shear and torsion. ⑤The even pressure intraoperatively can make the fracture ends well aligned and reduce the infiltration of synovial fluid, which could be conducive to callus formation and fracture healing. Biomechanical experiment and finite element analysis confirmed that DCLS had good fixed stability and biomechanical conductivity with no stress shielding, and was conducive to fracture healing .
At present, three cannulated screws are commonly used for internal fixation of stable femoral neck fractures, but many studies have significant differences in the position distribution and the clinical overcomes . The three cannulated screws have large differences in torsion resistance and fracture end stability, and the instability of the fracture end is not conducive to fracture healing . Weil et al  used three cannulated screws in an inverted triangle to treat 41 cases of femoral neck fractures, 71% of them had a significant femoral neck horizontal shortening greater than 5 mm, and 25% of them had severe shortening greater than 10 mm. Significant shortening occurred in 43% of patients in the vertical direction, and severe shortening occurred in 17% of patients. Screw pullout greater than 5 mm occurred in 41% of patients. 7 cases required late hip replacement. Gupta et al. studied hollow cancellous screws for femoral neck fractures for up to 4 years. The imaging healing time was 7.1 months, the healing rate was 82.22%, the osteonecrosis rate was 6.67%, and the Harris Hip Score was 88.65. Manohara et al.  studied cancellous screw fixation for undisplaced femoral neck fractures in elderly patients, Of the 96 patients followed up for a mean of 39 months, 8/96 (8.3%) underwent revision surgery for femoral head avascular necrosis (5/96, 5.2%) or non-union/implant failure (3/96, 3.1%). Overall, 30/96 (31.3%) patients had a decrease in their mobility status. Chen et al studied patients with femoral neck fractures treated with the dynamic hip system blade or MCCS for an average follow-up of 27 months. No statistically significant differences in the rates of nonunion (4.5% vs. 0) and femoral head avascular necrosis (9.1% vs. 7.1%) were observed. 15.9% of patients reported a femoral neck shortening greater than 10 mm. Other study has found that for femoral neck fractures treated with three hollow screws, nonunion and osteonecrosis were 42% and 17% in the displaced fracture group and 6% and 4% in the non-displaced fracture group .
However, this study also had the following limitations. The number of cases in this group was relatively small. It was a single-center prospective study and has not been completely randomized and double-blind. The results may be biased. Therefore, this study needs to be verified by further multicenter, randomized, controlled, double-blind clinical trials.