FNF is a common intra-articular fracture in elderly people, with the 3.6% of total fractures and 48%-54% of the hip fractures[12, 13]. Recently, it is the public method by using the closed reduction and internal cannulated screw fixation in the treatment of FNF. Due to a significant poor closed reduction and unstable fixation, the curative effect of FNF is still not satisfactory and the probability of postoperative nonunion and necrosis of femoral head fluctuates are 20%-40% although the bio-mechanical research of FNF and the improvement of fixation methods are constantly improved[15–17]. It is conducive to the stability and healing of fracture only when the fracture surface is compressed and the implanted internal fixation meets the requirements of uniform stress distribution and minimum fracture displacement under the maximum stress after the fracture fixation, which is necessary to consider the biomechanical characteristics after the implantation of the cannulated screw. As we all known, a good stable internal environment can promote the fracture healing and the evenly distributed stress of internal fixation can be effectively fixed to avoid the loosening and fracture of the internal fixation[19, 20]. There are many factors influencing the stress after the implantation of cannulated screws, such as the number, angle, position and etc of the implantation. In the present study, it is considered stability using the three cannulated screw with inverted triangular dispersion distribution. However, due to the irregular anatomical structure of the femoral neck and the complexity of human mechanics experiments, it is difficult to guarantee whether the angle and position of cannulated screw can meet the characteristics of biomechanical stability, so the reliability of fixation is also difficult to guarantee.
Recently, the finite element analysis was mostly used to discuss the researches on the biomechanical stability of FNF to evaluate the stress distribution[22, 23]. As a theoretical method. it can simulate geometric models of various structures and endow various tissue biomaterials with properties to reflect their biomechanical properties under non-invasive condition. The finite element analysis has its unique advantage on completing the complex intra-articular fractures which are difficult to complete the biomechanical research through human mechanical experiments. The results of our study demonstrated that the finite element analysis can simulate the mechanical test of the proximal femur to divide the complex whole into a collection of finite elements, which can provide help for the mechanical analysis of the proximal femur. Although there is a theoretical basis on discussing the best angle and position of cannulated screw implantation by the finite element analysis, it is still a difficulty that how to estimate the best position of cannulated screw implantation because of the instability of the operator, which needs to be completed through the nail channel. In this way, the emergence of 3D-printed navigation template can perfectly solve the above problem. The present study, as well as the results from the previous study [21, 23], 3D-printed navigation template was made at the proximal lateral femoral nail placement to complete the nail placement through the personalized finite element analysis of every FNF patient, which can not only shorten the operation time, reduce the number of intraoperative fluoroscope and improve the efficiency of the operation, but also be closer to the femoral neck cortex in the position and angle of the nail placement. In the line with the results of finite element analysis, the stress distribution was more diffuse, with smaller displacement peak under the maximum stress, so the biomechanics was more stable. Within the 12-month follow-up of all patients, it was also confirmed that the healing rate of FNF treated with 3D printing guide plate was higher and the necrosis rate of femoral head was lower in the study group than that in the control group. The reasons were that the angle and position of the cannulated screw needed to be constantly adjusted by the traditional surgical method to aggravate the trauma and destroy the blood supply of the fracture end and femoral head. Meanwhile, the replacement of the cannulated screw further destroyed the cancellous bone to reduce the holding strength of the screw to result in decreasing the fixation strength and increasing the fretting of the fracture end, which could affect the fracture healing.
Based on the experience of our study, we can summarize the advantages as follows: (1) The best channel for the cannulated screw placement to meet the biomechanical stability can be determined by the finite element analysis. (2) The cannulated screw placement can be completed at one time to reduce the secondary trauma by combined with 3D-printed navigation template, which is conducive to fracture healing and reduce the rate of femoral head necrosis. (3) Compared with the traditional method, the application of a personalized finite element analysis and 3D-printed navigation template in the treatment of FNF is more accurate and efficient on reducing the operation time, intraoperative fluoroscopy times and iatrogenic damage. In spite of the above advantages, there are a number of shortcomings as follow: (1) The finite element analysis is based on the ideal complete anatomical reduction, which is often different from the clinical complex fracture situation. (2) The finite element model is an ideal femoral neck model, not considering the soft tissue around the joint. (3) The production cost of the application of a personalized finite element analysis and 3D-printed navigation template in the treatment of FNF is relatively high and need more time to complete.