Previously, a lower limb shortening of more than 2cm was considered to cause lower limb dysfunction and even claudication. However, continuous study by scholars found that shortening of the proximal femur exceeding 5mm causes the dysfunction of the patient's hip joint. Intertrochanteric fractures are in the elderly. With continuous improvements in the internal fixation system, PFNA has obvious advantages in most types of intertrochanteric fractures. However, many scholars have found that proximal femoral shortening after intramedullary fixation of intertrochanteric fractures is common. Hou et al. found that the shortening rates of intertrochanteric fractures of the femur were 72.46% and 88.76% after treatment with a third generation gamma nail (TGN) and PFNA,respectively. Some scholars believe that due to the anteversion angle of the hip joint and the collodiaphyseal angle, gravity transmission from the pelvis to the lower limb occurs at a turning point. Therefore, when the proximal femur is fractured, fretting and bone loss of the fracture plane occur easily, resulting in proximal femur shortening. However, strict biomechanical experiments to verify the biomechanical effects of proximal femoral shortening after PFNA internal fixation on the proximal femur and internal fixation are lacking.
In this study, models of proximal femur shortening of different degrees were established, and it was found that the overall femur stress was concentrated on the medial aspect of the dorsal side of the proximal femur, that is, near the calcar femur. The calcar femur is the internal weight-bearing system of the proximal femur, and plays an important role in the fracture of the proximal femur. By decreasing the force on the posterior and medial femur and increasing the force on the anterior and lateral femur[10,11], we found that with the gradual shortening of the proximal femur, the maximum stress on all four surfaces of the proximal femur decreased gradually. This suggests that the stability of the whole femur gradually increased with the shortening of the proximal femur.
According to the stiffness changes of different proximal femur shortening in Fig. 3, it was found that the overall deformation resistance of the structure increased by 227.4N for every 1mm shortening of the proximal femur. In addition, the stress in the cancellous bone region and fracture plane of the proximal femur decreased with a gradual increase in the shortening degree, indicating that the stability of the fracture plane gradually improved after shortening, which was in great contrast to the adverse effect of proximal femur shortening on hip joint function. This may be due to the helical blade of the PFNA fracture compression effect. Because, the head end of the control force is strong enough, the compression effect of the fracture plane will gradually increase with shortening aggravation, so that the fracture surface stress decreases, which is conducive for fracture healing, and is also consistent with the clinical results. In clinical practice, shortening of the proximal femur is often caused by continuous compression of the fracture end, which often promotes fracture healing.
Bone around the end of a helical blade stress changes as shown in Fig. 6, the stress variation in the shortening of the different groups is not obvious, it can be presumed that after the shortening, the lateral femoral wall stress effect is reduce, but it can be found that the maximum stress on the front and rear sides of the helical blade is about three times the upper stress. This may lead to forward displacement of the bone after separation, and this may be related to the model setup. In this study, the intertrochanteric fracture of the femur was set as Evans-Jensen type III, and the fracture involved the lateral wall, which may lead to poor stability of the lateral wall in the front and rear directions. However, different degrees of shortening seem to have little effect on the lateral wall stress, but the specific reasons for this need to be explored further in the future.
By observing different degrees of proximal femoral shortening, the helical blade tip and the tail end, it can be found that with increase in shortening, the stress of the helical blade end increases gradually, while the stress of the helical blade tip decreases. At the same time, the shortening of the various groups of at top of the helical blade tip stress was significantly greater than that under stress. This could occur if shortening gradually increases, which may result in an increased chance of the helical blade pulling out the screw laterally and a decreased chance of cutting in the direction of the femoral head and acetabulum. This may be due to the fact that this experiment simulates a fracture involving the lateral wall. With an increase in shortening, the blocking effect on the tail end of the helical blade is weakened. Meanwhile, the stress difference between the tip and tail of the helical blade may indicate that if the helical blade cuts toward the femoral head, the probability of cutting toward the proximal end is very high. Helical blade cutting and nail removal are common complications of PFNA[13–17]. Previous studies have reported that the cutting rate of a helical blade after PFNA for intertrochanteric fracture is 0-7.9%.There are many influencing factors, including fracture type, reduction quality, apex distance, helical blade position, osteoporosis, incorrect functional exercise, and design defects of the PFNA itself. At present, the tip apex distance(TAD) is considered to be the most important predictor of helical blade cutting[16,18].Additionally, the integrity of the lateral wall is an important factor. Previous studies have shown that the presence of the lateral wall provides lateral support for the proximal main nail and head and neck screw, preventing the removal and withdrawal of the head and neck screw.In this study, intertrochanteric fractures of the femur involving the lateral wall may increase the probability of screw-blade nail withdrawal after proximal femoral shortening, but the probability of cutting toward the femoral head and acetabulum may decrease.
In conclusion, through finite element analysis, it was theoretically found that the stability of intertrochanteric fractures would gradually increase with proximal femoral shortening after PFNA internal fixation. Also, when intertrochanteric fractures involve the lateral wall, the probability of screw blade disengagement may increase as shortening increases, and the probability of cutting toward the femoral head and acetabulum may decrease.
This experimental study has some limitations: First, it simulates the mechanical analysis of a static hip joint. If finite element analysis can be carried out in the walking state of the hip joint, the data obtained will be closer to the mechanical characteristics of the human body. Second, only one type of femoral trochanter was simulated in this experiment. Third, the fracture type is relatively simple. These limitations will need to be addressed in further research.