What is the stable internal fixation for the unstable and osteoporotic supracondylar femoral fractures: a finite element analysis

Over the past 20 years, OSFF mainly caused by low-energy trauma has recorded an increase in incidence in the elderly population [1‐8]. Only osteosynthetic reconstruction with intramedullary nails or extramedullary plates can stabilize these fractures against forces and strains on the femur [9]. Intramedullary nail can provide greater stiffness but is limited by its torsion control. Due to the promotion and maturation of the MiPPO technique, plates have gradually become mainstream [10]. However, some clinical studies have shown that ULLP fixation fails or results in implant breakage when the fracture is accompanied by the severe comminution, bone loss, or osteopenia due to the thin cortex, poor bone quality, and limited bone stock available for screw purchase [11]. It has been reported that nonunion of the distal femoral after ULLP treatment occurs in 10%–23% of cases [12], and the adding of an medial locking plate is effective in preventing nonunion [13‐15].

Both excessive stress values at the support and excessive movement between the fracture blocks always appear in the failure of the ULLP internal fixation, especially in UOSFF. However, clinically quantifying three-dimension (3D) fracture-site motion and stress values remains impractical, and determining the location where the ULLP fixation failed or resulted in implant breakage is also largely limited by the inability to measure or predict fracture-site motion [16‐18]. Computational modeling, especially finite element analysis (FEM), permits parametric investigation at a lower cost. Such computational analysis has been proposed as a tool to study the stress, displacement, and stiffness. Besides, this analysis can help identify potential causes of internal fixation failure of locking plates and suggest improvement schemes [19].

From what has been discussed above, adding a medial locking plate (in the BLLP fixation) becomes crucial to fix the UOSFF. Therefore, we designed this study to explore the stable internal fixation method for UOSFF and to investigate the primary causes of implant breakage.

In the reduction of OSFF, acquiring support from the bilateral cortex at the fracture site immediately after surgery is important. However, for UOSFF, particularly those with large bone defects and treated with ULLP, fixation failure or implant breakage is more likely to occur. This may be due to the loss of continuity of bilateral cortex in such fractures, the obvious loss of bone quality, and the inability to effectively support the distal femur, when the lateral locking plate is applied alone, the medial side cannot maintain the stability of the inner femur due to the lack of sufficient support, resulting in a significant increase in the displacement of the medial fracture, increasing the risk of delayed fracture union or bone disunion, and long-term concentration of stress on the lateral bone plate will eventually lead to internal fixation failure [20, 21]. Our study was based on the FE method and found that BLLP of the femur can significantly reduce the probability of internal fixation failure (the number of convergences of ULLP model was 30, while that for BLLP models was 153, each group with a total of 154 models), providing a biomechanical evidence for treating UOSFF with BLLP. Currently, biomechanical analysis of UOSFF and implants is common. The stress distribution can be analyzed using FE method, by setting parameters that reflect actual conditions, simulating the original tissue using discrete finite elements, and calculating the stress values within it. If the stress is too high, it will result in the destruction of bone tissue. The von Mises stress distribution of the implant is an indicator of metal yield strength. The higher the stress value on the implant, the higher the probability of implant failure [22‐25].

The position selection of the distal fixation screw is often limited by clinical conditions and cannot be in the correct position [34]. The distal screws and plates in this paper were locked in the most ideal and stable state after referring to the relevant literature (Fig. 2). From the stress analysis of the plates and screws, it was found that the maximum stress values of the plates were most likely to occur around the fracture line, while the screws were most likely to occur at the contact points with the bone cortex. This is consistent with what has been stated in many previous articles [12, 22‐25]. Moreover, the stress values in the ULLP model were significantly higher than those in the BLLP model (P < 0.05). BLLP could significantly relieve the highly concentrated stress distribution caused by ULLP. The stress values increased with the increase in fracture line angle and bone defect degree. The ULLP model began to exhibit non-convergence when the fracture line type was “/,” and the fracture line angle was 30°. When the fracture line angle was 45°, the model did not converge at all. When the fracture line type was “\” and the fracture line angle was greater than 30°, the model did not converge at all. The BLLP models had only one non-converging case. For patients with a fracture line angle greater than 30° and accompanying bone defects, it is strongly recommended to use the BLLP internal fixation method instead of the ULLP internal fixation method. Additionally, surgeons should pay attention to the deformation of the screws at the contact points between the plate and bone cortex at the fracture line, and repair them in a timely manner.

The previous studies have found that shear movement (x-axis displacement) inhibits healing tissue formation, while proper longitudinal movement (z-axis displacement) promotes healing tissue formation [12]. In this study, we found a large displacement of the model in the x-axis direction when changing the degree of bone defect and a large displacement in the z-axis direction when changing the angle of the fracture line. Thus, we believe that if the fracture is accompanied by a large bone defect, attention should be paid to the formation of callus, and if the fracture line is large, more attention should be paid to the bearing of the plate. However, adding a medial plate can effectively reduce the longitudinal and shear movement of the femoral fracture site. We also found that the stress values on the femur and equivalent displacement values at the fracture site were significantly decreased in the 13H group compared to the 9H group, while the stress values on the screws were significantly higher in the 13H group than in the 9H group. However, the number of convergences in the 13H group was not significantly greater than that in the 9H group. These results suggest that a longer locking plate can disperse the stress concentration on the femur to a certain extent, thus avoiding excessive displacement and providing more stable support for the formation of callus [16, 26‐29].

The clinical validation of this paper came from a rare case in our hospital (Fig. 8), which was retrospectively analyzed. The angle of the fracture line was large (45°), and the fracture line type was “/,” the internal fixation (ULLP) was performed by an experienced orthopaedic surgeon. The operation was successful, there were no common complications such as infection and bone nonunion [35]. However, a sudden internal fixation rupture occurred 3 months after the operation during a normal walk. The reason for the failure of internal fixation was mainly because the ULLP could not be stabilized under this fracture after systematic evaluation, which was consistent with the concept proposed in this study.

This study has certain limitations, as it is not possible to simulate all situations in FE analysis. In our simulations, we did not consider patient-specific measurements of soft tissue thickness or muscle function. Moreover, there is no way to choose the right in vitro experiment to validate the results because of the sheer number of models.

Our data demonstrate that more caution should be exercised in patients in the following four cases: First, when the angle of the fracture line is large (30°, 45°). Second, when the fracture line type is “/.” Third, when the bone defect is large. Fourth, when the bone defect is medial. To sum up, adding a small locking plate to the medial side not only effectively reduces the postoperative stress concentration distribution in the femur, but also reduces the longitudinal and shear movement at the fracture end, which is of great significance for the guidance of clinical treatment.