Finite element analysis of biomechanics of two internal fixation methods for Pauwels type III fractures based on fatigue life calculation

Abstract

BACKGROUND: In patients with Pauwels type III femoral neck fracture who are unable to undergo closed reduction, the traditional cannulated compression screw internal fixation method cannot effectively counteract the large shear force. To solve this problem, this study personalized an internally supported plate and used it in conjunction with the cannulated compression screw internal fixation protocol to improve treatment outcomes. OBJECTIVE: To compare the biomechanical performance of two internal fixation methods for Pauwels type III femoral neck fractures under gait loading: an "inverted triangle" arrangement of three screws and an "inverted triangle" arrangement of three screws combined with a medial support plate by finite element calculation. METHODS: Based on the CT scan data, inverse modeling was first performed using Mimics software to generate a point cloud model of the femur. Subsequently, the model was refined using Geomagic software to optimize its geometry and ensure the accuracy of the model. Finally, the processed model was imported into NX software to establish a femoral neck fracture model with a Pauwels angle of 70°. The mechanical and fatigue life results of the 3-nail [fully threaded (model 1), unthreaded (model 2)] model, 3-nail + personalized internal support plate [fully threaded (model 3), unthreaded (model 4)] model were computed based on the Ansys software for the gait loading. RESULTS AND CONCLUSION: (1) Under gait loading, the introduction of an internal support plate reduced the mean femoral stress compared with the 3-nail fixation approach, including a decrease in fracture and stump stresses of 6.6 MPa and 11.0 MPa, respectively; a decrease in displacement of 0.24 mm and 0.12 mm, respectively, and a reduction in relative displacement of the fracture surface. (2) The internal fixation method reduced the fatigue life of the bone system, and the addition of the internal support plate further reduced fatigue life. (3) The finite element analysis was sensitive to thread parameters, so the thread type and characteristics of screws should be considered in model construction. Compared with the 3-screw fixation alone, the addition of an internal support plate reduced stress and deformation levels, providing a more stable mechanical environment for bone healing. From the perspective of fatigue life, internal fixation reduced the life of the femoral system, and the more implants, the lower the life; the fixation scheme with the internal support plate had the lowest fatigue life. (4) This indicates that in clinical design of fixation schemes, the impact of implants on long-term healing outcomes should be fully considered, and the number of implants and fixation method should be comprehensively evaluated.

1. Introduction

Femoral neck fracture is a common clinical fracture type. According to the angle between the fracture line and the acetabular superior rim in the coronal plane (<30°, 30°-50°, >50°), it corresponds to Pauwels types I, II, and III, respectively. Among them, Pauwels type III is difficult to treat due to vertical instability, leading to high clinical complication rates [1-4].

Internal fixation surgery is the first choice for clinical treatment of Pauwels type III femoral neck fractures, but the optimal internal fixation scheme remains controversial [5-6]. Different numbers and combinations of screw fixation are common fixation methods. Some studies have used finite element analysis to compare the fixation effects of 1, 2, 3, and 4 screws arranged in different patterns [7-10], and the results showed that three parallel screws arranged in an "inverted triangle" pattern had the best fixation effect. The combination of screws and a medial support plate is also widely used clinically. Mechanical analysis, experiments, and clinical treatment studies have shown that the combination of three screws in an "inverted triangle" arrangement with a medial support plate has good anti-torsion and anti-shear capabilities [11-14]. However, the comparison of fixation effects between these two methods has not been discussed.

For internal fixation methods, conventional finite element analysis mainly focuses on static loading and does not consider the long-term mechanical effects between bone and implants after functional loading. Liu Quanfeng et al. [15-16] performed fatigue life simulation analysis of the femur after hip arthroplasty, but reports on the life of internal fixation for Pauwels type III femoral neck fractures are rare. This study aims to evaluate the life of screws and screws combined with an internal support plate, comprehensively considering the long-term effects of different fixation methods on the femur, and to provide a theoretical basis for clinical implant selection.