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 decrease in relative displacement of the fracture surface. (2) The internal fixation method reduced the fatigue life of the bone system, and the life was reduced even more with the addition of the internal support plate. (3) It is concluded that finite element analysis is more sensitive to thread parameters, so the thread type and characteristics of the screws should be considered in model construction. The fixation method with the addition of an internal support plate reduces the level of stress and deformation and provides a more stable mechanical environment for bone healing compared with the threaded triple-nail fixation scheme only. In terms of fatigue life, the internal fixation method reduces the life span of the femoral system, and the higher the number of internal implants, the lower the life span, and the incorporation of the internal support plate resulted in the lowest longevity of the fixation scheme. (4) This suggests that the impact of implants on long-term healing should be fully considered when designing the fixation program, and the number of implants and fixation methods should be taken into account.

Key words: femoral neck fracture, finite element, fatigue life, inverted triangular cannulated compression screw internal fixation, internal support plate, biomechanics