Biomechanical Study Using the Finite Element Method of Internal Fixation in Pauwels Type III Vertical Femoral Neck Fractures

BACKGROUND: Several factors are known to influence osseous union of femoral neck fractures. Numerous clinical studies have reported different results, hence with different recommendations regarding treatment of Pauwels III fractures: femoral neck fractures with a more vertically oriented fracture line. The current study aimed to analyze biomechanically whether this fracture poses a higher risk of nonunion. OBJECTIVES: To analyze the influence of one designated factor, authors believe that a computerized fracture model, using a finite element Finite Element Method (FEM), may be essential to negate the influence of other factors. The current study aimed to investigate a single factor, i.e. orientation of the fracture line toward a horizontal line, represented by Pauwels classification. It was hypothesized that a model with a vertically oriented fracture line maintaining parity of all other related factors has a higher stress at the fracture site, which would delay fracture healing. This result can be applicable to other types of pinning. PATIENTS AND METHODS: The finite element models were constructed from computed tomography data of the femur. Three fracture models, treated with pinning, were constructed based on Pauwels classification: Type I, 30° between the fracture line and a horizontal line; Type II, 50°; and Type III, 70°. All other factors were matched between the models. The Von Mises stress and principal stress distribution were examined along with the fracture line in each model. RESULTS: The peak Von Mises stresses at the medial femoral neck of the fracture site were 35, 50 and 130 MPa in Pauwels type I, II, and III fractures, respectively. Additionally, the peak Von Mises stresses along with the fracture site at the lateral femoral neck were 140, 16, and 8 MPa in Pauwels type I, II, and III fractures, respectively. The principal stress on the medial femoral neck in Pauwels type III fracture was identified as a traction stress, whereas the principal stress on the lateral femoral neck in Pauwels type I fracture was a compression stress. CONCLUSIONS: The most relevant finding was that hook pinning in Pauwels type III fracture may result in delayed union or nonunion due to significantly increased stress of a traction force at the fracture site that works to displace the fracture. However, in a Pauwels type I fracture, increased compression stress contributes to stabilize it. Surgeons are recommended not to treat Pauwels type III femoral neck fractures by pinning.