Fluid-solid coupled biomechanical analysis of implants and the microenvironment in Pauwels type III fractures

Pauwels type III femoral neck fractures are characterized by a steep fracture orientation and substantial shear loading. Increasing knee flexion during loaded activities alters hip joint loading and may affect both fixation mechanics and the poroelastic behavior of the interfragmentary healing tissue. How these effects differ among fixation constructs and postoperative tissue states remains unclear. This study compared cannulated compression screws (CCS), the dynamic hip screw (DHS), and the femoral neck system (FNS) using poroelastic finite element models based on Biot's theory, with material properties representing two postoperative tissue states nominally corresponding to 1 and 12 weeks postoperatively. Hip joint reaction forces obtained from musculoskeletal simulations of squatting for the study subject were applied under five loading conditions corresponding to prescribed knee flexion targets of 60°, 75°, 90°, 105°, and 120°. The results showed that under the condition representing 1 week after surgery and at a prescribed knee flexion target of 120°, implant stresses were 1175.38, 928.77, and 634.35 MPa for CCS, DHS, and FNS, respectively, with FNS exhibiting lower stress than CCS and DHS. The early tissue state also produced large localized principal strain and pronounced pore pressure gradients. Under the 12-week condition, implant stresses decreased below 162 MPa and tissue strain remained below 0.013, although peak pore pressure was higher. The higher assigned tissue stiffness enhanced load sharing and reduced deformation. Overall, knee flexion and tissue state jointly affected fixation mechanics and the local poroelastic behavior, providing a biomechanical insight for postoperative load evaluation.