Biomechanical and finite element analysis of a novel annular multi-axial locking plate for patellar fracture fixation

Traditional tension band structures for patellar fractures have multiple complications such as wires lossening and soft tissue irritation. In response to the above problems, a new type of annular multi-axis locking plate has been designed for the treatment of patellar fractures. This study aims to compare the biomechanical performance of two methods for stabilizing patellar fractures.

A three-dimensional finite element model was established to evaluate the biomechanical performance of annular multi-axis locking steel plate fixation for patellar fractures at knee flexion angles of 20, 45, and 90°. Furthermore, four pairs of matched fresh-frozen cadaveric transverse patellar fracture models (AO/OTA 34B1) stabilized with annular multi-axis locking steel plates (AMALP, experimental group) and cable tension band (CTB, control group) were established to compare the biomechanical performance of the two internal fixation methods. Dynamic fatigue testing was performed by moving the knee from 90to 0° under a load of 6 kg until 200 cycles were completed or the internal device failures. Using a testing machine to evaluate the ultimate force required for the failure of each internal device at knee flexion angles of 20, 45, and 90°. The fracture displacement distance and the ultimate load were recorded.

The outcome of finite element analysis (FEA) demonstrated the max value of Von-Mises stress of the three knee flexion angles simulated were between 45.328 MPa to 109.191 MPa,which is lower than the yield strength value of the plate(351-429 MPa).The displacement value was between 0.1645 and 0.3487 mm, which is acceptable according to clinical criterion(< 2 mm).After 200 cycles of knee flexion fatigue testing, neither group showed any failure of internal fixation. After 100, 150, and 200 cycles of knee flexion fatigue testing, the displacement of the fracture on the anterior surface (0.36 ± 0.05 mm, 0.41 ± 0.04 mm, 0.49 ± 0.08 mm) and joint surface (0.29 ± 0.05 mm, 0.36 ± 0.07 mm, 0.44 ± 0.09 mm) in AMALP were significantly lower than those in the CTB (anterior surface: 0.54 ± 0.09 mm, 0.66 ± 0.11 mm, 0.73 ± 0.09 mm) (joint surface: 0.45 ± 0.06 mm, 0.52 ± 0.06 mm, 0.59 ± 0.08 mm) (P < 0.05).Across all three knee flexion angles (20, 45, and 90°), the AMALP exhibited significantly higher ultimate failure loads (812.40 ± 46.62 N, 546.59 ± 45.22 N, and 370.52 ± 36.41 N, respectively) compared to the CTB(669.60 ± 65.43 N, 423.71 ± 39.46 N, and 264.57 ± 48.15 N, respectively; P < 0.05).

The novel plate features an anatomically contoured profile that closely conforms to the patellar surface, adhering to the principles of tension-band fixation. The screw trajectories are designed with a centripetal orientation, generating intrinsic compressive forces that facilitate axial compression across the fracture site. This not only helps maintain patellar integrity but also enhances knee joint stability, aligning well with established biomechanical principles. Collectively, these design characteristics position the novel plate as a promising new internal fixation option for the treatment of patellar fractures.