To investigate the biomechanical properties of our self-designed 4 cannulated screws in 4 configurations for fixation of extremely unstable femoral neck fractures.

Twelve adult cadaveric femoral specimens were randomly divided into 4 equal groups (n=3) and made into models of extremely unstable femoral neck fracture combined with comminution (Pauwels type Ⅲ). Group A was subjected to fixation in configuration of "double axial compressions plus double stabilizations" , group B to configuration of "positive triangle parallel compression plus small angle screwing" , group C to configuration of "inverted triangle parallel compression plus small angle screwing" , and group D to configuration of "diamond pattern screwing" . Static compression tests, cyclic loading tests and limit load tests were carried out for the 4 groups on a biomechanical testing machine.

For groups A, B C and D, the axial compression stiffness was respectively 995.29±34.16 N/mm, 509.89±138.90 N/mm, 559.28±111.25 N/mm and 610.18±232.35 N/mm, and the limit load was respectively 3, 225.33±461.31 N, 2, 008.67±237.27 N, 2, 705.67±496.39 N and 2, 395.33±403.71 N, showing significant differences between the 4 groups (P<0.05). For groups A, B C and D, the displacement was respectively 0.46±0.10 mm, 1.47±0.72 mm, 1.14±0.24 mm and 1.22±0.22 mm, and the limit stiffness was respectively 1, 139.28±342.09 N/mm, 843.56±408.91 N/mm, 585.98±81.60 N/mm and 729.96±251.37 N/mm, showing no significant differences between the 4 groups (P>0.05).

In the fixation of extremely unstable femoral neck fracture with our self-designed 4 cannulated screws, the configuration of "double axial compressions plus double stabilizations" may lead to the greatest biomechanical advantage while the configuration of "positive triangle parallel compression plus small angle screwing" may result in the poorest biomechanical properties.

Femoral neck fractures; Fracture fixation, internal; Bone nails; Biomechanics