A biomechanical comparison of single- versus double-threaded cannulated screws for femoral neck fractures of Pauwels type III

Objective To compare the biomechanical performance between the single- versus double-threaded cannulated screws in the treatment of femoral neck fractures of Pauwels type Ⅲ. Methods Models of femoral neck fracture of Pauwels type Ⅲ (70°) were made of the Sawbone synthetic composite femurs. All specimens were divided into 2 groups (n=12). Group A was fixated with single-threaded cannulated screws and group B with double-threaded cannulated screws, both in an inverted triangle configuration. The screws ranged from 90 to 95 mm in length and from 7.3 to 7.5 mm in diameter. All the specimens were subjected to axial stiffness and failure load tests with 7° valgus (simulating normal two-legged weight-bearing stance) and 25° valgus (simulating normal one-legged weight-bearing stance) and torsion test as well. The 2 groups were compared in the torques at axial stiffness angles of 1°, 2°, 3°, 5° and 7°. Results Group B had significantly greater axial stiffness at 7° valgus and 25° valgus (89±26 N/mm and 128±37 N/mm) and failure load (1,154±368 N) than group A did (36±12 N/mm and 47±16 N/mm; 688±94 N) (P< 0.05). The torques increased with the increase in rotation angle in both groups. However, the torques in group B (3.26±0.96, 4.16±1.23, 4.64±1.13, 5.59±1.26 and 6.53±1.47 N·m) were all significantly larger than in group A (1.44±0.19, 2.03±0.41, 2.33±0.62, 2.74±0.87 and 3.05±1.07 N·m) (P<0.05). Conclusion Double-threaded cannulated screws may provide better biomechanical stability than single-threaded ones, due to their substantial improvement in anti-compression and anti-rotation performance. Key words: Femoral neck fractures; Fracture fixation, internal; Bone nails; Biomechanics; Pauwels Ⅲ