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OBJECTIVES
Displaced intracapsular neck of femur fractures (NOF) in younger patients are usually fixed with partially-threaded cannulated screws. However posterior comminution may lead to construct failure. We hypothesised that a posterior fully threaded positioning screw would enhance stability.

METHODS
A total of 16 left composite femora (Sawbone) were used for testing. To mimic a subcapital fracture with posterior comminution, a subcaptial osteotomy was performed and a posterior wedge was resected from the neck. Group A (n=8) was fixed using 3 partially threaded cancellous screws. In Group B (n=8), a fully threaded positioning screw instead of a partially threaded was used posteriorly. The specimens were tested for bending (antero-posterior=A-P) and axial stiffness. Finally, they were axially loaded up to failure or up to 10,000 cycles and the final displacement was measured at the site of the resected neck. More than 5mm of displacement was considered as a failure of the construct.

RESULTS
Group B showed significantly higher average A-P stiffness (665±17N/mm compared to 414±41N/mm, p=0.0004); whereas axial stiffness did not significantly differ between the two groups (p=0.301). In Group B, the mean final displacement after cyclic axial loading was 0.51±0.13mm and none of the specimens failed, whereas 7 of 8 constructs failed in Group A (p=0.001).

CONCLUSIONS
This biomechanical study points out a potential benefit of replacing the posterior partially threaded cancellous screw with a fully threaded positioning screw in subcapital NOF with posterior comminution. The construct with the fully threaded screw significantly improved the A-P stiffness and reduced the collapse of the fracture.