How Reliable Are The Threaded Locking Screws

Abstract

Objectives: A frequent problem for interlocking nailing, that affects the treatment of the fracture is locking screw deformation. The research question is whether bending resistance is different between high, low and unthreaded locking screws of interlocking femoral nails.
Materials and methods. : We used 90 screws for nine groups, ten screws for each group in this experimental study. We performed three-point bending tests on six group of 5 mm screws (titanium, stainless steel, crossed with unthreaded, low threaded and high threaded) and the same three thread types of 5.5 mm stainless steel screws in a steel tube that has 30 mm inner diameter, which imitates the level of lesser trochanter. We determined the yield points at three-point bending tests that permanent deformation started in the locking screws   using an axial compression testing machine.
Results: The mean yield point value at 3-point bending test of 5 mm low threaded stainless steel locking screws  was 2071N and 53% less than unthreaded ones (3169 N).The mean yield point value of 5 mm high threaded stainless steel locking screws was 556 N and 272% less than low threaded ones (2071 N).
Conclusion: To avoid locking screw deformation, high threaded screws must not be used as locking screws. Five mm low threaded screws in nailing of comminuted or oblique femur shaft fractures of unreliable patients should not be used. All 5 mm unthreaded screws and 5.5 mm low threaded stainless steel screws can be used safely in full weight conditions of unreliable patients.