Biomechanical comparison of three-point bending resistance of titanium and stainless steel locking screws in intramedullary nails
Ahmet Adnan Karaarslan1, Ahmet Karakaşlı2, Hakan Aycan1, Fatih Ertem2, Erhan Sesli1
1Department of Orthopedics and Traumatology, Medical Faculty of Şifa University, İzmir, Turkey
2Department of Biomechanics, Dokuz Eylül University Institute of Health Science, İzmir, Turkey
Keywords: Femur fractures; femur nail; locking screw; three-point bending.
Objectives: This study aims to investigate whether there is any significant difference in bending resistance between titanium and stainless steel locking screws of femur nails and to review deformation of locking screws which is a common problem in interlocking nailing. Materials and methods: In this study, a total of 60 pieces of 5 mm major diameter titanium and stainless steel locking screws were used as six groups in three different thread depth structures (high threaded, low threaded, and unthreaded). Three-point bending tests were conducted on steel screws placed inside stainless steel tube with 30 mm inner diameter, which imitated the level of lesser trochanter. We used an axial compression testing machine in order to determine the yield points that permanent deformation occurred in the locking screws.
Results: For low threaded locking screws, which are the most frequently used thread type for locking screws, the mean bending yield points were 1413 N on the titanium screws and this level was below 1922 N (2.8 BW) of level walking loading on femur for 70 kg person. On low threaded stainless screws, bending resistance was 2071 N, which was above the value of 1922 N. For high threaded locking screws, the mean bending yield points were 874 N on the titanium screws and 556 N on stainless screws.
Conclusion: In comminuted femur shaft fractures (in full load bearing conditions), using stainless steel locking screws is better instead of titanium screws to avoid locking screw deformation since low threaded stainless steel screws were 46.5% more resistant to bending deformation than titanium ones. Stainless steel or titanium high threaded locking screws may only be carefully used in non-comminuted fractures.