The effect of thread length in cancellous bone fixation under shear loading: A biomechanical study

The thread length of bone screws is a crucial design parameter directly related to the length of the thread engagement in the bone. For porous materials, like cancellous bone, the length of engagement is correlated with the holding power of screws. The objective of this study is to assess the effect of the thread length in fixation under shear loading, a relatively unexplored area of investigation. For this purpose, sawbone blocks were used as a substitute for cancellous bone, fixed with either a single 6.5 solid-core cancellous screw with a thread length of 16 mm or a single screw with a 32 mm thread length (lag by design). The experiments (shear loading of the constructs) were carried out using an MTS Insight frame. The stiffness, proportionality limit, conventional yield load, and yield displacement were determined for each test piece. The results indicated that the constructs fixed with longer threaded screws had statistically significantly greater stiffness, proportionality limit, and yield force compared to the constructs fixed with shorter threaded screws. This study suggests that the length of thread engagement may affect the resistance of fixation to shear loading.