The effect of the combination of locking screws and non-locking screws on the torsional properties of a locking plate construct

This thesis investigated the effects of replacing one non-locking screw with a locking screw on the torsional properties of a bone-plate construct. Sixty cadaveric canine femurs were divided into 3 groups. In the first group, the plate was affixed using three non-locking screws, in the second group only locking screws were used while a combination of one locking and two non-locking screws were used in the third group. These constructs were then subjected to torsion to failure using a specifically crafted jig on a servohydraulic materials testing machine. Torque, angle of torsion, and work were all calculated at the maximum failure point, as well as at 5 degrees of plastic deformation (offset failure point), which was thought to be more representative of clinical failure. At the maximum failure point, the locking and non-locking groups had a significantly different torque (51.83 Nm vs. 39.17 Nm, respectively), angle (47° vs. 31°), and work (1616.6 Nm-degree vs. 867.8 Nm-degree). The combination group was intermediate to the two other groups and had a significantly lower work compared to the locking group (1121.2 Nm-degree vs. 1616.6 Nm-degree), and a significantly higher torque compared to the non-locking group (47.43 Nm vs. 39.17 Nm). At 5 degrees of plastic deformation, the locking group required significantly higher torque and work than the non-locking group (45.14 Nm vs. 34.78 Nm and 626.37 Nm-degree vs. 461.68 Nm-degree, respectively). The combination group required a significantly higher torque compared to the non-locking group (40.6 Nm vs. 34.78 Nm). None of the slopes of the curves for the different constructs had a significantly different slope, meaning that the stiffness of all the constructs was similar. This study suggests that a construct composed of all locking screws will fail at a greater angle, at a greater torque value, and sustain greater work to failure in torsion compared to a construct composed of all non-locking screws. The addition of a single locking screw to an otherwise all non-locking construct will increase the torque at the offset failure point and may be of clinical value in constructs subjected to high torsional loads.