Volume 9, Issue 2 (February 2012)
Symposium on Static and Dynamic Spinal Implants: Are We Evaluating Them Appropriately? Effect of Tightening Torque on ASTM F1798 Axial Gripping Capacity
The objective of this study was to investigate how the axial gripping capacity can be affected by varying the tightening torques applied to the set screws in a pedicle screw ASTM F1798 subassembly. Although the current ASTM F1798 standard indicates that all tightening loads to the locking mechanism be applied as specified by the manufacturer, the torque limiting instruments used in the field can have nominal upper and lower torque limits. The interconnection assemblies were assembled using a multi-axial screw, spinal rod, and set screw. Six groups were assembled using different types of spinal rods. Each group was sub-divided into three groups each containing five interconnection assemblies. The three sub-groups of interconnection assemblies were locked to the nominal torque value as indicated by the manufacturer technique, lower limit, and higher limit respectively. Static axial grip testing was performed according to ASTM F1798 guidelines. Axial gripping capacity was influenced by the nominal tightening torque and lower tightening torque and holds true regardless of rod diameter and rod material. There is no significant difference in the axial gripping capacity between the nominal tightening torque and higher tightening torque. These results indicate that the set screw tightening torque has an effect on the gripping capacity and could potentially affect yield load, ultimate load, and failure modes when performing ASTM F1717 compression bending testing. With the wide range of materials used to manufacture pedicle screw systems and the processing techniques that are currently available, e.g., anodization and shot peening, critical thinking should be put into determining if the torque applied to the system represents a worst-case scenario.