Effect of insertion torque on bone screw pullout strength

The effect of insertion torque on the holding strength of 4.5-mm ASIF/AO co rtical bone screws was studied in vitro, Screw holding strength was determi ned using an Instron materials testing machine (Bristol, United Kingdom) on 55 lamb femora and 30 human tibiocortical bone sections. Holding strength was defined as tensile stress at pullout with rapid loading to construct fa ilure. Different insertion torques were tested, normalizing to the thicknes s of cortical bone specimen engaged. These represented low, intermediate, h igh, and thread-damaging insertion torque. All screws inserted with thread- damaging torque and single cortex engaging screws inserted to high torque t ightening moments showed diminished holding strength. This loss of strength amounted to 40%-50% less than screws inserted with less torque.