Non-locking screw insertion: No benefit seen if tightness exceeds 80% of the maximum torque

Millions of non-locking screws are manually tightened during surgery each year, but their insertion frequently results in overtightening and damage to the surrounding bone. We postulated that by calculating the torque limit of a screw hole, using bone and screw properties, the risk of overtightening during screw insertion could be reduced. Additionally, predicted maximum torque could be used to identify optimum screw torque, as a percentage of the maximum, based on applied compression and residual pullout strength.

Longitudinal cross-sections were taken from juvenile bovine tibial diaphyses, a validated surrogate of human bone, and 3.5 mm cortical non-locking screws were inserted. Fifty-four samples were used to define the association between stripping torque and cortical thickness. The relationship derived enabled prediction of insertion torques representing 40 to 100% of the theoretical stripping torque (T_str) for a further 170 samples. Screw-bone compression generated during insertion was measured, followed immediately by axial pullout testing.

Screw-bone compression increased linearly with applied torque up to 80% of T_str (R² = 0.752, p < 0.001), but beyond this, no significant further compression was generated. After screw insertion, with all screw threads engaged, more tightening did not create any significant (R² = 0.000, p = 0.498) increase in pullout strength.

Increasing screw tightness beyond 80% of the maximum did not increase screw-bone compression. Variations in torques below T_str, did not affect pullout forces of inserted screws. Further validation of these findings in human bone and creation of clinical guidelines based on this research approach should improve surgical outcomes and reduce operative costs.