Pull-out strength is a critical parameter to judge screw fixation in orthopedic implants. However, the insertion torque is the main tactile sensation for surgeons, relating the strength of synthesis.

The purpose of a new study published in Clinical Biomechanics was to determine the correlation between pull-out strength and torque during orthopedic screw fixation, with the objective to clarify whether torque can be considered a valid parameter to judge the quality of fixation.

In this experiment, three screws differing only in pitch (1.5, 2.1, 2.8 mm pitches) were tested in three foam-block densities (10, 15, and 20 pcf), using the ASTM F543 as reference. The correlation was investigated by assessing the role of density and screw geometry.

The results revealed that torque was related to pull-out strength in all configurations. No difference in pull-out strength was observed when screws were tightened to a range of 71.6%, SD = 7.6, of the torque to fail. Meanwhile, torque and pull-out strength were stratified according to density that influenced the two parameters up to 524%. Besides, the pitch determined pull-out strength up to 33% and the 2.1 mm screw pitch showed the highest pull-out strength and torque in all configurations.

In conclusion, it was stated that the insertion torque is a valid parameter to judge the quality of bone under fixation, as well as the strength of the synthesis. Surgeons should not tighten the screws to values approaching torque to fail to obtain the highest pull-out strength. Moreover, density is the main factor influencing the pull-out strength and torque while pitch is another parameter deciding the screw holding capacity.

Source: Clinical Biomechanics. 2019 Dec 17;72:130-135. doi: 10.1016/j.clinbiomech.2019.12.004