Screw insertion torque as parameter to judge the fixation. Assessment of torque and pull-out strength in different bone densities and screw-pitches

Background: Pull-out strength is a critical parameter to judge screw fixation in orthopaedic implants. However, the insertion torque is the main feeling in the hand of a surgeon relating to the strength of synthesis. The correlation between pull-out strength and torque is not completely understood. This creates uncertainty about the key-question: Should the torque be considered a valid parameter to judge the quality of fixation? Methods: Using the ASTM F543 as reference, 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). The correlation was investigated by assessing the role of density and screw geometry. Findings: Torque was related to pull-out strength in all configurations (R = 0.979, P = 0.000). No difference in pull-out strength was found when screws were tightened to a range of 71.6%, SD = 7.6, of torque to fail (P > 0.05). Torque and pull-out strength were stratified according to density that influenced the two parameters up to 524% (P < 0.000). Pitch determined pull-out strength up to 33% (P < 0.000) while the 2.1 mm screw pitch showed the highest pull-out strength and torque in all configurations. Interpretation: Insertion torque was demonstrated to be a valid parameter to judge the quality of bone under fixation and therefore, the strength of the synthesis. Surgeons should not tighten the screws to values approaching torque to fail to obtain the highest pull-out strength. Density was the main factor influencing pull-out strength and torque. Pitch is another parameter deciding screw holding capacity.