Can the "turn-of-the-Nut" method improve cortical screw fixation?

OBJECTIVES:: To determine the angular rotation that maximizes screw tension without sacrificing screw pullout force yet minimizing loss of screw purchase. METHODS:: Three pairs of human humeri in each of the 3 groups (osteopenic, osteoporotic, and normal) underwent screw tension and pullout protocols. For screw tension, 3.5-mm screws were tightened into a strain gauge-instrumented plate until screw stripping occurred. Insertion torque, screw tension, and screw rotation were measured. For pullout, 3.5-mm screws were inserted until the head contacted the plate, additionally rotated (90, 180, 270, or 360 degrees), and then pulled out. A generalized linear and latent mixed model was used to check for significant associations (P < 0.05). RESULTS:: Mean (95% confidence interval) peak screw tension occurred at 286 degrees (95% confidence interval, 261-311 degrees) beyond screw seating. Screw tension significantly increased at 90-135 degrees but not after 180 degrees. At 270 degrees, 39% of the screws had already reached their peak ability to compress the plate. Peak screw torque lagged behind peak screw tension by 31 ± 50 degrees, and in seeking peak screw torque, a loss of 104 ± 115 N in screw tension resulted. Screw pullout force was greatest at 90 degrees, but it was not significantly different from that of the other angle groups. CONCLUSIONS:: Screw rotation at 180 degrees provides screw tension and pullout strength statistically similar to those at greater rotations but without the loss of purchase associated with greater rotations.