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.