OTA 2006 Posters

Scientific Poster #85 Basic Science

Biomechanical Analysis of Cross-Threaded Locking Screws
John Boudreau, MD (a-Stryker, Synthes);
Robert O'Toole, MD (a,e-Stryker, a-Synthes);
Oleg Vesnovsky, PhD (a-Stryker, Synthes);
Tim Topoleski, PhD (a-Stryker, Synthes);
Marcus Sciadini, MD (a,e-Stryker, a-Synthes);
R. Adams Crowley Shock Trauma Center, University of Maryland
School of Medicine, Baltimore, Maryland, USA

Purpose: The purpose of this study was to evaluate the biomechanical performance of locking screws in proximal tibial locking plates inserted perpendicular to the plate compared to those inserted off axis (ie, cross threaded). Our hypothesis is that screws inserted perpendicular and off axis will both provide sufficient stability for clinical use.

Materials/Methods: Large fragment proximal tibial locking plates from Stryker and Synthes were used. Stryker uses a click-lock system for the shaft screws while Synthes uses the combi-hole system. Each screw was used for only one test, and each hole of the plate was only used for one test cycle. There were two test groups for each plate: 0 off axis and 5 off axis. All constructs were tested to failure with a load applied perpendicular to the screw at 11 mm from the undersurface of the plate. A second set of identical constructs were then subjected to cyclic loading from 0 N to 72 N at 5 Hz for 90,000 cycles, or until failure The maximum load (72 N) was the statically equivalent load applied by our test setup, assuming that a single screw would sustain 60% of the patient's body weight (667 N). Plate-screw constructs that survived cyclic loading were then tested quasistatically to failure. Six specimens were tested in each group.

Results: All constructs were able to resist the minimum moment of 1.1 Nm, which we theorize is equivalent to the load each unicortical screw must sustain to support an 890-N person standing on one limb. For all groups the mean yield moment was greater than 3.5 Nm. For both plate constructs, there was little change in yield strength with off-axis insertion of the screws. While all of the perpendicular constructs survived cyclic loading, two of the Synthes off-axis constructs failed during cylic loading.

Conclusions: Although there were differences in biomechanical properties between our test groups, all plate-screw interfaces were able to sustain clinically relevant loads before failure, even when the screws were cross threaded. There was little change in biomechanical properties after cyclic loading; however, some of the off-axis constructs failed during loading. This study demonstrates that cross-threaded screws may have biomechanical properties that are adequate for some clinical applications, but should be used with caution.

If noted, the author indicates something of value received. The codes are identified as a-research or institutional support; b-miscellaneous funding; c-royalties; d-stock options; e-consultant or employee; n-no conflicts disclosed, and *disclosure not available at time of printing.
· The FDA has not cleared this drug and/or medical device for the use described in this presentation (i.e., the drug or medical device is being discussed for an "off label" use). · · FDA information not available at time of printing.