OTA 2006 Posters

Scientific Poster #84 Basic Science

Biomechanical Evaluation of Locking Plate Fixation in a Sawbones Radius Model
Jason W. Roberts, MD (a-Synthes); Brandon Rebholz, BS (n);
Mei Wang, PhD (n); Steven I. Grindel, MD (a-Synthes);
Medical College of Wisconsin, Department of Orthopaedic Surgery, Milwaukee, Wisconsin, USA

Purpose: Compression plating is a commonly accepted technique for treating diaphyseal forearm fractures. With the introduction of locked plating constructs, there has been interest in using them in clinical situations in which locking may be advantageous. Previously, we compared plates fixed with locked unicortical screws to compression plates fixed with unlocked bicortical screws in a cadaver radius model. While the locked devices resisted bending forces equally, they were found to be inferior in torsion. The purpose of this study was to compare the stability of plate fixation using mixed bicortical and unicortical screws in locked devices to that of unicortical locked fixation and bicortical unlocked fixation.

Methods: 16 composite radius models were equally divided into four groups to receive midshaft osteotomy and plate fixation on the volar surface with three unlocked bicortical screws on each side (3Bi), three locked unicortical screws on each side (3Uni), or with two unicortical locked screws near the fracture and one bicortical unlocked (2Uni+unlockBi) or locked (2Uni+lockBi) screw distant from the fracture on each end (LCP System, Synthes). Specimens were tested in nondestructive 4-point bending (anteroposterior [AP] and lateral directions) and torsion on a servohydraulic, bimodal materials testing system (Model 809, MTS Systems). Three cycles of preconditioning were performed and the construct stiffness was obtained from the linear portion of the load-displacement curves. The results from all groups were compared using ANOVA and post-hoc Bonferroni tests.

Results: The 2Uni+lockBi (57.6%, P = 0.0004), 2Uni+unlockBi (51.6%, P = 0.0009), and 3Bi (57.1%, P = 0.0004) groups all show significantly higher torsional stiffness than the 3Uni group. Under AP bending forces, the construct stiffness of 2Uni+lockBi was significantly higher than the 3Uni (42.9%, P <0.0001), 2Uni+unlockBi (28.3%, P =0.001), and 3Bi (20.4%, P = 0.007). The construct stiffness was comparable among all groups in lateral bending.

Conclusion/Significance: Replacing a single set of unicortical locked screws with locked or unlocked bicortical screws distant from the fracture site improves torsional stability of the construct by over 50%, giving stability equal to standard unlocked plating. Unicortical locked constructs have stability equal to unlocked plates in bending. Locked bicortical screws can even further improve strength in AP bending.

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.