OTA 2005 Posters

Scientific Poster #67 Basic Science

Locked versus Unlocked Plating for Humeral Shaft Non-unions: A Biomechanical Study of Four Techniques

Jonathan C. Levy, MD (b-Plates, Screws and Sawbones provided by Synthes); Loren Latta, PhD (n); Ted Milne, PhD (n);

Steven P. Kalandiak, MD (n); James J. Hutson, MD (n);

Gregory Zych, MD (n);

Department of Orthopaedics and Rehabilitation, University of Miami School of Medicine, Miami, Florida, USA

Purpose: This study was conducted to evaluate the biomechanical performance of four different plate constructs for the treatment of humeral shaft nonunions: a single limited contact dynamic compression plate (SC) placed posterior, a double plate construct (DC) posterior and lateral, a posterior locking plate using bicortical screw (BLP) purchase, and a posterior locking plate using unicortical screw (ULP).

Methods: A midshaft humeral defect model was created using left humeri models (Generation III Sawbones), simulating an unstable humeral nonunion. Each of the four constructs was tested for varus/valgus and flexion/extension with cyclic ± 5 Nm moments and combined 100 N axial compression load. Torsional stiffness was then measured with cyclic ± 10 Nm torque and combined 100 N axial compression, and finally a ramp torque to failure. The stiffness and torsional strength measurements of each construct were measured using Selspot LEDs in a configuration capable of measuring 0.3-mm movements to monitor the gap movements in each humerus. Statistical analysis was performed using Student t-test with Bonferonni correction for multiple comparisons, to compare individual constructs to each other.

Results: The DC was significantly more rigid than all single posterior plate constructs for structural stiffness in flexion, extension, and torque, but not in varus or valgus loading. Additionally, DC was significantly stronger than all other constructs when tested to torsional failure. BLP was significantly more rigid in torque than unlocked screws, but not in any of the other loading directions. When tested to torsional failure, however, there was no statistical difference. BLP was significantly more rigid than ULP in torque; in fact, the ULP screws loosened in each specimen when torsional loads reached only 4 Nm.

Conclusions/Significance: Double plating provides significant advantages in both stiffness and torque to failure when compared to all other constructs, at the potential cost of greater soft-tissue dissection. Bicortical locked plating may provide a slight biomechanical advantage over conventional unlocked plating, but the clinical significance of this finding is unclear. Unicortical locked plating provides a very weak construct and should be avoided. This study provides biomechanical support for the use of double plating in managing humeral nonunions; unicortical plating techniques should be avoided.