OTA 2005 Posters

Scientific Poster #64 Basic Science

Strain Distribution in Bone for Conventional and Locked Plate Constructs

Amer J. Mirza, MD; Larry Ehmke, MS; James Krieg, MD;

Steven Madey, MD; Michael Bottlang PhD (all authors a-Synthes USA);

Legacy Emanual Biomechanics Laboratory, Portland, Oregon, USA

Purpose: Conventional and locked plating techniques employ different load transfer mechanisms for fracture fixation. This study investigated load transfer by assessing the bone strain distribution along plate constructs in an osteoporotic gap osteotomy model. We hypothesized that locked plate constructs induce a significantly different distribution of bone strains compared to a conventional plate construct.

Methods: Nine surrogate foam specimens were custom manufactured to simulate osteoporotic femoral diaphyseal bone. Each specimen was fixed with a 4.5-mm narrow 10-hole locking plate with screws in holes 1, 3, and 5 on one side of a 1-cm fracture gap (hole 1 being closest to the fracture gap). Conventional nonlocking 4.5-mm cortical screws (NL) were used in the first group of three specimens. In the second group, locking unicortical 5-mm screws (LU) were used. In the third group, 5.0-mm locking bicortical screws (LB) were used. Five strain gauges were mounted on the foam specimens between screw holes and adjacent to the plate, with the fifth strain gage being applied after screw hole 5. The foam specimens were then loaded in compression, four-point bending, and torsion. Strain distribution along the foam specimens was measured for each screw configuration at 100 N for compression and at 1 Nm for bending and torsion. The location and amount of maximum strain was determined in each configuration for comparison.

Results: In compression, all constructs exhibited the greatest bone strain at the fifth strain gage. Strain continuously decreased toward the fracture gap, with the lowest strain present at strain gauge 1. LU constructs exhibited significantly higher maximum strain as compared to both NL and LB constructs. In four-point bending, the greatest bone strain was consistently present at the screwhole furthest from the fracture gap. LB constructs exhibited significantly higher maximum strain than both NL and LU constructs. In torsion, LB and NL constructs had significantly lower maximum strain in comparison to LU constructs. Maximum strain for LU constructs was observed between screwholes 4 and 5.

Conclusion/Significance: In a fracture gap model for bridge plating, we found that unicortical locked screws can induce higher bone strain than locked or conventional bicortical screws under specific loading scenarios. Such elevated strain peaks are indicative of stress risers, which can contribute to stress fractures in osteoporotic bone.