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Interfragmentary Compression Variation for Differing Simulated Fracture Locations and Scaphoid Bone Screws in Polyurethane Foam
Samik Patel, MD; Nathan Tiedeken, MD; Lars Qvick, MD; John R. Fowler, MD; Richard Debski, PhD
University of Pittsburgh, Pittsburgh, PA
Introduction: Proximal pole scaphoid fractures present a clinical challenge due to their lower success rate with internal fixation. Internal fixation of scaphoid fractures with headless compression screws (HCS) provides a compressive force at the fracture site to promote healing. Differences in geometrical design between HCS lead to differences in compression that vary based on the location of the fracture. The objective of this study is to analyze compression forces based on the screw geometry utilized for fixation and fracture location.
Methods: Eighty-eight (n=4 for 11 simulated fracture locations and two screws) polyurethane blocks were cut to 28mm x 10mm x 10mm. A fracture was simulated with a transverse cut from the top edge of the block through 22mm, incrementing by 2mm for each possible fracture site along the 28mm length of the block. The fully threaded 24mm Mini Acutrak 2 (Screw A) HCS (Acumed, Hillsboro, OR) or double threaded 24mm Fixos (Screw B) HCS (Stryker, Kalamazoo, MI) were inserted according to manufacturer's protocol. The compression force at the fracture site was measured with sensors and a rubber/metal jig that provided rigid support; this resulted in a simulated fracture thickness of 2.54mm. Two K-wires were placed around the center of the screw to prevent rotation. The HCS was inserted into the polyurethane block until the screw head was flush with the polyurethane block. A paired t-test and two-way analysis of variance were performed with significance set at p<0.05.
Results: A significant effect of fracture location on compression was found. For a fracture located 2mm from the top edge of the top block, Screw B generated significantly more compression than Screw A with ~3.6 times more compression. Screw A generated significantly greater compression than Screw B with a 12mm and 18mm top block fixation.
Discussion: Screw B generated significantly greater compression for a simulated proximal pole fracture repaired through the dorsal approach. Screw A generated significantly greater compression for fractures that could be repaired at the waist of the scaphoid. Consideration of compression maybe useful in determining the optimal fixation technique for a scaphoid fracture based on the location of the fracture.
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