Don't Believe What You See: Cross-sectional Areas and Volume Replacement of Commonly Used Screws for Scaphoid Fixation
Frances Sharpe, MD1,2, Colin P McCarty, BS3, Connor Goo, BS4, Nicolas Kim, BS4, Sang-Hyun Park, PhD4 and Edward Ebramzadeh, PhD4, (1)University of Southern California Keck School of Medicine, Los Angeles, CA, (2)Southern California Permanente Medical Group, Fontana, CA, (3)JVL Research Center, Orthopaedic Institiute for Children, UCLA, Los Angeles, CA, (4)JVL Research Center, Orthopaedic Institute for Children, UCLA, Los Angeles, CA
Purpose: This study determined the volume of bone replaced by the implant in the proximal and distal pole of simulated scaphoid fractures. It also measured the cross-sectional area of the implant relative to the cross-sectional area of the scaphoid at two different simulated fracture locations.
Methods: Micro-CT scans of seven cadaveric scaphoids were used to create 3-dimensional models in which transverse proximal pole and mid-waist fractures were simulated. The volume occupied by five commonly used implants as well as the cross-sectional area occupied at the fracture surface was measured using computer modeling software. Both antegrade and retrograde screw placement was modeled.
Results: Calculations for implant volume replacement based on planar (2-dimensional) imaging demonstrated an 11-30% surface area replacement by the implant, while the implant width occupied 12-38% of the width of the scaphoid. By comparison, in the three-dimensional model, in simulated proximal pole fractures, the implants replaced between 1.5% and 7.4% of the fracture cross-sectional area, and 1.2% to 6.4% of the proximal fragment bone volume. For mid-waist fractures, the implants replaced between 1.5% and 6.8% of the fracture cross-sectional area, and 1.8% to 4.6% of the proximal pole volume. Antegrade versus retrograde screw placement had a negligible effect on cross-sectional area replaced by the implant. Although the different implant designs replaced statistically significant different areas and volumes, these differences were small and all below 4%.
Conclusions: This study provides data as it relates to one aspect of fracture healing, specifically the surface area taken up by five different implants at proximal and mid-waist scaphoid fractures, as well as the volume of bone replaced by the implant.
Clinical Relevance: As opposed to the impression provided by 2-dimensional planar imaging, when shown in a 3-dimensional model, the volume and surface areas replaced by the implant represent a minimal percentage of scaphoid bone, suggesting a negligible clinical effect
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