Deviations in positioning variable pitch screws-scaphoid waist fractures

Introduction: Operative therapy using a headless cannulated variable pitch compression screw is the gold standard for the treatment of instable scaphoid fractures. Hypothesis: Deviation from the central placement is associated with a loss of stability and stiffness. Material and methods: An artificial bone model was manufactured and different screw positions (central, 10 degrees and 20 degrees to the long axis) were assessed. A shearing test with axial force on the 45 degrees flexed scaphoid was applied. Results: The inserted variable pitch screw showed the highest stiffness and failure force in a position in the long axis. At 10 degrees, a slight decrease in stiffness (32.7 N/mm +/- 9.3 N/mm) and failure force (41.6N +/- 13.2 N) was observed, while a significant reduction in stiffness (29.3 N/mm +/- 4.6 N/mm) and failure force (50.3N +/- 19.5N) was measured at 20 degrees. Discussion: Deviations in the angle of insertion of the compression screw cause loss in failure force, thus deviations from the central placement is associated with less stability and stiffness. (C) 2019 Elsevier Masson SAS. All rights reserved.