Evaluation of the Effect of Ununion Percentage of Scaphoid on Stress and Displacement of Fragment Part of Scaphoid in Various Degrees of Wrist Extension with and without External Force.

Background: Scaphoid fractures are almost two-thirds of different kinds of carpal fractures. Surgeons usually have challenges in deciding if the wrist should be mobilized or immobilized for the patients with different percentiles of partial union of scaphoid. This study's aim was to investigate different percentiles of union (25-50-75) in waist and proximal pole of scaphoid fracture to find the union percentile which can tolerate normal daily activities to help surgeons in taking appropriate decisions in this regard. Materials and Methods: A model of wrist joint was developed in this study based on computed tomography scan images of wrist joint. Various percentage of union in waist and proximal poles of scaphoid with and without internal fixation were tested in this study. The stress applied on scaphoid and displacement of fragment parts were evaluated in Abaqus software during motion of wrist from neutral to 40 degrees of extension. Results: Although Von Mises stress of scaphoid bone increased following use of external force, the difference was significant for conditions 3 and 6 (75% of union in middle and distal parts). The stress applied on scaphoid increased depends on conditions (ununion percentage of bone). Moreover, its stress depends on the angle of wrist extension. Use of internal fixation screw decreased stress of scaphoid in most of conditions. Conclusion: The stress developed in scaphoid depends on the nonunion percentage of scaphoid and the amount of motion of wrist joint. It is better to use external fixation screw especially for conditions 3 and 6 (75% of union in middle and distal parts) to decrease the displacement of fragment parts and to decrease the stress applied on scaphoid in wrist extension. Type of Study and Level of Evidence: Case study, Level of evidence III.