The Mechanic Responses Of Reduced Supracondylar Humeral Fracture With Residual Displacements—a Finite Element Study

ObjectiveThe purpose of this study was to investigate the alterations in stress value at the fracture site and the Baumann angle of affected elbow by performing external loading to finite element models of reduced pediatric supracondylar humeral fracture with various degrees of residual displacements, so as to provide biomechanical evidences in optimizing current principle of manipulative reduction.MethodsA fresh cadaveric humerus of a seven-year-old was scanned and the original CT images of the fracture site were imported into Mimics and further processed. Following3-D construction, meshing and material attribute assignment, the model was then imported into Solidworks to simulate different types of residual displacements and record corresponding coordinate values after moving the distal fragment. By duplicating these coordinate values, Abaqus was used to assemble models of reduced supracondylar humeral fracture with residual displacements and apply loading tests on them. SPSS was used to collect and analysis data of changes in stress and Baumann angle.ResultsTests of between-subjects effects for stress in both posterior and anterior margin of the fracture site indicated that external load reduced the variation explained by both of two main effects (displace direction and contact area). After removing the effects of external load, the outcomes showed that the significance value for the two terms and their interaction effect are less than0.05for stress in both posterior and anterior margin of the fracture site. Post hoc tests for stress in the posterior margin of the fracture site demonstrated that posteromedial displacement of the distal fragment had the greatest effects on stress value while anteromedial displacement had the least. Fracture with75%contact area at the fracture site significantly increased stress value in the posterior margin when compared to the other two groups, simple effect test indicated posteromedial displacement of the distal fragment enhanced the effect of different contact area on the stress in the posterior margin of the fracture site. Analysis of stress in the anterior margin of the fracture site had opposite results. Anteromedial displacement of the distal fragment had greatest impact on the stress value thought significant difference only emerged between anteromedial and posteromedial groups, fracture with85%contact area had a greater influence on stress value than the other two groups. Simple effect test showed that both anteromedial and medial displacement enhance the effect of contact area on stress value. Baumann angle of the group with medial displacement and75%contact area was distinctly larger than81°after loading. In other groups, fracture with medial and anteromedial displacement also generated greater than81°Baumann angle under200N loading or higher when there was85%contact area at the fracture site.ConclusionContact area of the fracture site had a greater effect on stress value than displace direction of the distal fragment. The main influence of displace direction on stress was the distribution of stress concentration. Posterior displacement increased stress value in the posterior margin and anterior displacement increased that in the anterior margin. There was a consistent trend that stress increased as contract area reduced in regardless of displace direction. This suggests that it is essential to fully restore anatomic relationship of the fracture site both posteriorly and medially when reducing extension type of supracondylar humeral fracture with ulnar deviation. Meanwhile, this study provides evidences supporting the viewpoint that medial inclination has the most impact on Baumann angle. General standard of reduction in Orthopedics of Chinese medicine may not completely apply to supracondylar humeral fracture. In terms of manipulative treatment, it is preferable to persuade anterior and radial displacement of the distal fragment rather than posterior and ulnar displacement. The whole procedure should be carried out precisely and specifically.