Computational modelling of long bone fractures fixed with locking plates : how can the risk of non-union/implant failure be reduced?

The Locking Compression Plate (LCP) is part of a new plate generation requiring an adapted surgical technique and new thinking about commonly used concepts of internal fixation using plates. Knowledge of the fixation stability provided by locking compression plates is very limited and further research is necessary to determine how mechanical stability and risk of implant failure can best be controlled. The results of a study based on finite element analysis using locking compression plate for diaphyseal fractures are presented followed by 3 patient specific finite element analysis case studies.

Several factors were shown to influence stability in compression. Axial stiffness was mainly influenced by the working length. On omission of the two innermost screws near the fracture site, axial stiffness decreased by 39%. Construct stability was also affected by the distance from the plate to the bone. Axial stiffness of the construct decreased by increasing the boneplate distance.

Increasing the post-fixation fracture angle and the fracture translation reduced construct stability, whereas fracture gap had no effect on construct stability when no bone contact occurred during loading.

Stress analysis of the LCP demonstrated that the maximum von Mises stresses were found in the innermost screws at the screw-head junction. Screw stresses decreased significantly with the removal of the two innermost screws.

When the stresses in the plate were isolated, the maximum von Mises stresses were concentrated at the outer edges of the two outermost empty screw holes. Maximum plate stresses also decreased with the removal of the two innermost screws, a pair at a time. Reduction in stresses was more pronounced when the first pair of innermost screws were removed.

Stress concentrations were localized to a point or a specific region of the implant. The majority of stresses were below the yield stress and would nothave led to permanent deformation. Despite this, these stress concentrations can indicate where fatigue failure might occur.