Failure Analysis of PHILOS Plate Construct Used for Pantalar Arthrodesis Paper II—Screws and FEM Simulations

A fractured stainless steel 3.5 mm proximal humerus internal locking system (PHILOS) plate and screws were investigated in this paper. This plate was used for ankle arthrodesis of a 68-year-old female with a right ankle deformity. Both the plate and screws were considered in this investigation. Optical and scanning electron microscopes (SEM) were used to document fracture surface characteristics, such as extensive scratching, plastic deformation, rubbed surfaces, discoloration, and pitting, along with cleavage, secondary cracking, deposits of debris, striations, and dimples. Indications of these features show that the plate failed by corrosion fatigue, however, overloading separated the screw(s) in two parts. Radiographic evidence shows that the screws failed ahead of the plate from the proximal end. Three-dimensional models of the plate and the screws: cortical, locking, and cannulated, were constructed using Solidworks and imported in ANSYS Workbench 16.2 to simulate the loading conditions and regions of stress development. Statistical analysis was conducted to understand the impact of different factors on the maximum von Mises stresses of the locking compression plate. These factors were the load, screw design pattern, coefficient of friction between the plate and screws, and cortical screw displacement. In summary, the finite element simulation of the plate validates the fractographic examination results. The following observations were made: (a) as the angle between the screws and the plates increased, the von Mises stresses increased in the cortical screws; and (b) the stress in the locking screws was lower than that of the cortical screws, which may be due to locking the screws with fixed angles onto the plate. Finally, fractographic examination of the cortical and locking screws supports the mechanism of corrosion-fatigue fracture from crack initiation sites, pits, due to the presence of inclusion bodies for this material (ASTM standards F138-03 and F139-03) documented for the plate in Paper I