Hitting the mark: probing at the initiation site allows for accurate prediction of a thin shell's buckling load.

Geometric imperfections are understood to play an essential part in the buckling of a thin shell, but how multiple defects interact to control the onset of failure remains unclear. Here, we examine the failure of real cylindrical shells by experimentally poking soda cans with a large imparted dimple. By high-speed imaging of the can's surface, the initiation of buckling from axial loading is directly observed, revealing that larger dimples tend to set the initial buckling location. However, the influence of the shell's background geometric imperfections can still occasionally dominate, causing initiation to occur far from the dimple. In this situation, probing at the dimple leads to an over-prediction of the axial capacity. Using finite-element simulations, we understand our experimental results as a competition between the large dimple and the shell's inherent defect structure. In our simulations, we empirically observe a deformation-based criterion that connects the ideal poking location to the initiation site. This article is part of the theme issue 'Probing and dynamics of shock sensitive shells'.

Application of the lambda plate on condylar fractures: Finite element evaluation of the fixation rigidity for different fracture patterns and plate placements.

PURPOSE: The treatment challenges of condylar fractures necessitated the production of several plate designs. Among the relatively new plate designs is the lambda plate, for which biomechanical and clinical data are lacking. The purpose of this study is to examine the rigidity of fixation achieved when the lambda plate is applied to different fractures of the condylar neck and base. METHODS: Five fractures of the condylar area were designed on a virtual model of a healthy mandible obtained from a CT scan. The fractures were reduced using the lambda plate. For the same fractures, alternative placements of the plate were simulated. The generated models were analysed using the finite element analysis for a 500 N bite load. The displacement of the two condylar fragments along the fracture line was calculated as an indicator of the rigidity of the fixation. RESULTS: The displacement along the fracture was less than 0.144 mm for the neck fractures and greater than 0.165 mm for the fractures of the condylar base. A more cranial placement of the plate for the neck fractures further reduced the displacement, while a more anterior placement of the plate for the base fractures resulted in displacements greater than 0.330 mm. CONCLUSION: According to our study, the lambda plate offers better rigidity when applied as cranially as possible for condylar neck fractures. The lambda plate did not provide adequate fixation for base fractures. A second plate at the sigmoid notch should be considered to achieve better stabilization along the fracture if the lambda plate is eventually used.

Finite element analysis of different titanium miniplates: Evaluation of three-dimensional designs applied on condylar neck fractures.

INTRODUCTION: The fixation of condylar neck fractures is raising difficulties and for this reason, many plate designs have been developed. The current study compares the performance of four miniplates used in the condylar neck. MATERIAL AND METHODS: A virtual condylar neck fracture in a mandible obtained from a CT scan was fixed with four miniplates (two straight miniplates, lambda, strut, and trapezoidal). Using finite element analysis, we examined the fragments' displacements and stress distribution in the titanium material and bone. The models were analyzed under two loading conditions: a reduced bite force of 135 N and a force of 500 N. RESULTS: No risk of material failure was observed. For a load of 135 N, all four plates offer an adequate fixation. For 500 N of applied loading, the lambda and strut plates showed the best rigidity and lowest bone strains. The two parallel plates showed the lowest rigidity and the trapezoidal plate the highest bone strains around all screws. DISCUSSION: These findings imply that three-dimensional miniplates (lambda, strut) perform better when higher loads are applied. On the other hand, the trapezoidal plate has an increased risk of screw loosening and the two straight plates higher mobility of the fragments.

Finite Element Analysis of Different Titanium Plates for Internal Fixation of Fractures of the Mandibular Condylar Neck.

PURPOSE: The purpose of the present study is to compare the performance of 4 titanium miniplates (alpha, kappa, rhomboidal, and trapezoidal) used for the fixation of condylar neck fractures by implementing computational finite element analysis. METHODS: Three-dimensional models of the plates were used to reduce a virtually created condylar neck fracture in a mandible obtained from a computed tomography scan of a healthy adult. The developed models were analyzed, making use of the finite element method under 2 loading scenarios: a reduced postoperative bite force of 135 N and a clenching force of 500 N were examined. The plating designs' performance was assessed based on displacements along the fracture area, bone strains, and plate stresses. RESULTS: For a loading limited to 135 N, all 4 plates offer an adequate fixation with a small risk of screw loosening for the rhomboidal and trapezoidal plates. For an applied force of 500 N, the alpha and kappa plates showed better results, distributing more homogeneously the strains in the bone and offering better rigidity. CONCLUSIONS: These findings implicate that the alpha and kappa plates performed better when bigger loads are applied. On the other hand, the trapezoidal and rhomboidal plates are not recommended for condylar fractures, especially if bigger functional loads are expected.