Tomáš Návrat

Background and objective: The use of bone allograft reconstructions after tumor resection can introduce significant complications. Stable fixation is required to decrease the incidence of mechanical complications of segmental bone allografts. The purpose of the present study is to compare plating fixation methods of diaphyseal allografts after intercalary resection of the femur. Methods: We created four defined fixation models using plates and/or intramedullary polymethylmethacrylate (PMMA) to simulate typical bone tumor resection with intercalary allograft reconstruction. One angularly stable plate (DFP) with 13 locking screws and fresh frozen allografts (labeled “I”) were used for bone reconstruction. Three modified reconstructions were created: “II” included a supplementary plate (SP) with four locking screws, “III” was augmented with intramedullary PMMA in the allograft, and “IV” combined intramedullary PMMA and both plates. We applied a load model that simulates partial weight bearing on the lower limb to simulate the load during postoperative rehabilitation. Results: The highest stress in the DFP occurred at the allograft-bone transition, with variant IV reaching 297 MPa. PMMA augmentation reduced median interfragmentary motion (IFM) and sliding distances, with variant III achieving the lowest distal sliding distance (0.9 μm) in the distal area. Supplementary plate fixation reduced maximal and median proximal IFM distances (86.9 μm in variant II vs. 116.0 μm in variant I) but increased sliding distances (23.7 μm in variant II vs. 0.6 μm in variant I). Conclusions: PMMA augmentation reduces IFM and sliding distances, enhancing rigidity, particularly in the distal area. Supplementary plate fixation decreases IFM distances in the proximal area but increases sliding distances in the same region. Variants III and IV demonstrate lower IFM and sliding distances in the distal area overall. Variant III shows very low sliding distances in both distal and proximal areas. Variant IV combines improved firmness with slightly higher stress levels.

Background and Objectives: Curettage is the removal of a tumor from the bone while preserving the surrounding healthy cortical bone, and is associated with higher rates of local recurrence. To lower these rates, curettage should be combined with local adjuvants, although their use is associated with damage to nearby healthy bone. Objective: The purpose of this analysis is to determine the effect of local adjuvants on cortical porcine bone by using micro-computed tomography (micro-CT) along with histological and mechanical examination. Methods: Local adjuvants were applied to porcine specimens under defined conditions. To assess changes in bone mineral density (BMD), a micro-CT scan was used. The pixel gray values of the volume of interest (VOI) were evaluated per specimen and converted to BMD values. The Vickers hardness test was employed to assess bone hardness (HV). The depth of necrosis was measured histologically using hematoxylin and eosin-stained tissue sections. Results: A noticeable change in BMD was observed on the argon beam coagulation (ABC) sample. Comparable hardness values were measured on samples following electrocautery and ABC, and lowering of bone hardness was obtained in the case of liquid nitrogen. Extensive induced depth of necrosis was registered in the specimen treated with liquid nitrogen. Conclusion: This study determined the effect of local adjuvants on cortical bone by using micro-CT along with histological and mechanical examination. Phenolization and liquid nitrogen application caused a decrease in bone hardness. The bone density was affected in the range of single-digit percentage values. Liquid nitrogen induced extensive depth of necrosis with a wide variance of values.

This paper focuses on the analysis of the plasticity effect in the measurement of the residual stress by the hole-drilling method. Relaxed strains were evaluated by the computational simulation of the hole-drilling experiment using the finite element method. Errors induced by the yielding were estimated for uniaxial tension, plane shear stress state and equi-biaxial stress state at various magnitudes of residual stress uniformly distributed along the depth. The correction of the plasticity effect in the evaluation of residual stress was realized according to the method proposed by authors from the University in Pisa, which was coded in MATLAB. Results obtained from the MATLAB script were compared to the original input data of the hole-drilling simulation and discussed. The analyses suggested that the plasticity effect is negligible at the ratio of applied equivalent stress to yield stress, being 0.6, and that the correction of the plasticity effect is very successful at the previous ratio, being 0.9. Failing to comply with the condition of the strain gauge rosette orientation according to the principal stresses directions causes an increase in the relative error of corrected stresses only for the case of uniaxial tension. It affects the relative error negligibly for the plane shear and equi-biaxial stress states.

The ductile failure predictions have been an issue in many engineering applications. It begins with a design of machines and tools, continues with an evaluation of manufacturing processes, and last but not least ends with the assessment of various structures. The paper deals with a predictability of used criteria for a random structure of aluminum alloy 2024-T351, which was performed under the conditions of room temperature three-point bending. The bi-failure mode creates a space for the numerical studies of various approaches and gives an insight into the model performance. The plasticity was described by Lode-dependent yield criterion, which was coupled with several pressure and Lode-dependent fracture models to form a continuum damage mechanics approach via the material weakening. It was incorporated through a nonlinear damage accumulation, which was finally implemented using Fortran 77 subroutine into abaqus/explicit. All the models exhibited a good ability of crack onset prediction in terms of the force responses and realistic predictability of the crack propagation. The field of deformations was successfully compared with experimental data obtained by an optical method.