C. Relvas

Straight, semi-anatomic and anatomic TMJ implants: The influence of condylar geometry and bone fixation screws

A 3D finite element model of in vitro intact and implanted mandibles with different temporomandibular joints (TMJ) was analyzed. Three TMJ implant geometries were assessed. The displacements, stress and strain fields on the condyle were obtained for both simulated cases. Strains were also assessed near the screws that fixate the implant to the mandible. The geometry of the mandible was obtained through 3D digitalization of a synthetic model. The TMJ implants studied were modelled considering a commercial implant which was also used to create semi-anatomic and anatomic implants that were analyzed and to assess the influence of the geometry. Numerical finite element models were built and the implants were positioned by an experienced orofacial surgeon. All implants were fixed by four screws which were placed in the same position on the mandible. The boundary conditions were simulated considering the support on the incisive tooth, the loads of the five most important muscular forces and a 5mm mouth aperture. This study indicates that the deformation on the intact mandible was similar when an anatomic implant was considered in the implanted mandible. However, the anatomic geometry presented some problems concerning the implant integrity due to geometric variations. The geometry of TMJ implant also played a role relatively to the screws structural integration and bone fixation. The geometry of TMJ implant defines the necessary number of screws and position in the mandible fixation.

The influence of finishing milling strategies on texture, roughness and dimensional deviations on the machining of complex surfaces

Abstract The objective of the study hereby presented consisted on the analysis of different finishing milling strategies of a complex geometry part containing concave and convex surfaces. The machining quality was assessed through the comparison of surface roughness, surface texture and dimensional control parameters. Three typical milling strategies were studied: radial, raster and 3D offset. The tool-cutting parameters were maintained constant for all strategies tested. The 3D offset machining strategy evidenced to be the suitable one for the geometry model used within the study.

Biomechanical evaluation of proximal tibia behaviour with the use of femoral stems in revision TKA: An in vitro and finite element analysis

BACKGROUND Recognized failure mechanisms after revision total knee arthroplasty include failure of fixation, instability and loosening. Thus, extended stems have been used to improve fixation and stability. In clinical cases where the stem is only applied in the femur, a question concerning the structural aspect of tibia may arise: Does a stemmed femur changes the structural behaviour of proximal tibia? It seems, that question has not yet been fully answered and the use of stems in the opposite bone structure requires further analysis. METHODS Proximal cortex strains were measured with tri-axial strain gauges in synthetic tibias for three different types of implanted femurs, with two constrained implants. To assess the strains at the cancellous bone under the tibial tray, it was considered a closest physiological load condition with the use of finite element models. FINDINGS No significant differences of the mean of the tibial cortex strains for the stemmed femur relatively to the stemless femur were observed. The R(2) and slopes values of the linear regressions between experimental and finite element strains were close to one indicating good correlations. The strain behaviour of cancellous bone under the tibial tray is not completely immune to the use of femoral stem extensions. However, the level of this alteration is relatively small when compared with the strain magnitudes. INTERPRETATION The main insight given by the present study could probably lie in the fact that the use of femoral stems does not contribute to an increase of the risk of failure of the tibia.

Vacuum casting with room temperature vulcanising rubber and aluminium moulds for rapid manufacturing of quality parts: a comparative study

The purpose of this study was to compare costs and dimensional control parameters between same geometry parts obtained by same prototyping techniques using different material moulds. Vacuum casting was used to obtain prototypes of a three‐blade helix of a motor boat propeller. Aluminium and room temperature vulcanising (RTV) rubber moulds were manufactured and tested within the study. Manufacturing costs and dimensional control parameters were used within the comparison study. For the dimensional deviation performance factor used, no significant differences were observed relatively to the dimensional control parameters measured. However, the average deviations were smaller for the RTV rubber mould prototype.

Biomechanical evaluation of different reconstructive techniques of proximal tibia in revision total knee arthroplasty: An in-vitro and finite element analysis

BACKGROUND Bone loss and subsequent defects are often encountered in revision total knee arthroplasty. In particular, when the cortical rim of proximal tibia is breached, the surgical decision on the reconstructive options to be taken is challenging due to the variety of defects and the lack of data from clinical or experimental studies that can support it. The purpose of this study is to assess how different reconstructive techniques, when applied to an identical defect and bone condition, can be associated to dissimilar longevity of the revision procedure, and the role of a stem in this longevity. METHODS Proximal cortex strains and implant stability were measured in ten reconstructive techniques replicated with synthetic tibiae. The cancellous bone strains under each construct were assessed with finite element models which were validated against experimental strains. FINDINGS The measured strains and stability showed that the proximal cortex is not immune to the different reconstructive techniques when applied to an identical defect. The largest cancellous strain differences between modular and non-modular techniques indicate a distinct risk between reconstructive techniques, associated to the supporting capacity of cancellous bone at long term. INTERPRETATION The main finding of the present study is the observation that modular augments increases, on a long term basis, the potential risk of bone resorption relative to the non-modular techniques. In addition, the use of a press-fit stem in the scope of non-modular techniques can lead to improved stability and load transfer, which can contribute positively to the life expectancy of these techniques.

A new press-fit stem concept to reduce the risk of end-of-stem pain at revision TKA: A pre-clinical study

PURPOSE Revision total knee arthroplasty presents numerous technical challenges, with lower patient outcomes compared with those obtained in primary surgery. Extended stems have been used in revision total knee arthroplasty to improve component alignment and fixation. Hybrid fixation with cemented tibial tray and press-fit stem has shown good results. One of the disadvantages of this technique is pain related to the presence of a cementless diaphyseal engaging stem, often designated as end-of-stem pain. Patients with this pain have reported a decrease in overall satisfaction, as well as demonstrate a lower clinical outcome score. Clinical findings suggest that stem material and design are important factors in the development of end-of-stem pain. Therefore, a question can be raised: can a novel press-fit stem concept minimize bone strain changes at the stem tip? The hypothesis here considered lies upon the fact, that if periosteal cortex strain changes are minimized at the stem tip comparatively to the intact situation, the risk of end-of-stem pain might be minimized. SCOPE This pre-clinical study was accomplished using synthetic tibiae to experimentally predict the periosteal cortex strains at the proximal and stem tip regions, with a commercial press-fit stem and a new stem concept. CONCLUSIONS The results demonstrated that the new stem concept has the ability to minimize strain changes induced by the stem tip at the distal periosteal cortex and consequently, at the periosteal layer of bone tissue, which is highly pain sensitive, probably contributing to the reduction of the risk of end-of-stem pain.

A systematic approach for an accuracy level using rapid prototyping technologies

Nowadays there has emerged a series of rapid prototyping processes with great potential, and designers and engineers need to know the accuracy performance of these processes to compare and select the best solution. There is a significant lack of published data related to rapid prototyping processes and feature accuracy. This research was conducted to minimize this gap and provide much needed accuracy in terms of dimensional and geometric information. The methodology includes the summarization of previous studies and definition of a benchmarking part that is composed of elementary shapes representative of different features most likely to be found in a final product. The benchmarking part was controlled in terms of dimensional accuracy, geometric precision and freeform deviation. The sources of errors controlled by the final user were analysed, like Standard Tessellation Language (STL) file format resolution and build direction. Four custom rapid prototyping processes have been used and compared: stereolithography, selective laser sintering, fused deposition modelling and three-dimensional printer. Computer numerically controlled machining has been used as an alternative prototyping process in this study as a standard to compare costs and accuracy. This work assessed measures that can be used to quantify the accuracy performance for a given part so that the choices for prototyping can be made based on scientific knowledge and best working practices. These results are very useful for designing products to be prototyped or manufactured through direct methods. The results can be used to improve the functionality of prototypes and the decision process through the best systematic approach.

Biomechanical analysis of total elbow replacement with unlinked iBP prosthesis: An in vitro and finite element analysis

BACKGROUND Numerous models of elbow prostheses are being used and can be divided into two categories: one being a semi-constrained, linked type; and the other being non-constrained, unlinked type. Recent reports of National Elbow Arthroplasty Registers reveal no significant differences in the survival rates between linked and unlinked prosthesis brands, and the main cause appointed for revision for both types is loosening. Some previous biomechanical studies confirm the presence of abnormal bone stresses for the linked type, which can be associated with the risk of loosening. However for the unlinked type, biomechanical studies are not available that corroborate a loosening risk. It seems, that issue has not yet been fully answered and requires further analysis. METHODS Cortex strains adjacent to the elbow joint were measured with strain gauges in synthetic humeri and ulnae, before and after replacement. To assess cancellous bone strains and cement stresses around the implant finite element models validated relative to measured strains were used. FINDINGS Bone strains adjacent to the implant tip increased several times in the humerus and ulna. At the epiphyseal regions a generalised cancellous bone strain reduction was observed for both humerus and ulna relatively to the intact bones. INTERPRETATION The unlinked elbow prostheses can be associated with the risk of bone fatigue failure by overload, particularly in the ulna, and bone resorption by stress-shielding at the epiphyseal regions. The identical structural behaviour relative to linked prostheses associated with the same loosening risks corroborates the results of recent arthroplasty published register reports.

The design of a washing machine prototype

The design process for a washing machine prototype is described. The prototype was developed within the final-year project of the Mechanical Engineering course at the University of Aveiro. Professional designers at the School of Arts and Design of Matosinhos carried out conceptual studies and the CAD modelling. To develop the appliance, different design and engineering aspects were carefully studied, namely concept studies, scaled model prototyping, structural engineering, materials selection, CAD/CAM mould making, advanced sandwich composite manufacturing structures, automation (programming and control) and testing. Within the project, pedagogic objectives were also set. In fact, these types of projects incorporate extra teaching and learning values because they involve students in more practical learning of engineering topics.

The influence of acetabular bone cracks in the press-fit hip replacement: Numerical and experimental analysis

BACKGROUND The press-fit hip acetabular prosthesis implantation can cause crack formation in the thin regions surrounding the acetabular. As a consequence the presence of cracks in this region can lead to poor fixation and fibrous tissue formation. METHODS Numerical and experimental models of commercial press-fit hip replacements were developed to compare the behavior between the intact and implanted joints. Numerical models with an artificial crack and without crack were considered. The iliac and the femur were created through 3D geometry acquisition based on composite human replicas and 3D-Finite Element models were generated. FINDINGS The mechanical behavior was assessed numerically and experimentally considering the principal strains. The comparison between Finite Element model predictions and experimental measurements revealed a maximum difference of 9%. Similar distribution of the principal strains around the acetabular cavity was obtained for the intact and implanted models. When comparing the Von Mises stresses, it is possible to observe that the intact model is the one that presents the highest stress values in the entire acetabular cavity surface. INTERPRETATION The crack in the posterior side changes significantly the principal strain distribution, suggesting bone loss after hip replacement. Relatively to micromotions, these were higher on the superior side of the acetabular cavity and can change the implant stability and bone ingrowth.