A numerical investigation into the effects of overweight and obesity on total knee arthroplasty

Overweight and obesity increase risks of knee osteoarthritis, which is a major cause of disability. Severe knee osteoarthritis can be treated by knee arthroplasty. Total knee arthroplasty has been used in overweight and obese patients; however, clinical reports showed that the outcome of this group of patients was not good as normal-weight patients. Two computer models were created in this paper to simulate the effect of excess loads on the distal femoral bone and contact pressures in total knee arthroplasty during a gait cycle. The numerical results showed increased stress in periprosthetic distal femoral bones and higher contact pressure on tibial polyethylene insert during the stance phase. Based on the computer simulation results and published research work, cementless total knee arthroplasty with thicker tibial polyethylene insert may be a better option for overweight patients

Functional Relationship between Skull Form and Feeding Mechanics in Sphenodon, and Implications for Diapsid Skull Development

The vertebrate skull evolved to protect the brain and sense organs, but with the appearance of jaws and associated forces there was a remarkable structural diversification. This suggests that the evolution of skull form may be linked to these forces, but an important area of debate is whether bone in the skull is minimised with respect to these forces, or whether skulls are mechanically “over-designed” and constrained by phylogeny and development. Mechanical analysis of diapsid reptile skulls could shed light on this longstanding debate. Compared to those of mammals, the skulls of many extant and extinct diapsids comprise an open framework of fenestrae (window-like openings) separated by bony struts (e.g., lizards, tuatara, dinosaurs and crocodiles), a cranial form thought to be strongly linked to feeding forces. We investigated this link by utilising the powerful engineering approach of multibody dynamics analysis to predict the physiological forces acting on the skull of the diapsid reptile Sphenodon. We then ran a series of structural finite element analyses to assess the correlation between bone strain and skull form. With comprehensive loading we found that the distribution of peak von Mises strains was particularly uniform throughout the skull, although specific regions were dominated by tensile strains while others were dominated by compressive strains. Our analyses suggest that the frame-like skulls of diapsid reptiles are probably optimally formed (mechanically ideal: sufficient strength with the minimal amount of bone) with respect to functional forces; they are efficient in terms of having minimal bone volume, minimal weight, and also minimal energy demands in maintenance

Finite element analysis of total knee replacement considering gait cycle load and malalignment

A thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of PhilosophyThis research has investigated the influence of gait cycle, malalignment and overweight on total knee replacements using a finite element method. Dynamic and finite element models of fixed- and mobile-bearing implants have been created and solved; the fixed- and mobile-bearing implants demonstrated different performance on movement and contact pressure distribution in the tibio-femoral contact surfaces. More contact areas were found in the mobilebearing implant than in the fixed-bearing implant, but the maximum contact pressures were almost the same in both. The thickness of the tibial bearing component influenced the fixed- and mobile-bearing implants differently. A dynamic model of an implanted knee joint has been developed using MSC/ADAMS and MSC/MARC software. Stress shielding was found in the distal femur in the implanted knee joint. The stresses and strains in the distal femur were found to increase with body weight, especially during the stance phase. Serious stress shielding and more bone loss appear in condition of overweight. The increase of bone loss rate and stress in the distal femur with increase of body weight will result in a higher risk of migration of femoral component after total knee replacement. The peg size effect has been studied using this dynamic model; a longer peg with smaller diameter was found to be the best. Varus/valgus malalignment redistributed the tibio-femoral contact force and stress/strain distribution in the distal femur. The difference between contact forces on the medial and lateral condyle decreased in the valgus malalignment condition. Contact pressure increased in the varus/valgus malalignment condition in the dynamic models of both the fixed- and mobile-bearing implant. However, the mobile-bearing implant performed better in conditions of malalignment, especially malrotation. Body weight had less influence on the maximum contact pressure in the mobile-bearing implant

Sensitivity analysis of a cemented hip stem to implant position and cement mantle thickness

Patient-specific finite element models of the implanted proximal femur can be built from pre-operative computed tomography scans and post-operative X-rays. However, estimating three-dimensional positioning from two-dimensional radiographs introduces uncertainty in the implant position. Further, accurately measuring the thin cement mantle and the degree of cement–bone interdigitation from imaging data is challenging. To quantify the effect of these uncertainties in stem position and cement thickness, a sensitivity study was performed. A design-of-experiment study was implemented, simulating both gait and stair ascent. Cement mantle stresses and bone–implant interface strains were monitored. The results show that small variations in alignment affect the implant biomechanics, especially around the most proximal and most distal ends of the stem. The results suggest that implant position is more influential than cement thickness. Rotation around the medial–lateral axis is the dominant factor in the proximal zones and stem translations are the dominant factors around the distal ti

Developing a musculoskeletal model of the primate skull: Predicting muscle activations, bite force, and joint reaction forces using multibody dynamics analysis and advanced optimisation methods

An accurate, dynamic, functional model of the skull that can be used to predict muscle forces, bite forces, and joint reaction forces would have many uses across a broad range of disciplines. One major issue however with musculoskeletal analyses is that of muscle activation pattern indeterminacy. A very large number of possible muscle force combinations will satisfy a particular functional task. This makes predicting physiological muscle recruitment patterns difficult. Here we describe in detail the process of development of a complex multibody computer model of a primate skull (Macaca fascicularis), that aims to predict muscle recruitment patterns during biting. Using optimisation criteria based on minimisation of muscle stress we predict working to balancing side muscle force ratios, peak bite forces, and joint reaction forces during unilateral biting. Validation of such models is problematic; however we have shown comparable working to balancing muscle activity and TMJ reaction ratios during biting to those observed in vivo and that peak predicted bite forces compare well to published experimental data. To our knowledge the complexity of the musculoskeletal model is greater than any previously reported for a primate. This complexity, when compared to more simple representations provides more nuanced insights into the functioning of masticatory muscles. Thus, we have shown muscle activity to vary throughout individual muscle groups, which enables them to function optimally during specific masticatory tasks. This model will be utilised in future studies into the functioning of the masticatory apparatus

The application of muscle wrapping to voxel-based finite element models of skeletal structures

Finite elements analysis (FEA) is now used routinely to interpret skeletal form in terms of function in both medical and biological applications. To produce accurate predictions from FEA models, it is essential that the loading due to muscle action is applied in a physiologically reasonable manner. However, it is common for muscle forces to be represented as simple force vectors applied at a few nodes on the model's surface. It is certainly rare for any wrapping of the muscles to be considered, and yet wrapping not only alters the directions of muscle forces but also applies an additional compressive load from the muscle belly directly to the underlying bone surface. This paper presents a method of applying muscle wrapping to high-resolution voxel-based finite element (FE) models. Such voxel-based models have a number of advantages over standard (geometry-based) FE models, but the increased resolution with which the load can be distributed over a model's surface is particularly advantageous, reflecting more closely how muscle fibre attachments are distributed. In this paper, the development, application and validation of a muscle wrapping method is illustrated using a simple cylinder. The algorithm: (1) calculates the shortest path over the surface of a bone given the points of origin and ultimate attachment of the muscle fibres; (2) fits a Non-Uniform Rational B-Spline (NURBS) curve from the shortest path and calculates its tangent, normal vectors and curvatures so that normal and tangential components of the muscle force can be calculated and applied along the fibre; and (3) automatically distributes the loads between adjacent fibres to cover the bone surface with a fully distributed muscle force, as is observed in vivo. Finally, we present a practical application of this approach to the wrapping of the temporalis muscle around the cranium of a macaque skull

Exploring inter-subject anatomic variability using a population of patient-specific femurs and a statistical shape and intensity model

This paper is motivated by the need to accurately and efficiently measure key periosteal and endosteal parameters of the femur, known to critically influence hip biomechanics following arthroplasty. The proposed approach uses statistical shape and intensity models (SSIMs) to represent the variability across a wide range of patients, in terms of femoral shape and bone density. The approach feasibility is demonstrated by using a training dataset of computer tomography scans from British subjects aged 25-106 years (75 male and 34 female). For each gender, a thousand new virtual femur geometries were generated using a subset of principal components required to capture 95% of the variance in both female and male training datasets. Significant differences were found in basic anatomic parameters between females and males: anteversion, CCD angle, femur and neck lengths, head offsets and radius, cortical thickness, densities in both Gruen and neck zones. The measured anteversion for female subjects was found to be twice as high as that for male subjects: 13 ± 6.4° vs. 6.3 ± 7.8° using the training datasets compared to 12.96 ± 6.68 vs. 5.83 ± 9.2 using the thousand virtual femurs. No significant differences were found in canal flare indexes. The proposed methodology is a valuable tool for automatically generating a large specific population of femurs, targeting specific patients, supporting implant design and femoral reconstructive surgery

Bite forces and jaw joint forces predicted by the MDA.

<p>Total forces are shown for bilateral bites, therefore the force on each side of the skull is approximately half that presented. Working refers to the force on the same side as the bite occurs, while balancing refers to the opposite side to which biting occurs. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0029804#pone-0029804-g002" target="_blank">Figure 2</a> for explanation of bite locations.</p