The design of commercially available fixation plates and the materials used for their fabrication lead to the plates being stiffer than bone. Consequently, commercial plates are prone to induce bone stress shielding. In this study, three-dimensional fixation plates are designed using topology optimisation aiming to reduce the risk of bone stress shielding. Fixation plate designs were optimised by minimising the strain energy for three levels of volume reduction (i.e. 25%, 45% and 75%). To evaluate stress shielding, changes in bone stress due to the different fixation plate designs were determined on the fracture plane of an idealised shaft of a long bone under a four-point bending load considering the effect of a patient walking with crutches of a transverse fractured tibia. Topology optimisation is a viable approach to design less stiff plates with adequate mechanical strength considering high volume reductions, which consequently increased the stress transferred to the bone fracture plane minimising bone stress shielding.

The inverse dynamics technique applied to musculoskeletal models, and supported by optimisation techniques, is used extensively to estimate muscle and joint reaction forces. However, the solutions of the redundant muscle force sharing problem are sensitive to the detail and modelling assumptions of the models used. This study presents four alternative biomechanical models of the upper limb with different levels of discretisation of muscles by bundles and muscle paths, and their consequences on the estimation of the muscle and joint reaction forces. The muscle force sharing problem is solved for the motions of abduction and anterior flexion, acquired using video imaging, through the minimisation of an objective function describing muscle metabolic energy consumption. While looking for the optimal solution, not only the equations of motion are satisfied but also the stability of the glenohumeral and scapulothoracic joints is preserved. The results show that a lower level of muscle discretisation provides worse estimations regarding the muscle forces. Moreover, the poor discretisation of muscles relevant to the joint in analysis limits the applicability of the biomechanical model. In this study, the biomechanical model of the upper limb describing the infraspinatus by a single bundle could not solve the complete motion of anterior flexion. Despite the small differences in the magnitude of the forces predicted by the biomechanical models with more complex muscular systems, in general, there are no significant variations in the muscular activity of equivalent muscles.

Although the upper limb is needed nearly for all commonly performed activities it is still one of the lesser studied parts of the human musculoskeletal system. In this work three musculoskeletal models of the upper limb, based on multibody formulations, are presented. The aim of this work is to compare the performance and applicability of three biomechanical models with different levels of complexity. The models are based on data published by Garner and Pandy [1,2] and by the Delft Shoulder group [3,4]. The simpler model (Model 1) is defined by 4 rigid bodies – thorax, humerus, ulna and radius – and three anatomical articulations – glenohumeral (GH), humeroulnar (HU) and radioulnar (RU). The remaining two are more complex and include 7 rigid bodies – thorax, rib cage, clavicle, scapula, humerus, ulna and radius – constrained by the sternoclavicular, acromioclavicular, scapulothoracic, GH, HU and RU articulations. The muscular system supporting the skeletal system is different for each of the biomechanical models: Model 1 is defined by 15 muscles modeled by 24 bundles and since the thorax, clavicle and scapula are considered as one body, all muscles between these are neglected; Model 2 is an extension of Model 1. The inclusion of the shoulder girdle leads to a total of 21 muscles modeled by 37 bundles; Model 3 uses the muscle data set published by the Delft Shoulder group. All data was scaled to our skeletal system making a total of 20 muscles modeled by 127 bundles. The muscle contraction dynamics is simulated by the Hill-type muscle model. Being the activation of each muscle unknown the whole problem of force sharing is redundant. This indeterminacy is overcome by an optimization technique applied through the minimization of an objective function related with muscle metabolic energy consumption. In models 2 and 3, while looking for the optimal solution, not only the equations of motion must be satisfied but also the stability of the glenohumeral and scapulothoracic joints must be assured. The input for the model analysis comprises the data for an abduction motion, kinematically consistent with the biomechanical models developed, acquired using video imaging at the Laboratory of Biomechanics of Lisbon. Taking into account that Model 1 is only applicable in a small range of motion all three models gave results consistent with the literature

Although the primary stability of joint implants is fundamental for successful osseointegration, little is know about this issue in the context of stemless shoulder implants. Considering 3D finite element models, the purpose of this study was to evaluate the primary stability of five stemless designs, based on the Sidus, SMR, Simpliciti, Eclipse, and Global Icon stemless systems. Three alternative bone quality conditions were considered for cancellous bone. For the Sidus, SMR, and Simpliciti designs, which do not possess a collar that sits on the cortical rim of the humeral resected surface, contact and no contact conditions were considered between the bone surface and the humeral head components. Micromotions at bone-implant interfaces promoting osseointegration were computed as a measure of primary stability for eight load cases consisting of peak in vivo joint loads measured during selected upper limb activities. Under good bone quality conditions, all stemless designs presented micromotions below 150 μm. The Eclipse-based and Global-Icon based designs were the least sensitive to bone quality. Stemless designs presenting a solid collar or contact between the humeral head component and bone provided more stability. Overall, the Eclipse-based and Global Icon-based designs presented the best performance from the primary stability point of view. However, if bone adaptation data available in the literature are considered along with the primary stability data computed here, the Global Icon-based design, as well as other designs, might be considered superior long-term options due to their better compromise between primary stability and impact on bone adaptation.