J. Ojeda

Finite element analysis of the human mastication cycle

The aim of this paper is to propose a biomechanical model that could serve as a tool to overcome some difficulties encountered in experimental studies of the mandible. One of these difficulties is the inaccessibility of the temporomandibular joint (TMJ) and the lateral pterygoid muscle. The focus of this model is to study the stresses in the joint and the influence of the lateral pterygoid muscle on the mandible movement. A finite element model of the mandible, including the TMJ, was built to simulate the process of unilateral mastication. Different activation patterns of the left and right pterygoid muscles were tried. The maximum stresses in the articular disc and in the whole mandible during a complete mastication cycle were reached during the instant of centric occlusion. The simulations show a great influence of the coordination of the right and left lateral pterygoid muscles on the movement of the jaw during mastication. An asynchronous activation of the lateral pterygoid muscles is needed to achieve a normal movement of the jaw during mastication.

Reliability and Validity Study of the Chamorro Assisted Gait Scale for People with Sprained Ankles, Walking with Forearm Crutches.

Objective The aim of this study was to design and validate a functional assessment scale for assisted gait with forearm crutches (Chamorro Assisted Gait Scale—CHAGS) and to assess its reliability in people with sprained ankles. Design Thirty subjects who suffered from sprained ankle (anterior talofibular ligament first and second degree) were included in the study. A modified Delphi technique was used to obtain the content validity. The selected items were: pelvic and scapular girdle dissociation(1), deviation of Center of Gravity(2), crutch inclination(3), steps rhythm(4), symmetry of step length(5), cross support(6), simultaneous support of foot and crutch(7), forearm off(8), facing forward(9) and fluency(10). Two raters twice visualized the gait of the sample subjects which were recorded. The criterion-related validity was determined by correlation between CHAGS and Coding of eight criteria of qualitative gait analysis (Viel Coding). Internal consistency and inter and intra-rater reliability were also tested. Results CHAGS obtained a high and negative correlation with Viel Coding. We obtained a good internal consistency and the intra-class correlation coefficients oscillated between 0.97 and 0.99, while the minimal detectable changes were acceptable. Conclusion CHAGS scale is a valid and reliable tool for assessing assisted gait with crutches in people with sprained ankles to perform partial relief of lower limbs.

On the Use of Bone Remodelling Models to Estimate the Density Distribution of Bones. Uniqueness of the Solution.

Bone remodelling models are widely used in a phenomenological manner to estimate numerically the distribution of apparent density in bones from the loads they are daily subjected to. These simulations start from an arbitrary initial distribution, usually homogeneous, and the density changes locally until a bone remodelling equilibrium is achieved. The bone response to mechanical stimulus is traditionally formulated with a mathematical relation that considers the existence of a range of stimulus, called dead or lazy zone, for which no net bone mass change occurs. Implementing a relation like that leads to different solutions depending on the starting density. The non-uniqueness of the solution has been shown in this paper using two different bone remodelling models: one isotropic and another anisotropic. It has also been shown that the problem of non-uniqueness is only mitigated by removing the dead zone, but it is not completely solved unless the bone formation and bone resorption rates are limited to certain maximum values.

Analysis of spatial and temporal step parameters during assisted gait as a dual-task: a pilot study

The main objective was to analyze the changes in the spatial and temporal step parameters during a dual-task: walking with a forearm crutch to partially unload the body weight of the subject. The secondary objective was to determine the influence of the use of the crutch with the dominant or nondominant hand in the essential gait parameters. Seven healthy subjects performed gait without crutches (GWC) and unilateral assisted gait (UAG) with the crutch carried out by dominant hand (dominant crutch (DC)) and nondominant hand (nondominant crutch (NDC)). Gait was recorded using a Vicon System; the GCH System 2.0 and the GCH Control Software 1.0 controlled the loads. The variables were step length, step period, velocity, step width, and step angle. The Wilcoxon signed-rank test compared GWC and UAG while also analyzing the parameters measured for both legs with DC and NDC in general and in each subject. Wilcoxon test only found significant differences in 1 of the 15 general comparisons between both legs. In the analysis by subject, step length, step period, and velocity showed significant differences between GWC and UAG. These parameters obtained less differences in DC. The effect of a forearm crutch on UAG caused a reduction in step length and velocity, and an increase in step period. However, it did not entail changes in step angle and step width. UAG was more effective when the DC carried the crutch. The unloading of 10% body weight produced an assisted gait which closely matched GWC.

A New Approach to Estimate a Subject-Specific Set of Muscle Parameters

Two main difficulties are presented when a study of muscle forces is carried out. First, the complexity of modeling muscle behavior. Second, the impossibility of measuring all the needed parameters in order to tune the model for a specific subject. Since the first problem has been widely studied the aim of this work is to propose a new approach to estimate muscle parameters in a non-invasive way and with no additional measure. Thus, a general methodology to estimate parameters valid for any task is proposed. A particular task is performed in order to compare different set of parameters and to validate the new approach. Results show a good agreement between the forces got with the new set of parameters and the EMG records.

Influence of the Temporomandibular Joint in the Estimation of Bone Density in the Mandible through a Bone Remodelling Model

The temporomandibular joint (TMJ) plays a key role in the distribution of stresses in the mandible during mastication and consequently in the distribution of bone density, due to the interconnection between both variables through bone remodelling. Two finite element models of the mandible were compared to study the influence of the redistribution of stresses produced by the joint: a model without TMJ, but with simplified boundary conditions to replace the joint, as done in previous models; a more realistic model including the articular disc and some ligaments present in the TMJ. The stresses and strains in both models were compared through the strain energy density, used in many bone remodelling models as a measure of the mechanical stimulus. An anisotropic bone remodelling model was used to simulate the behaviour of mandible bone and to estimate its density distribution. The results showed that the TMJ strongly affects the stress distribution, the mechanical stimulus, and eventually the bone density, and not only locally in the condyle, but also in the whole mandible. It is concluded that it is utterly important to include a detailed model of the TMJ to estimate more correctly the stresses in the mandible during mastication and, from them, the bone density and anisotropy distribution.

Influence of the load exerted over a forearm crutch in spatiotemporal step parameters during assisted gait: pilot study

BackgroundAssisted gait with forearm crutches is frequently performed during the recovery of musculoskeletal injuries of the lower limb. The amount of body weight applied to the crutch or crutches depends on the pathology and the treatment phase. The transition from assisted gait with two crutches to a single crutch is usually recommended when the subject is able to load the 50% of the body weight upon the affected member. An altered assisted gait will cause biomechanic alterations and, therefore, longer treatments and relapses. The aim of this study was to analyze the influence of 10, 25 and 50% of body weight applied to a forearm crutch during a unilateral assisted gait in the spatial and temporal step parameters to determine the load that produces alterations in gait biomechanics and the load that does not.MethodsEleven healthy subjects performed normal gait (NG) and assisted gait with a forearm crutch, in which the applied loads were: comfortable (C), 25 and 50% of their body weight. Vicon System was employed for gait recording. GCH System 2.0 and GCH Control Software 1.0 controlled the loads. The variables were: step length, step period, velocity, step width and step angle. Friedman test compared all the gait modalities: NG and the different loads. Wilcoxon signed-rank test analyzed ipsilateral and contralateral step parameters to the crutch globally and for each subject.ResultsFriedman test showed significant differences between NG, C, 25 and 50%, especially for step period and velocity. Wilcoxon test had significant differences only in 4 of the 20 general comparisons between ipsilateral and contralateral steps to the crutch. In the analysis by subjects, step length, step period and velocity showed 79/132, 110/132 and 58/66 significant differences, respectively.ConclusionsThe increase in the load exerted over a forearm crutch produced an increase in the step period, accompanied by a reduction of step length and gait velocity. Step width and step angle were not modified. The unloading of 25 and 50% of body weight on a single crutch is incorrect from the biomechanical point of view. Two crutches should be employed when the body weight to unload exceeds 10%.

STUDY, OPTIMIZATION OF PARAMETERS AND VALIDATION OF THE BEHAVIOR OF A WHOLE HUMAN FEMUR DURING NORMAL GAIT

Many studies have been carried out to analyze the behaviour of the femur under walking loads and quite good estimations of the density distribution have been obtained by application of a bone remodelling algorithm [Bisatkos, 2005] (among others). However, obvious difficulties have forced these models to make important assumptions about geometry and loads. Regarding geometry, a standard model is usually employed to overcome this difficulty [Sommer, 1982]. Secondly, the loads applied to the model are usually taken from the literature [Brand, 1986]. Thirdly, the kind of analysis. Because of the complexity of bone remodelling models, static analysis of gait have been usually done to estimate local stresses in the femur, needed for the bone remodelling algorithm. Finally, the validation of results was only qualitative. In this work all these limitations have been overcome. As far as the authors know, this is the first study to analyze the mechanical behaviour of a human femur considering the gait dynamics and taking all the data from the same subject (geometry, load data, bone density obtained through CT scanning). The goal of the work has been to adjust the parameters of the bone remodelling model to this specific femur.

Cost Function in Muscle Redundancy Problems: Computational Aspects

Several methods have been proposed to solve the “redundancy problem in biomechanics,” optimization techniques that are being most widely used. In posing the optimization problem, a variety of cost functions can be found in the literature, designed to meet various criteria, whether to minimize muscle fatigue, active or passive instant muscle strength, the level of muscle activation, or metabolic cost. The aim of this paper is to compare muscles forces generated by different cost functions. We have studied the forces produced by the muscles of the human leg during flexion, using a static optimization algorithm, accounting for the dynamics of muscle contraction.