Juan Fernando Ramírez

Analysis of bone demineralization due to the use of exoprosthesis by comparing Young’s Modulus of the femur in unilateral transfemoral amputees

Background: There is a relation between Hounsfield units obtained from computed tomography (CT) scans and bone density. The density of the bones can be used to establish its mechanical properties and therefore to assess the bone mechanical condition using CT images. Objectives: To identify the effect of the transfemoral amputation and the use of external lower limb prosthesis in the bone properties, by comparing Young’s modulus. Study Design: Young’s modulus comparison. Methods: Comparison of bone density between the healthy femur and the amputated bone of 20 unilateral transfemoral amputees was done by generating three histograms of the Hounsfield units at different parts of the femur. The histograms were created based on images obtained by CT and the Hounsfield units were translated to Young’s modulus to establish the comparison. Results: The results show a significant difference (p-value <0.05) between the mean value of Young’s modulus of healthy and amputated bone. Conclusions: There is clearly a direct association between the use of external prosthesis and the bone demineralization due the stress shielding phenomenon. The Young’s modulus comparison using information from CT images can be a suitable tool to analyze the bone demineralization due to the use of exoprosthesis.

Long term three dimensional tracking of orthodontic patients using registered cone beam CT and photogrammetry

The measurements from registered images obtained from Cone Beam Computed Tomography (CBCT) and a photogrammetric sensor are used to track three-dimensional shape variations of orthodontic patients before and after their treatments. The methodology consists of five main steps: (1) the patients bone and skin shapes are measured in 3D using the fusion of images from a CBCT and a photogrammetric sensor. (2) The bone shape is extracted from the CBCT data using a standard marching cube algorithm. (3) The bone and skin shape measurements are registered using titanium targets located on the head of the patient. (4) Using a manual segmentation technique the head and lower jaw geometry are extracted separately to deal with jaw motion at the different record visits. (5) Using natural features of the upper head the two datasets are then registered with each other and then compared to evaluate bone, teeth, and skin displacements before and after treatments. This procedure is now used at the University of Alberta orthodontic clinic.

Experimental assessment of friction coefficient between polypropylene and human skin using instrumented sclerometer

Prosthetic devices are used to restore as much as possible not only the functionality, but also the self-esteem of patients who have been submitted to amputation surgeries. Typically, lower limb prostheses need a socket to act as a link with the human stump, so the contact stresses at the socket–stump interface are critical for the recovery process and the subsequent comfort perception of the patient as well. In this work, a broad experimentation to establish the coefficient of friction (COF) between socket material (polypropylene) and human skin was developed with the aid of an instrumented sclerometer, which was adapted to put in contact a polypropylene probe with human forearms. Seven factors were considered, but only sweat and hair skin were found to have a significant effect on COF, which varied from 0.22 to 0.45 in the tests. Lower values of COF were obtained when sweat was present at the interface, while the absence of both sweat and hair skin led to the highest value. The results are believed to be relevant for developing reliable finite element (FE) models for socket–stump interaction since the relation between normal and shear stresses at the interface of two interacting bodies is strongly determined by the COF.

Incidence of the boundary condition between bone and soft tissue in a finite element model of a transfemoral amputee

Background: Many finite element investigations have been made in the field of lower limb prosthetics; however, friction between bone and soft tissues as a boundary condition has not been considered. Objectives: To establish whether the change in the contact boundary condition between bone and soft tissues in a transfemoral amputee affects the stress-strain state on the residual limb. Study Design: Finite element analysis comparison. Methods: Finite element models of four transfemoral amputees were developed. In these models the socket, soft tissues and femur were included and two simulations were made for each model, in one of them the interaction between bone and soft tissues was defined as tied (there is no relative displacement between surfaces) and in the other it was defined as a friction boundary condition. Results: The von Mises stress and strain peaks are higher when the friction definition is used than for tied contact definition. The distribution pattern of stresses and strains also change when the contact definition varies from tied to friction. Conclusions: It was concluded that the friction between bone and soft tissues have a significant impact on the results of finite element models of lower limb prosthetic systems, and therefore in its predictive capabilities. Clinical relevance Understanding the bone-soft tissue interaction can lead to more realistic and accurate finite element models used to predict the stress-strain state in the residual limb of prosthetic users and therefore predict the occurrence of deep tissue injuries.

Thinning effect on Euterpe oleracea population dynamics in the Choco biogeographic region of Colombia

Key messageIn the Chocó biogeographic region, forest thinning decreases competition and increases growth evenness among individuals ofEuterpe oleracea, which could be important to the development of sustainable species management.AbstractThe main goal of this study was to characterize and compare the effect of disturbance and competitive thinning in structuring the size–abundance relationship, growth, and mortality of populations of E. oleracea inhabiting forests subjected to clearing. The study was conducted in permanent plots located in the Atrato River valley, Chocó biogeographic region in Colombia. In each of these plots, we took repeated measurements of E. oleracea height and density during 1997–2008. Two treatments were implemented in each plot: unthinned (control) and thinned. We found a significant change over time in the height–abundance relationship of E. oleracea populations due to forest thinning. In unthinned forest plots, E. oleracea exhibits asymmetric intraspecific competition for light. However, this relationship is reversed in thinned forest plots where plant uptake of aboveground resources becomes more evenly distributed among individuals. Forest thinning promotes a switch in the developmental state of the E. oleracea populations. The decrease in the asymmetric light competition enhances the relative performance of smaller stems in relation to the larger ones, diminishing the inequality in the size–abundance distribution of unthinned forest. Although more research is still required, this study offers an insight into new management techniques for seeking alternative sustainable uses of tropical rain forests, but particularly for the quite degraded forests in the Chocó biogeographic region.

Pores distribution statistical analysis for metal foams obtained by casting-dissolution process

Interesting properties of metal foams such as light weight, good energy absorption and low thermal conductivity, have promoted the development of new processes to improve properties without sacrificing productivity. This study aims to verify the uniformity pore distribution in an aluminum alloy foam obtained by Casting - Dissolution Process, using a nondestructive analysis. In order to evaluate mechanical properties of a metal foam using computational numerical models, the use of a small reconstructed section of the sample representing an entire volume of metal foam, was validated. In conclusion, it was possible to determine that all parts of the sample have the equivalent superficial area and volume.