A novel method of determining the active drag profile in swimming via data manipulation of multiple tension force collection methods

A novel method aimed at evaluating the active drag profile during front-crawl swimming is proposed. Fourteen full trials were conducted with each trial using a stationary load cell set-up and a commercial resistance trainer to record the tension force in a rope, caused by an athlete swimming. Seven different stroke cycles in each experiment were identified for resampling time dependent data into position dependent data. Active drag was then calculated by subtracting resistance trainer force data away from the stationary load cell force data. Mean active drag values across the stroke cycle were calculated for comparison with existing methods, with mean active drag values calculated between 76 and 140 N depending on the trial. Comparing results with established active drag methods, such as the Velocity Perturbation Method (VPM), shows agreement in the magnitude of the mean active drag forces. Repeatability was investigated using one athlete, repeating the load cell set-up experiment, indicating results collected could range by 88 N depending on stroke cycle position. Variation in results is likely due to inconsistencies in swimmer technique and power output, although further investigation is required. The method outlined is proposed as a representation of the active drag profile over a full stroke cycle.

Processing-property relationships of biaxially stretched poly(L-lactic acid) sheet for application in coronary stents

The development of coronary stents from poly(L-lactic acid) requires knowledge of its mechanical properties and the effects of manufacturing processes on those properties. The effects of the biaxial stretching procedure on the mechanical and microstructural properties of poly(L-lactic acid) are hereby investigated. The mechanical properties were evaluated before and after biaxial stretching, with a Design of Experiments methodology employed to identify processing parameters that had the most significant effect on the elastic modulus and yield strength of the biaxially stretched sheets. Microstructural characterisation was performed using differential scanning calorimetry to evaluate crystallinity and thermal transitions of the biaxially stretched sheets. The results show that the mechanical properties of the stretched sheets are highly dependent on the extent of stretch ratio applied during processing; however, neither the elastic modulus nor yield strength are directly attributable to crystallinity, but are affected by the degree of amorphous orientation. The results of this study have the potential to be applied in the design of high stiffness, thin-strut polymeric expandable scaffolds for the application of coronary stents.

Effect of combined flexion and external rotation on measurements of the proximal femur from anteroposterior pelvic radiographs

INTRODUCTION

Fixed flexion and external rotation contractures are common in patients with hip osteoarthritis and, in particular, before total hip replacement (THR). We aimed to answer the following question: how does combined flexion and external rotation of the femur influence the radiographic assessment of (1) femoral offset (FO) (2) neck-shaft angle (NSA) and (3) distance (parallel to the femoral axis) from greater trochanter to femoral head center (GT-FHC)?

HYPOTHESIS

Combined flexion and external rotation impact the accuracy of two-dimensional (2D) proximal femur measurements.

MATERIALS AND METHODS

Three-dimensional (3D) CT segmentations of the right femur from 30 male and 42 female subjects were acquired and used to build a statistical shape model. A cohort (n=100; M:F=50:50) of shapes was generated using the model. Each 3D femur was subjected to external rotation (0°-50°) followed by flexion (0°-50°) in 10° increments. Simulated radiographs of each femur in these orientations were produced. Measurements of FO, NSA and GT-FHC were automatically taken on the 2D images.

RESULTS

Combined rotations influenced the measurement of FO (p<0.05), NSA (p<0.001), and GT-FHC (p<0.001). Femoral offset was affected predominantly by external rotation (19.8±2.6mm [12.2 to 26.1mm] underestimated at 50°); added flexion in combined rotations only slightly impacted measurement error (20.7±3.1mm [13.2 to 28.8mm] underestimated at 50° combined). Neck-shaft angle was reduced with flexion when external rotation was low (9.5±2.1° [4.4 to 14.2°] underestimated at 0° external and 50° flexion) and increased with flexion when external rotation was high (24.4±3.9° [15.7 to 31.9°] overestimated at 50° external and 50° flexion). Femoral head center was above GT by 17.0±3.4mm [3.9 to 22.1mm] at 50° external and 50° flexion. In contrast, in neutral rotation, FHC was 12.2±3.4mm [3.9 to 22.1mm] below GT.

DISCUSSION

This investigation adds to current understanding of the effect of femoral orientation on preoperative planning measurements through the study of combined rotations (as opposed to single-axis). Planning measurements are shown to be significantly affected by flexion, external rotation, and their interaction.

LEVEL OF EVIDENCE

IV Biomechanical study.

Computer simulating a clinical trial of a load-bearing implant: an example of an intramedullary prosthesis

Computational modelling is becoming ever more important for obtaining regulatory approval for new medical devices. An accepted approach is to infer performance in a population from an analysis conducted for an idealised or 'average' patient; we present here a method for predicting the performance of an orthopaedic implant when released into a population--effectively simulating a clinical trial. Specifically we hypothesise that an analysis based on a method for predicting the performance in a population will lead to different conclusions than an analysis based on an idealised or 'average' patient. To test this hypothesis we use a finite element model of an intramedullary implant in a bone whose size and remodelling activity is different for each individual in the population. We compare the performance of a low Young's modulus implant (E=20 GPa) to one with a higher Young's modulus (200 GPa). Cyclic loading is applied and failure is assumed when the migration of the implant relative to the bone exceeds a threshold magnitude. The analysis for an idealised of 'average' patient predicts that the lower modulus device survives longer whereas the analysis simulating a clinical trial predicts no statistically-significant tendency (p=0.77) for the low modulus device to perform better. It is concluded that population-based simulations of implant performance-simulating a clinical trial-present a very valuable opportunity for more realistic computational pre-clinical testing of medical devices.

How can cells sense the elasticity of a substrate? An analysis using a cell tensegrity model

A eukaryotic cell attaches and spreads on substrates, whether it is the extracellular matrix naturally produced by the cell itself, or artificial materials, such as tissue-engineered scaffolds. Attachment and spreading require the cell to apply forces in the nN range to the substrate via adhesion sites, and these forces are balanced by the elastic response of the substrate. This mechanical interaction is one determinant of cell morphology and, ultimately, cell phenotype. In this paper we use a finite element model of a cell, with a tensegrity structure to model the cytoskeleton of actin filaments and microtubules, to explore the way cells sense the stiffness of the substrate and thereby adapt to it. To support the computational results, an analytical 1D model is developed for comparison. We find that (i) the tensegrity hypothesis of the cytoskeleton is sufficient to explain the matrix-elasticity sensing, (ii) cell sensitivity is not constant but has a bell-shaped distribution over the physiological matrix-elasticity range, and (iii) the position of the sensitivity peak over the matrix-elasticity range depends on the cytoskeletal structure and in particular on the F-actin organisation. Our model suggests that F-actin reorganisation observed in mesenchymal stem cells (MSCs) in response to change of matrix elasticity is a structural-remodelling process that shifts the sensitivity peak towards the new value of matrix elasticity. This finding discloses a potential regulatory role of scaffold stiffness for cell differentiation.

Computational simulation methodologies for mechanobiological modelling: a cell-centred approach to neointima development in stents

The design of medical devices could be very much improved if robust tools were available for computational simulation of tissue response to the presence of the implant. Such tools require algorithms to simulate the response of tissues to mechanical and chemical stimuli. Available methodologies include those based on the principle of mechanical homeostasis, those which use continuum models to simulate biological constituents, and the cell-centred approach, which models cells as autonomous agents. In the latter approach, cell behaviour is governed by rules based on the state of the local environment around the cell; and informed by experiment. Tissue growth and differentiation requires simulating many of these cells together. In this paper, the methodology and applications of cell-centred techniques--with particular application to mechanobiology--are reviewed, and a cell-centred model of tissue formation in the lumen of an artery in response to the deployment of a stent is presented. The method is capable of capturing some of the most important aspects of restenosis, including nonlinear lesion growth with time. The approach taken in this paper provides a framework for simulating restenosis; the next step will be to couple it with more patient-specific geometries and quantitative parameter data.

The relationship between cement fatigue damage and implant surface finish in proximal femoral prostheses

The majority of cemented femoral hip replacements fail as a consequence of loosening. One design feature that may affect loosening rates is implant surface finish. To determine whether or not surface finish effects fatigue damage accumulation in a bone cement mantle, we developed an experimental model of the implanted proximal femur that allows visualisation of damage growth in the cement layer. Five matte surface and five polished surface stems were tested. Pre-load damage and damage after two million cycles was measured. Levels of pre-load (shrinkage) damage were the same for both matte and polished stems; furthermore damage for matte vs. polished stems was not significantly different after two million cycles. This was due to the large variability in damage accumulation rates. Finite element analysis showed that the stress is higher for the polished (assumed debonded) stem, and therefore we must conclude that either the magnitude of the stress increase is not enough to appreciably increase the damage accumulation rate or, alternatively, the polished stem does not debond immediately from the cement. Significantly (P=0.05) more damage was initiated in the lateral cement compared to the medial cement for both kinds of surface finish. It was concluded that, despite the higher cement stresses with debonded stems, polished prostheses do not provoke the damage accumulation failure scenario.

Residual stress due to curing can initiate damage in porous bone cement: experimental and theoretical evidence

Residual stress due to shrinkage of polymethylmethacrylate bone cement after polymerisation is possibly one factor capable of initiating cracks in the mantle of cemented hip replacements. No relationship between residual stress and observed cracking of cement has yet been demonstrated. To investigate if any relationship exists, a physical model has been developed which allows direct observation of damage in the cement layer on the femoral side of total hip replacement. The model contains medial and lateral cement layers between a bony surface and a metal stem; the tubular nature of the cement mantle is ignored. Five specimens were prepared and examined for cracking using manual tracing of stained cracks, observed by transmission microscopy; cracks were located and measured using image analysis. A mathematical approach for the prediction of residual stress due to shrinkage was developed which uses the thermal history of the material to predict when stress-locking occurs, and estimates subsequent thermal stress. The residual stress distribution of the cement layer in the physical model was then calculated using finite element analysis. Results show maximum tensile stresses normal to the observed crack directions, suggesting a link between residual stress and pre-load cracking. The residual stress predicted depends strongly on the definition of the reference temperature for stress-locking. The highest residual stresses (4-7 MPa) are predicted for shrinkage from maximum temperature; in this case, magnitudes are sufficiently high to initiate cracks when the influence of stress raisers such as pores or interdigitation at the bone/cement interface are taken into account (up to 24 MPa when calculating stress around a pore according to the method of Harrigan and Harris (J. Biomech. 24(11) (1991) 1047-1058). We conclude that the damage accumulation failure scenario begins before weight-bearing due to cracking induced by residual stress around pores or stress raisers.

Evaluation of cement stresses in finite element analyses of cemented orthopaedic implants

Stress analysis of the cement fixation of orthopaedic implants to bone is frequently carried out using finite element analysis. However the stress distribution in the cement layer is usually intricate, and it is difficult to report it in a way that facilitates comparison of implants for pre-clinical testing. To study this problem, and make recommendations for stress reporting, a finite element analysis of a hip prosthesis implanted into a synthetic composite femur is developed. Three cases are analyzed: a fully bonded implant, a debonded implant, and a debonded implant where the cement is removed distal to the stem tip. In addition to peak stresses, and contour and vector plots, a stressed volume and probability-of-failure analysis is reported. It is predicted that the peak stress is highest for the debonded stem, and that removal of the distal cement more than halves this peak stress. This would suggest that omission of the distal cement is good for polished prostheses (as practiced for the Exeter design). However, if the percentage of cement stressed above a certain threshold (say 3 MPa) is considered, then the removal of distal cement is shown to be disadvantageous because a higher volume of cement is stressed to above the threshold. Vector plots clearly demonstrate the different load transfer for bonded and debonded prostheses: A bonded stem generates maximum tensile stresses in the longitudinal direction, whereas a debonded stem generates most tensile stresses in the hoop direction, except near the tip where tensile longitudinal stresses occur due to subsidence of the stem. Removal of the cement distal to the tip allows greater subsidence but alleviates these large stresses at the tip, albeit at the expense of increased hoop stresses throughout the mantle. It is concluded that a thorough analysis of cemented implants should not report peak stress, which can be misleading, but rather stressed volume, and that vector plots should be reported if a precise analysis of the load transfer mechanism is required.