Radial Mismatch Research Articles

Interface micromotions increase with less-conforming cementless glenoid components

The optimal degree of conformity between the glenoid and humeral components in total shoulder arthroplasty for best performance and durability is still a matter of debate. The main aim of this study is to evaluate the influence of joint conformity on the bone-implant interface micromotions in a cementless glenoid implant. Polyethylene inlays with different degrees of conformity (radial mismatch of 0, 2, 4, and 6 mm) were mounted on a cementless metal back and then implanted in a bone substitute. These glenoid components were loaded by a prosthetic humeral head during a force-controlled experiment. Normal-to-interface micromotions and bone substitute deformations were measured at different points of the interface. Rim displacement and humeral head translation were also measured. A finite element (FE) model of the experiments was implemented to estimate the normal- and tangent-to-interface micromotions in the entire bone-implant interface. All measured variables increased with less-conforming PE inlays. Normal-to-interface micromotions were significantly larger (P < .05) when the radial mismatch was 6 mm compared with the fully conforming inlay. The FE model was in agreement and complemented the experimental results. FE model-predicted interface micromotions were already significantly larger when the radial mismatch was equal to 4 mm. In a force-controlled experiment with a cementless glenoid component, a non-conforming PE inlay allows larger interface micromotions than a conforming inlay, reaching a magnitude that may hamper local bone ingrowth in this type of component. This is mainly because of the larger humeral head translation that boosts the effects of the so-called rocking-horse phenomenon.

Stress transfer around a broken fiber in unidirectional fiber-reinforced composites considering matrix damage evolution and interface slipping

A shear-lag model is applied to study the stress transfer around a broken fiber within unidirectional fiber-reinforced composites (FRC) subjected to uniaxial tensile loading along the fiber direction. The matrix damage and interfacial debonding, which are the main failure modes, are considered in the model. The maximum stress criterion with the linear damage evolution theory is used for the matrix. The slipping friction stress is considered in the interfacial debonding region using Coulomb friction theory, in which interfacial clamping stress comes from radial residual stress and mismatch of Poisson’s ratios of constituents (fiber and matrix). The stress distributions in the fiber and matrix are obtained by the shear-lag theory added with boundary conditions, which includes force continuity and displacement compatibility constraints in the broken and neighboring intact fibers. The result gives axial stress distribution in fibers and shear stress in the interface and compares the theory reasonably well with the measurement by a polarized light microscope. The relation curves between damage, debonding and ineffective region lengths with external strain loading are obtained.

Long-Term Outcome following Stent Reconstruction of the Aortic Bifurcation and the Role of Geometric Determinants

We assessed the long-term patency of kissing stent reconstruction of the aortoiliac bifurcation and identified variables that may influence it. We retrospectively reviewed our experience with stent-reconstruction procedures of the aortoiliac bifurcation from January 1998 through June 2005. The impact of demographic variables, vascular risk factors, disease location and characteristics, stent material and design, and stenting configuration on stent patency was assessed using univariate and multivariate analysis. In particular, we evaluated the effect of geometric mismatch between the protruding segment of the stents and the distal aortic lumen. Sixty-six patients underwent aortobi-iliac stent reconstruction. Indications were bifurcation or bilateral proximal iliac disease in 52 patients and unilateral ostial disease requiring contralateral protection in 14 patients. Limited disease (TASC A and B) was present in 40 limbs in 19 patients; extensive/diffuse disease (TASC C and D) was present in 78 limbs in 47 patients. Complete occlusions were present in 37 limbs in 28 patients (bilateral in nine patients). Self-expanding stents were used in 56 procedures and balloon-expandable stents in 10. Crossing configuration was used in 43 procedures, while abutting configuration was used in 23 procedures. Technical success was achieved in 62 patients (94%), with all four failures due to inability to cross a chronically occluded limb. Three of these patients underwent aortomono-iliac stenting with a crossover femoral-femoral bypass graft, with the remaining one opting for no further interventions. Median combined follow-up was 37 +/- 27 months (range 0-102). Hemodynamically significant restenosis developed in nine patients (14%). The management of restenosis was endovascular in eight patients and was successful in all (balloon dilation in four, restenting in three, thrombolysis and stenting in one) and operative in one patient who developed aortic occlusion and underwent aortobifemoral grafting. Survival table analysis showed primary and assisted patency rates at 4 years of 81% and 94%, respectively. The mortality rate during follow-up was 19 (cardiac cause in eight, pulmonary cause in three, and malignancy in five). Univariate analysis showed radial mismatch (aortic lumen dead space around the protruding segment of the stents), female gender, prior occlusion, and residual stenosis to be significant predictors of restenosis. Multivariate logistic regression analysis showed radial mismatch to be the only significant determinant of restenosis, although the statistical power of the model was limited by the small number of restenoses. Stent reconstruction of the aortoiliac bifurcation for occlusive disease is effective and durable, even with complex aortoiliac disease and long segment occlusions. Most restenoses are amenable to endovascular treatment, with excellent long-term assisted patency. Geometric variables related to individual aortic anatomy and disease pattern (patient-dependent) and stenting configuration (operator-dependent) may have an impact on long-term patency.

Influence of glenohumeral conformity on glenoid stresses after total shoulder arthroplasty

Glenohumeral conformity has been reported to be one of the most critical implant-related features that may affect the occurrence of glenoid loosening. This study evaluated the mechanical effects of this parameter with a 3-dimensional finite element model of a prosthetic shoulder, which included the scapula, the humerus, and the rotator cuff muscles. Aequalis humeral and glenoid components were implanted numerically according to manufacturer's recommendations for 2 different orientations of the glenoid component (0 degrees and 15 degrees of retroversion). Different values of glenohumeral conformity (1-15 mm of radial mismatch) were tested by a progressive flattening of the glenoid surface. Free and countered rotation movements were simulated. Glenohumeral contact pressure, cement stress, shear stress, and micromotions at the bone-cement interface were calculated. At 0 degrees of retroversion, conformity had only a slight effect, whereas at 15 degrees of retroversion, all quantities increased by more than 200% and exceeded critical values above 10 mm of mismatch.

Arthroplasty of the shoulder

Although the first shoulder arthroplasty was implanted in 1893 by the French surgeon Jules-Emile Pean,[1][1] the development of the procedure came in the 1950s when Neer[2][2] described the results using a vitallium prosthesis to treat comminuted fractures of the head of the humerus. About 20

Changes in the Mechanical Environment of Stenotic Arteries During Interaction With Stents: Computational Assessment of Parametric Stent Designs

Clinical studies have identified factors such as the stent design and the deployment technique that are one cause for the success or failure of angioplasty treatments. In addition, the success rate may also depend on the stenosis type. Hence, for a particular stenotic artery, the optimal intervention can only be identified by studying the influence of factors such as stent type, strut thickness, geometry of the stent cell, and stent-artery radial mismatch with the wall. We propose a methodology that allows a set of stent parameters to be varied, with the aim of evaluating the difference in the mechanical environment within the wall before and after angioplasty with stenting. Novel scalar quantities attempt to characterize the wall changes inform of the contact pressure caused by the stent struts, and the stresses within the individual components of the wall caused by the stent. These quantities are derived numerically and serve as indicators, which allow the determination of the correct size and type of the stent for each individual stenosis. In addition, the luminal change due to angioplasty may be computed as well. The methodology is demonstrated by using a full three-dimensional geometrical model of a postmortem specimen of a human iliac artery with a stenosis using imaging data. To describe the material behavior of the artery, we considered mechanical data of eight different vascular tissues, which formed the stenosis. The constitutive models for the tissue components capture the typical anisotropic, nonlinear and dissipative characteristics under supra-physiological loading conditions. Three-dimensional stent models were parametrized in such a way as to enable new designs to be generated simply with regard to variations in their geometric structure. For the three-dimensional stent-artery interaction we use a contact algorithm based on smooth contact surfaces of at least C-continuity, which prevents numerical problems known from standard facet-based contact algorithm. The proposed methodology has the potential to provide a scientific basis for optimizing treatment procedures and stent geometries and materials, to help stent designers examine new stent designs "virtually," and to assist clinicians in choosing the most suitable stent for a particular stenosis.

The influence of glenohumeral prosthetic mismatch on glenoid radiolucent lines: results of a multicenter study.

In shoulder arthroplasty, mismatch is defined as the difference in the radius or diameter of curvature between the humeral head and glenoid components. Recommendations for mismatch have not been substantiated scientifically. The purpose of this study was to evaluate the effect of mismatch on glenoid radiolucent lines. The results of 319 total shoulder arthroplasties performed for the treatment of primary osteoarthritis were evaluated. All of the arthroplasties were performed with a single type of prosthesis (Aequalis; Tornier, Montbonnot, France) that included a cemented, all-polyethylene glenoid component. Three sizes of glenoid components and seven humeral head diameters were utilized. Radial mismatch was categorized as < or = 4 mm, 4.5 to 5.5 mm, 6 to 7 mm, or >7 to 10 mm. Radiographs were evaluated at a mean of 53.5 months (range, twenty-four to 110 months) postoperatively. Glenoid radiolucent lines were scored with a scale ranging from 0 points for no radiolucency to 18 points for radiolucent lines exceeding 2 mm in six zones. Variance, linear contrasts polynomial, quadratic polynomial contrast statistical, and linear regression analyses were performed to evaluate the relationship between radial mismatch and glenoid radiolucent lines. A significant linear relationship was found between mismatch and the glenoid radiolucency score (p < 0.0001), with significantly lower (better) radiolucency scores associated with radial mismatches of >5.5 mm. In this study of glenohumeral prosthetic mismatch ranging from 0 to 10 mm, the mismatch had a significant influence on the scores for the glenoid radiolucent lines, which were best when the radial mismatch was between 6 and 10 mm. The theoretical risk of prosthetic instability with larger mismatch values was not demonstrated within the range of mismatch values evaluated in this series.

Total shoulder arthroplasty biomechanics: a study of the forces and strains at the glenoid component.

The objective of this study was to examine how changes in glenohumeral joint conformity and loading patterns affected the forces and strains developed at the glenoid. After removal of soft tissue (muscles, ligaments, and labrum), force-displacement data were collected for both natural and prosthetically reconstructed joints. Joints were shown to develop higher forces for a given translation as joint conformity increased. A rigid body model of joint contact forces was used to determined the so-called effective radial mismatch of each joint. For the purposes of this study, the effective radial mismatch is defined as the mismatch required for a rigid body joint to have the same force-displacement relationship as the joint in question. This parameter is an indication of the deformation at the articular surface. The effective radial mismatch dramatically increased with increasing medial loads, indicating that under physiological loads, the effective radial mismatch of a joint is much greater than its measured mismatch at no load. This increase in effective mismatch as medial loads were increased was found to be threefold greater in cartilaginous joints than in reconstructed joints. Rosette strain gages positioned at the midlevel of the glenoid keel in the reconstructed joints revealed that anterior/posterior component loading leads to fully reversible cyclic keel strains. The highest compressive strains occurred with the head centered in the glenoid, and were larger for nonconforming joints (epsilon = 0.23 percent). These strains became tensile just before rim loading and were greater for conforming joints (epsilon = 0.15 percent). Although recorded peak strains are below the yield point for polyethylene, the fully reversed cyclic loading of the component in this fashion may ultimately lead to component toggling and implant failure.

TRANSVERSE STRAIN RESPONSE OF HI-NICALON FIBRE REINFORCED SILICON NITRIDE MATRIX COMPOSITES

The relation between the longitudinal stress {sigma} and the transverse strain {var_epsilon}{sub tr} during monotonic tension testing of unidirectional fiber toughened ceramic matrix composites (UD-CMCs) has been studied experimentally and theoretically by several authors. Matrix cracking and interface debonding generally causes {var_epsilon}{sub tr} to increase or to become positive. Recently, this has been theoretically explained by a model taking into account the Poisson contraction of fiber and matrix, the redistribution of mechanical stress and the relief of residual thermal strain upon damage development, and the built-up of compressive radical stresses at the interface caused by the radial interfacial mismatch after interface debonding and sliding. In this paper, the {var_epsilon}{sub tr} response of a series of Hi-Nicalon (SiC) fiber reinforced silicon nitride matrix composites is reported and described by a previously developed model. This paper focuses on the extension of the model to composites with a radial gap {delta} after interface debonding, which was necessary to model the present CMC materials.

Glenohumeral joint translations before and after total shoulder arthroplasty. A study in cadavera.

The purpose of the present study was to examine the motions of natural and prosthetically reconstructed glenohumeral joints without capsular contracture in the laboratory to obtain a better understanding of joint motion as it may relate to failure of the implant. Seven joints from fresh-frozen human cadavera were instrumented with a six-degrees-of-freedom magnetic tracking device to study patterns of translation and rotation with and without the application of simulated muscle forces (active and passive models). The specimens were tested before and after reconstruction of the joint with use of modified operative techniques and implants that had from zero to five millimeters of radial mismatch between the humeral head and glenoid components. The natural and reconstructed joints had similar patterns of translation, with larger rotations and translations observed in the passive model. On the average, the active translations of the natural joints were best reproduced by the reconstructed joints with less conforming articulations. The mean active translation in the natural joints was 1.5 millimeters along the anterior-posterior axis and 1.1 millimeters along the superior-inferior axis. The active translations in the reconstructed joints were observed to increase consistently as the conformity of the components decreased: the mean active anterior-posterior translations ranged from 0.3 millimeter for conforming components to 1.7 millimeters for components with a five-millimeter radial mismatch, and the mean active superior-inferior translations ranged from 0.4 to 1.1 millimeters. The patterns of translation during passive motions were less consistent, presumably because of the influence of capsular ligaments and because the translations were large enough for the articular surface of the humeral head to lose contact with the articular surface of the glenoid component. Glenohumeral translations during active motions were found to depend on articular conformity, which indicates that the choice of implants may have important consequences for normal motions. Totally conforming designs may impose a degree of restraint to translations that is higher than that in the natural joint. The patterns of translation may influence the longevity of the joint replacement with respect to stability, loosening of the glenoid component, and wear of the components.

The relationship between longitudinal stress and transverse strain during tensile testing of unidirectional fibre toughened ceramic matrix composites

A comprehensive, micromechanical model, relating the longitudinal stress and transverse strain during tensile testing of unidirectional fibre toughened ceramic matrix composites, has been developed. The model uses different unit-cells to describe the composite and considers all relevant damage as it develops throughout a tensile test. Specifically, the proposed model takes into account the Poisson contraction of fibre and matrix, the redistribution of mechanical stress and the relief of thermal stress due to the development of damage, and the build-up of compressive radial stresses at the interface due to the radial mismatch between fibre and matrix after interface debonding and sliding. Consequently, the modelled transverse strain response depends on a wide variety of structural and mechanical parameters. The followed approach has been assessed by comparing the simulated and experimentally observed response of a unidirectional SiC CAS composite. Theory and experiment are in excellent agreement for an experimentally determined set of constituent properties, but a parametric study shows the important effect of some of these, such as the radial interfacial mismatch, which are difficult to determine experimentally.

Dislocation emission at fibers—II. Experiments and microstructure of thermal punching

A decoration technique is used to observe dislocations produced by thermal stresses between glass or alumina fibers and silver chloride, used as a model material for metal matrix composites. Fibers ends are found to punch prismatic loop rows, the length of which is compared with a model presented in Part I of this paper as well as another model based on a mismatching spheroid. The geometry of dislocations punched at fiber ends is discussed for the case where the fiber axis is not coaxial with the glide direction. Elongated loops are also observed to be punched radially by the fiber and their number is correlated to the thermal radial mismatch. A simple model is proposed for the nucleation of such loops, which are usually entangled in a plastic zone surrounding the fiber.