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PubMed2004

Finite-element modal analysis of large-core multimode optical fibers.

Plastic optical fibers that are a typical large-core multimode optical fiber support a great number of modes compared with conventional silica-glass multimode optical fibers. So far the WKB method hasbeen used for most of the modal analyses of these fibers because of a great number of guided modes. We describe the accurate eigenmodal analysis of large-core multimode optical fibers with the finite-element method (FEM) and compute the propagation constants of all LP modes. In addition, the FEM has a strong advantage for arbitrary core profiles whereas the WKB method is not suitable fornonmonotonic profiles. To demonstrate the advantage, we apply the FEM to the fiber having sinusoidal fluctuations.

Journal Article↗
PubMed1998

Prosthetic restoration after coronoradicular resection: mechanical behavior of the distal root remaining and surrounding bone.

PURPOSE: This 3-dimensional finite element analysis evaluated: (1) the mechanical behavior of the distal root remaining after resection of the mesial root in a mandibular molar; (2) the consequences of the remaining distal root displacements on the bone; and (3) the role of the radicular post in the transmission of stress to the remaining root. METHODS: Three-dimensional finite element analysis was performed. RESULTS: The most reliable solution for the remaining root and surrounding bone consisted of fitting a conventional fixed partial denture with abutments on either side of the amputated root. The solution provided a single cantilever supported only by the residual root. CONCLUSIONS: This analysis makes it possible to quantify, in the model situation, the risks taken for each prosthetic option.

Alveolar Process↗
PubMedDate not supplied

Relative stiffness and stress of type I and type II external fixators: acrylic versus stainless-steel connecting bars--a theoretical approach.

OBJECTIVE: To compare the stiffness and pin stresses of three sizes of external fixator systems with stainless-steel and acrylic connecting bars. STUDY DESIGN: Finite element analysis. METHODS: Small, medium, and large external fixator systems of type I and type II configurations were modeled for finite element analysis. Each model was evaluated with a standard stainless-steel and three different diameters of acrylic connecting bar. Displacements and stresses were calculated for the loading modes of axial compression, medio-lateral bending, cranio-caudal bending, and torsion. The location of the pin experiencing maximum stress was determined for all configurations and loading modes. RESULTS: Acrylic column diameters of 9.53 mm for the small external fixator system and 15.9 mm for the medium external fixator system provide equivalent stiffness and maximum pin stresses to those provided by the standard stainless-steel connecting bars (3.2- and 4.8-mm diameter, respectively). The largest diameter acrylic column tested (31.75-mm) produced lower stiffness and higher maximum pin stresses than the standard stainless-steel connecting bar (11.1-mm diameter). CONCLUSIONS: When applying a small or medium external fixator, an acrylic column of 9.53-mm or 15.9-mm diameter, respectively, can be used. For a large external fixator system, an acrylic column of diameter >31.75 mm is required. CLINICAL RELEVANCE: The sizes of acrylic connecting bars for use in small and medium external fixator systems have been determined. Large systems should incorporate the standard stainless-steel connecting bar.

Acrylic Resins↗
PubMed2002

Strains in the marginal ridge during occlusal loading.

BACKGROUND: The marginal ridge is considered fundamental to the ability of the tooth to resist functional and parafunctional occlusal loads without damage. Despite this role, very little is known of patterns of stress and strain in marginal ridges under load. This study investigated strains in proximal enamel of mandibular premolars using finite element analysis (FEA) and strain gauge measurements. METHODS: A three-dimensional FEA model of a human mandibular premolar was developed using commercial FEA software, and strains were computed in response to loads simulating clenching and chewing functions. Strains were measured in extracted premolars using strain gauges mounted on the proximal surfaces, under similar occlusal loading conditions. RESULTS: Strains in the vicinity of contact areas and marginal ridges were lower than near the cemento-enamel junction and on buccal and lingual surfaces. The magnitude of proximal strains increased with oblique loading on cuspal inclines. Finite element analysis results correlated well with strain gauge measurements and can be used to predict strain directions and magnitudes. CONCLUSIONS: At least for mandibular premolars, the marginal ridges are not highly stressed areas during simulated occlusal loading.

Analysis of Variance↗
PubMed2006

Dental arch shape and size in Malay schoolchildren with Class II malocclusion.

AIM: To quantify and localise differences in Class I and Class II dental arches in Malay schoolchildren. METHODS: The subjects were 50 Malay schoolchildren (Mean age: 15 + 0.7 years) with either Class I (N = 25) or Class II (N = 25) malocclusions. Fourteen homologous landmarks on the upper and lower study models of the subjects were digitised using MorphoStudio software, and the Procrustes means were computed. The mean Class I and Class II configurations were subjected to ftests and finite element analysis. RESULTS: Normalised upper arch Class I and Class II configurations were statistically different (p < 0.05), but the lower arches were not. However, normalised Class II linear parameters were statistically smaller in the upper arch and larger in the lower arch. The upper Class II dental arch was narrower by 15 per cent and asymmetric changes in size (11-20 per cent) were localised in the labial and buccal regions of the lower Class II arch. CONCLUSIONS: Size and shape differences between Class I and Class II dental arches can be identified with finite element analysis.

Adolescent↗
PubMed2002

Hemodynamics and wall mechanics of a compliance matching stent: in vitro and in vivo analysis.

PURPOSE: Evidence is emerging that the abrupt compliance mismatch that exists at the junction between the stent ends and the host arterial wall disturbs both the vascular hemodynamics and the natural wall stress distribution. These stent-induced alterations are greatly reduced by smoothing the compliance mismatch between the stent and host vessel. A stent that provides this smooth transition in compliance, the compliance matching stent (CMS), has been developed. This study attempts to evaluate the hemodynamics and wall mechanical consequences of the CMS both in vitro and in vivo. MATERIALS AND METHODS: Finite element analysis was used to assess the solid mechanical behavior (compliance and stress) of the CMS in a stent/artery hybrid structure. A similar analysis was performed with a Palmaz stent. In vivo hemodynamics and wall mechanical changes induced by the CMS were investigated in a swine model from direct measurements of flow, pressure, diameter, and histology in the stented segment of superficial femoral arteries after 7 days. RESULTS: Finite element analysis showed that the abrupt compliance mismatch was substantially smoothed between the vessel portions with and without the stent with CMS segments. Circumferential stress was also markedly reduced with the CMS compared to other stent. The in vivo results showed that the CMS was efficient in compliance matching and did not dampen flow or pressure waves downstream the stent. Concurrent histology showed limited thrombus and inflammatory cell accumulation around the stent struts. CONCLUSION: These results indicate that the stent/artery hybrid structure can be compliance matched with proper stent design and that this structure limits solid mechanical stress and hemodynamic disturbances. It remains to be seen whether compliance-matched vascular stents reduce in-stent restenosis.

Animals↗
PubMedDate not supplied

Finite element method analysis of craniofacial morphology in unilateral cleft lip and palate prior to palatoplasty.

The craniofacial morphology in unilateral cleft lip and palate (UCLP) prior to palatoplasty was analyzed with the finite element method (FEM). The study examined cross-sectional data sets of affected and control males aged 4.5 to 6.0 years from Czechoslovakia. The facial skeleton and cranial base were discretized into 26 two-dimensional, triangular finite elements. Cranial base morphology is altered in UCLP. The extensive nature of most craniofacial deformities means that almost any registration is likely to be distorted. The fundamental advantage of the FEM is that biological descriptions are not dependent upon the selected reference plane. The FEM localizes changes in form to discrete anatomical regions. Significant shape and size differences characterize UCLP prior to palatoplasty. Dramatic deformations associated with cheiloplasty are localized to the lips and maxillary alveolar process. Nasal deformation reflects loss of structural support for the nose.

Child↗
PubMed2005

Fractures of distally-fixed femoral stems after revision arthroplasty.

We identified five (2.3%) fractures of the stem in a series of 219 revision procedures using a cementless, cylindrical, extensively porous-coated, distally-fixed femoral stem. Factors relating to the patients, the implant and the operations were compared with those with intact stems. Finite-element analysis was performed on two of the fractured implants. Factors associated with fracture of the stem were poor proximal bone support (type III-type IV; p = 0.001), a body mass index > 30; (p = 0.014), a smaller diameter of stem (< 13.5 mm; p = 0.007) and the use of an extended trochanteric osteotomy (ETO 4/5: p = 0.028). Finite-element analysis showed that the highest stresses on the stem occurred adjacent to the site of the fracture. The use of a strut graft wired over an extended trochanteric osteotomy in patients lacking proximal femoral cortical support decreased the stresses on the stem by 48%.We recommend the use of a strut allograft in conjunction with an extended trochanteric osteotomy in patients with poor proximal femoral bone stock.

Adult↗
PubMed2003

Alignment in total knee arthroplasty following failed high tibial osteotomy.

In total knee arthroplasty (TKA) following failed high tibial osteotomy, the mechanical axis does not intersect the center of the tibial component if the tibia has been resected perpendicular to the anatomical axis. Therefore, tibial resection referencing the predicted postoperative mechanical axis instead of the tibial shaft axis is advocated. To obtain the optimal tibial resection, characteristics of the tibial proximal deformity were measured radiographically and predicted postoperative lower limb alignment was calculated using full-length, weight-bearing, lower limb anteroposterior radiographs. Two finite element analysis models also were examined. The proximal tibia was resected perpendicular to the tibial shaft axis in model 1, and perpendicular to the predicted postoperative tibial mechanical axis in model 2. When the proximal tibia was resected perpendicular to the tibial shaft axis, the predicted lower limb mechanical axis was significantly shifted medially to the center of the tibial joint surface. The results of the finite element analysis reflected the medial shift of the lower limb mechanical axis in model 1, where stresses were increased in the medial tibial compartment. Tibial resection referencing the predicted postoperative tibial mechanical axis, instead of the tibial shaft axis, should be performed, especially in cases with a deformed tibia.

Aged↗
PubMed2003

Reconstruction after total sacrectomy using a new instrumentation technique: a biomechanical comparison.

STUDY DESIGN: The stresses exerted on the instrumentation and adjacent bone were evaluated for three reconstruction methods after a total sacrectomy: a modified Galveston reconstruction (MGR), a triangular frame reconstruction (TFR), and a novel reconstruction (NR). OBJECTIVE: To perform finite-element analysis of reconstruction methods used after a total sacrectomy. SUMMARY OF BACKGROUND DATA: When a sacral tumor involves the first sacral vertebra, a total sacrectomy is necessary. It is mandatory to reconstruct the continuity between the spine and the pelvis after a total sacrectomy. However, no previous reports have described a biomechanical study of the reconstructed lumbosacral spine. METHODS: A finite-element model of the lumbar spine and pelvis was constructed. Then three-dimensional MGR, TFR, and NR models were reconstructed, and a finite-element analysis was performed to account for the stresses on the bones and instrumentation. RESULTS: With excessive stress concentrated at the spinal rod in MGR, there is a strong possibility that the rod between the spine and the pelvis may fail. Although there was no stress concentration on the instruments in TFR, excessive stress on the iliac bones around the sacral rod was above the yield stress of the iliac bone. Such stress may cause a loosening of the sacral rod from the iliac bone. In NR, excessive stress concentration was not detected in the rod or the bones. This reconstruction has a low risk of instrument failure and loosening. CONCLUSIONS: If the patient were to stand or sit immediately after MGR or TFR instrumentation, failure or loosening may occur. The NR has a low risk of instrument failure and loosening after a total sacrectomy.

Adult↗
PubMed1992

Finite element stress analysis of a push-out test. Part 1: Fixed interface using stress compatible elements.

In this first part of a two-part paper, interelement stress compatible finite elements are developed and used to perform the stress analysis of a push-out test with a fixed interface. In the formulation, the required continuity of some of the stresses along either a specific interface or all interelement interfaces is enforced by a penalty procedure. The model is axisymmetric and consists of two cylinders attached to each other through the interface. Various relative material properties and boundary conditions are simulated in order to examine their effects on the interface stresses. Both loadings of axial compression force and axial torque are considered. The predicted results exhibit identical interelement stresses and displacements even when highly dissimilar materials are used. They also exhibit a complex state of interface stresses which depend on the geometry, material arrangement, boundary conditions, and loading. The variation of the shear stress is often highly nonuniform and the radial normal stresses are likely to be large. The present results, therefore, disagree with the common assumptions made in the pull-out tests in the orthopaedic applications. Finally, stress analysis of a number of possible testing configurations could lead to the design of an optimal pull-out test which maximizes the usefulness of the measured results in terms of the interface bond strength and factors affecting it.

Biomechanical Phenomena↗
PubMed2001

Bone density adjacent to press-fit acetabular components. A prospective analysis with quantitative computed tomography.

BACKGROUND: The status of periprosthetic bone stock is an important concern when revision total hip arthroplasty is undertaken. Remodeling of periprosthetic femoral bone after total hip arthroplasty has been studied extensively, and the phenomenon of femoral stress-shielding has been well characterized. Finite element analysis and computer-simulated remodeling theory have predicted that retroacetabular bone-mineral density decreases after total hip arthroplasty; however, remodeling of periprosthetic pelvic bone in this setting has yet to be well defined. This study was conducted to evaluate the short-term natural history of periacetabular bone-mineral density following primary total hip arthroplasty. METHODS: Periacetabular bone-mineral density was studied prospectively in a group of twenty-six patients who underwent primary hybrid total hip arthroplasty for the treatment of advanced osteoarthritis. Density within the central part of the ilium (directly cephalad to a press-fit acetabular component) was assessed with serial quantitative computed tomography. Baseline density was measured within the first five days following the total hip arthroplasty. Ipsilateral density measurements were repeated at an average of 1.28 years postoperatively. Density values at corresponding levels of the contralateral ilium were obtained at both time-points in all patients to serve as internal controls. RESULTS: Bone-mineral density decreased significantly (p< or =0.001) between the two time-points on the side of the operation. The mean absolute magnitude of the interval density reduction (75 mg/cc) was greatest immediately adjacent to the implant (p<0.001), but it was also significantly reduced (by 35 mg/cc) at a distance of 10 mm cephalad to the implant (p = 0.001). Relative declines in mean density ranged from 33% to 20% of the baseline values. No focal bone resorption (osteolysis) was detected at the time of this short-term follow-up study. With the numbers available, no significant interval alteration in bone-mineral density was found on the untreated (internal control) side (p> or =0.07). CONCLUSIONS: We suggest that the observed decline in bone-mineral density represents a remodeling response to an altered stress pattern within the pelvis that was induced by the presence of the acetabular implant. This finding corroborates the predictions of finite element analysis and computer-simulated remodeling theory. It remains to be seen whether this trend of atrophy of retroacetabular bone stock will continue with longer follow-up or will ultimately affect the long-term stability of press-fit acetabular components.

Acetabulum↗
PubMed1998

Effect of precise mechanical loading on fibroblast populated collagen lattices: morphological changes.

The contraction of a collagen lattice by resident fibroblasts causes strains to be developed within that lattice. These strains can be increased or decreased by altering the aspect ratio (ratio of length/width/thickness) of the fibroblast populated collagen lattice, as the cross-sectional area resisting the strain is changed and by the application of an external load. The fibroblasts align themselves with the direction of the maximum principle strain; in effect, these cells are "hiding" from the perceived strain. The direction of the maximum principle strain can be predetermined by the use of a computational finite element analysis. Using the tensioning-Culture Force Monitor to apply pre-determined loading patterns of known repeatable magnitudes, as calculated by the finite element analysis, we have succeeded in aligning fibroblasts into a deliberate predicted orientation. This study has shown that the resident fibroblast population will respond to changes in strain resulting from the most subtle of mechanical loads. This may be an important mechanism in development and repair of connective tissue.

Biomechanical Phenomena↗
PubMed2004

On morphometric measurement of oxygen diffusing capacity in middle ear gas exchange.

An accurate mathematical model of transmucosal gas exchange is prerequisite to understanding middle ear (ME) physiology. Current models require experimentally measured gas species time constants for all extant conditions as input parameters. However, studies on pulmonary gas exchange have shown that a morphometric model that incorporates more fundamental physiochemical and anatomic parameters accurately simulates transport from which the species time constants can be derived for all extant conditions. Here, we implemented a variant of that model for ME gas exchange that requires the measurement of diffusional length (tau) for the ME mucosa. That measure contributes to the mucosal diffusing capacity and reflects the resistance to gas flow between air space and capillary. Two methods for measuring tau have been proposed: linear distance between the air-mucosal boundary and capillary and the harmonic mean of all contributing pathway lengths. Oxygen diffusing capacity was calculated for different ME mucosal geometries by using the two tau measures, and the results were compared with those predicted by a detailed, two-dimensional finite element analysis. Predictive accuracy was improved by incorporating the harmonic tau measure, which captures important information regarding variations in capillary shape and distribution. However, compared with the oxygen diffusing capacity derived from the finite element analysis, both measures yielded nonlinear, positively biased estimates. The morphometric techniques underestimate diffusion length by failing to account for the curvilinear gas flow pathways predicted by the finite element model.

Animals↗
PubMed2006

Biomechanics of the rostrum in crocodilians: a comparative analysis using finite-element modeling.

This article reports the use of simple beam and finite-element models to investigate the relationship between rostral shape and biomechanical performance in living crocodilians under a range of loading conditions. Load cases corresponded to simple biting, lateral head shaking, and twist feeding behaviors. The six specimens were chosen to reflect, as far as possible, the full range of rostral shape in living crocodilians: a juvenile Caiman crocodilus, subadult Alligator mississippiensis and Crocodylus johnstoni, and adult Caiman crocodilus, Melanosuchus niger, and Paleosuchus palpebrosus. The simple beam models were generated using morphometric landmarks from each specimen. Three of the finite-element models, the A. mississippiensis, juvenile Caiman crocodilus, and the Crocodylus johnstoni, were based on CT scan data from respective specimens, but these data were not available for the other models and so these--the adult Caiman crocodilus, M. niger, and P. palpebrosus--were generated by morphing the juvenile Caiman crocodilus mesh with reference to three-dimensional linear distance measured from specimens. Comparison of the mechanical performance of the six finite-element models essentially matched results of the simple beam models: relatively tall skulls performed best under vertical loading and tall and wide skulls performed best under torsional loading. The widely held assumption that the platyrostral (dorsoventrally flattened) crocodilian skull is optimized for torsional loading was not supported by either simple beam theory models or finite-element modeling. Rather than being purely optimized against loads encountered while subduing and processing food, the shape of the crocodilian rostrum may be significantly affected by the hydrodynamic constraints of catching agile aquatic prey. This observation has important implications for our understanding of biomechanics in crocodilians and other aquatic reptiles.

Alligators and Crocodiles↗
PubMed1993

Bone and bones, architecture and stress, fossils and osteoporosis.

The combined use of architectural and stress technologies in osteological studies is starting to provide the basic biomechanical underpinnings to both evolutionary and applied medical investigations of bone. The architectural investigations, though tested using invasive methods, are aimed at non-invasive ways of obtaining information from radiographs of bones, fossils and people. They include optical (Fourier) data analysis (ODA) and computational Fourier transformations (FFT). The stress studies, though initially involving older techniques such as photoelastic stress analysis, now employ finite element analysis (FEA) and, most recently, fast Lagrangian analysis of continua (FLAC). Taken together, these methods are capable of providing more detailed knowledge of bone form and function that is important (a) in revealing functional adaptation in evolutionary studies of fossils and (b) for making early diagnosis and understanding pathological fractures in the late stages of osteoporosis.

Animals↗
PubMed1998

Biomechanical aspects of the insect wing: an analysis using the finite element method.

Insect wings appear as highly functional and largely optimized mechanical constructions. A series of stabilizing constructional elements have been 'designed' to cope with loading during flight. One such element is the expenditure of material in constructing the wing, i.e. the vein system of the wing and its arrangement. It functions like a zig-zag folding framework which stiffens the wing against aerodynamic bending moments. To quantify the quality of material distribution, models of a dragonfly wing and of a fly wing were calculated using the finite element method (FEM).

Animals↗
PubMed2001

The dynamic behaviour of a lower complete denture during unilateral loads: analysis using the finite element method.

The purpose of this study was to analyse the correlation between the location and/or direction of unilateral load and the dynamic behaviour of a mandibular complete denture using the three-dimensional finite element method (FEM). The FE model consisted of the body of mandible, alveolar mucosa, and a complete denture, and it could simulate the condition of contact between the surface of the mucosa and mucosal surface of a denture. Load vectors, which simulated premature contacts, were calculated on the supposition that a premature contact on an occlusal facet causes a vertical load on it. Load-A, a load vertical to the anterior occlusal facet, caused the least displacement of the denture and less concentrated stress distribution. Load-P, a load vertical to the posterior occlusal facet, was characterized by the stress distributed in the lingual area of the pre-molar lesion on the contra-lateral side of the load. Load-B, a load vertical to the balancing occlusal facet, caused the largest displacement of the denture. The relationship between loads and stress distribution was agreed approximately with the observations previously reported, and lead to some suggestions about occlusion for complete dentures.

Alveolar Process↗