Principal Strain Magnitudes Research Articles

Orbital Stress Analysis

The purpose was to study the biomechanics of bone fracture repair, of the orbital floor, using osteosynthetic bioresorbable implant and how to improve the implant design. A finite element model of the orbit and the globe of 1 patient who experienced orbital blowout fracture and treated with bioresorbable poly-L/DL-lactide (P[L/DL]LA 70:30) implant (PolyMax; Synthes, Oberdorf, Switzerland) was generated based on computed tomographic scans. Simulations were performed with a computer using a commercially available finite element software. The effects of changing the geometry, bony support, and method of fixation of the implant on the finite element model predictions were investigated. The factor that had the biggest impact on the predicted principal strain magnitudes was absence of bony support of the implant (up to 65%). Applying elastic fixation reduced stresses (up to 40%) posteriorly. The principal stresses inside the bone and the implant were evenly distributed when elastic fixation was applied to the implant. Applying rigid fixation increased stresses (up to 50% and 80% anteriorly and posteriorly, respectively). The resulting stress values indicated a likely rapid failure of the osteosynthetic implant when rigid fixation was applied. Applying rigid fixation induced a significant increase in stress patterns. Principal stresses were reduced remarkably when elastic fixation was applied to the implant. The role of fixation becomes more prominent when there is no bony support posteriorly and/or medially. It is recommended to avoid rigid fixation and to apply elastic fixation when using bioresorbable P(L/DL)LA 70:30 implants to reconstruct inferior orbital wall bony defects.

Sensitivity and ex vivo validation of finite element models of the domestic pig cranium

A finite element (FE) validation and sensitivity study was undertaken on a modern domestic pig cranium. Bone strain data were collected ex vivo from strain gauges, and compared with results from specimen-specific FE models. An isotropic, homogeneous model was created, then input parameters were altered to investigate model sensitivity. Heterogeneous, isotropic models investigated the effects of a constant-thickness, stiffer outer layer (representing cortical bone) atop a more compliant interior (representing cancellous bone). Loading direction and placement of strain gauges were also varied, and the use of 2D membrane elements at strain gauge locations as a method of projecting 3D model strains into the plane of the gauge was investigated. The models correctly estimate the loading conditions of the experiment, yet at some locations fail to reproduce correct principal strain magnitudes, and hence strain ratios. Principal strain orientations are predicted well. The initial model was too stiff by approximately an order of magnitude. Introducing a compliant interior reported strain magnitudes more similar to the ex vivo results without notably affecting strain orientations, ratios or contour patterns, suggesting that this simple heterogeneity was the equivalent of reducing the overall stiffness of the model. Models were generally insensitive to moderate changes in loading direction or strain gauge placement, except in the squamosal portion of the zygomatic arch. The use of membrane elements made negligible differences to the reported strains. The models therefore seem most sensitive to changes in material properties, and suggest that failure to model local heterogeneity in material properties and structure of the bone may be responsible for discrepancies between the experimental and model results. This is partially attributable to a lack of resolution in the CT scans from which the model was built, and partially due to an absence of detailed material properties data for pig cranial bone. Thus, caution is advised when using FE models to estimate absolute numerical values of breaking stress and bite force unless detailed input parameters are available. However, if the objective is to compare relative differences between models, the fact that the strain environment is replicated well means that such investigations can be robust.

The impact of bone and suture material properties on mandibular function in Alligator mississippiensis: testing theoretical phenotypes with finite element analysis

The functional effects of bone and suture stiffness were considered here using finite element models representing three different theoretical phenotypes of an Alligator mississippiensis mandible. The models were loaded using force estimates derived from muscle architecture in dissected specimens, constrained at the 18th and 19th teeth in the upper jaw and 19th tooth of the lower jaw, as well as at the quadrate-articular joint. Stiffness was varied systematically in each theoretical phenotype. The three theoretical phenotypes included: (i) linear elastic isotropic bone of varying stiffness and no sutures; (ii) linear elastic orthotropic bone of varying stiffness with no sutures; and (iii) linear elastic isotropic bone of a constant stiffness with varying suture stiffness. Variation in the isotropic material properties of bone primarily resulted in changes in the magnitude of principal strain. By comparison, variation in the orthotropic material properties of bone and isotropic material properties of sutures resulted in: a greater number of bricks becoming either more compressive or more tensile, changing between being either dominantly compressive or tensile, and having larger changes in the orientation of maximum principal strain. These data indicate that variation in these model properties resulted in changes to the strain regime of the model, highlighting the importance of using biologically verified material properties when modeling vertebrate bones. When bones were compared within each set, the response of each to changing material properties varied. In two of the 12 bones in the mandible, varied material properties within sutures resulted in a decrease in the magnitude of principal strain in bricks adjacent to the bone/suture interface and decreases in stored elastic energy. The varied response of the mandibular bones to changes in suture stiffness highlights the importance of defining the appropriate functional unit when addressing relationships of performance and morphology.

Orientation and Magnitude of Left Ventricular Principal Strains as Indicator of Infarct Expansion

Orientation and Magnitude of Left Ventricular Principal Strains as Indicator of Infarct Expansion

The influence of prior hamstring injury on lengthening muscle tissue mechanics

Hamstring strain injuries often occur near the proximal musculotendon junction (MTJ) of the biceps femoris. Post-injury remodeling can involve scar tissue formation, which may alter contraction mechanics and influence re-injury risk. The purpose of this study was to assess the affect of prior hamstring strain injury on muscle tissue displacements and strains during active lengthening contractions. Eleven healthy and eight subjects with prior biceps femoris injuries were tested. All previously injured subjects had since returned to sport and exhibited evidence of residual scarring along the proximal aponeurosis. Subjects performed cyclic knee flexion–extension on an MRI-compatible device using elastic and inertial loads, which induced active shortening and lengthening contractions, respectively. CINE phase-contrast imaging was used to measure tissue velocities within the biceps femoris during these tasks. Numerical integration of the velocity information was used to estimate two-dimensional tissue displacement and strain fields during muscle lengthening. The largest tissue motion was observed along the distal MTJ, with the active lengthening muscle exhibiting significantly greater and more homogeneous tissue displacements. First principal strain magnitudes were largest along the proximal MTJ for both loading conditions. The previously injured subjects exhibited less tissue motion and significantly greater strains near the proximal MTJ. We conclude that localized regions of high tissue strains during active lengthening contractions may predispose the proximal biceps femoris to injury. Furthermore, post-injury remodeling may alter the in-series stiffness seen by muscle tissue and contribute to the relatively larger localized tissue strains near the proximal MTJ, as was observed in this study.

Subject-specific finite element model of knee: experimental validation using composite and bovine specimens

This work describes the experimental validation of lower limb finite element models using a composite femur, tibia and bovine tibia. Experimental and predicted equivalent strains and principal strain magnitudes and directions were compared to provide all the necessary information for validation studies. Inaccurate geometry, loading conditions and material properties are frequent errors occurring in validation studies; a sensitivity analysis was done to investigate how the latter two affect the validation. Results were most affected when the directions of the loads were not correctly implemented due to slight misalignment between test apparatus loading axis and the vertical axis in the FE environment.

Component mode synthesis approach to estimate tibial strains in gait

This paper presents a component mode synthesis approach to estimate tibial strains in gait. First, 3D models of the human musculoskeletal system were constructed based on the China Visible Human (CVH) dataset. Then an experiment was carried out to capture the subject gait motion. An inverse dynamic algorithm was developed to predict joint moments and an optimization algorithm was used to predict muscle forces in subject gait. Finally, a finite element model of the tibia was built. The muscle forces were input into the tibia finite element model as boundary conditions. Through the proposed component mode synthesis approach, 12 mode shapes and strains of tibia were estimated. The maximum and minimum principal strain magnitudes of tibia are 499 microstrain and −612 microstrain respectively. The maximum and minimum strain rates of tibia are 4130 microstrain s−1 and –3970 microstrain s−1 respectively. These figures are in line with literature values from in vivo measurements using an invasive strain gauge. In conclusion, the component mode synthesis approach may be used as a surrogate for experimental bone strain measurements and thus be of use in detailed strain estimation of bones in different applications.

Strain distribution in the proximal human femoral metaphysis

There is significant interest in the stress-strain state in the proximal femoral metaphysis, because of its relevance for hip fractures and prosthetic replacements. The scope of this work was to provide a better understanding of the strain distribution, and of its correlation with the different directions of loading, and with bone quality. A total of 12 pairs of human femurs were instrumented with strain gauges. Six loading configurations were designed to cover the range of directions spanned by the hip joint force. Inter-specimen variability was reduced if paired specimens were considered. The principal strain magnitude varied greatly between loading configurations. This suggests that different loading configurations need to be simulated in vitro. The strain magnitude varied between locations but, on average, was compatible with the strain values measured in vivo. The strain magnitudes and the direction of principal tensile strain in the head and neck were compatible with the spontaneous fractures of the proximal femur reported in some subjects. The principal tensile strain was significantly larger where the cortical bone was thinner; the compressive strain was larger where the cortical bone was thicker. The direction of the principal strain varied significantly between measurement locations but varied little between loading configurations. This suggests that the anatomy and the distribution of anisotropic material properties enable the proximal femur to respond adequately to the changing direction of daily loading.

Functional Loads of the Tongue and Consequences of Volume Reduction

This study was conducted to evaluate functional loads of the tongue on its surrounding bones and investigate how tongue volume reduction affects these loads. Masticatory bone strains and pressures on facial bones directly contacted by the tongue were measured in 12 (6 sibling pairs) 12-week-old Yucatan miniature pigs. One of each sibling pair underwent surgery to reduce the tongue volume by 23% to 25% (reduction group); the other underwent identical tongue incisions without tissue removal (sham group). Rosette strain gauges were bonded to the palatal surface of premaxilla (PM), the lingual surface of mandibular alveolar bones between the second and third decidious incisors (MI), and below the third decidious molar (MM). Single-element stain gauges were placed across the palatal surface of the premaxillary stuture (PMS) and the lingual surface of the mandibular symphysis (MSP). Pressure tranducers were placed on the hard palatal surface of the maxillary (PAL) and the lingual surface of the mandible (MAN) posterior to the deciduous canine. Animals were allowed to feed unrestrainedly after surgery and device placement. Data from bone strain, pressure, and electromyographic (EMG) activity of bilateral masseter muscles were recorded during natural mastication (pig chow). In the sham animals, the principal bone surface strains were less than 100 microepsilon in all measures. The principal strains were greater compressive than tensile strains at the PM and greater tensile than compressive strains at the MI and MM. Tensile strains were significantly greater at the MM than at the PM (P < .01). Strains were tensile at the PMS and compressive at the MSP, with significantly higher magnitude (> 100 microepsilon) at the PMS (P < .05). Pressures ranged from 2.12 to 8.04 kPa, with higher readings at the MSP compared with the PAL (P < .05). Tongue volume reduction did not affect strain polarity at any site but did diminish principal strain magnitudes significantly at the MI (P < .05). At the PM and MI, the principal tensile orientation was significantly altered from the lateroanterior to the lateroposterior direction (P < .05 to .001). Strains at the MM and MSP showed little change. Compared with the sham animals, tensile strain at the PMS and pressures at the PAL and MAN were decreased by > or = 50% (P < .01). These results suggest that 1) the tongue produces greater functional loads (strains and pressures) on mandibular lingual surfaces than on maxillary/premaxillary palatal surfaces; 2) tongue volume reduction decreases these loads, specifically those in the anterior mouth; and 3) masticatory loads produced by the tongue on the lingual mandibular and palatal maxillary/premaxillary surfaces are much smaller than those produced by masticatory muscles on the dorsal surfaces of these bones and temporomandibular joint structures.

The current anatomical description of the inferior glenohumeral ligament does not correlate with its functional role in positions of external rotation

The objective of this study was to determine the strain distribution in the inferior glenohumeral ligament at 0 degree, 30 degrees, and 60 degrees of external rotation with an anterior load applied to the joint. Five cadaver shoulders were dissected free of all soft tissue except the glenohumeral capsule and a 7 x 11 grid of strain markers were affixed to the inferior glenohumeral ligament. The location of these strain markers was then determined for a reference strain configuration and while a 25 N anterior load was applied to simulate a clinical exam for instability. The magnitude and direction of the maximum principal strains were then determined at each joint position. For all specimens, the magnitude of the maximum principal strains were significantly greater for 30 degrees and 60 degrees of external rotation when compared to 0 degree of external rotation. Furthermore, when comparing 30 degrees to 60 degrees of external rotation, three of the five specimens were significantly different. Additionally, the previously described regions of the inferior glenohumeral ligament could not be identified with a qualitative evaluation of the strain distribution pattern for each specimen at all external rotation angles. This indicates that our current description of the three regions of the inferior glenohumeral ligament does not correspond to its functional role. Additionally, the directions of the maximum principal strains across the inferior glenohumeral ligament became more aligned with one another as external rotation was increased. The complex strain distributions observed indicates that future studies should treat the inferior glenohumeral capsule as a continuous sheet of fibrous tissue.

A Musculoskeletal Model of the Equine Forelimb for Determining Surface Stresses and Strains in the Humerus—Part II. Experimental Testing and Model Validation

The first objective of this study was to experimentally determine surface bone strain magnitudes and directions at the donor site for bone grafts, the site predisposed to stress fracture, the medial and cranial aspects of the transverse cross section corresponding to the stress fracture site, and the middle of the diaphysis of the humerus of a simplified in vitro laboratory preparation. The second objective was to determine whether computing strains solely in the direction of the longitudinal axis of the humerus in the mathematical model was inherently limited by comparing the strains measured along the longitudinal axis of the bone to the principal strain magnitudes and directions. The final objective was to determine whether the mathematical model formulated in Part I [Pollock et al., 2008, ASME J. Biomech. Eng., 130, p. 041006] is valid for determining the bone surface strains at the various locations on the humerus where experimentally measured longitudinal strains are comparable to principal strains. Triple rosette strain gauges were applied at four locations circumferentially on each of two cross sections of interest using a simplified in vitro laboratory preparation. The muscles included the biceps brachii muscle in addition to loaded shoulder muscles that were predicted active by the mathematical model. Strains from the middle grid of each rosette, aligned along the longitudinal axis of the humerus, were compared with calculated principal strain magnitudes and directions. The results indicated that calculating strains solely in the direction of the longitudinal axis is appropriate at six of eight locations. At the cranial and medial aspects of the middle of the diaphysis, the average minimum principal strain was not comparable to the average experimental longitudinal strain. Further analysis at the remaining six locations indicated that the mathematical model formulated in Part I predicts strains within +/-2 standard deviations of experimental strains at four of these locations and predicts negligible strains at the remaining two locations, which is consistent with experimental strains. Experimentally determined longitudinal strains at the middle of the diaphysis of the humerus indicate that tensile strains occur at the cranial aspect and compressive strains occur at the caudal aspect while the horse is standing, which is useful for fracture fixation.

Modulation of mandibular loading and bite force in mammals during mastication

Modulation of force during mammalian mastication provides insight into force modulation in rhythmic, cyclic behaviors. This study uses in vivo bone strain data from the mandibular corpus to test two hypotheses regarding bite force modulation during rhythmic mastication in mammals: (1) that bite force is modulated by varying the duration of force production, or (2) that bite force is modulated by varying the rate at which force is produced. The data sample consists of rosette strain data from 40 experiments on 11 species of mammals, including six primate genera and four nonprimate species: goats, pigs, horses and alpacas. Bivariate correlation and multiple regression methods are used to assess relationships between maximum (epsilon(1)) and minimum (epsilon(2)) principal strain magnitudes and the following variables: loading time and mean loading rate from 5% of peak to peak strain, unloading time and mean unloading rate from peak to 5% of peak strain, chew cycle duration, and chew duty factor. Bivariate correlations reveal that in the majority of experiments strain magnitudes are significantly (P<0.001) correlated with strain loading and unloading rates and not with strain loading and unloading times. In those cases when strain magnitudes are also correlated with loading times, strain magnitudes are more highly correlated with loading rate than loading time. Multiple regression analyses reveal that variation in strain magnitude is best explained by variation in loading rate. Loading time and related temporal variables (such as overall chew cycle time and chew duty factor) do not explain significant amounts of additional variance. Few and only weak correlations were found between strain magnitude and chew cycle time and chew duty factor. These data suggest that bite force modulation during rhythmic mastication in mammals is mainly achieved by modulating the rate at which force is generated within a chew cycle, and less so by varying temporal parameters. Rate modulation rather than time modulation may allow rhythmic mastication to proceed at a relatively constant frequency, simplifying motor control computation.

Development of a three‐dimensional finite element model of a human mandible containing endosseous dental implants. II. Variables affecting the predictive behavior of a finite element model of a human mandible

The purpose of this study was to propose a systematic approach to validate a finite element model (FEM) of the human mandible and to investigate the effects of changing the geometry and orthotropic material properties on the FEM predictions. Thirty-eight variables affecting the material properties, boundary conditions, and the geometry of a FEM of a human mandible, including two dental implants, were systematically changed, creating a number of FEMs of the mandible. The effects of the variations were quantified as differences in the principal strain magnitudes modeled by the original FEM (gold standard), prior to the sensitivity analyses, and those generated by the changed FEMs. The material properties that had the biggest impact on the predicted cortical principal strain were the shear moduli (up to 31% in difference from the unchanged state), and the absence of cancellous bone (up to 34%). Alterations to the geometry of the mandibular cross section, such as an increase in corpus dimensions, had the greatest effect on principal strain magnitudes (up to 16%). Changes in the cortical thickness in relation to the width of the corpus section modified strain more than alterations to the corpus depth (14% and 5%, respectively). The relatively small difference (up to 13.5%) between the predicted and measured interimplant distances indicates the accuracy of the FEM. Changes in geometry and orthotropic material properties could induce significant changes in strain patterns. These values must therefore be chosen with care when using finite element techniques for predicting stresses, strains, and displacements.

Mandibular Mechanics Following Osteotomy and Appliance Placement II: Bone Strain on the Body and Condylar Neck

The purpose of this investigation was to determine if the mechanical environment of the mandible is changed by osteotomy and fixation, as assessed by the measurement of bone strain on the condylar neck and mandibular corpus. Immediately following unilateral mandibular osteotomy and distractor placement, strain gauges were attached directly to the corpus and condylar neck in a sample of domestic pigs. Bone strains were recorded during mastication and muscle stimulation. Comparisons of principal strain magnitudes and orientations were made between sides and between the osteotomy sample and a control database. The animals preferred to chew on the non-osteotomy side. Corpus strains were higher for osteotomy-side chewing but were comparable to the control database, regardless of chewing side. For the condyle, compared with the control database and the non-osteotomy side, the osteotomy side was underloaded in compression. Furthermore, the orientation of compressive strain was highly variable and more horizontally oriented than that of control and non-osteotomy condyles. Stimulation of the masseter and medial pterygoid loaded the mandible to normal levels. Masticatory behavior was altered, probably as a combined result of disruption of the occlusion, changes in muscle recruitment, and probable loss of sensory feedback. However, neither these changes nor damage to the muscles explain the decrease and reorientation of compressive strain on the condylar neck. Alternatively, the modified strain pattern could have arisen from positional instability of the proximal bone fragment.

Shear Does Not Necessarily Inhibit Bone Healing

Interfragmentary shear has been perceived as inhibitory to bone healing. We think this is because of inadequate balance between stimulatory and disruptive interfragmentary displacement magnitudes in the shear direction. We hypothesized that pure shear is not necessarily detrimental to bone healing. This was investigated by comparing bone healing under interfragmentary torsional shear, axial compression, and no applied motion. Applied motion was controlled carefully with similar interfragmentary principal strain magnitudes found to stimulate healing under axial compression. The observation period was 8 weeks. Torsional rotation stimulated intercortical mineralized callus formation with greater area than the group without applied motion, and led to a stiffness and rate of bony bridging similar to that of the no motion group. Axial compression stimulated less intercortical mineralized callus of a lower density than the no motion group, and there also was little bridging. These results support the hypothesis that interfragmentary shear does not necessarily inhibit bone healing.

Torsional Strains in the Proximal Fifth Metatarsal: Implications for Jones and Stress Fracture Management

Reports of nonunion of proximal fifth metatarsal fractures treated by internal fixation indicate that current fixation methods do not always adequately address the stresses to which the bone is subjected during ambulation. In particular, the insertion sites of the peroneus brevis and peroneus tertius tendons on the fifth metatarsal suggest that their actions can impose torsional stresses on the areas of the bone in which Jones fractures and stress fractures occur. Intramedullary screw fixation, however, offers little resistance to rotation of the proximal and distal fragments relative to one another. To determine the potential for the existence of torsional stresses in the fifth metatarsal during post-operative ambulation, a simplified cadaver model of single-limb stance was used in which cadaver feet were subjected to concurrent axial and tendon forces while monitoring the outputs of stacked rosette strain gauges placed at the typical sites of Jones and stress fractures. Principal strain and shear strain magnitudes and directions were measured. The shear strain magnitudes and strain axis directions indicated the presence of torsional stresses in the underlying bone potentially capable of causing internal rotation of the proximal fragment relative to the distal end of the bone. This finding has implications for the treatment of both Jones fractures and stress fractures of the proximal fifth metatarsal. An internal fixation device that has the capability to resist torsion as well as tension and bending would appear optimal to treat these fractures.

Trabecular Bone Tissue Strains in the Healthy and Osteoporotic Human Femur

Quantitative information about bone tissue-level loading is essential for understanding bone mechanical behavior. We made microfinite element models of a healthy and osteoporotic human femur and found that tissue-level strains in the osteoporotic femoral head were 70% higher on average and less uniformly distributed than those in the healthy one. Bone tissue stresses and strains in healthy load-adapted trabecular architectures should be distributed rather evenly, because no bone tissue is expected to be overloaded or unused. In this study, we evaluate this paradigm with the use of microfinite element (microFE) analyses to calculate tissue-level stresses and strains for the human femur. Our objectives were to quantify the strain distribution in the healthy femur, to investigate to what extent this distribution is affected by osteoporosis, to determine if osteoporotic bone is simply bone adapted to lower load levels, and to determine the "safety factor" for trabecular bone. microFE models of a healthy and osteoporotic proximal femur were made from microcomputed tomography images. The models consisted of over 96 and 71 million elements for the healthy and osteoporotic femur, respectively, and represented their internal and external morphology in detail. Stresses and strains were calculated for each element and their distributions were calculated for a volume of interest (VOI) of trabecular bone in the femoral head. The average tissue-level principal strain magnitude in the healthy VOI was 304 +/- 185 microstrains and that in the osteoporotic VOI was 520 +/- 355 microstrains. Calculated safety factors were 8.6 for the healthy and 4.9 for the osteoporotic femurs. After reducing the force applied to the osteoporotic model to 59%, the average strain compared with that of the healthy femur, but the SD was larger (208 microstrains). Strain magnitudes in the osteoporotic bone were much higher and less uniformly distributed than those in the healthy one. After simulated joint-load reduction, strain magnitudes in the osteoporotic femur were very similar to those in the healthy one, but their distribution is still wider and thus less favorable.

The mechanical environment of the chondrocyte: a biphasic finite element model of cell–matrix interactions in articular cartilage

Mechanical compression of the cartilage extracellular matrix has a significant effect on the metabolic activity of the chondrocytes. However, the relationship between the stress–strain and fluid-flow fields at the macroscopic “tissue” level and those at the microscopic “cellular” level are not fully understood. Based on the existing experimental data on the deformation behavior and biomechanical properties of articular cartilage and chondrocytes, a multi-scale biphasic finite element model was developed of the chondrocyte as a spheroidal inclusion embedded within the extracellular matrix of a cartilage explant. The mechanical environment at the cellular level was found to be time-varying and inhomogeneous, and the large difference (∼3 orders of magnitude) in the elastic properties of the chondrocyte and those of the extracellular matrix results in stress concentrations at the cell–matrix border and a nearly two-fold increase in strain and dilatation (volume change) at the cellular level, as compared to the macroscopic level. The presence of a narrow “pericellular matrix” with different properties than that of the chondrocyte or extracellular matrix significantly altered the principal stress and strain magnitudes within the chondrocyte, suggesting a functional biomechanical role for the pericellular matrix. These findings suggest that even under simple compressive loading conditions, chondrocytes are subjected to a complex local mechanical environment consisting of tension, compression, shear, and fluid pressure. Knowledge of the local stress and strain fields in the extracellular matrix is an important step in the interpretation of studies of mechanical signal transduction in cartilage explant culture models.

Microvascular obstruction and left ventricular remodeling early after acute myocardial infarction.

The presence of microvascular obstruction (MO) within infarcted regions may adversely influence left ventricular (LV) remodeling after acute myocardial infarction. This study examined whether the extent of MO directly alters the mechanical properties of the infarcted myocardium. Seventeen dogs underwent 90 minutes of balloon occlusion of the left anterior descending coronary artery, followed by reperfusion. Gadolinium-enhanced perfusion MRI and 3D-tagging were performed 4 to 6 and 48 hours (8 animals) and 10 days (9 animals) after reperfusion. Early increase in LV end-diastolic volume (from 42+/-9 to 54+/-14 mL, P<0.05) between 4 to 6 and 48 hours after reperfusion was predicted by both extent of MO (r=0.89, P<0.01) and infarct size (r=0.83, P<0.01), defined as MRI hypoenhanced and hyperenhanced regions, respectively. Multivariate analysis demonstrated that extent of MO had better and independent value to predict LV volume than overall infarct size. A strong inverse relationship existed between magnitude of first principal strain (r=-0.80, P<0.001) and relative extent of MO within infarcted myocardium. Also, infarcted myocardium involved by extensive areas of MO demonstrated reductions of circumferential (r=-0.61, P<0.01) and longitudinal (r=-0.53, P<0. 05) stretching. Furthermore, significant reductions of radial thickening (9+/-6% versus 14+/-3%, P<0.01) occurred in noninfarcted regions adjacent to infarcts that had increased (>35%) amounts of MO. In the early healing phase of acute myocardial infarction, the extent of MO in infarcted tissue relates to reduced local myocardial deformation and dysfunction of noninfarcted adjacent myocardium. Such strain alterations might explain the increased remodeling observed in patients with large regions of MO.

In vivo locomotor strain in the hindlimb bones ofAlligator mississippiensisandIguana iguana: implications for the evolution of limb bone safety factor and non-sprawling limb posture

ABSTRACTLimb postures of terrestrial tetrapods span a continuum from sprawling to fully upright; however, most experimental investigations of locomotor mechanics have focused on mammals and ground-dwelling birds that employ parasagittal limb kinematics, leaving much of the diversity of tetrapod locomotor mechanics unexplored. This study reports measurements of in vivo locomotor strain from the limb bones of lizard (Iguana iguana) and crocodilian (Alligator mississippiensis) species, animals from previously unsampled phylogenetic lineages with non-parasagittal limb posture and kinematics. Principal strain orientations and shear strain magnitudes indicate that the limb bones of these species experience considerable torsion during locomotion. This contrasts with patterns commonly observed in mammals, but matches predictions from kinematic observations of axial rotation in lizard and crocodilian limbs. Comparisons of locomotor load magnitudes with the mechanical properties of limb bones in Alligator and Iguana indicate that limb bone safety factors in bending for these species range from 5.5 to 10.8, as much as twice as high as safety factors previously calculated for mammals and birds. Limb bone safety factors in shear (3.9–5.4) for Alligator and Iguana are also moderately higher than safety factors to yield in bending for birds and mammals. Finally, correlations between limb posture and strain magnitudes in Alligator show that at some recording locations limb bone strains can increase during upright locomotion, in contrast to expectations based on size-correlated changes in posture among mammals that limb bone strains should decrease with the use of an upright posture. These data suggest that, in some lineages, strain magnitudes may not have been maintained at constant levels through the evolution of a non-sprawling posture unless the postural change was accompanied by a shift to parasagittal kinematics or by an evolutionary decrease in body size.