Biomechanical Material Properties Research Articles

Identifiability of tissue material parameters from uniaxial tests using multi-start optimization

Determining tissue biomechanical material properties from mechanical test data is frequently required in a variety of applications. However, the validity of the resulting constitutive model parameters is the subject of debate in the field. Parameter optimization in tissue mechanics often comes down to the “identifiability” or “uniqueness” of constitutive model parameters; however, despite advances in formulating complex constitutive relations and many classic and creative curve-fitting approaches, there is currently no accessible framework to study the identifiability of tissue material parameters. Our objective was to assess the identifiability of material parameters for established constitutive models of fiber-reinforced soft tissues, biomaterials, and tissue-engineered constructs and establish a generalizable procedure for other applications. To do so, we generated synthetic experimental data by simulating uniaxial tension and compression tests, commonly used in biomechanics. We then fit this data using a multi-start optimization technique based on the nonlinear least-squares method with multiple initial parameter guesses. We considered tendon and sclera as example tissues, using constitutive models that describe these fiber-reinforced tissues. We demonstrated that not all the model parameters of these constitutive models were identifiable from uniaxial mechanical tests, despite achieving virtually identical fits to the stress-stretch response. We further show that when the lateral strain was considered as an additional fitting criterion, more parameters are identifiable, but some remain unidentified. This work provides a practical approach for addressing parameter identifiability in tissue mechanics.

Practical aspects of finite element method applications in dentistry

The use of numerical methods, such as finite element method (FEM), has been widely adopted in solving structural problems with complex geometry under external loads when analytical solutions are unachievable. Basic idea behind FEM is to divide the complex body geometry into smaller and simpler domains, called finite elements, and then to formulate solution for each element instead of seeking a solution for the entire domain. After finding the solutions for all elements they can be combined to obtain a solution for the whole domain. This numerical method is mostly used in engineering, but it is also useful for studying the biomechanical properties of materials used in medicine and the influence of mechanical forces on the biological systems. Since its introduction in dentistry four decades ago, FEM became powerful tool for the predictions of stress and strain distribution on teeth, dentures, implants and surrounding bone. FEM can indicate aspects of biomaterials and human tissues that can hardly be measured in vivo and can predict the stress distribution in the contact areas which are not accessible, such as areas between the implant and cortical bone, denture and gingiva, or around the apex of the implant in trabecular bone. Aim of this paper is to present - using results of several successful FEM studies - the usefulness of this method in solving dentistry problems, as well as discussing practical aspects of FEM applications in dentistry. Some of the method limitations, such as impossibility of complete replication of clinical conditions and need for simplified assumptions regarding loads and materials modeling, are also presented. However, the emphasis is on FE modelling of teeth, bone, dentures and implants and their modifications according to the requirements. All presented studies have been carried out in commercial software for FE analysis ANSYS Workbench.

Multimodal biomechanical methods supporting abdominal aortic aneurysm management

In clinical practice, management of abdominal aortic aneurysms (AAA) is predominantly based on the diameter of the lumen. State-of-the-art fluid structure interaction (FSI) simulations have proven to be superior in estimating rupture risk, although they still have considerable shortcomings. In this paper we address one such unresolved problem, and present the outline of biomechanical material parameter distribution identification via inverse finite element (FE) analysis. Our method is based on the non invasive approximation of the displacement field using electrocardiogram-gated computer tomography angiography (ECG-gated CT-angiography, CTA) and estimating the load field with usual computational fluid dynamics (CFD) simulations. Parameters of our model connecting the two abovementioned sets of variables result from an optimization algorithm minimizing a work and energy related variational functional. Consequently, also supported by experimental measurements, we are able to assess local distribution of the biomechanical material properties of the arterial wall and give a functional characterization of vessel wall degeneration. DOI: 10.17489/biohun/2015/2/06 Normal 0 21 false false false HU X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:Normal tablazat; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:Calibri,sans-serif; mso-bidi-font-family:Times New Roman;}

Effect of material property heterogeneity on biomechanical modeling of prostate under deformation

Biomechanical model based deformable image registration has been widely used to account for prostate deformation in various medical imaging procedures. Biomechanical material properties are important components of a biomechanical model. In this study, the effect of incorporating tumor-specific material properties in the prostate biomechanical model was investigated to provide insight into the potential impact of material heterogeneity on the prostate deformation calculations. First, a simple spherical prostate and tumor model was used to analytically describe the deformations and demonstrate the fundamental effect of changes in the tumor volume and stiffness in the modeled deformation. Next, using a clinical prostate model, a parametric approach was used to describe the variations in the heterogeneous prostate model by changing tumor volume, stiffness, and location, to show the differences in the modeled deformation between heterogeneous and homogeneous prostate models. Finally, five clinical prostatectomy examples were used in separately performed homogeneous and heterogeneous biomechanical model based registrations to describe the deformations between 3D reconstructed histopathology images and ex vivo magnetic resonance imaging, and examine the potential clinical impact of modeling biomechanical heterogeneity of the prostate. The analytical formulation showed that increasing the tumor volume and stiffness could significantly increase the impact of the heterogeneous prostate model in the calculated displacement differences compared to the homogeneous model. The parametric approach using a single prostate model indicated up to 4.8 mm of displacement difference at the tumor boundary compared to a homogeneous model. Such differences in the deformation of the prostate could be potentially clinically significant given the voxel size of the ex vivo MR images (0.3 × 0.3 × 0.3 mm). However, no significant changes in the registration accuracy were observed using heterogeneous models for the limited number of clinical prostatectomy patients modeled and evaluated in this study.

Comparison of Clinical and Radiological Parameters with Two Different Surgical Methods for Anterior Cruciate Ligament Reconstruction

Objectives:We investigated the effects anatomic or nonanatomic femoral tunnel positions and tunnel fixation methods obtained using two different surgery methods on tunnel widening and clinical results in anterior cruciate ligament (ACL) reconstructions.Methods:Patients with isolated anterior cruciate ligament rupture are included to study who don’t have intra-and extra-articular additional pathology of the knee, without previously a history of operations of both knees. 2 groups were created. Group 1 Aperfix implant were used which can be able to perform non anatomical femoral tunnel and intra tunnel fixation with transtibial technique. In Group 2 Endobutton CL implant were used which can make fixation from outside the cortex with anatomic femoral tunnel in use of anteromedial portal techniques. 27 patients (average age 29,33, range 18 to 55 years) in group 1 and 27 patients (average age 27,51, range 16 to 45 years) in group 2 totally 54 patients were performed surgery. All patients were assessed using the IKDC (International knee documentation committee), Tegner Activity Scala and Lysholm II Functional Scores. Muscle strength measurements in both groups compared to intact knee was measured with an isokinetic dynamometer Biodex System 3 Pro. The location of the femoral tunnel aperture and tunnel widening were imaged with 3D reconstructive computed tomography. All measurements were performed using the same software application by the same radiologist.Results:The two groups were similar with respect to age and sex distribution, operated side, the size of the tunnel created, and follow-up period (p>0.05). After surgery in both groups, the clinical scores showed significant improvement compared to preoperative (p=0,0001). However, postoperative clinical outcomes in the two groups did not show a difference significantly (p>0,005). Isokinetic muscle strength study showed significant differences between the two groups (p=0,0001). Location of femoral tunnel aperture on the medial wall of the lateral femoral condyle showed a significant differences in the two groups (p=0,0001). The expansion of proximal and distal femoral tunnel in two groups showed significant differences (p=0,0001). There was relationship between distal femoral tunnel widening and location of femoral tunnel aperture.Conclusion:Although there is no statistically significant difference between the two groups clinically, difference noticed in terms of isokinetic muscle strength may be due to differences in the degree of shift as a result of multiple loading depending on the biomechanical properties of materials. We thought that the difference seen in the widening of tunnel in the proximal or distal may be due to, the technique of graft fixation, the distance between the fixation point and the joint, and to the location of the femoral tunnel aperture on medial wall of lateral condyle from anatomical or non anatomical region. There is no golden standard in neither surgical technique nor material of fixation. Proper theoretical knowledge and extensive clinical experience are important in the light of an accurate surgical technique applied. We thought that information we have reached in our study should be supported by biomechanical studies

Evaluation of a novel poly N-acetyl glucosamine (pGlcNAc) hydrogel for treatment of the degenerating intervertebral disc

AimsThe early stages of degenerative disc disease (DDD) primarily affect the disc nucleus pulposus (NP). Tissue-engineered strategies may enhance intervertebral disc (IVD) functionality. The aim of this study was to develop and evaluate a novel deacetylated poly-N-acetyl glucosamine (pGlcNAc) hydrogel characterizing its biochemical effect on human IVD cells as well as material biomechanical properties. Main methodsA novel deacetylated derivative of a marine diatom-derived glycosaminoglycan was developed into a hydrogel formulation as a potential therapy to treat degenerating IVD NP. In vitro biochemical studies were conducted using primary human disc cell cultures to evaluate cell viability, metabolic activity, proteoglycan and extracellular matrix protein expression. The biomechanical hydration kinetics and viscoelastic behavior of the hydrogel were determined and compared with the behavior of human lumbar NP. Key findingsDisc cell viability, metabolic activity, and proteoglycan content of the treated cells were observed to be significantly greater in experimental samples when compared to untreated control groups. RT-PCR and immunohistochemical data corroborated the expression of characteristic NP disc markers, aggrecan and type II collagen in cultured cells. Rheological data demonstrated that the elastic component of the hydrogel dominated the viscous component over a frequency range of 0.1 to 15.85rad/s. Of several formulations evaluated, a sulphated, deactylated derivative of the nanofiber derived pGlcNAc hydrogel demonstrated the most robust biologic effects on cell viability, metabolic activity, and proteoglycan expression. SignificanceThis in vitro study using human disc cells demonstrates that a sulphated deacetylated glycosaminoglycan derivative hydrogel possesses promising characteristics motivating further evaluation as a potential therapy for NP degeneration.

Carbon fiber reinforced PEEK Optima—A composite material biomechanical properties and wear/debris characteristics of CF-PEEK composites for orthopedic trauma implants

BackgroundThe advantageous properties of carbon fiber reinforced polyetheretherketone (CF-PEEK) composites for use as orthopedic implants include similar modulus to bone and ability to withstand prolonged fatigue strain. MethodsThe CF-PEEK tibial nail, dynamic compression plate, proximal humeral plate and distal radius volar plate were compared biomechanically (by four-point bending, static torsion of the nail, and bending fatigue) and for wear/debris (by amount of the debris generated at the connection between the CF-PEEK plate and titanium alloy screws) to commercially available devices. ResultsFour-point bending stress of the tibial nail and dynamic and distal radius plates yielded characteristics similar to other commercially available devices. The distal volar plate bending structural stiffness of the CF-PEEK distal volar plate was 0.542Nm2 versus 0.376Nm2 for the DePuy's DVR anatomic volar plate. The PHILOS proximal humeral internal locking system stainless steel plate was much stronger (6.48Nm2) than the CF-PEEK proximal humeral plate (1.1Nm2). Tibial nail static torsion testing showed similar properties to other tested nails (Fixion, Zimmer and Synthes). All tested CF-PEEK devices underwent one million fatigue cycles without failure. Wear test showed a lower volume of generated particles in comparison to the common implants in use today. InterpretationThus, these tested implants were similar to commercially used devices and can be recommended for use as implants in orthopedic surgery.

A comparison of the shock-absorbing properties of cervical disc prosthesis bearing materials

Background Data Cervical arthroplasty offers theoretical advantages over traditional spinal fusion, including elimination of adjacent segment disease and elimination of the risk of pseudoarthrosis formation. Initial studies of cervical arthroplasty have shown promising results, however, the ideal design characteristics for disc replacement constructs have not been determined. The current study seeks to quantify the differences in the shock absorption characteristics of three commonly used materials in cervical disc arthroplasty. Methods Three different nucleus materials, polyurethane (PU), polyethylene (PE) and a titanium-alloy (Ti) were tested in a humidity- and temperature-controlled chamber. Ten of each nucleus type underwent three separate mechanical testing protocols to measure 1) dynamic stiffness, 2) quasi-static stiffness, 3) energy absorption, and 4) energy dissipation. The results were compared using analysis of variance. Results PU had the lowest mean dynamic stiffness (435 ± 13 N/mm, P < .0001) and highest energy absorption (19.4 ± 0.1 N/mm, P < .0001) of all three nucleus materials tested. PU was found to have significantly higher energy dissipation (viscous damping ratio 0.017 ± 0,001, P < .0001) than the PE or TI nuclei. PU had the lowest quasi-static stiffness (598 ± 23 N/mm, P < .0001) of the nucleus materials tested. A biphasic response curve was observed for all of the PU nuclei tests. Conclusions Polyurethane absorbs and dissipates more energy and is less stiff than either polyethylene or titanium. Level of Evidence Basic Science/Biomechanical Study. Clinical Relevance This study characterizes important differences in biomechanical properties of materials that are currently being used for different cervical disc prostheses.

Effects of the combination treatment of raloxifene and alendronate on the biomechanical properties of vertebral bone

Raloxifene (RAL) and alendronate (ALN) improve the biomechanical properties of bone by different mechanisms. The goal here was to investigate the effects of combination treatment of RAL and ALN on the biomechanical properties of vertebral bone. Six-month-old Sprague-Dawley rats (n = 80) were randomized into five experimental groups (sham, OVX, OVX + RAL, OVX + ALN, and OVX + RAL + ALN; n = 16/group). Following euthanization, structural and derived material biomechanical properties of vertebral bodies were assessed. Density and dynamic histomorphometric measurements were made on cancellous bone. The results demonstrate that the structural biomechanical properties of vertebral bone are improved with the combination treatment. Stiffness and ultimate load of the OVX + RAL and OVX + ALN groups were significantly lower than those of sham animals, but the combination treatment with RAL + ALN was not significantly different from sham. Furthermore, the OVX + RAL + ALN group was the only agent-treated group in which the ultimate load was significantly higher than that in OVX animals (p < .05). Cancellous bone fractional volume (BV/TVcanc) and bone mineral density (aBMD) also were improved with the combination treatment. BV/TVcanc of the OVX + RAL + ALN group was 6.7% and 8.7% greater than that of the OVX + RAL (p < .05) and OVX + ALN (p < .05) groups, respectively. Areal BMD of the OVX + RAL or OVX + ALN groups was not significantly different from that in OVX animals, but the value in animals undergoing combination treatment was significantly higher than that in OVX or OVX + RAL animals alone and not significantly different from that in sham-operated animals. Turnover rates of both the RAL + ALN and ALN alone groups were lower than in the RAL-treated alone group (p < .05). We conclude that the combination treatment of raloxifene and alendronate has beneficial effects on bone volume, resulting in improvement in the structural properties of vertebral bone. © 2011 American Society for Bone and Mineral Research.

Pseudotumor Cerebri: A New Concept of Pathophysiology

The diagnosis of pseudotumor cerebri has many hypotheses about its pathophysiological pathways. A review of the literature about this diagnosis has led to a revision of pathophysiological pathways regarding the pseudotumor cerebri as nonthrombotic theory. The author studied the physiological phenomena of bridging veins and brain outflow. Biomechanical material properties of the bridging brain veins were investigated experimentally and theoretically. The main goal of developed theory is to formulate the biomechanical conditions (geometrical dimensions, viscoelastic properties of veins, and blood fluid flow conditions) at which an unstable behavior or even the vein collapse can occur. The Neo-Hook's material model was used to determine the analytical formula for the collapse conditions. It was proved that for the brain-vein contraction (approximately 5%), the vein collapse can occur even under normal physiological conditions into vessels-the angiosynizesis. The weak formulation is based on the virtual work principle. To describe the wider class of problems, three material models were simulated: the Neo-Hook, the isotropic Gent, and the anisotropic Gent to include the influence of collagen structure of blood vessels. Simultaneous clinical observations (histological findings), in vitro experiments, and numerical modeling give sufficient data to predict biomechanical conditions of the angiosynizesis. The fluid structure interaction is studied experimentally on the special experimental line. The biomechanical, histological study and biomechanical and the mathematical modeling of the function of bridging veins and venous sinuses allowed development of new pathways for this diagnosis.

Simulated evolution of the vertebral body based on basic multicellular unit activities

A numerical model based on the theory of bone remodeling is proposed to predict the evolution of trabecular bone architecture within the vertebral body and to investigate the process of degeneration in vertebral bone. In this study, particular attention is paid on the description of microstructure changes during the aging process. To take into account the effect of basic multicellular units (BMUs), a set of computational algorithms has been developed. It is assumed that BMU activation probability depends on the state of damaged bone tissue (damage accumulation, ω), which is evaluated according to previous research concerning bone fatigue damage. Combining these algorithms with the finite-element method (FEM), the microstructure of vertebral bone has been predicted for up to 8 simulated years. Moreover, biomechanical material properties have been monitored to investigate the changes of vertebral bone with age. This study shows that the simulation based on BMU activities has the potential to define and predict the morphological evolution of the vertebral body. It can be concluded that the novel algorithms incorporating the coupled effects of both adaptive remodeling and microdamage remodeling could be utilized to gain greater insight into the mechanism of bone loss in the elderly population.

Bisphosphonate effects on bone turnover, microdamage, and mechanical properties: What we think we know and what we know that we don't know

The bisphosphonates (BPs) have been useful tools in our understanding of the role that bone remodeling plays in skeletal health. The purpose of this paper is to outline what we know, and what is still unknown, about the role that BPs play in modulating bone turnover, how this affects microdamage accumulation, and ultimately what the effects of these changes elicited by BPs are to the structural and the material biomechanical properties of the skeleton. We know that BPs suppress remodeling site-specifically, probably do not have a direct effect on formation, and that the individual BPs vary with respect to speed of onset, duration of effect and magnitude of suppression. However, we do not know if these differences are meaningful in a clinical sense, how much remodeling is sufficient, the optimal duration of treatment, or how long it takes to restore remodeling to pre-treatment levels following withdrawal. We also know that suppression is intimately tied to microdamage accumulation, which is also site-specific, that BPs impair targeted repair of damage, and that they can reduce the energy absorption capacity of bone at the tissue level. However, the BPs are clearly effective at preventing fracture, and generally increase bone mineral density and whole bone strength, so we do not know whether these changes in damage accumulation and repair, or the mechanical effects at the tissue level, are clinically meaningful. The mechanical effects of BPs on the fatigue life of bone, or BP effects on bone subject to an impact, are entirely unknown. This paper reviews the literature on these topics, and identifies gaps in knowledge that can be addressed with further research.

Role of genetic background in determining phenotypic severity throughout postnatal development and at peak bone mass in Col1a2 deficient mice ( oim)

Osteogenesis imperfecta (OI) is a genetically and clinically heterogeneous disease characterized by extreme bone fragility. Although fracture numbers tend to decrease post-puberty, OI patients can exhibit significant variation in clinical outcome, even among related individuals harboring the same mutation. OI most frequently results from mutations in type I collagen genes, yet how genetic background impacts phenotypic outcome remains unclear. Therefore, we analyzed the phenotypic severity of a known proα2(I) collagen gene defect ( oim) on two genetic backgrounds (congenic C57BL/6J and outbred B6C3Fe) throughout postnatal development to discern the phenotypic contributions of the Col1a2 locus relative to the contribution of the genetic background. To this end, femora and tibiae were isolated from wildtype (Wt) and homozygous ( oim/oim) mice of each strain at 1, 2 and 4 months of age. Femoral geometry was determined via μCT prior to torsional loading to failure to assess bone structural and material biomechanical properties. Changes in mineral composition, collagen content and bone turnover were determined using neutron activation analyses, hydroxyproline content and serum pyridinoline crosslinks. μCT analysis demonstrated genotype-, strain- and age-associated changes in femoral geometry as well as a marked decrease in the amount of bone in oim/oim mice of both strains. Oim/oim mice of both strains, as well as C57BL/6J (B6) mice of all genotypes, had reduced femoral biomechanical strength properties compared to Wt at all ages, although they improved with age. Mineral levels of fluoride, magnesium and sodium were associated with biomechanical strength properties in both strains and all genotypes at all ages. Oim/oim animals also had reduced collagen content as compared to Wt at all ages. Serum pyridinoline crosslinks were highest at two months of age, regardless of strain or genotype. Strain differences in bone parameters exist throughout development, implicating a role for genetic background in determining biomechanical strength. Age-associated improvements indicate that oim/oim animals partially compensate for their weaker bone material, but never attain Wt levels. These studies indicate the importance of genetic background in determining phenotypic severity, but the presence of the proα2(I) collagen gene defect and age of the animal are the primary determinants of phenotypic severity.

Parathyroid Hormone and Bone Biomechanics

Parathyroid hormone (PTH) treatment, either in the form of teriparatide or recombinant human PTH(1–34), reduces the fracture risk of osteoporotic women by enhancing both structural and material biomechanical properties. Cortical bone thickness and cross-sectional moment of inertia increase because of new bone formation on periosteal and endocortical surfaces. Intracortical porosity is increased yet preferential localization near the endocortical surface limits any negative biomechanical effects. Greater trabecular bone volume results from a positive basic multicellular unit bone balance and renewed modeling. Initial increases in trabecular thickness are eventually reduced as a result of trabecular tunneling, increasing trabecular number, and connectivity. These geometrical and architectural alterations are the predominant mechanism by which PTH increases structural strength, stiffness, and energy absorption. PTH also positively alters material-level strength, modulus, and toughness. These changes are counterintuitive based on known effects of decreasing average tissue mineralization and increasing tissue heterogeneity. This combination of enhanced structural and material biomechanical properties makes PTH an effective treatment for reducing fracture risk despite smaller gains in bone mineral density compared to other osteoporosis agents.

Biomechanical properties of materials used in static facial suspension.

To compare the biomechanical properties of the superficial (human acellular dermis); (AlloDerm; LefeCell Corp, Branchburg, NJ) and deep layers of cadaveric dermis and expanded polytetrafluoroethylene (ePTFE); (Gore-Tex; W. L. Gore & Associates, Flagstaff, Ariz). Sixteen samples of superficial dermis (AlloDerm), 12 samples of deep dermis, and 12 samples of ePTFE were axial loaded on a materials testing machine. Maximum load to failure and stiffness were calculated and statistical analysis was performed to compare the materials. Dermis samples had statistically greater mean stiffness compared with ePTFE samples. There was no statistical difference of maximum load to failure comparing ePTFE with superficial dermis. There was a statistical difference in maximum load to failure between ePTFE and deep dermis. There was no statistical difference between the superficial and deep layers of the dermis with respect to stiffness or maximum load to failure. Cadaveric dermis has some biomechanical properties to be a superior material for static facial suspension. There was larger than expected variability in both parameters (stiffness and maximum load to failure) tested in dermis samples, which may correlate with occasional clinical failure.

Effects of different exercise modes on mineralization, structure, and biomechanical properties of growing bone

Weight bearing during exercise plays an important role in improving the mechanical properties of bone. The effect on bone of non-weight-bearing exercise such as swimming remains controversial. To investigate the effects of exercise mode on growing bone, 29 male Wistar rats (7 wk old) were randomly assigned to a running exercise group (Run, n = 9), a swimming exercise group (Swim, n = 10), or a nonexercise control group (Con, n = 10). During an 8-wk training session (20-60 min/day, 5 days/wk), the Run rats were trained at progressively increasing running speeds (12-22 m/min), and weights attached to the tail of the Swim rats were progressively increased from 0 to 2% of their body weight. The bone mineral density of the proximal tibiae of the Run rats was significantly higher than in the Swim (P < 0.05). Femoral wet weights of the two exercise groups were significantly higher than in the control group (P < 0.05). Interestingly, the percent difference between the tissue wet weight and dry weight (water content ratio), which is related to bone mechanical properties, was significantly higher in the tibiae of the Swim rats and the femora of both exercise groups compared with controls (P < 0.05). Extrinsic as well as intrinsic biomechanical material properties were measured in a three-point bending test. Bone mechanical properties of the tibiae and femora of rats in the Swim and Run groups were significantly greater than those in the control group (P < 0.05). In summary, different modes of exercise may benefit bone mechanical properties in different ways. The specific effects of swimming exercise (non-weight-bearing exercise) on bone require further study.

Evolution of the biomechanical material properties of the femur.

The biomechanical performance of long bones is dictated by four key factors: element size, element shape, loading conditions, and material properties. Our understanding of the latter of these has been mostly limited to eutherian mammals and birds, which show similarity. Whether their possession of comparable material properties reflects common ancestry or independent evolution is uncertain. In the present analysis, we tested the bending strength, modulus, and failure strains of the femur and its pterygiophore homolog in actinpterygian fish. Sixty-nine specimens representing basal character states in seven major vertebrate crown clades were tested. These data were then coupled with avian and mammalian data from the literature and analyzed in an evolutionary context using phylogenetic character analysis. Mean values of 188 MPa for yield strength, 22.4 GPa for Young's modulus, and 8,437 mu epsilon for yield strain were obtained for the long bones. Analysis of variance (ANOVA) revealed comparable values between clades that span a 30,000-fold range of body mass. We conclude that material properties of the first long bones 475 million years ago were conserved throughout evolution. Major locomotory challenges to femora during vertebrate evolution were almost solely accomplished by modifications of element size and shape.

Healing of Canine Femoral Osteotomies

The effects of rigid self-compressing plates were compared with the effects of less rigid internal contact splints on the healing characteristics of osteotomized canine femora. Radiographically, the rigid fixation afforded by the compression plate produced accelerated healing as compared with the less rigid fixation obtained with splints at two to four months. At six months there was little or no difference between the two groups. Comparison of the material biomechanical properties of the healing femora suggested that the repair tissue formed early after compression plating was stronger than the callus tissue that surrounded the less rigidly fixed femora. A comparison of the structural biomechanical properties of the bones suggested, conversely, that a stronger whole bone resulted from the less rigid fixation splint. The histomorphometric data showed that some cortical bone was lost under both types of fixation plates. However, the patterns of loss were not comparable. Use of rigid internal fixation led to loss of cortical bone after four to six months, while the less rigid fixation obtained with the Eggers splint produced mild intracortical resorption only. These observations point to the desirability of rigid internal fixation early in the healing process and the possible benefit of decreasing the rigidity of the fixation device as healing proceeds.