STRUCTURAL BASIS FOR THE STATIC MECHANICAL PROPERTIES OF THE AORTIC MEDIA

The arrangement and interrelation of the structural components of the rabbit aorta were studied by light and electron microscopy. Segments of abdominal aorta were restored to in vivo length and fixed in formalin or osmic acid while intraluminal pressures ranging from 0 to 200 mm Hg were maintained by a constant pressure perfusion apparatus. Transverse, longitudinal, and tangential sections of vessels fixed at various distending pressures were examined. Micrometric measurements included vessel diameters and wall thickness, thickness and waviness of elastin lamellae and interlamellar distances.

With increasing pressures below the diastolic value, aortic radius increased and wall thickness decreased rapidly. Waviness of the tubular elastin lamellae decreased uniformly throughout the wall. Interlamellar distances decreased uniformly and markedly. Lamellar thicknesses decreased uniformly but much less than interlamellar distances. A fine fibrillary elastin network connected the thick lamellae. Collagen fibers showed no definite pattern of orientation.

At and above diastolic pressure radius and wall thickness changed little with increasing pressures. Elastin lamellae were straight and interlamellar distances were uniform; the fibrils of the interlamellar elastin net were arranged obliquely. Collagen fibers were arranged nearly circumferentially. Collagen and elastin fibers were closely intermingled in the narrow interlamellar space but no collagen-elastin connections were obserevd.

The mechanical properties and organization of the collagen and elastin components of the aortic media indicate that the wall normally functions as a "two-phase" material. At and above physiological pressures, circumferentially aligned collagen fibers of high tensile strength and relatively high modulus of elasticity bear most of the stressing force. Elastin lamellae and fibrils of relatively low modulus of elasticity distribute stressing forces uniformly.

Attempts to assess the role of medial pressure and tension gradients in the pathogenesis of aortic disease must take into account the special mechanical properties of this "two-phase" material.

Elastic fibres in health and disease

Elastic fibres are a major class of extracellular matrix fibres that are abundant in dynamic connective tissues such as arteries, lungs, skin and ligaments. Their structural role is to endow tissues with elastic recoil and resilience. They also act as an important adhesion template for cells, and they regulate growth factor availability. Mutations in major structural components of elastic fibres, especially elastin, fibrillins and fibulin-5, cause severe, often life-threatening, heritable connective tissue diseases such as Marfan syndrome, supravalvular aortic stenosis and cutis laxa. Elastic-fibre function is also frequently compromised in damaged or aged elastic tissues. The ability to regenerate or engineer elastic fibres and tissues remains a significant challenge, requiring improved understanding of the molecular and cellular basis of elastic-fibre biology and pathology, and ability to regulate the spatiotemporal expression and assembly of its molecular components.

Fluid-structure interaction models of the mitral valve: function in normal and pathological states

Successful mitral valve repair is dependent upon a full understanding of normal and abnormal mitral valve anatomy and function. Computational analysis is one such method that can be applied to simulate mitral valve function in order to analyse the roles of individual components and evaluate proposed surgical repair. We developed the first three-dimensional finite element computer model of the mitral valve including leaflets and chordae tendineae; however, one critical aspect that has been missing until the last few years was the evaluation of fluid flow, as coupled to the function of the mitral valve structure. We present here our latest results for normal function and specific pathological changes using a fluid-structure interaction model. Normal valve function was first assessed, followed by pathological material changes in collagen fibre volume fraction, fibre stiffness, fibre splay and isotropic stiffness. Leaflet and chordal stress and strain and papillary muscle force were determined. In addition, transmitral flow, time to leaflet closure and heart valve sound were assessed. Model predictions in the normal state agreed well with a wide range of available in vivo and in vitro data. Further, pathological material changes that preserved the anisotropy of the valve leaflets were found to preserve valve function. By contrast, material changes that altered the anisotropy of the valve were found to profoundly alter valve function. The addition of blood flow and an experimentally driven microstructural description of mitral tissue represent significant advances in computational studies of the mitral valve, which allow further insight to be gained. This work is another building block in the foundation of a computational framework to aid in the refinement and development of a truly non-invasive diagnostic evaluation of the mitral valve. Ultimately, it represents the basis for simulation of surgical repair of pathological valves in a clinical and educational setting.

The dominant and recessive forms of cutis laxa

Images

Relationship of radius to pressure along the aorta in living dogs

The instantaneous pressure-radius relationship and mean dimensions were studied at various sites along the aorta in 30 dogs. The pressure-radius relationship was studied in the living state using an electrical caliper to measure the radius continuously through the cardiac cycle. The relative dimensions of the aortic tree at branching sites were calculated from Jeltrate casts made immediately post mortem. The results indicate: 1) The value of ΔR/ΔP and the mean radius decreased with distance along the aorta. 2) The calculated value for Ep, the “pressure-strain elastic modulus” increased monotonically from the ascending aorta to the bifurcation. 3) An “average” arterial tree was constructed from the mean values of the radius obtained from the caliper data and the branching ratios obtained from the casts.

elastic properties, aorta; dimensions, dog aorta

Submitted on March 5, 1963

Elastic properties of single elastic fibers

The elastic properties of elastic tissue were studied in a situation which minimized the effects of extraneous connective tissue and of the position of fibers in the elastic network. Single elastic fibers were dissected free from the ligamentum nuchae of the ox and were stretched under conditions of constant temperature and salinity. The strain was an exponential function of the applied tension. Single fibers were found somewhat less stretchable than the ligaments from which they were taken. The data given can be used to calculate the contribution of such elastic fibers to the behavior of an elastic system in which they are incorporated.

Submitted on August 7, 1961

Acellular dermal matrix allografts to achieve increased attached gingiva. Part 2. A histological comparative study

BACKGROUND

In part 1 of this study, we compared the clinical efficacy of freeze-dried acellular dermal matrix (ADM) allograft in 6 patients with autogenous free gingival graft (FGG) in 6 patients for increasing the width of attached gingiva in the mandibular anterior area. The purpose of the present study was to histologically compare the microstructure of ADM and FGG treated sites from the same group.

METHODS

Biopsies were harvested from all 12 patients at 6 months postsurgery. The biopsies included the grafted sites with adjacent alveolar mucosa and gingiva propria and also donor palatal mucosa saved at the time of surgery. The 5 microm thick, neutral buffered formalin fixed, paraffin-embedded tissue sections were stained with hematoxylin and eosin (H&E), Masson's trichrome, and Verhoeff-van Gieson stains in order to investigate the density of collagen and elastic fibers. Additional sections were stained with periodic acid-Schiff (PAS) and Papanicolaou's stain to identify the presence of glycogen granules in the epithelial layer and to highlight the keratin layer respectively.

RESULTS

The unique appearance of ADM-derived tissue did not parallel any known oral mucosa. The connective tissue portion contained dense to extremely dense collagen fibers along with scattered elastic fibers. The demarcations between the ADM graft and the coronal gingiva as well as the apical alveolar mucosa were usually not very defined. A moderate to thin epithelial layer, with heterogeneous expression of keratinization and flat epithelium-connective tissue interface, covered the lamina propria. Both the thickness of the epithelium and the degree of keratinization decreased in apical direction, being mostly para- or orthokeratinized in the area close to gingiva and non-keratinized adjacent to the alveolar mucosa. In the FGG-treated sites, the density of collagen fibers was less than in ADM-derived tissue, palatal mucosa, and gingiva. Elastic fibers were very sparse, comparable to gingiva, but much less than in ADM-derived tissue. The epithelium was moderate, somewhat thinner but the shape of the rete ridges resembled that of palatal mucosa. Similar to gingiva epithelium, the epithelium of the FGG-treated area was relatively uniform in both thickness and keratinization, mostly para-keratinized with a well defined border to the non-keratinized alveolar mucosa. Underneath the FGG-alveolar mucosa junction, a scar band composed of extremely dense collagen fibers consistently existed.

CONCLUSIONS

The results of this 6-month histological evaluation suggest that: 1) the resultant tissue types of ADM grafts were similar to "scar" tissue; 2) the non-vital dermal matrix of ADM allograft lacked the capability of directing cyto-differentiation of the covering epithelium; 3) autogenous FGG-derived tissue was neither identical to donor palatal mucosa nor to adjacent gingiva propria; 4) the connective tissue of donor palatal mucosa only partially contributed to the differentiation of the epithelium covering the FGG-treated area; and 5) the epithelium/connective tissue microenvironment surrounding the recipient site influenced the epithelial differentiation of the graft; this may play a more critical role in ADM grafting than in the grafting of autogenous FGG.

In vitro measurement of corneal strain, thickness, and curvature using digital image processing

A video camera, a microscope, and a PC-based digital image processing board were assembled for in vitro studies of corneal biomechanics. Central tangential corneal strain was determined by placing pairs of tiny mercury droplets on the epithelial surface and on the endothelial surface of the cornea. A distance of 3.2 mm could be determined with a standard deviation of 1.2 micron. Central corneal radius of curvature was measured from digital traces of the corneal surface contour. The standard deviation on estimating the radius of an 8 mm steel sphere was 15 microns. Corneal thickness was measured by digital optical pachometry. The standard deviation on measuring a thickness of 1 mm was 4.6 microns. The corneal extensibility was investigated in a total of 10 enucleated human eyes with increased corneal hydration. Tension was applied by varying the intraocular pressure from 2 to 100 mmHg with a column of isotonic saline. The epithelial side corneal strain and the increase in corneal curvature were approximately 1% for a change in intraocular pressure from 2 to 100 mmHg. When a high intraocular pressure was kept constant for 2 h the corneal thickness, radius of curvature, and the epithelial side strain gradually decreased, whereas the endothelial side strain increased. The elastic and visco-elastic behaviour of the human cornea was found to be closely related to changes in corneal hydration. We found digital image processing useful for in vitro biomechanical studies of the cornea.

Early changes in optic disc compliance and surface position in experimental glaucoma

PURPOSE

To detect changes in the compliance and baseline position (position at the baseline time point of a compliance test) of the monkey optic disc after the onset of chronic experimental glaucoma.

METHODS

Sixty-six compliance tests were performed on 26 eyes of 13 monkeys. Longitudinal Study. In seven normal monkeys, compliance tests were performed three times in one eye (study eye) and once in the contralateral eye. In the study eye of five of these monkeys, chronic experimental glaucoma was then induced and compliance tests were performed at some or all of the following postglaucoma testing intervals: 1 to 2 weeks, 3 to 4 weeks, 5 to 8 weeks, 9 to 12 weeks, 13 to 18 weeks, and more than 18 weeks after the onset of elevated intraocular pressure (IOP). In the study eye of the remaining two monkeys, the optic nerve was transected, and compliance was tested at 5, 9, and 13 weeks after transection. An analysis of variance (ANOVA) was performed to detect an increase (hypercompliance) or decrease (rigidity) in the compliance of the glaucomatous eyes at each testing interval. A second ANOVA was performed to detect the onset of chronic posterior deformation of the baseline position of each disc. Cross-Sectional Study. In six additional monkeys with pre-existing experimental glaucoma, the glaucomatous study eye was compliance tested at one of the postglaucoma testing intervals used in the longitudinal study. The contralateral normal eye was compliance tested once. These data were then added to the data from the five longitudinally studied monkeys at the appropriate preglaucoma and postglaucoma testing intervals. A third ANOVA was done to compare the compliance of the expanded group of glaucomatous eyes at each postintervention testing interval with the compliance of the 13 normal contralateral eyes.

RESULTS

Compliance. In the longitudinally (Pr > F = 0.0005) and cross-sectionally (Pr > F = 0.0001) studied glaucomatous eyes, optic disc compliance increased significantly by 1 to 2 weeks and then returned to a level statistically indistinguishable from normal within 13 to 18 weeks after the onset of glaucoma. In the transection eyes, the optic discs were significantly less compliant (more rigid) at 5 and 9 weeks after transection compared with the discs in either the normal or the glaucomatous eyes (Pr > F < 0.05). Baseline Optic Disc Position. Chronic posterior deformation of the disc was detected in one of three eyes tested 1 to 2 weeks and three of four eyes tested 3 to 4 weeks after the onset of glaucoma (Pr > F < 0.05). Chronic posterior deformation was not detected in the discs of either of the transection eyes at any of the post-transection testing intervals.

CONCLUSION

Changes in optic disc compliance and surface position were detected by digitized image analysis within 2 to 4 weeks of the onset of experimental glaucoma in the monkey eye. These findings are unlikely to be due to axon loss alone, because they did not occur in optic nerve transection eyes (which constitute a model of axon loss in which intraocular pressures remain normal). The results suggest that IOP-related damage to the load-bearing connective tissues of the optic nerve head may occur early in the course of experimental glaucoma.

Multiscale Disordered Porous Fibers for Self-Sensing and Self-Cooling Integrated Smart Sportswear

Smart clothing has demonstrated potential applications in a wide range of wearable fields for human body monitoring and self-adaption. However, current wearable sensors often suffer from not seamlessly integrating with normal clothing, restricting sensing ability, and a negative wearing experience. Here, integrated smart clothing is fabricated by employing multiscale disordered porous elastic fibers as sensing units, which show the capability of inherently autonomous self-sensing (i.e., strain and temperature sensing) and self-cooling. The multiscale disordered porous structure of the fibers contributes to the high transparency of mid-infrared human body radiation and backscatter of visible light, which allows the microenvironment temperature between the skin and clothing to drop at least ∼2.5 °C compared with cotton fabrics. After the capillary-assisted adsorption of graphene inks, the modified porous fibers could also possess real-time strain and temperature-sensing capacities with a high gauge factor and thermal coefficient of resistance. As a proof of concept, the integrated smart sportswear achieved the measuring of body temperature, the tracking of large-scale limb movements, and the collection of subtle human physiological signals, along with the intrinsic self-cooling ability.

The structure of elastic tissue as studied with the electron microscope

Electron microscope examination of fragmented elastic tissue obtained from fish swim bladder, bovine ligamentum nuchae, and aortas of various mammals, including man, reveals characteristically formed fibers and much amorphous material. Boiling in dilute acid destroys the associated collagen but does not obviously alter the elastic tissue. Digestion in crystalline trypsin of either boiled or unheated tissue from any of the above-mentioned sources causes the release of thin threads ranging in length from 0.1 micro to many microns. A large proportion of these threads are evenly and tightly coiled double helices formed from at least two interlacing filaments and measuring about 120 A in width. The distance between coils ranges from about 470 to 590 A. The individual smooth filaments, many of which are present in parallel pairs, measure approximately 70 A in width. Raising the pH of a neutral suspension of threads from ligamentum nuchae lowers the ratio of helical threads to uncoiled filaments, whereas lowering the pH with acetic acid results in clumping of threads with complete loss of identity at about pH 3.6. Threads and filaments obtained from all sources studied were destroyed in the temperature range 75-85 degrees C. at pH 7. It is concluded that the elastic fiber is a two component system composed of bundles of trypsin-resistant threads of characteristic form and size plus a trypsin-sensitive, heat-resistant "amorphous" binding matrix. The possible relationship of this structure to the elastic properties of the tissue is discussed.

Central corneal thickness and corneal hysteresis during corneal swelling induced by contact lens wear with eye closure

PURPOSE

To determine if corneal hysteresis (CH) was associated with increased central corneal thickness (CCT) induced by wearing soft contact lenses during eye closure.

DESIGN

A prospective laboratory investigation.

METHODS

CCT was measured with a modified optical coherence tomography (OCT), and CH was measured with a Reichert Ocular Response Analyzer (ORA) [Reichert Ophthalmic Instruments, Depew, New York, USA]. The ORA also determined values for intraocular pressure (IOP), corneal compensated IOP (IOPcc), and corneal resistance factor (CRF). One randomly selected eye of 20 non-contact lens wearers (four males and 16 females, age 19.7 +/- 1.1 years) was patched during three hours of soft contact lens wear. Measurements were made before lens insertion, immediately upon removal, and every 20 minutes thereafter for 100 minutes.

RESULTS

Immediately after contact lens removal, CCT was increased by 13.1 +/- 2.2% (mean +/- SD) compared with baseline (post hoc, P = .001). After 100 minutes, it remained elevated by 2.4 +/- 1.6% (post hoc, P = .001). However, there were no significant differences of CH at any time after lens wear (analysis of variance [ANOVA], P = .9). Immediately after lens removal, there were significant increases in IOP (post hoc, P = .003) and corneal resistance factor (CRF) (post-hoc, P = .015), but not in IOPcc (post hoc, P = .07). After lens wear, there were significant but weak correlations between the percentage change of CCT (CCT%) and IOP (r = 0.32, P = .001) and IOPcc (r = 0.29, P = .001). However, there was no significant correlation between CCT% and CH (r = 0.07, P = .458).

CONCLUSION

CH as measured by ORA was not associated with corneal swelling induced by soft contact lens wear in this study group.