Joan E. Sanders

Tissue response to single-polymer fibers of varying diameters: evaluation of fibrous encapsulation and macrophage density.

An in vivo study was conducted to assess the sensitivity of fibrous capsule thickness and macrophage density to polymer fiber diameter. Single polypropylene fibers of diameters ranging from 2.1 to 26.7 microm were implanted in the subcutaneous dorsum of Sprague-Dawley rats. Results at 5 weeks demonstrated reduced fibrous capsule thickness for small fibers. Capsule thickness was 0.6 (+/-1.8) microm, 11.7 (+/-12.0) microm, 20.3 (+/-11.6) microm, and 25.5 (+/-10.0) microm for fibers in the ranges of 2.1 to 5.9, 6.5 to 10.6, 11.1 to 15.8, and 16.7 to 26.7 microm, respectively. Fibers very near to blood vessels had smaller capsules than did those with local vasculature further away. The macrophage density in tissue with fiber diameters 2.1 to 5.9 microm (23.03 +/- 8.67%) was comparable to that of unoperated contralateral control skin (18.72+/-10.06%). For fibers with diameters in the ranges of 6.5 to 10.6, 11.1 to 15.8, and 16.7 to 26.7 microm, macrophage densities were 33.90+/-13.08%, 34.40+/-15.77%, and 41.68+/-13.98%, respectively, all of which were significantly larger (p<0.002) than that for the control. The reduced fibrous capsule thickness and macrophage density for small fibers (<6 microm) compared with large fibers could be due to the reduced cell-material contact surface area or to a curvature threshold effect that triggers cell signaling. A next step will be to extend the analysis to meshes to evaluate fiber-spacing effects on small-fiber biomaterials.

Energy storage and return prostheses: does patient perception correlate with biomechanical analysis?

The development and prescription of energy storage and return prosthetic feet in favor of conventional feet is largely based upon prosthetist and amputee experience. Regretfully, the comparative biomechanical analysis of energy storage and return and conventional prosthetic feet is rarely a motivation to either the technical development or clinical prescription of such devices. The development and prescription of prosthetic feet without supportive scientific evidence is likely due to the conflicting or non-significant results often presented in the scientific literature. Despite the sizeable history of comparative prosthetic literature and continued analysis of prosthetic components, the link between clinical experience and scientific evidence remains largely unexplored.A review of the comparative analysis literature evaluating energy storage and return and conventional prosthetic feet is presented to illustrate consistencies between the perceptive assessments and the objective biomechanical data. Results suggest that while experimental methodologies may limit the statistical significance of objective gait analysis results, consistent trends in temporal, kinetic, and kinematic parameters correlate well with perceptive impressions of these feet. These correlations provide insight to subtle changes in gait parameters that are deemed neither clinically nor statistically significant, yet are perceived by amputees to affect their preference for and performance of prosthetic feet during locomotion. Acknowledging and targeting areas of perceptive significance will help researchers develop more structured protocols for energy storage and return prosthesis evaluation as well as provide clinicians with information needed to enhance the appropriateness of their clinical recommendations. Expanding test environments to measure activities of perceived improvement such as high-velocity motions, stair ascent/descent, and uneven ground locomotion will provide a more appropriate assessment of the conditions for which energy storage and return prosthetic feet were designed. Concentrating research to specific test populations by age or amputation etiologies can overcome statistical limitations imposed by small study samples. Finally, directing research toward the areas of gait adaptation, heel performance, and the temporal release of energy in energy storage and return feet may reinforce the selection and utilization of advanced prosthetic components. These enhancements to current biomechanical analyses may serve to reduce the boundaries of perceptive significance and provide clinicians, designers, and researchers with the supportive data needed to prescribe, design, and evaluate energy storage and return prosthetic feet.

Skin response to repetitive mechanical stress : A new experimental model in pig

OBJECTIVES To develop a new animal model for investigating the relations between interface stresses at the skin, adaptation, and breakdown. There were two hypotheses. (1) In skin subjected to varying types of repetitive mechanical stress, the tissue response depends on the direction and magnitude of the load. As the shear stress increases, tissue breakdown occurs earlier. (2) In skin subjected to repetitive mechanical stress of longer duration, there will be evidence of tissue adaptation. DESIGN Multiple case control, single-blind. INTERVENTIONS Varying combinations of normal and shear mechanical loads are applied to pigs skin for short durations (breakdown studies) or longer durations (adaptation studies). MAIN OUTCOME MEASURES Gross evidence of breakdown (visual inspection of skin) and microscopic changes (eg, histologic features of breakdown; thickness of epidermis and dermis; the length and shape of the basement membrane; concentration of inflammatory cells, mast cells, and fibroblasts; and quantity of elastin fibers). RESULTS The instrumentation was reliable and a significant improvement over past models in that shear forces were delivered and measured in a controlled manner. The animal model and tissue methodology provided consistent results, and it was found that skin breakdown occurred earlier as shear forces were increased. Evidence of tissue adaptation occurred in the long-term experiments, although corresponding morphologic changes have been difficult to elucidate. CONCLUSIONS To address the problem of skin breakdown, new animal models are strongly needed to better understand basic biologic processes related to pressure ulcer development.

Tissue Engineering of Skeletal Muscle Using Polymer Fiber Arrays

The purpose of this study was to assess a new scaffold design for muscle tissue engineering: arrays of parallel-oriented polymer microfibers. First, C2C12 skeletal myoblasts were seeded onto single, laminin-coated polypropylene fibers and their growth and alignment were characterized. With the aim of creating skeletal muscle sheets, it was then investigated whether cell layers of single fibers merged when in close proximity to neighboring fibers. The optimal fiber spacing needed to achieve cell alignment with the lowest possible content of scaffold material was established. Further, it was assessed whether such a cell sheet became contractile and whether it survived in vitro for extended periods of time. C2C12 cells, cultured on fibers 10 to 15 microm in diameter, formed up to 50-microm-thick layers of longitudinally aligned cells. Four different groups based on fiber spacing (30 to 35, 50 to 55, 70 to 75, and 90 to 95 microm) were evaluated. Complete cell sheets formed between fibers that were spaced 55 microm apart or less; larger spacing led to no or incomplete sheets. C2C12 cells, seeded onto a 10 x 20 mm fiber array, formed a contractile cell sheet that was maintained in vitro for 70 days. Larger, three-dimensional structures might be created by arranging fibers in several layers or by stacking cellular sheets.

Tissue engineering of perfused microvessels

One major obstacle toward the creation and survival of larger, three-dimensional tissues is the lack of a vascular network that provides transport of oxygen, nutrients, and metabolic byproducts. Although attempts to create microvasculature in vitro have been described previously (Microcirculation 2 (1995), 377; Tissue Eng. 6 (2000), 105; Ann. NY Accd. Sci. 944 (2001), 443), these methods depend on vascularization of void spaces within the tissue-construct or on the utilization of empty capillary networks by host vessels. In the present study, we examined the possibility of creating perfused microvessels in vitro that can be included in an artificial tissue. First, strands of nylon line with their ends fit into microtubing were positioned within small perfusion chambers. Vascular smooth muscle cells (SMCs) were then seeded onto the nylon strands and tubing. The cells multiplied to form concentric layers. Layer thickness was approximately 100 microm after 21 days and 150 microm after 28 days of culture. The lines were then extracted and the chambers connected to a perfusion system. The vessels were continuously perfused with culture medium over 7 days without failure. Artificial microvessels may prove useful in tissue engineering and as models for vascular research.

Collagen fibril diameters increase and fibril densities decrease in skin subjected to repetitive compressive and shear stresses

Understanding microstructural changes that occur in skin subjected to repetitive mechanical stress is crucial towards the development of therapies to enhance skin adaptation and load tolerance in patients at risk of skin breakdown (e.g. prosthesis users, wheelchair users). To determine if collagen fibril diameter, collagen fibril density, dermal thickness, epidermal thickness, basement membrane length, and dermal cell density changed in response to repetitive stress application, skin subjected to moderate cyclic compressive and shear stresses for 1h/d, 5d/week, for 4 weeks was compared with skin from an unstressed contralateral control. The lateral aspects of the hind limbs of 12 Landrace/Yorkshire pigs were used. Skin from under the stressed site and a contralateral control site was processed for electron microscopy and light microscopy analysis. Electron microscopy results demonstrated significant (p<0.01) increases in collagen fibril diameter of 15.9%, 22.4%, and 22.9% for the upper, mid, and lower layers of the dermis, respectively, for the stressed skin compared with the control skin. Collagen fibril density (fibrils/unit cross-sectional area) decreased significantly for stressed vs. control by 19.8%, 29.2%, and 31.8% for the upper, mid, and lower layers, respectively. Light microscopy results demonstrated trends of a decrease in dermal thickness and an increase in cell density for stressed vs. control samples, but the differences were not significant. Differences in epidermal thickness and basement membrane length were not significant. These results demonstrate that quantifiable changes occur in collagen fibril architecture but not in the gross tissue morphology following in vivo cyclic loading of pig skin.

Residual limb volume change: Systematic review of measurement and management

Management of residual limb volume affects decisions regarding timing of fit of the first prosthesis, when a new prosthetic socket is needed, design of a prosthetic socket, and prescription of accommodation strategies for daily volume fluctuations. This systematic review assesses what is known about measurement and management of residual limb volume change in persons with lower-limb amputation. Publications that met inclusion criteria were grouped into three categories: group I: descriptions of residual limb volume measurement techniques; group II: studies investigating the effect of residual limb volume change on clinical care in people with lower-limb amputation; and group III: studies of residual limb volume management techniques or descriptions of techniques for accommodating or controlling residual limb volume. We found that many techniques for the measurement of residual limb volume have been described but clinical use is limited largely because current techniques lack adequate resolution and in-socket measurement capability. Overall, limited evidence exists regarding the management of residual limb volume, and the evidence available focuses primarily on adults with transtibial amputation in the early postoperative phase. While we can draw some insights from the available research about residual limb volume measurement and management, further research is required.

Melt electrospinning of biodegradable polyurethane scaffolds.

Electrospinning from a melt, in contrast to from a solution, is an attractive tissue engineering scaffold manufacturing process as it allows for the formation of small diameter fibers while eliminating potentially cytotoxic solvents. Despite this, there is a dearth of literature on scaffold formation via melt electrospinning. This is likely due to the technical challenges related to the need for a well-controlled high-temperature setup and the difficulty in developing an appropriate polymer. In this paper, a biodegradable and thermally stable polyurethane (PU) is described specifically for use in melt electrospinning. Polymer formulations of aliphatic PUs based on (CH(2))(4)-content diisocyanates, polycaprolactone (PCL), 1,4-butanediamine and 1,4-butanediol (BD) were evaluated for utility in the melt electrospinning process. The final polymer formulation, a catalyst-purified PU based on 1,4-butane diisocyanate, PCL and BD in a 4/1/3M ratio with a weight-average molecular weight of about 40kDa, yielded a nontoxic polymer that could be readily electrospun from the melt. Scaffolds electrospun from this polymer contained point bonds between fibers and mechanical properties analogous to many in vivo soft tissues.

Interface shear stresses during ambulation with a below-knee prosthetic limb.

Shear stresses on a residual limb in a prosthetic socket are considered clinically to contribute to tissue breakdown in below-knee amputees. When applied simultaneously with normal stresses, they can cause injury within the skin or can generate an abrasion on the surface. To gain insight into shear stresses and parameters that affect them, interface stresses were recorded on below-knee amputee subjects during walking trials. On the tibial flares, resultant shear ranged from 5.6 kPa to 39.0 kPa, while on the posterior surface it ranged from 5.0 kPa to 40.7 kPa. During stance phase, anterior resultant shears on a socket were directed toward the apex while posterior resultant shears were directed downward approximately perpendicular to the ground. Waveform shapes were usually double-peaked, with the first peak at 25% to 40% into stance phase and the second peak at 65% to 85% into stance. Application of these results to residual limb tissue mechanics and prosthetic design is discussed.

Effects of alignment changes on stance phase pressures and shear stresses on transtibial amputees: measurements from 13 transducer sites

Interface pressures and shear stresses were measured at 13 sites on two unilateral below-knee amputee subjects ambulating with lower-limb patellar-tendon-bearing prostheses. Interface stresses at the time of the first peak in the shank axial force-time curve were investigated at different socket-shank alignment settings. Stress magnitudes ranged from 1.2 to 214.7 kPa for pressure and 0.4 to 79.6 kPa for resultant shear stress, and changes in stress due to misalignment ranged from 1.3 to 80.7 kPa for pressure and from 0.2 to 38.0 kPa for resultant shear stress. For both subjects interface stress changes were much greater in the anterior socket region than in the lateral or posterior regions. Thus, alignment changes had a localized effect on interface stresses. Plots of alignment versus pressure or resultant shear stress were nonlinear for both subjects, in a number of cases maximizing or minimizing at the nominal alignment, indicating complex interface stress-alignment relationships. Variation (standard deviation/mean) was not significantly different for nominal versus misaligned steps, indicating that the subjects adapted well to the alignment changes. Session to session differences in interface stresses were typically larger than interface stress differences induced by alignment modifications. Thus, while these subjects compensated well for alignment changes to maintain consistent interface stresses within a session, they did not do so for different sessions conducted weeks apart.