R. Reihsner

Mechanical properties of peripheral nerves.

Peripheral nerve trunks are viscoelastic tissues with unique mechanical characteristics. Tensile strength, which includes elastic limit and mechanical failure, concerns surgeons. This study shows that measurements of the load necessary to achieve certain elongations on specimens outside the body do not correspond with in situ measurements. The necessary load is influenced by the presence or absence of branches and by the amount of fibrosis. Because of transverse contraction, elongation beyond a certain limit substantially decreases intrafascicular volume, leading to increased intrafascicular pressure. Stress relaxation is effective only if the nerve repair site is maintained under constant tension. Its beneficial effect disappears after 10 minutes if the repaired nerve is returned to a relaxed state. Therefore, tension at the repair site should be minimized.

Alterations of biochemical and biomechanical properties of rat tail tendons caused by non-enzymatic glycation and their inhibition by dibasic amino acids arginine and lysine

SummaryThe influence of dibasic amino acids arginine and lysine on non-enzymatic glycation of tail tendon fibers from old (900-day-old) and young (61-day-old) rats was investigated in vitro. The biomechanical changes in tendon fibers of young rats after an incubation interval of 7 or 14 days in a glucose solution were abolished by the addition of arginine or lysine (molar ratio amino acid:glucose 1∶10). Glucose incorporation into rat tail tendon fibers as well as Amadori product formation was decreased significantly in the presence of the amino acids. The inhibitory effect of arginine was further confirmed by measurement of the amount of ketoamine formed during the glycation reaction using soluble albumin as a protein target. The effective inhibition of non-enzymatic glycation by arginine or lysine suggests their potential use in vivo as a means of controling protein over-glycation.

Alterations of biochemical and two-dimensional biomechanical properties of human skin in diabetes mellitus as compared to effects of in vitro non-enzymatic glycation.

OBJECTIVE The aim of this study was to evaluate whether multiaxial analysis of diabetes-specific biomechanical changes generated in vitro by non-enzymatic glycation of human skin samples from healthy subjects reflect the changes seen in skin from subjects with diabetes mellitus. DESIGN Descriptive study. BACKGROUND Non-enzymatic glycation of skin in vitro causes an increased stiffness comparable to in vivo changes seen in diabetic patients. These changes are probably due to increased cross-linking of collagen molecules. METHODS Skin specimens from 7 subjects with diabetes mellitus and 7 controls (age range: 74-90) were analyzed for biomechanical changes using a multiaxial tensile testing device. Control skins from healthy individuals in the age range of 50-65 yr were artificially glycated. One part of these samples was coincubated with the glycation inhibitor aminoguanidine. Glycation of tissues was determined by measuring fluorescence of solubilized samples. Multiaxial biomechanical analysis allows the determination of maximum (a(I)) and minimum elastic modulus (a(II)). These parameters describe the amplitude of the elastic stress response, which is exponentially related to strain. RESULTS Principal stresses, both maximum and minimum, were increased in skins from diabetic subjects as compared to controls. The increases of the principal stresses were comparable to those obtained by in vitro glycation of normal skins. CONCLUSION These results, which can be detected unequivocally with the multiaxial test mode, show that our in vitro model closely reflects changes in skin samples from individuals with diabetes mellitus. Aminoguanidine partially inhibited these as well as biochemical changes. RELEVANCE Multiaxial testing of in vitro glycated skin samples can be used as a model for in vivo changes caused by diabetes mellitus. In addition, therapeutical effects of aminoguanidine, an inhibitor of non-enzymatic glycation, can be monitored in this model.

Two-dimensional stress-relaxation behavior of human skin as influenced by non-enzymatic glycation and the inhibitory agent aminoguanidine

In order to simulate the in vivo stress alterations of diabetic skin in an in vitro model, we examined the viscoelastic properties of long-term glycated human skin samples. Since skin is subjected to biaxial tension, we used two-dimensional multiaxial testing which better reflects the in vivo situation than the uniaxial testing mode. For native skin samples from the abdominal region we found a direction-dependent elastic stress strain behavior. The viscous stress component was separated from the elastic stress component by relaxation tests at consecutive incremental steps of radial strains. We hypothesize that glycation-induced changes in the tissue stiffness are generated in a direction-dependent mode. A marked increase of the direction-dependent stiffness was found upon long-term incubation with glucose-6-phosphate. This increase was statistically significant for the maximum principal elastic stress component which was highly correlated with the degree of non-enzymatic collagen modification. The viscous fractions obtained from two-dimensional relaxation tests at consecutive radial strains were inversely correlated with non-enzymatic modification. Only at 30% radial strain a significant decrease of the viscous fraction engendered by glucose-6-phosphate was observed together with a direction-dependent significant increase of the expectation value of the time constant. The biomechanical and biochemical effects of long-term glycation could be partially reversed by aminoguanidine, a potential therapeutic agent for patients with diabetes mellitus. Our findings suggest that additional cross-links generated by long-term glycation cause two-dimensional biomechanical alterations in human skin, which can be unequivocally detected by multiaxial testing.

Biomechanical properties of normal tendons, normal palmar aponeuroses, and tissues from patients with Dupuytren's disease subjected to elastase and chondroitinase treatment

Normal tendons, normal palmar aponeuroses and palmar aponeuroses from patients with Dupuytrens disease were subjected to elastase or chondroitinase treatment. Youngs modulus was derived from the linear portion of stress-strain graph. It showed the lowest value for the apparently normal palmar aponeuroses and the highest value for tendon samples. Elastase treatment caused an increase of extensibility and a reduction of Youngs modulus of normal palmar aponeuroses and tendons, but not of contracture bands. In normal tendons, normal palmar aponeuroses and apparently normal palmar aponeuroses residual strain and hysteresis loop increased significantly as a linear function of the amount of digested elastin. In contrast these biomechanical parameters were not affected significantly in contracture bands. In normal and apparently normal areas incubation with chondroitinase ABC resulted in a significant increase of residual strain and, as opposed to elastase, a decrease of normalized hysteresis loop. In contracture bands, however, these biomechanical parameters remained unchanged. RELEVANCE: The increasing evidence of a correlation between morphological changes of palmar elastin and ground substance with the progress of Dupuytrens disease emphasizes the need to determine the relative importance of these connective tissue components for the pathogenesis of Dupuytrens disease.

On the Orthogonal Anisotropy of Human Skin as a Function of Anatomical Region

Skin samples were obtained from 8 anatomical sites of 6 human deceased at ages ranging from 30 to 80 years 24 hours post mortem. As shown by biochemical analysis the collagen content varied between 71% and 78% depending on the anatomical location of the skin samples. The content of collagen type III was in the range of 19.2% to 22.2% of the total collagen concentration. As to the biomechanical analysis the axes of minimum and maximum shrinkage after excision were determined and correlated with Langer cleavage line drawn on the specimen with a marker after incision. Two-dimensional biomechanical tests were conducted with a multiaxial tensile testing device consisting of 12 loading axes. The in vivo configuration was a circle with 30 mm diameter. The in vivo stresses were determined by restoring the original shape of the specimen. According to the nonlinear stress-strain relationship incremental strains were applied to the sample with the in vivo configuration and states of uniform extension as reference. The corresponding stresses were recorded after stress relaxation was completed and the equilibrium stresses were regarded as the elastic contribution to the viscoelastic biomechanical behavior. The elastic parameters as a function of the initial strain level were calculated using a set of different incremental strains and stresses. The highest in vivo stresses were found in patella, and upper and lower back. The maximum deviation of the direction of maximum in vivo stress from the Langer cleavage line was found in upper back, the volar part of thigh, and sternum. In vivo orthogonal anisotropy was most pronounced in patella and hollow of the knee.

Atomic short-range order in CuMn alloys

Abstract The establishment of atomic short-range order (SRO) has been investigated in CuMn alloys (5, 8,13,16,20 at. % Mn) by measurement of the electrical resistivity during isochronal and isothermal annealing. An increasing degree of SRO is accompanied by a reduction of resistivity; this effect increases with Mn concentration. For 8,13 and 16 at. % Mn SRO kinetics turn out to deviate significantly from single exponential behavior with a maximum at 13 at. % Mn, whereas at 5 and 20 at. % Mn SRO is adjusted in a single exponential process. For data analysis three methods are used: a sum of two and three exponentials as well as a log-normal spectrum of relaxation times. The strong interaction between second-nearest-neighbor atoms in CuMn seems to be essential for SRO kinetics but of minor importance for the value of SRO-induced resistivity change.

Comparison of the Effect of Different Inhibitors on the Non-Enzymatic Glycation of Rat Tail Tendons and Bovine Serum Albumin

The biomechanical and biochemical properties of collagen are changed by non-enzymatic glycation culminating in increased cross-linking. We have previously shown that dibasic amino acids such as L-arginine inhibit in vitro the non-enzymatic glycation of soluble proteins and insoluble connective tissue macromolecules. In the present in vitro study we obtained evidence that the nucleophilic hydrazine derivative aminoguanidine and the non-steroidal antirheumatic drug ibuprofen inhibit the formation of fluorescent advanced glycation end products (AGEs) to a comparable extent, while arginine is ineffective as a consequence of its tendency to form AGEs itself. Periodic replacement of glycated arginine in the rat tail tendon system, however, engendered an inhibition of fluorescence similar to that obtained by the other inhibitors. Long-term glycation of rat tail tendons caused a significant increase in Youngs modulus, which could also be inhibited by periodically renewed arginine. In contrast to ibuprofen, aminoguanidine and arginine-lysine inhibited the marked increase in maximum contraction force of long-term glycated rat tail tendons. As opposed to other inhibitors, aminoguanidine also reduced the thermal contraction force of native tendons, shifted the maximum contraction temperature to markedly lower values and solubilized a significant part of the rat tail tendon collagen. These findings indicate that the in vitro alterations of rat tail tendon collagen induced by non-enzymatic glycation can be prevented by arginine, arginine-lysine and aminoguanidine. However, collagen structure is seriously affected by aminoguanidine.

Biomechanical Properties of Elastase Treated Palmar Aponeuroses

Human palmar aponeurosis was treated with elastase in the presence or absence of soybean trypsin inhibitor. The removal of elastic fibers was complete as proved by electron microscopy. Cyclic loading was performed at a constant strain rate. Residual strain was measured and the stiffness and the fraction of dissipated energy of strain energy was calculated and compared to that of untreated samples of palmar aponeurosis. Residual strain and dissipated energy showed a dramatic increase after enzyme treatment, both in presence and absence of soybean trypsin inhibitor. Stiffness, on the other hand was reduced. The remaining collagen fibers show a more viscous behavior. Our results support the hypothesis that the elastin fibers are responsible for the elastic recovery, observed on specimens of untreated palmar aponeurosis.

Comparison of palmar aponeuroses from individuals with diabetes Mellitus and Dupuytren's contracture

It is well known that Dupuytrens contracture is often associated with diabetes mellitus. Palmar fascia from individuals with diabetes mellitus and/or Dupuytrens contracture as well as controls were subjected to differential scanning calorimetry, biomechanical and biochemical analysis. The collagen denaturation temperature of the palmar aponeurosis from individuals with diabetes mellitus in the presence (71.0°C) or absence of Dupuytrens contracture (70.6°C) was increased as compared with controls (68.5°C), while this parameter was significantly reduced (about 3.5°C) in contracture bands of Dupuytrens contracture. Stress relaxation experiments revealed that the viscous fraction was slightly reduced in diabetes mellitus (6.5%) vs. controls (8.3%), whereas in Dupuytrens contracture, irrespective of additional diabetes mellitus, a pronounced increase of this parameter was seen (36.5% vs. 24.5%) in the presence of diabetes mellitus. The time constants were significantly elevated by both disorders, this increase being more pronounced in Dupuytrens contracture. Taken together, these changes can be explained by increased cross‐linking in diabetes mellitus, while in Dupuytrens contracture other structural changes, such as increased collagen type III content and loss of fascicular organization, play an additional role besides the finding of reduced cross linking. Anat Rec 255:401–406, 1999.