Anisotropy of bovine cortical bone tissue damage properties.

Bone is a heterogeneous, anisotropic natural composite material. Several studies have measured human cortical bone elastic properties in different anatomical directions and found that the Young's modulus was highest in the longitudinal, followed by the tangential and then by the radial direction. This study compared the Young's modulus, the accumulated microdamage and local strains related to the failure process in these three anatomical directions. Cortical bone samples (≈360 µm×360 µm) were mechanically tested in three-point bending and concomitantly imaged to assess local strains using digital image correlation technique. The bone whitening effect was used to detect microdamage formation and propagation. No statistically significant difference was found between the Young's modulus of longitudinal (9.4±2.0 GPa) and tangential (9.9±1.8 GPa) bovine bone samples, as opposed to previous findings on human bone samples. The same similarity was found for the whitening values (5000±1900 pix/mm(2) for longitudinal, 5800±2600 pix/mm(2) for tangential) and failure strains (16.8±7.0% for longitudinal, 19.1±3.2% for tangential) as well. However, significantly lower values were observed in the radial samples for Young's modulus (5.92±0.77 GPa), whitening (none or minimal) and failure strain (10.8±3.8%). For strains at whitening onset, no statistically significant difference was seen for the longitudinal (5.1±1.6%) and radial groups (4.2±2.0%), however, the tangential values were significantly greater (7.0±2.4%). The data implies that bovine cortical bone tissue in long bones is designed to withstand higher loads in the longitudinal and tangential directions than in the radial one. A possible explanation of the anisotropy in the mechanical parameters derived here might be the structure of the tissues in the three directions tested.

Similar damage initiation but different failure behavior in trabecular and cortical bone tissue.

The mechanical properties of bone tissue are reflected in its micro- and nanostructure as well as in its composition. Numerous studies have compared the elastic mechanical properties of cortical and trabecular bone tissue and concluded that cortical bone tissue is stiffer than trabecular bone tissue. This study compared the progression of microdamage leading to fracture and the related local strains during this process in trabecular and cortical bone tissue. Unmachined single bovine trabeculae and similarly-sized cortical bovine bone samples were mechanically tested in three-point bending and concomitantly imaged to assess local strains using a digital image correlation technique. The bone whitening effect was used to detect microdamage formation and propagation. This study found that cortical bone tissue exhibits significantly lower maximum strains (trabecular 36.6%±14% vs. cortical 22.9%±7.4%) and less accumulated damage (trabecular 16100±8800 pix/mm2 vs. cortical 8000±3400 pix/mm2) at failure. However, no difference was detected for the maximum local strain at whitening onset (trabecular 5.8%±2.6% vs. cortical 7.2%±3.1%). The differences in elastic modulus and mineral distribution in the two tissues were investigated, using nanoindentation and micro-Raman imaging, to explain the different mechanical properties found. While cortical bone was found to be overall stiffer and more highly mineralized, no apparent differences were noted in the distribution of modulus values or mineral density along the specimen diameter. Therefore, differences in the mechanical behavior of trabecular and cortical bone tissue are likely to be in large part due to microstructural (i.e. orientation and distribution of cement lines) and collagen related compositional differences.

Local strain and damage mapping in single trabeculae during three-point bending tests.

The use of bone mineral density as a surrogate to diagnose bone fracture risk in individuals is of limited value. However, there is growing evidence that information on trabecular microarchitecture can improve the assessment of fracture risk. One current strategy is to exploit finite element analysis (FEA) applied to 3D image data of several mm-sized trabecular bone structures obtained from non-invasive imaging modalities for the prediction of apparent mechanical properties. However, there is a lack of FE damage models, based on solid experimental facts, which are needed to validate such approaches and to provide criteria marking elastic-plastic deformation transitions as well as microdamage initiation and accumulation. In this communication, we present a strategy that could elegantly lead to future damage models for FEA: direct measurements of local strains involved in microdamage initiation and plastic deformation in single trabeculae. We use digital image correlation to link stress whitening in bone, reported to be correlated to microdamage, to quantitative local strain values. Our results show that the whitening zones, i.e. damage formation, in the presented loading case of a three-point bending test correlate best with areas of elevated tensile strains oriented parallel to the long axis of the samples. The average local strains along this axis were determined to be (1.6±0.9)% at whitening onset and (12±4)% just prior to failure. Overall, our data suggest that damage initiation in trabecular bone is asymmetric in tension and compression, with failure originating and propagating over a large range of tensile strains.

Mechanical properties of single bovine trabeculae are unaffected by strain rate.

For a better understanding of traumatic bone fractures, knowledge of the mechanical behavior of bone at high strain rates is required. Importantly, it needs to be clarified how quasistatic mechanical testing experiments relate to real bone fracture. This merits investigating the mechanical behavior of bone with an increase in strain rate. Various studies examined how cortical and trabecular bone behave at varying strain rates, but no one has yet looked at this question using individual trabeculae. In this study, three-point bending tests were carried out on bovine single trabeculae excised from a proximal femur to test the trabecular material's strain rate sensitivity. An experimental setup was designed, capable of measuring local strains at the surface of such small specimens, using digital image correlation. Microdamage was detected using the bone whitening effect. Samples were tested through two orders of magnitude, at strain rates varying between 0.01 and 3.39 s(-1). No linear relationship was observed between the strain rate and the Young's modulus (1.13-16.46 GPa), the amount of microdamage, the maximum tensile strain at failure (14.22-61.65%) and at microdamage initiation (1.95-12.29%). The results obtained in this study conflict with previous studies reporting various trends for macroscopic cortical and trabecular bone samples with an increase in strain rate. This discrepancy might be explained by the bone type, the small sample geometry, the limited range of strain rates tested here, the type of loading and the method of microdamage detection. Based on the results of this study, the strain rate can be ignored when modeling trabecular bone.

Antioxidant properties of calcium dobesilate in ischemic/reperfused diabetic rat retina.

Calcium dobesilate possesses antioxidant properties and protects against capillary permeability by reactive oxygen species in the rat peritoneal cavity, but whether a similar action can take place in the diabetic rat retina is unknown. We investigated the oral treatment of diabetic rats with calcium dobesilate on the prevention of free radical-mediated retinal injury induced by ischemia/reperfusion (90 min ischemia followed by 3 min and/or 24 h of reperfusion). Streptozotocin-induced diabetic rats were orally treated with 50 and 100 mg/kg of calcium dobesilate for 10 days (n=12 in each group). In the first series of studies, calcium dobesilate was found to significantly reduce the maldistribution of ion content in diabetic ischemic/reperfused rat retina. Thus, in diabetic rats treated with 100 mg/kg/day calcium dobesilate, ischemia/reperfusion provoked: (i) 27.5% increase in retinal Na(+) content compared to 51.8% in the vehicle-treated group (P<0.05), and (ii) 59.6% increase in retinal Ca(2+) content compared to 107.1% in vehicle-treated animals (P<0.05). In the second series of studies, calcium dobesilate was found to significantly protect diabetic rat retina against inhibition of Na(+)/K(+)-ATPase and Ca(2+)/Mg(2+)-ATPase activities by ischemia/reperfusion (54% and 41% reduction, respectively, with 100 mg/kg of calcium dobesilate) and also against changes in retinal ATP, reduced glutathione (GSH), and oxidized glutathione (GSSG) contents. In the third series of experiments, rats treated with 100 mg/kg of calcium dobesilate reduced the hydroxyl radical signal intensity to 41% (measured by electron paramagnetic resonance), induced by ischemia/reperfusion in diabetic rat retina. Finally, 100 mg/kg calcium dobesilate significantly reduced retinal edema (measured by the thickness of the inner plexiform layer) in diabetic rats. In conclusion, oral treatment with calcium dobesilate significantly protected diabetic rat retina against oxidative stress induced by ischemia/reperfusion. Whether the antioxidant properties of calcium dobesilate explain, at least in part, its beneficial therapeutic effects in diabetic retinopathy deserves further investigation.

Direct measurement of free radicals in ischemic/reperfused diabetic rat retina.

Electron paramagnetic resonance (EPR) spectroscopy was used to directly measure free radical generation in ischemic/reperfused diabetic rat retina. Tissue was frozen at 77 degrees K after 90 min ischemia, and 90 min ischemia followed by 1 min, 3 min, 5 min, and 24 hours reperfusion, respectively. After 90 min of ischemia followed by 1 min, 3 min, 5 min, and 24 hours of reperfusion (n = 10 in each group), free radical signal intensity was increased from its diabetic nonischemic control value of 12 +/- 3 arbitrary units to 58 +/- 6 (P < 0.05), 62 +/- 7 (P < 0.05), 32 +/- 5 (P < 0.05), and 14 +/- 4 arbitrary units, respectively. The peak intensity of free radical production was observed after 90 min ischemia followed by 3 min of reperfusion; therefore, this time point was selected to study the retinal free radical production in superoxide dismutase (conjugated with polyethylene glycol, PEG-SOD) and EGb 761 (Ginkgo biloba extract)-treated groups. With 7,500, 15,000, and 30,000 U/liter of SOD, and 25, 50, and 100 mg/kg of EGb 761, a dose-dependent reduction in oxygen free radical production was detected, respectively, which may be responsible for the attenuation of abnormal postischemic function in ischemic and reperfused diabetic retina.

EGb 761 and the recovery of ion imbalance in ischemic reperfused diabetic rat retina.

We studied the effects of a free-radical scavenger, EGb 761, on electrolyte shifts (Na+, Ca2+, and K+) induced by ischemia and reperfusion in the retinas obtained from streptozotocin-induced diabetic rats. Eyes were subjected to 90 min ischemia followed by 24 h of reperfusion by clamping and releasing the central retinal artery. Ten days before the induction of ischemia and reperfusion, diabetic rats received a daily dose of 25, 50, and 100 mg/kg p.o. of EGb 761, respectively (n = 12 in each group). In the drug-free diabetic control group, 90 min ischemia followed by 24 h of reperfusion resulted in an increase in retinal Na+ and Ca2+ (measured by atomic absorption spectrophotometry) compared to nonischemic control values of 73 +/- 4 and 2.6 +/- 0.3 mumol/g dry weight to 113 +/- 5 (p < 0.05) and 5.3 +/- 0.3 mumol/g dry weight (p < 0.05), respectively. Tissue K+ content was significantly reduced compared to its nonischemic diabetic control value of 268 +/- 7 to 213 +/- 6 mumol/g dry weight (p < 0.05). EGb 761 dose-dependently reduced reperfusion-induced ion imbalance, improving the recovery of ion content in diabetic rat retina. EGb 761 did not reduce blood glucose in streptozotocin-induced diabetic rats. Therefore we may conclude that these protective effects of EGb 761 are independent of blood glucose content or of the severity of diabetes and protect against electrolyte shifts directly in retinal cells.

Modification of ischemia/reperfusion-induced ion shifts (Na+, K+, Ca2+ and Mg2+) by free radical scavengers in the rat retina.

We investigated the contribution of scavenging of oxygen free radicals to retinal ion contents during ischemia and reperfusion with the use of superoxide dismutase (SOD, Sigma), allopurinol (Sigma), EGB 761 (extract of Ginkgo biloba, Tanakan, IPSEN, Paris, France) and allopurinol plus EGB 761 in the rat. SOD (15,000 U/kg/day), allopurinol (50 mg/kg/day), EGB 761 (100 mg/kg/day) and allopurinol (50 mg/kg/day) plus EGB 761 (100 mg/kg/day) were administered for 10 days, respectively. Then, the eyes were subjected to 90 min of ischemia followed by 4 and 24 h of reperfusion, respectively. Retinal Na+, K+, Ca2+ and Mg2+ contents were measured by atomic absorption spectrophotometry after the washing out of blood and extracellular fluid from the vasculature. SOD, EGB 761 and the combination of EGB 761 with allopurinol significantly reduced the ischemia/reperfusion-induced Na+ and Ca2+ accumulation and K+ loss in ischemic/reperfused retinal tissue. Allopurinol alone failed to reduce the maldistribution of Na+, Ca2+ and K+ induced by ischemia/reperfusion in the retina. Neither intervention inhibited the cell Mg2+ loss which was observed during ischemia and reperfusion. Despite the responsible mechanisms remaining controversial, many studies confirmed that ischemia/reperfusion could trigger very sudden metabolic, electrophysiologic, morphologic and functional changes. There is general agreement that major ionic shifts are implicated; what triggers these changes is unclear, although many investigators believe that free radicals and oxidant stress may be important.(ABSTRACT TRUNCATED AT 250 WORDS)

Ischaemia and reperfusion-induced injury in rat retina obtained from normotensive and spontaneously hypertensive rats: effects of free radical scavengers.

The authors have studied the effects of free radical scavengers, superoxide dismutase (SOD) and extract of Ginkgo biloba (EGb 761, flavone-rich extract) on ion shifts (Na, K and Ca) induced by ischaemia and reperfusion in rat retina obtained from normotensive and spontaneously hypertensive rats. Eyes were subjected to 90 min of ischaemia by occlusion of the retinal artery, followed by 4 and 24 hours of reperfusion. SOD (15,000 U/kg, i.v.) or EGb 761 (50 mg/kg, per os) was administered in a daily dose for 10 days. In the drug-free control groups, 90 min of ischaemia significantly increased tissue Na gains from their pre-ischaemic control values of 63 +/- 7 microM/g dry weight (in retina obtained from normotensive rats) and 76 microM/g dry weight (in retina obtained from hypertensive rats) to 89 +/- 9 microM/g dry weight and 101 +/- 7 microM/g dry weight, respectively. During reperfusion, a further elevation was found in retinal Na in both the normotensive and hypertensive groups. Probably, because of the ischaemia-induced inhibition of Na-K-ATPase, retinal K loss was detected after ischaemia and reperfusion, respectively. An accumulation of retinal Ca was measured after ischaemia and reperfusion in the normotensive and spontaneously hypertensive groups. Both free radical scavengers significantly reduced the maldistribution of ions induced by ischaemia and reperfusion, but the effectiveness of drugs was more evident in normotensive than hypertensive groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Modification of reperfusion-induced ionic imbalance by free radical scavengers in spontaneously hypertensive rat retina.

We studied the effects of free radical scavengers, superoxide dismutase (SOD), vitamin E, and EGB 761, on ion shifts (Na+, K+, and Ca2+) induced by ischemia reperfusion in rat retina obtained from spontaneously hypertensive rats. Eyes were subjected to 90 min of retinal ischemia followed by 24 h of reperfusion. Two basic protocols were used: (1) chronic application, in which rats received SOD (7500, 15,000, and 30,000 U/kg, i.v.), vitamin E (50, 100, and 200 mg/kg, i.v.), and EGB 671 (50, 100, and 200 mg/kg, orally) for 10 d, respectively; and (2) acute administration, in which 7500, 15,000, and 30,000 U/kg of SOD, 50, 100, and 200 mg/kg of vitamin E, and 50, 100, and 200 mg/kg of EGB 761 were administered after an ischemic episode, at the onset of reperfusion, respectively. In the drug-free control group, 90 min ischemia followed by 24 h of reperfusion resulted in an accumulation of retinal sodium and calcium from their nonischemic control values of 76 +/- 4 and 3.2 +/- 0.1 mumol/g dry weight to 112 +/- 6 (p < .001) and 6.2 (p < .001) mumol/g dry weight, respectively. Tissue potassium loss was also observed in this model of retinal ischemia reperfusion, and after 90 min ischemia followed by 24 h of reperfusion potassium content was significantly reduced from its nonischemic control value of 266 +/- 5 to 207 +/- 6 (p < .001) mumol/g dry weight. The chronic administration of SOD, vitamin E, and EGB 761 dose dependently reduced the reperfusion-induced ionic imbalance and improved the recovery of retinal ion contents.(ABSTRACT TRUNCATED AT 250 WORDS)

Ischaemia- and reperfusion-induced Na+, K+, Ca2+ and Mg2+ shifts in rat retina: effects of two free radical scavengers, SOD and EGB 761.

Using Sprague-Dawley rats with transient (90-min) regional ischaemia induced by retinal artery occlusion in the eye, we have shown that superoxide dismutase (SOD) and EGB 761 (IPSEN, France), two free radical scavengers, can dramatically reduce the reperfusion-induced sodium and calcium gains, and potassium loss in retinal tissue. Investigating whether this was a 'direct' protective effect, operating during reperfusion, or an 'indirect' effect arising from the action of SOD or EBG 761 on the tissue during ischaemia. SOD (15,000 U kg-1) and EGB 761 (100 mg kg-1) were added to the rats at the moment of reperfusion (after an ischaemic insult). Eyes were subjected to 90 min ischaemia followed by 4 and 24 hr of reperfusion, respectively. In the drug-free control group, 90 min of ischaemia resulted in an accumulation of retinal sodium (2-fold) and calcium (3-fold), and a loss of cell potassium (by 40%) and magnesium (by 40%). During the first 4 hr of reperfusion the ionic imbalance was unchanged, while after 24 hrs of reperfusion a normalization was observed and the ion content of the retina almost returned to their preischaemic values. SOD and EGB 761 treatment significantly reduced the reperfusion-induced ionic imbalance (magnesium was an exception) and improved the recovery of retinal ion contents. Our results indicate that the elimination of oxygen radicals by free radical scavengers may reduce the reperfusion-induced ionic imbalance and improve the ionic homeostasis in the injured retinal cells.

Ischemia and reperfusion-induced histologic changes in the rat retina. Demonstration of a free radical-mediated mechanism.

Histologic alterations of ischemia- and reperfusion-induced retinal damage are critically dependent on the duration of the period of ischemia. Male Sprague Dawley rats were anesthetized, and a suture was placed behind the globe including the central retinal artery. Because it was desirable that untreated eyes show a great histologic change due to reperfusion-induced damage (in order that maximum scope would exist for demonstration of any protective effect of a drug treatment), a preliminary series of studies established the time-induced characteristics for the retina with transient regional ischemia. Eyes (n = 6-12 in each group) were subjected to 30, 60, or 90 min of ischemia followed by 0.5, 1, 2, 4, and 24 hr of reperfusion, respectively. The 30-min ischemia followed by reperfusion did not result in any histologic changes; 60-min ischemia followed by reperfusion induced a moderate retinal edema which returned to the preischemic value after 24 hr of reperfusion. The 90-min ischemia followed by reperfusion further aggravated retinal edema and increased the migration of neutrophil leukocytes. Even after 24 hr of reperfusion, the retinal edema had not disappeared although an attenuation was observed. In this study, the rats were treated with superoxide dismutase (SOD-PEG, 15 x 10(3) U/kg) or EGB 761 (100 mg/kg) for 10 days (chronic treatment). The SOD and EGB 761 significantly reduced the development of reperfusion-induced retinal edema and significantly prevented the neutrophil leukocyte infiltration. Both also had a protective effect against reperfusion-induced injury when these agents were administered just before reperfusion ("late" administration).(ABSTRACT TRUNCATED AT 250 WORDS)

Free radical-mediated effects in reperfusion injury: a histologic study with superoxide dismutase and EGB 761 in rat retina.

In Sprague-Dawley rats, retinal ischemia was induced by occlusion of the central retinal artery, while reperfusion was initiated by unclamping and removing the occluder. Ninety minutes of regional ischemia followed by 24 h of reperfusion resulted in a development of retinal edema in the inner plexiform layer and a migration of neutrophils into the retinal tissue. Oxygen free radicals have been implicated as inducers of cell damage in different tissues. This finding has led us to speculate that, if oxygen free radicals play an important role in the development of reperfusion injury, superoxide dismutase (SOD) and EGB 761 (Tanakan, extract of Ginkgo biloba, IPSEN) should be protective against reperfusion-induced injury. Under our experimental conditions, SOD dose-dependently reduced the development of edema formation (which was expressed in micrometers, measuring the thickness of the inner plexiform layer). Thus, 3,750, 7,500 and 15,000 U/kg of SOD reduced the reperfusion-induced edema formation from its drug-free ischemic value of 112 +/- 4 to 107 +/- 7, 91 +/- 6 (p less than 0.05) and 85 + 4 microns (p less than 0.001), respectively. Furthermore, SOD significantly reduced the migration of neutrophils which can also contribute to the development of reperfusion-induced injury. The same protective effect was observed, concerning the edema formation and neutrophil migration, in the EGB 761-treated groups. Our results indicate that free radicals play an important role in the development of reperfusion-induced injury, and the treatment of ischemic and reperfused retina with free radical scavengers may reduce the severity of reperfusion damage.