Banking of osteochondral allografts, Part II. Preservation of Chondrocyte Viability During Long-Term Storage

One of the most important factors concerning the successful clinical outcome after transplantation of osteochondral allografts is the viability of the cartilage.The viability of cryopreserved cartilage is quite poor, 20–30% cell survival has been published. The purpose of this study was to develop a new storage method which improves the chondrocyte viability. The talus of cadaveric donors was used as a model tissue to compare human osteochondral allograft cartilage viability following cryopreservation with that remaining after prolonged refrigerated storage. Full-thickness cartilage punch biopsies had been cryopreserved, and tali were divided into two matched groups and stored in TCM for 60 days at +4 °C, either with or without regular medium replacement. The cartilage of each graft was biopsied and assayed for viability on every third day by the MTT reduction assay. During 4 °C storage, a recurring pattern of large fluctuations in apparent cartilage viability was observed in every stored graft, with or without medium replacement. These fluctuations did not appear in control specimens of either fresh or cryopreserved human skin that were assayed in parallel with the cartilage biopsies, so the viability fluctuation seems an intrinsic property of the cartilage in these conditions. Cartilage stored for 60 days at +4 °C showed significantly higher viability (35.2 ± 3.3 %) than fresh cartilage that had been cryopreserved (21.6 ± 1.8 %). This was true even when cryopreserved and thawed cartilage was subjected to a 3 day post thaw incubation under presumably favorable conditions (17.7 ± 1.6 %). These viability assay results, (reflective of intracellular metabolic activity), were corroborated by the fluorescent dye mixture SYTO-16 and propidium iodide. The data indicate that long-term stored refrigerated cartilage appears to retain a viability higher than that of cryopreserved cartilage for up to and perhaps beyond 60 days of storage. There was no viability index difference between the medium replaced and non-replaced groups. Although an exceptional result, in one individual case, more than 65% viable cells could be detected in the talar cartilage after 60 days storage at +4 °C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of low dose and moderate dose gamma irradiation on the mechanical properties of bone and soft tissue allografts

The increased use of allograft tissue for musculoskeletal repair has brought more focus to the safety of allogenic tissue and the efficacy of various sterilization techniques. Gamma irradiation is an effective method for providing terminal sterilization to biological tissue, but it is also reported to have deleterious effects on tissue mechanics in a dose-dependent manner. At irradiation ranges up to 25 kGy, a clear relationship between mechanical strength and dose has yet to be established. The aim of this study was to investigate the mechanical properties of bone and soft tissue allografts, irradiated on dry ice at a low absorbed dose (18.3–21.8 kGy) and a moderate absorbed dose (24.0–28.5 kGy), using conventional compressive and tensile testing, respectively. Bone grafts consisted of Cloward dowels and iliac crest wedges, while soft tissue grafts consisted of patellar tendons, anterior tibialis tendons, semitendinosus tendons, and fascia lata. There were no statistical differences in mechanical strength or modulus of elasticity for any graft irradiated at a low absorbed dose, compared to control groups. Also, bone allografts and two soft tissue allografts (anterior tibialis and semitendinosus tendon) that were irradiated at a moderate dose demonstrated similar strength and modulus of elasticity values to control groups. The results of this study support the use of low dose and moderate dose gamma irradiation of bone grafts. For soft tissue grafts, the results support the use of low dose irradiation.     

Cultured epidermal allografts: Quantitative evaluation of their healing effect in deep dermal burns

This study summarizes the Brno Burn Centre experience with the application of cultured epidermal allografts (CEAl) in the treatment of deep dermal burns. In a prospective randomised trial on 30 patients with deep dermal burns CEAl obtained from young healthy and examined donors and fixed on tulle grass carrier (Grasolind) were compared with empty Grasolind as the lowest layer of dressing. All the other layers were identical. Both kinds of dressing were applied simultaneously on the same deep dermal burn wound between 6th and 10th day after burn. Six days later the non-healed wound areas were recorded through painting on cellophane membrane and scanned in the computer. The percentage of wound reduction was calculated and statistically evaluated. The reduction of the non-epithelialized wound area was 86.5% when covered through CEAl and only 71.2% when covered with tulle grass (Grasolind) only. This difference is statistically significant. In conclusion it can be stated that cultured epidermal allografts strongly stimulate reepithelialisation in deep dermal burns. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of freeze-drying and gamma irradiation on biomechanical properties of bovine pericardium

Freeze-drying and gamma irradiation are the techniques widely use in tissue banking for preservation and sterilization of tissue grafts respectively. However, the effect of these techniques on biomechanical properties of bovine pericardium is poorly known. A total of 300 strips of bovine pericardium each measured 4 cm × 1 cm were used in this study to evaluate the effect of freeze-drying on biomechanical properties of fresh bovine pericardium and the effect of gamma irradiation on biomechanical properties of freeze-dried bovine pericardium. The strips were divided into three equal groups, which consist of 100 strips each group. The three groups were fresh bovine pericardium, freeze-dried bovine pericardium and irradiated freeze-dried bovine pericardium. The biomechanical properties of the pericardial strips were measured by a computer controlled instron tensiometer while the strips thickness was measured by Mitutoyo thickness gauge. The results of the study revealed that freeze-drying has no significant (p > 0.05) effect on the tensile strength, Youngs modulus (stiffness) and elongation rate of fresh bovine pericardium. Irradiation with 25 kGy gamma rays caused significant decreased in the tensile strength, Youngs modulus and elongation rate of the freeze-dried pericardium. However, gamma irradiation has no significant effect on the thickness of freeze-dried bovine pericardium, while freeze-drying caused significant decreased in the thickness of the fresh bovine pericardium. The outcome of this study demonstrated that freeze-drying has no significant effect on the biomechanical properties of fresh bovine pericardium, and gamma irradiation caused significant effect on the biomechanical properties of freeze-dried bovine pericardium.     

The biomechanical properties of canine skin measured in situ by uniaxial extension

Introduction

A uniaxial extension system was setup to analyze the mechanical properties of dog skin.

Material and methods

Pads were glued onto dog skin with constant reproducible geometrical parameters and the extension force was measured as a function of the extension values. Forty-one sites (82 cycling tests) were investigated in situ on 11 canine cadavers, half of them after surgically isolating the test area from the surrounding skin. Series of loading-unloading cycles of up to 5 N or 10 N or both loads were performed on each site. The elastic properties and the dissipative effects were characterized respectively by evaluating the secant Rigidity at maximum loads and the Fraction of dissipated energy.

Results

A hysteresis phenomenon, implying the need for preconditioning to attain equilibrium cycles, was apparent during mechanical characterization. Polynomial expressions were used to relate the measured Rigidities and the Fractions of dissipated energy with or without sample isolation. The latter were less affected by isolation. The ratios between the Rigidities at 5 N to those at 10 N displayed non-linearity in the investigated extension range in contrary to the Fractions of dissipated energy.

Discussion/conclusion

The parameters confirming the dissipative non-linear elastic behavior of dog skin were identified and the correlation between Rigidity and Fraction of dissipated energy on isolated and non-isolated skin samples was quantitatively determined. This extension setup can now be used as a “true in vivo” mapping tool to determine the mechanical characteristics of the skin during healing processes or during the study of Human skin disease with the dog as an animal model.     

Evaluation of dynamic response and biomechanical properties of isolated blood vessels

In this study we present the experimental and mathematical model for a precise assessment of isolated blood vessels dynamic response under a sudden change of blood pressure. Only the end points within the time interval of the considered dynamic response of the blood vessel, or so-called “alternate steady states” of the processes, were usually considered in various studies. These studies do not provide an insight how the process variables change between these alternate steady states. Isolated blood vessels (rat abdominal aorta) were used to determine how the process dynamics can be described in detailed quantitative terms by mathematical parameters. The experimental model and mathematical procedures presented in this study describe precisely (at a high sensitivity level) the time history of the pressure and the diameter change in between alternate steady states, when an abrupt change of blood pressure occurs at the vessel outlet. Also, the experimental model and mathematical procedures were used to determine changes in the stress–strain law, caused by the action of L-arginine. The presented experimental design and mathematical model can be used for assessment of isolated blood vessel dynamic responses under different stimuli, such as drug effects, electrostimulation etc.     

Effect of freezing and thawing on the biomechanical characteristics of porcine ocular tissues

PurposeTo evaluate the effect of freezing and thawing on the biomechanical properties of ex-vivo porcine ocular tissue.MethodsThirty-six porcine eyes (18 pairs) were obtained fresh from a local abattoir and split into two groups of nine pairs to study the effect of storage at −20 °C and −80 °C. A randomly-selected eye from each pair (Control Group, CG) was tested fresh while the fellow eyes were frozen for 14 days, either at −20 °C and −80 °C (Frozen Group, FG) before thawing and testing. Seventy-two strips were extracted from the corneas and scleras of eye globes and subjected to uniaxial tension tests under loads up to 1.0 N. Following five preconditioning cycles, the load and elongation data obtained experimentally were analysed to derive the tissue’s stress-strain and tangent modulus-strain behaviour.ResultsCorneal tissue subjected to freezing at −20 °C exhibited significant increases in tangent modulus (mechanical stiffness) by 13 ± 17% (p = 0.003) at 1% strain and 14 ± 12% (p < 0.001) at 2% strain. In contrast, the increases in corneal stiffness at −80 °C were insignificant (6 ± 14%, p = 0.099 at 1% strain, 6 ± 15%, p = 0.091 at 2% strain). The corresponding increases in tangent modulus in the sclera were all insignificant (for −20 °C: 4 ± 14%, p = 0.265 at 1% strain, 3 ± 9%, p = 0.186 at 2% strain; for −80 °C: 3 ± 18%, p = 0.537 at 1% strain and 3 ± 18%, p = 0.491 at 2% strain).ConclusionsThe study provided evidence that freezing and thawing led to insignificant changes in ocular tissue stiffness except in corneal tissue that was frozen at −20 °C.     

Effects of freeze-thaw on the biomechanical and structural properties of the rat Achilles tendon

Rodent models are commonly used to investigate tendon healing, with the biomechanical and structural properties of the healed tendons being important outcome measures. Tendon storage for later testing becomes necessary when performing large experiments with multiple time-points. However, it is unclear whether freezing rodent tendons affects their material properties. Thus the aim of this study was to determine whether freezing rat Achilles tendons affects their biomechanical or structural properties. Tendons were frozen at either −20 °C or −80 °C directly after harvesting, or tested when freshly harvested. Groups of tendons were subjected to several freeze-thaw cycles (1, 2, and 5) within 3 months, or frozen for 9 months, after which the tendons were subjected to biomechanical testing. Additionally, fresh and thawed tendons were compared morphologically, histologically and by transmission electron microscopy. No major differences in biomechanical properties were found between fresh tendons and those frozen once or twice at −20 °C or −80 °C. However, deterioration of tendon properties was found for 5-cycle groups and both long-term freezing groups; after 9 months of freezing at −80 °C the tear resistance of the tendon was reduced from 125.4 ± 16.4N to 74.3 ± 18.4N (p = 0.0132). Moreover, tendons stored under these conditions showed major disruption of collagen fibrils when examined by transmission electron microscopy. When examined histologically, fresh samples exhibited the best cellularity and proteoglycan content of the enthesis. These properties were preserved better after freezing at −80 °C than after freezing at −20 °C, which resulted in markedly smaller chondrocytes and less proteoglycan content. Overall, the best preservation of histological integrity was seen with tendons frozen once at −80 °C. In conclusion, rat Achilles tendons can be frozen once or twice for short periods of time (up to 3 months) at −20 °C or −80 °C for later testing. However, freezing for 9 months at either −20 °C or −80 °C leads to deterioration of certain parameters.     

Effects of prolonged freezing stress on the photosynthetic apparatus of moderately hardy leaves as assayed by chlorophyll fluorescence kinetics

Abstract Moderately frost-hardy leaves of the wintergreen broadleaf woody shrubs Pyracantha coccinea and Ligustrum ovalifolium and the winter annual herb Spinacia oleracea were subjected to extended freezing stress up to 15 d at temperatures 2–8°C above the mean lethal temperature (LT₅₀). After thawing, the fast kinetics of in vivo chlorophyll fluorescence of photosystem II (PSII) and the potential of linear photosynthetic electron transport of isolated thylakoid membranes was measured at room temperature. The lower the minimum freezing temperature and the longer the time of exposure, the greater was the suppression of the fluorescence signals of the leaves and decrease of the electron transport capacity of the thylakoid membranes. The pattern of inactivation of PSII -mediated electron flow, i.e. inhibition of photoreaction to photochemistry and/or electron donation to the photochemical reaction, during long-term freezing at temperatures somewhat above the LT₅₀ of the leaves was similar to that observed earlier after relatively brief exposure of leaves and isolated thylakoid membranes to more severe freezing stress. As injury occurred during freezing in complete darkness, it is likely that prolonged winter stress under natural environmental conditions causes changes in the photosynthetic apparatus of moderately hardy leaves which are not due to photoinhibition.     

The effects of Mn2+ and Ca2+ on the prolonged depolarising after-potential in barnacle photoreceptor

We have studied the effects on the PDA of modifying intracellular and extracellular concentrations of Ca²⁺ and Mn²⁺. The effect of decreased Ca²⁺ concentration or addition of EGTA is mainly an increase in the PDA amplitude and length. Raising Ca²⁺ concentration using ruthenium red or high external Ca²⁺ has the opposite effect. The effect of Mn²⁺ is much more striking: In the presence of 50–100 mM Mn²⁺ the PDA is initially greatly depressed but can rise slowly for up to 20 or 30 s (in the dark) until it approaches its original amplitude and time course. Bridge measurements showed that the depression of the PDA corresponds to a depressed conductance and so is not due to an increase in K⁺ conductance. The Mn²⁺ effect is potentiated by decreased Ca²⁺. Appropriate stimulation suppresses the rising PDA as promptly as it does a normal PDA, suggesting that if lateral diffusion is the source of the slow rise, the PDA and PDA-depressing processes must be spatially linked. The action of the anti-PDA is apparently prolonged by both Ca²⁺ and Mn²⁺.Based on material presented at the European Neurosciences Meeting, Florence, September 1978     

Osmotic stress effects on the freezing tolerance of the antarctic nematodePanagrolaimus davidi

 The freezing and freezing survival of the Antarctic nematode Panagrolaimus davidi after exposure to solutions of different osmotic concentrations has been examined using a thermoelectric cooling stage and multi-specimen cooling block to see if there is any evidence that freeze-induced desiccation prevents inoculative freezing. The nematodes froze in all the test solutions used (up to 1138 mosmol ⋅ l^-1) and at all cooling rates and nucleation temperatures tested. Freezing survival was at its maximum in 0.1 mol ⋅ l^-1 NaCl in artificial tap water after 1 h exposure to the test solution and in artificial tap water after 24 h exposure. Hyperosmotic and hyposmotic stress adversely affected the nematodes’ ability to survive freezing. In non-frozen controls survival declined with increasing osmolality of the test solution. Measurements of the osmolality of water extracted from a variety of moss samples indicate that the nematodes are exposed to an osmotic concentration of about 9 mosmol ⋅ l^-1 in their natural habitat. This is close to that of artificial tap water. Our experiments, and measurements of freeze concentration effects in the literature, indicate that freeze-induced desiccation is unlikely to prevent inoculative freezing and the survival of nematodes over the winter. Accepted: 5 May 1996     

The effect of undissolved air on isochoric freezing

This study evaluates the effect of undissolved air on isochoric freezing of aqueous solutions. Isochoric freezing is concerned with freezing in a constant volume thermodynamic system. A possible advantage of the process is that it substantially reduces the percentage of ice in the system at every subzero temperature, relative to atmospheric freezing. At the pressures generated by isochoric freezing, or high pressure isobaric freezing, air cannot be considered an incompressible substance and the presence of undissolved air substantially increases the amount of ice that forms at any subfreezing temperature. This effect is measurable at air volumes as low as 1%. Therefore eliminating the undissolved air, or any separate gaseous phase, from the system is essential for retaining the properties of isochoric freezing.     

A freeze-etch study of the effects of extracellular freezing on cellular membranes of wheat

Seedlings of Triticum aestivum L. cv. Lennox were grown in different environments to obtain different hardiness. Pieces of laminae and leaf bases were slowly cooled to sub-zero temperatures and the damage caused was assessed by an ion-leakage method. Comparable pieces of tissue were slowly cooled to temperatures between 2° and-14°C and were then freeze-fixed and freeze-etched. Membranes generally retained their lamellar structures indicated by the abundance of typical membrane fracture faces in all treatments, and some membrane fracture faces had patches which lacked the usual scattering of intramembranous particles (IMP). These IMP-free areas were present in the plasma membrane of tissues given a damaging freezing treatment, but were absent from the plasma membrane of room-temperature controls, of supercooled tissues, and of tissues given a non-damaging freezing treatment. The frequency of IMP-free areas and the proportion of the plasma membrane affected increased with increasing damage. In the most damaged tissue (79% damage; leaf bases exposed to-8°C), 20% of the plasma membrane was IMP-free. The frequencies of IMP at a distance from the IMP-free areas were unaffected by freezing treatments. There was a patchy distribution of IMP in other membranes (nuclear envelope, tonoplast, thylakoids, chloroplast envelope), but only in the nuclear envelope did it appear possible that their occurrence coincided with damage. The IMP-free areas of several membranes were sometimes associated together in stacks. Such membranes lay both to the outside and inside of the plasma membrane, indicating that at least some of the adjacent membrane fragments arose as a result of membrane reorganization induced by the damaging treatment. Occasional views of folded IMP-free plasma membrane tended to confirm this conclusion. The following hypothesis is advanced to explain the damage induced by extracellular freezing. Areas of plasma membrane become free of IMP, probably as a result of the freezing-induced cellular dehydration. The lipids in these IMP-free patches may be in the fluid rather than the gel phase. The formation of these IMP-free patches, especially in the plasma membrane, initiates or involves proliferation and possibly fusion of membranes, and during or following this process, the cells become leaky.Abbreviations EF exoplasmatic fracture face - IMP intramembranous particles - PF protoplasmatic fracture face     

Effects of refrigeration and freezing on the electromechanical and biomechanical properties of articular cartilage

In vitro electromechanical and biomechanical testing of articular cartilage provide critical information about the structure and function of this tissue. Difficulties obtaining fresh tissue and lengthy experimental testing procedures often necessitate a storage protocol, which may adversely affect the functional properties of cartilage. The effects of storage at either 4°C for periods of 6 days and 12 days, or during a single freeze-thaw cycle at -20°C were examined in young bovine cartilage. Non-destructive electromechanical measurements and unconfined compression testing on 3 mm diameter disks were used to assess cartilage properties, including the streaming potential integral (SPI), fibril modulus (Ef), matrix modulus (Em), and permeability (k). Cartilage disks were also examined histologically. Compared with controls, significant decreases in SPI (to 32.3±5.5% of control values, p<0.001), Ef (to 31.3±41.3% [corrected] of control values, p=0.046), Em (to 6.4±8.5% of control values, p<0.0001), and an increase in k (to 2676.7±2562.0% of control values, p=0.004) were observed at day 12 of refrigeration at 4°C, but no significant changes were detected at day 6. A trend toward detecting a decrease in SPI (to 94.2±6.2% of control values, p=0.083) was identified following a single freeze-thaw cycle, but no detectable changes were observed for any biomechanical parameters. All numbers are mean±95% confidence interval. These results indicate that fresh cartilage can be stored in a humid chamber at 4°C for a maximum of 6 days with no detrimental effects to cartilage electromechanical and biomechanical properties, while one freeze-thaw cycle produces minimal deterioration of biomechanical and electromechanical properties. A comparison to literature suggested that particular attention should be paid to the manner in which specimens are thawed after freezing, specifically by minimizing thawing time at higher temperatures.     

The biomechanical model of the long finger extensor mechanism and its parametric identification

The extensor mechanism of the finger is a structure transmitting the forces from several muscles to the finger joints. Force transmission in the extensor mechanism is usually modeled by equations with constant coefficients which are determined experimentally only for finger extension posture. However, the coefficient values change with finger flexion because of the extensor mechanism deformation. This induces inaccurate results for any other finger postures. We proposed a biomechanical model of the extensor mechanism represented as elastic strings. The model includes the main tendons and ligaments. The parametric identification of the model in extension posture was performed to match the distribution of the forces among the tendons to experimental data. The parametrized model was used to simulate three degrees of flexion. Furthermore, the ability of the model to reproduce how the force distribution in simulated extensor mechanism changes according to the muscle forces was also demonstrated. The proposed model could be used to simulate the extensor mechanism for any physiological finger posture for which the coefficients involved in the equations are unknown.     

Numerical simulation of selective freezing of target biological tissues following injection of solutions with specific thermal properties

Recently, we proposed a method for controlling the extent of freezing during cryosurgery by percutaneously injecting some solutions with particular thermal properties into the target tissues. In order to better understand the mechanism of the enhancement of freezing by these injections, a new numerical algorithm was developed to simulate the corresponding heat transfer process that is involved. The three-dimensional phase change processes in biological tissues subjected to cryoprobe freezing, with or without injection, were compared numerically. Two specific cases were investigated to illustrate the selective freezing method: the injection of solutions with high thermal conductivity; the injection of solutions with low latent heat. It was found that the localized injection of such solutions could significantly enhance the freezing effect and decrease the lowest temperature in the target tissues. The result also suggests that the injection of these solutions may be a feasible and flexible way to control the size of the ice ball and its direction of growth during cryosurgery, which will help to optimize the treatment process.     

A theoretical model to study the effects of cellular stiffening on the damage evolution in deep tissue injury

Pressure induced deep tissue injury (DTI) is a severe form of pressure ulcers that is hard to detect in early stages and difficult to prevent and treat. High prevalence figures are partly due to a lack of understanding of pathological pathways involved in DTI. The aim of this study was to investigate, whether changes in material properties of damaged tissue can play a role in DTI aetiology. A numerical model was developed based on muscle microstructure and tissue engineering experiments. A time dependent damage law was proposed and stiffening of dead cells incorporated. The results obtained in the microstructural investigations were used to include the stiffening information in a pre-existing macroscopic model based on animal experiments, which correlated strains to tissue damage measured in the tibialis anterior muscle in rat limbs. With the modelling approach employed in this paper, the damaged area in the rat limb models increased up to 1.65-fold and the rate of damage progression was up to 2.1 times higher in microstructural simulations when stiffening was included.     

The effects of freezing on membrane electric potential in winter oilseed rape leaves

Extracellular ice formation in winter oilseed rape leaf discs (Brassica napus L. var. oleifera L. cv. Jantar) at different temperatures resulted in a transient membrane depolarization, which was followed by a decrease in membrane electric potential. In discs which underwent supercooling (no extracellular ice was formed), no membrane depolarization was observed. The inhibitors of calcium ion channels, gadolinium and lanthanum, decreased to some extend the amplitude of the frost-induced (−6 °C) depolarization and completely eliminated the decrease in membrane potential. Changes in membrane potential were associated with the increased electrolyte efflux, measured after thawing of the discs. No efflux from supercooled discs was observed. Application of calcium channel blockers decreased the level of the efflux induced by freezing at −6°C. It is suggested that membrane depolarization is one of the primary events induced by ice formation at a leaf surface. The possible reasons for changes in the membrane electric potential and their physiological consequences are discussed.     

The effects of total hip arthroplasty on the structural and biomechanical properties of adult bone

The responsiveness of bone to mechanical stimuli changes throughout life, with adaptive potential generally declining after skeletal maturity is reached. This has led some to question the importance of bone functional adaptation in the determination of the structural and material properties of the adult skeleton. A better understanding of age-specific differences in bone response to mechanical loads is essential to interpretations of long bone adaptation. The purpose of this study is to examine how the altered mechanical loading environment and cortical bone loss associated with total hip arthroplasty affects the structural and biomechanical properties of adult bone at the mid-shaft femur. Femoral cross sections from seven individuals who had undergone unilateral total hip arthroplasty were analyzed, with intact, contralateral femora serving as an approximate internal control. A comparative sample of individuals without hip prostheses was also included in the analysis. Results showed a decrease in cortical area in femora with prostheses, primarily through bone loss at the endosteal envelope; however, an increase in total cross-sectional area and maintenance of the parameters of bone strength, I(x), I(y), and J, were observed. No detectable differences were found between femora of individuals without prostheses. We interpret these findings as an adaptive response to increased strains caused by loading a bone previously diminished in mass due to insertion of femoral prosthesis. These results suggest that bone accrued through periosteal apposition may serve as an important means by which adult bone can functional adapt to changes in mechanical loading despite limitations associated with senescence.     

The effects of simulated weightlessness on bone biomechanical and biochemical properties in the maturing rat

Histomorphometric and biomechanical changes in bone resulting from hypogravity (simulated weightlessness) were examined in this study. Using a head-down hindlimb suspension model, three groups of six male rats underwent simulated weightlessness for periods of one, two and three weeks while a fourth recovery group was suspended for two weeks followed by two weeks of normal activity. Biomechanical data were collected during static and dynamic bending and torsion tests on intact femora. Histomorphometric values were determined from midshaft bone cross sections and material properties were obtained using ash and calcium assays. The experimental groups exhibited significantly lower geometric and material properties than the controls, resulting in structural hypotrophy; geometric and material changes contributed equally to the structural changes. Recovery following a return to normal activity was indicated, although full recovery may take longer than the weightlessness period. In the rat, altered maturation and reduced bone strength were the sequelae of weightlessness.