Biomedical subjects

Stephen D Waldman

Publications and source records attributed to Stephen D Waldman.

12 recordsLinked to original sources

PubMed2006

Specimen size effect in the volumetric shrinkage of cancellous bone measured at two levels of dehydration.

Water is commonly removed from bone to study its effect on mechanical behaviour; however, dehydration also alters the bone structure. To make matters worse, measuring structural changes in cancellous bone is complicated by a number of factors. Therefore, the goals of this study were to address these issues by (1) comparing Archimedes' method and a helium pycnometer as methods for measuring cancellous bone volume; (2) measuring the apparent dimensional and volumetric tissue shrinkage of cancellous bone at two levels of dehydration; and, (3) identifying whether a size effect exists in cancellous bone shrinkage. Cylindrical specimens (3, 5 and 8.3 mm diameters) of cancellous bone were taken from the distal bovine femur. The apparent dimensions of each cylindrical specimen were measured in a fully hydrated state (HYD), after drying at room temperature (AIR), and after oven drying at 105 degrees C (OVEN). Tissue volume measurements for those three hydration states were obtained using both a helium pycnometer and Archimedes' method. Aluminium foams, which mimic the cancellous structure, were used as controls. The results suggest that the helium pycnometer and Archimedes' method yield identical results in the HYD and AIR states, but that Archimedes' method under-predicts the nominal OVEN volume by incorporating the collagen-apatite porosity. A distinct size effect on volumetric shrinkage is observed (p<0.025) using the pycnometer in both AIR and OVEN states. Apparent dimensional shrinkage (2% and 7%) at the two dehydration levels is much smaller than the measured volumetric tissue shrinkage (16% and 29%), which results in a reduced dehydrated bone volume fraction.

Animals↗

PubMed2006

Sex differences in long bone fatigue using a rat model.

Stress fractures can occur because of prolonged exercise and are associated with cyclic loading. Fatigue is the accumulated damage that results from cyclic loading and bone fatigue damage is of special concern for athletes and army recruits. Existing literature shows that the rates of stress fracture for female athletes and female army recruits are higher than their male counterparts. In this study, we used an ex vivo rat model to investigate the fatigue response of female and male bones. We determined the strain versus number of cycles to failure (S/N) for each sex and found that for a certain initial strain (5,000-7,000 microepsilon) female bones have shorter fatigue life. To further characterize the bone response to fatigue, we also determined the creep that occurred during the fatigue test. From the creep data, for a certain strain range, female bones accumulated greater residual strains and reached the critical strain at a faster rate. In summary, this study demonstrates that female rat bones have a lower resistance to fatigue in the absence of a physiological response such as muscle fatigue or osteogenic adaptation. From these results, we hypothesized that creep was the underlying mechanism that accounted for the fast deterioration of female bones during fatigue.

Animals↗

PubMed2005

Effect of sample geometry on the apparent biaxial mechanical behaviour of planar connective tissues.

Mechanical testing methodologies developed for engineering materials may result in artifactual material properties if applied to soft planar connective tissues. The use of uniaxial tissue samples with high aspect ratios or biaxial samples with slender cruciform arms could lead to preferential loading of only the discrete subset of extracellular fibres that fully extend between the grips. To test this hypothesis, cruciform biaxial connective tissue samples that display distinctly different material properties (bovine pericardium, fish skin), as well as model textile laminates with predefined fibrous orientations, were repeatedly tested with decreasing sample arm lengths. With mechanical properties determined at the sample centre, results demonstrated that the materials appeared to become stiffer and less extensible with less slender sample geometries, suggesting that fibre recruitment increases with decreasing sample arm length. Alterations in the observed shear behaviour and rigid body rotation were also noted. The only truly reliable method to determine material properties is through in vivo testing, but this is not always convenient and is typically experimentally demanding. For the in vitro determination of the biaxial material properties, appropriate sample geometry should be employed in which all of the fibres contribute to the mechanical response.

Animals↗

PubMed2004

Tissue engineered nucleus pulposus tissue formed on a porous calcium polyphosphate substrate.

STUDY DESIGN: This study describes the formation of nucleus pulposus tissue using a novel tissue engineering approach. OBJECTIVES: To determine if a construct composed of nucleus pulposus tissue on the surface of a calcium polyphosphate substrate could be formed in vitro with properties similar to native nucleus pulposus tissue. SUMMARY OF BACKGROUND DATA: There is no optimal treatment for the persistent pain associated with intervertebral disc degeneration. Disc replacement using artificial intervertebral discs has met with some success, and biologic transplantation is limited by the availability of donor tissues. METHODS: Nucleus pulposus cells were isolated from bovine caudal intervertebral discs. Cells were seeded at high density on the upper surface of a porous bone substitute material (calcium polyphosphate) and maintained up to 6 weeks in culture. In vitro formed tissue was compared to native nucleus pulposus for histologic appearance, biochemical composition (tissue cellularity, proteoglycan and collagen accumulation), and compressive mechanical properties. RESULTS: When maintained on the surface of a three-dimensional substrate, nucleus pulposus cells formed a continuous layer of tissue with a proteoglycan content equivalent to the native tissue. Although collagen accumulation attained only 26% than that of the native tissue, there was no difference in tissue stiffness, viscosity, or weight-bearing capacity of the in vitro formed tissue when compared with the native tissue. CONCLUSION: Nucleus pulposus-like tissue formed in vitro on the surface of a calcium polyphosphate substrate resembles the native tissue in terms of proteoglycan content and compressive mechanical properties. These studies are the first step toward developing a functional spinal unit in vitro.

Animals↗

PubMed2003

Mesenchymal progenitor self-renewal deficiency leads to age-dependent osteoporosis in Sca-1/Ly-6A null mice.

The cellular and molecular mechanisms that underlie age-dependent osteoporosis, the most common disease in the Western Hemisphere, are poorly understood in part due to the lack of appropriate animal models in which to study disease progression. Here, we present a model that shows many similarities to the human disease. Sca-1, well known for its expression on hematopoietic stem cells, is present on a subset of bone marrow stromal cells, which potentially include mesenchymal stem cells. Longitudinal studies showed that Sca-1(-/-) mice undergo normal bone development but with age exhibit dramatically decreased bone mass resulting in brittle bones. In vivo and in vitro analyses demonstrated that Sca-1 is required directly for the self-renewal of mesenchymal progenitors and indirectly for the regulation of osteoclast differentiation. Thus, defective mesenchymal stem or progenitor cell self-renewal may represent a previously uncharacterized mechanism of age-dependent osteoporosis in humans.

Aging↗

PubMed2003

The use of specific chondrocyte populations to modulate the properties of tissue-engineered cartilage.

Tissue engineering of articular cartilage is a promising alternative to the conventional approaches for cartilage repair. However, recent attempts to develop articular cartilage in vitro have proven to be difficult. The tissue formed in vitro may not accumulate enough extracellular matrix, and the resulting mechanical properties are only a fraction of the native tissue. We investigated whether using specific populations of chondrocytes would improve the properties of the cartilaginous tissue that was generated in vitro. Full-thickness (FT), mid-and-deep zone (MD), and deep-zone (DEEP) chondrocytes were isolated, placed on the surface of porous ceramic substrates and maintained in culture for eight weeks. Tissue developed from DEEP chondrocytes was thicker (FT: 0.94+/-0.03, MD: 0.88+/-0.04, DEEP: 2.4+/-0.1 mm) and had accumulated larger amounts of extracellular matrix (FT: 1.61+/-0.05, MD: 1.5+/-0.1, DEEP: 3.8+/-0.2 mg dry weight) than the tissues formed by the FT and MD chondrocytes. The tissue formed by the FT chondrocytes accumulated the greatest amount of collagen (FT: 211+/-14, MD: 185+/-8, DEEP: 178+/-5 microg/mg dry weight) whereas the tissue formed by the MD chondrocytes accumulated significantly more proteoglycans (FT: 198+/-10, MD: 265+/-10, DEEP: 215+/-5 microg/mg dry weight). Interestingly, MD chondrocytes produced tissue that had compressive mechanical properties up to four times greater than the cartilaginous tissues formed by cells from either the FT or DEEP of cartilage. Thus, a combined population of intermediate and DEEP chondrocytes might be more suitable for the tissue engineering of articular cartilage.

Animals↗

PubMed2003

Long-term intermittent shear deformation improves the quality of cartilaginous tissue formed in vitro.

The formation of cartilaginous tissue in vitro is a promising alternative to repair damaged articular cartilage. However, recent attempts to tissue-engineer articular cartilage that has similar properties to the native tissue have proven to be difficult. The in vitro-formed cartilaginous tissue typically has a similar proteoglycan content to native cartilage, but has a reduced collagen content and only a fraction of the mechanical properties. In this study, we investigated whether the intermittent application of cyclic shearing forces during tissue formation would improve the tissue quality. Chondrocyte cultures were stimulated at a 2% shear strain amplitude at a frequency of 1 Hz for 400 cycles every 2nd day. At one week, both collagen and proteoglycan synthesis increased (23+/-6% and 20+/-6%, respectively) over the unstimulated, static controls. At four weeks, an increased amount of tissue formed (stimulated: 1.85+/-0.08, unstimulated: 1.58+/-0.07 mg dry wt.). This tissue contained approximately 40% more collagen (stimulated: 511+/-23, unstimulated: 367+/-24 microg/construct) and 35% more proteoglycans (stimulated: 376+/-21, unstimulated: 279+/-26 microg/construct). Tissues that formed in the presence of shearing forces also displayed a 3-fold increase in compressive load-bearing capacity (stimulated: 16+/-5, unstimulated: 5+/-1 kPa max. equilibrium stress) and a 6-fold increase in stiffness (stimulated: 112+/-36, unstimulated: 20+/-6 kPa max. equilibrium modulus) compared to the static controls. These results demonstrate that intermittent application of dynamic shearing forces over a four-week period improves the quality of cartilaginous tissue formed in vitro. Interestingly, low amplitudes of shear stimulation for short periods of time (6 min of stimulation applied every 2nd day) produced these changes.

Animals↗

PubMed2003

Effect of biomechanical conditioning on cartilaginous tissue formation in vitro.

BACKGROUND: Although tissue engineering of articular cartilage is a promising approach for cartilage repair, it has been difficult to develop cartilaginous tissue in vitro that mimics the properties of native cartilage. Isolated chondrocytes grown in culture typically do not accumulate enough extracellular matrix, and the generated tissue possesses only a fraction of the mechanical properties of native cartilage. One potential explanation for this might be that the cells are grown in an environment that lacks the mechanical stimuli to which the chondrocytes are exposed in vivo. In this study, we compared the long-term effects of both dynamic compressive and shearing forces on cartilaginous tissue formation in vitro. METHODS: Bovine articular chondrocytes were grown on the surface of porous ceramic substrates and were maintained under static, free-swelling conditions for a period of four weeks. Cultures were then subjected to six minutes of mechanical stimulation every other day, in either compression or shear, for an additional four-week period. RESULTS: Cartilaginous tissues cultured in the presence of intermittent compression or shear were significantly thicker (p < 0.05) and had accumulated more extracellular matrix (p < 0.01) compared with the unstimulated controls. However, when normalized by the wet weight of the tissue, cultures stimulated in the presence of shearing forces contained more proteoglycans and collagen compared with compression-stimulated cultures. These cultures also displayed the largest increase in mechanical properties, with a threefold increase in equilibrium stress and a fivefold increase in equilibrium modulus. CONCLUSIONS AND CLINICAL RELEVANCE: The results of this study demonstrate that a brief application of mechanical forces applied periodically over a long duration can improve the quality of cartilaginous tissue formed in vitro. However, the changes in tissue composition and mechanical properties were dependent on the specific mode of the applied mechanical forces, with shear stimulation eliciting the greater effect. This finding suggests that chondrocytes may respond differently to different modes of applied forces.

Animals↗

PubMed2002

Characterization of cartilagenous tissue formed on calcium polyphosphate substrates in vitro.

Successful joint resurfacing by tissue-engineered cartilage has been limited, in part, by an inability to secure the implant to bone. To overcome this, we have developed the methodology to form a cartilage implant in vitro consisting of a layer of cartilagenous tissue overlying a porous, biodegradable calcium polyphosphate (CPP) substrate. As bone will grow into the CPP after implantation, it will result in anchorage of the cartilage. In this study, the cartilagenous tissue formed in vitro after 8 weeks in culture was characterized and compared to native articular cartilage. Light microscopic examination of histological sections showed that there was a continuous layer of cartilagenous tissue on, and integrated with the subsurface of, the CPP substrate. The in vitro-formed tissue achieved a similar thickness to native articular cartilage (mean +/- SEM: in vitro = 0.94 +/- 0.03 mm; ex vivo = 1.03 +/- 0.01 mm). The cells in the in vitro-formed tissue synthesized large proteoglycans (Kav +/- SEM: in vitro = 0.27 +/- 0.01; ex vivo = 0.27 +/- 0.01) and type II collagen similar to the chondrocytes in the ex-vivo cartilage. The in vitro-formed tissue had a similar amount of proteoglycan (GAG microg/mg dry wt.: in vitro = 198 +/- 10; ex vivo = 201 +/- 13) but less collagen than the native cartilage (hydroxyproline microg/mg dry wt.: in vitro = 21 +/- 1; ex vivo = 70 +/- 8). The in vitro-formed tissue had only about 3% of the load-bearing capacity and stiffness of the native articular cartilage, determined from unconfined mechanical compression testing. Although low, this was within the range of properties reported by others for tissue-engineered cartilage. It is possible that the limited load-bearing capacity is the result of the low collagen content and further studies are required to identify the conditions that will increase collagen synthesis.

Animals↗

PubMed2002

Boundary conditions during biaxial testing of planar connective tissues. Part 1: dynamic behavior.

Current mechanical testing methods used to determine the biaxial properties of planar connective tissues may lead to artifactual observations of material behavior. The method of sample gripping affects the constraint on the extracellular fibers at the bounds of the sample. This applied constraint not only affects how the load is transferred to the sample, but also how the load is transmitted throughout the rest of the material - thereby influencing the resulting mechanical behavior of the tissue. In this study, we compared the dynamic biaxial mechanical response of pericardial tissue samples under two different gripping methods: (i) the common method of suturing sample edges and (ii) a new biaxial clamping method. Tissue samples were repeatedly testing using both gripping methods under the same conditions. The tissue samples appeared to be stiffer and less extensible when mechanically tested with clamped sample edges, as opposed to when tested with sutured sample edges. Thus, the influence of the sample boundaries affected the response of the material - precisely the situation to be avoided for reliable material testing. This casts doubt on whether any in vitro mechanical testing method can used to determine the "real" properties of the tissue since the boundary conditions of the tissue in situ are presently unknown.

Journal Article↗

PubMed2002

Mechanical characterization of a novel cell stimulating system (CSS) to apply dynamic, uniform and isotropic biaxial strains to cells in vitro.

Mechanical loading alters cellular responses. While in vitro mechanical stimulation is a powerful tool for exploration of mechanotransduction, very little has been published documenting techniques for validation of such devices. We have developed an in vitro experimental system that imposes well-defined temporal and spatial strain profiles using a pressure-actuated, tethered diaphragm substrate (Bioflex dishes). More importantly we have accurately characterized the strain and strain rate performance of this system and herein describe that methodology. The prototype CSS deflected cell substrates over cylindrical platens, producing dynamic biaxial strains. Dynamic studies at 1 Hz were conducted at 8.0, 9.0, 10.0 and 13.0 kPa peak transmural pressures for a total of 1000 loading cycles. To study the effects of frequency, experiments were also run at 0.5 and 1.5 Hz at 8 and 13 kPa. A series of 33 dots were placed collinearly in rings on the membrane. Dot motions were monitored via a CCD video camera and acquisition was performed using an 8-bit gray-scale video board and N1H Image software. Strain fields and rates were subsequently calculated using Mathematica software. Results confirmed that the strains were biaxially uniform over the frequencies and pressures examined: e.g., at 9.0 KPa, max radial & circumferential strain = 0.009 +/- 0.001. It was also shown that, as transmural pressure was increased, both membrane strains and strain rates increased; however biaxial strain isotropy was preserved. While we cannot measure out-of-plane deflections, video-based image analysis is a very useful technique for validation of dynamic planar biaxial strains in cell stimulation systems.

Anisotropy↗

PubMedDate not supplied

Long-term intermittent compressive stimulation improves the composition and mechanical properties of tissue-engineered cartilage.

Tissue engineering of articular cartilage is a promising alternative for cartilage repair. However, it has been difficult to develop tissue in vitro that mimicks native cartilage. Cartilaginous tissue formed in vitro does not accumulate enough extracellular matrix, is deficient in collagen, and possesses only a fraction of the mechanical properties of native cartilage. In this study, we investigated whether long-term intermittent compressive stimulation would improve the quality of the generated tissue. Chondrocyte cultures were established on the surface of porous calcium polyphosphate substrates and allowed to form cartilaginous tissue. In vitro-formed tissues were subjected to different stimulation protocols for 1 week. The optimal mechanical stimulation parameters identified in this short-term study were then applied to the cultures for up to 4 weeks. Mechanical stimulation applied at a 5% compressive amplitude at a frequency of 1 Hz for 400 cycles every second day resulted in the greatest increase in collagen synthesis (37 +/- 9% over control) while not significantly affecting proteoglycan synthesis (2 +/- 8% over control). This condition, applied to the chondrocyte cultures for 4 weeks, resulted in a significant increase in the amount of tissue that formed (stimulated, 2.4 +/- 0.2 mg dry wt; unstimulated, 1.61 +/- 0.08 mg dry wt). Stimulated tissues contained approximately 40% more collagen (stimulated, 590 +/- 58 microg; unstimulated, 420 +/- 42 microg), and 30% more proteoglycans (stimulated, 393 +/- 34 microg; unstimulated, 302 +/- 32 microg) as well as displaying a 2- to 3-fold increase in compressive mechanical properties (maximal equilibrium stress: stimulated, 10 +/- 1 kPa; unstimulated, 5 +/- 1 kPa; maximal equilibrium modulus: stimulated, 80 +/- 23 kPa; unstimulated, 24 +/- 6 kPa). The results of this study demonstrate that intermittent mechanical stimulation can increase collagen synthesis and, when applied over a 4-week period, can accelerate extracellular matrix accumulation as well as improve the material properties of the developed tissue. Interestingly, only short periods of mechanical stimulation (6 min every second day) were needed to affect the quality of cartilaginous tissue formed in vitro.

Adaptation, Physiological↗