Biomedical subjects

R Vanderby

Publications and source records attributed to R Vanderby.

At least 19 recordsLinked to original sources

PubMed2004

Response of the osteocyte syncytium adjacent to and distant from linear microcracks during adaptation to cyclic fatigue loading.

Cyclic loading induces fatigue in bone and initiates a complex, functionally adaptive response. We investigated the effect of a single period of fatigue on the histologic structure and biomechanical properties of bone. The ulnae of 40 rats were subjected to cyclic fatigue (-6000 microepsilon) unilaterally until 40% loss of stiffness developed, followed by 14 days of adaptation. The contralateral ulna served as a treatment control (n = 20 rats), and a baseline loaded/non-loaded group (n = 20 rats/group) was included. Bones from 10 rats/group were examined histologically and the remaining bones (10 rats/group) were tested mechanically. The following measurements were collected: volumetric bone mineral density (vBMD); ultimate force (Fu); stiffness (S); energy-to-failure (U); cortical area (Ct.Ar); microcrack density (Cr.Dn); microcrack mean length (Cr.Le); microcrack surface density (Cr.S.Dn); osteocyte density (Ot.N/T.Ar and Ot.N/TV); bone volume fraction (B.Ar/T.Ar); resorption space density (Rs.N/Ct.Ar); and maximum and minimum area moments of inertia (IMAX and IMIN). Using confocal microscopy, the bones were examined for diffuse matrix injury, canalicular disruption, and osteocyte disruption. The adapted bones had increased B.Ar, IMAX, and IMIN in the mid-diaphysis. Fatigue loading decreased structural properties and induced linear microcracking. At 14 days, adaptation restored structural properties and microcracking was partially repaired. There was a significant nonlinear relationship between Ot.N/T.Ar and B.Ar/T.Ar during adaptation. Disruption of osteocytes was observed adjacent to microcracks immediately after fatigue loading, and this did not change after the period of adaptation. In fatigue-loaded bone distant from microcracks, diffuse matrix injury and canalicular disruption were often co-localized and were increased in the lateral (tension) cortex. These changes were partially reversed after adaptation. Loss of canalicular staining and the presence of blind-ends in regions with matrix injury were suggestive of rupture of dendritic cell processes. Taken together, these data support the general hypothesis that the osteocyte syncytium can respond to cyclic loading and influence targeted remodeling during functional adaptation. Changes in the appearance of the osteocyte syncytium were found in fatigue-loaded bone with and without linear microcracks. We hypothesize that the number of dendritic cell processes that experience load-related disruption may determine osteocyte metabolic responses to loading and influence targeted remodeling.

Adaptation, Physiological↗

PubMed2003

Degradation of bone structural properties by accumulation and coalescence of microcracks.

Failure of bone adaptation to protect the skeleton from fatigue fracture is common, and site-specific accumulation and coalescence of microcracking in regions of high strain during cyclic loading is considered a key factor that decreases the resistance of whole bones to fracture. We investigated the effect of cyclic fatigue loading on the monotonic structural properties of the rat ulna during accumulation and coalescence of microcracks. Cyclic end-loading of the ulna was performed at 4 Hz ex vivo at an initial peak strain of -6000 muepsilon to 20% loss of stiffness (n = 7) or 40% loss of stiffness (n = 7) bilaterally. A 0% loss of stiffness monotonically loaded control group (n = 7) was also included. Volumetric bone mineral density (vBMD), ultimate strength (F(u)), stiffness (S), and energy-to-failure (U) were determined in one ulna and in the contralateral ulna vBMD, cortical bone area (B.Ar), maximum and minimum second moments of inertia (I(MAX) and I(MIN)), microcrack density (Cr.Dn), microcrack mean length (Cr.Le), and microcrack surface density (Cr.S.Dn) were determined. In two additional groups of rats, cyclic end-loading of the ulna was also performed ex vivo unilaterally to 20% loss of stiffness (n = 10) and 40% loss of stiffness (n = 10) and then vBMD, F(u), S, U, B.Ar, I(MAX), and I(MIN) were determined bilaterally. Fatigue loading had incremental degradative effects on ulna structural properties. This decreased resistance to fracture was associated with accumulation and coalescence of branching arrays of microcracks within the cortex of the ulna. Microcracking was most prominent in the middiaphysis and corresponded to the region of the bone that fractured during monotonic structural testing. Fatigue loading influenced the relationship between bone cross-sectional geometry and vBMD and ulna structural properties. At 40% loss of stiffness, F(u), S, and U were all significantly correlated with cross-sectional bone geometry and vBMD, whereas this was not the case at 20% loss of stiffness and with the 0% loss of stiffness monotonic control ulnae. We also found a biologically significant individual animal effect. Larger ulnae required a higher number of load cycles for fatigue to develop, retained higher strength, and accumulated a greater amount of microcracking at the end of the cyclic fatigue testing. Small increases in bone size and density can substantially improve the resistance of whole bones to fracture as microcracking accumulates and coalesces during cyclic fatigue loading.

Animals↗

PubMed2002

Application of nonlinear viscoelastic models to describe ligament behavior.

Recent experiments in rat medial collateral ligament revealed that the rate of stress relaxation is strain dependent and the rate of creep is stress dependent. This nonlinear behavior requires a more general description than the separable quasilinear viscoelasticity theory commonly used in tissue biomechanics. The purpose of this study was to determine whether the nonlinear theory of Schapery or the modified superposition method could adequately model the strain-dependent stress-relaxation behavior of ligaments. It is shown herein that both theories describe available nonlinear experimental ligament data well and hence can account for both elastic and viscous nonlinearities. However, modified superposition allows for a more direct interpretation of the relationship between model parameters and physical behavior, such as elastic and viscous nonlinearities, than does Schapery's theory. Hence, the modified superposition model is suggested to describe ligament data demonstrating both elastic nonlinearity and strain-dependent relaxation rate behavior. The behavior of the modified superposition model under a sinusoidal strain history is also examined. The model predicts that both elastic and viscous behaviors are dependent on strain amplitude and frequency.

Animals↗

PubMed2001

The effect of recombinant human bone morphogenetic protein-2 on femoral reconstruction with an intercalary allograft in a dog model.

This study compared the effect of augmentation of allograft host bone junctions with recombinant human bone morphogenetic protein-2 (rhBMP-2) on an absorbable collagen sponge (ACS), autogenous cancellous bone graft (CBG), and a collagen sponge alone in a canine intercalary femoral defect model repaired with a frozen allograft. Outcome assessment included serial radiographs, dual energy X-ray absorptiometry scans, and gait analyses, and mechanical testing and histology of post-mortem specimens. The distal junction healed more quickly and completely with rhBMP-2 than ACS alone based on qualitative radiography and histologic evaluations. The primary tissue in the unhealed gaps in the ACS group was fibrous connective tissue. The proximal allograft host bone junction had complete bone union in the three treatment groups. There was significantly greater new bone callus formation at both junctions with rhBMP-2 than with CBG or ACS alone that resulted in increased bone density around the allograft host bone junctions. All dogs shifted their weight from the treated leg to the contralateral pelvic limb immediately after surgery. Weight bearing forces were redistributed equally between the pelvic limbs at 12 weeks after surgery with rhBMP-2, at 16 weeks after surgery with CBG, and at 24 weeks after surgery with ACS alone. Bending and compressive stiffnesses of the whole treated femora were equal to the contralateral control femora in all treatment groups, whereas torsional rigidities of the whole treated femora for the CBG and ACS groups were significantly less than the control. Both the proximal and distal junctions the treated with rhBMP-2 had torsional stiffnesses and strengths equal to intact control bones. Ultimate failure torques of the proximal junctions of the CBG group and of both junctions of the ACS group were significantly less than the BMP-treated bones. Augmentation of the allograft host bone junctions with rhBMP-2 on an ACS gave results for all parameters measured that equaled or exceeded autogenous graft in this canine intercalary femoral defect model.

Animals↗

PubMed2001

Nonlinear ligament viscoelasticity.

Ligaments display time-dependent behavior, characteristic of a viscoelastic solid, and are nonlinear in their stress-strain response. Recent experiments (25) reveal that stress relaxation proceeds more rapidly than creep in medial collateral ligaments, a fact not explained by linear viscoelastic theory but shown by Lakes and Vanderby (17) to be consistent with nonlinear theory. This study tests the following hypothesis: nonlinear viscoelasticity of ligament requires a description more general than the separable quasilinear viscoelasticity (QLV) formulation commonly used. The experimental test for this hypothesis involves performing both creep and relaxation studies at various loads and deformations below the damage threshold. Freshly harvested, rat medial collateral ligaments (MCLs) were used as a model. Results consistently show a nonlinear behavior in which the rate of creep is dependent upon stress level and the rate of relaxation is dependent upon strain level. Furthermore, relaxation proceeds faster than creep; consistent with the experimental observations of Thornton et al. (25) The above results from rat MCLs are not consistent with a separable QLV theory. Inclusion of these nonlinearities would require a more general formulation.

Animals↗

PubMed2001

Microstructural morphology in the transition region between scar and intact residual segments of a healing rat medial collateral ligament.

This study used a rat model to investigate the microstructural organization of collagen through the transition from scar to intact residual segments of a healing medial collateral ligament (MCL). Twenty-two male retired breeder Sprague-Dawley rats were randomly separated into two groups. Eleven underwent surgical transections of both MCLs and were allowed unrestricted cage activity until euthanized two weeks post surgery. The remaining eleven rats were used as normal controls. All 44 MCLs were harvested including intact femoral and tibial insertions and prepared for scanning electron microscopy (SEM) imaging. At harvest the scar region in the healing ligaments was more translucent than the normal tissue. Ligaments were viewed from femoral to tibial insertions at magnifications of 100X through 20,000X. Tissue away from the scar region in the transected MCLs was indistinguishable from normal tissue in uninjured ligaments. Collagen fibers and fibrils in these tissues were more aligned along the longitudinal axis of the ligament than in the scar tissue. Continuity of collagen fibers and fibrils were consistently observed from the residual portions of the transected ligament through the scar region. Bifurcations/fusions, but no anastomoses, in fibers and fibrils were observed in both normal and scar tissues of ligaments. Qualitatively, bifurcations were encountered more frequently in scar tissue. In the transition region, larger diameter fibers from the residual tissue bifurcated into smaller diameter fibrils in the scar. This connection between larger diameter and smaller diameter fibers and fibrils indicates that bifurcations/fusions are likely to be the dominant way in which force is transmitted from a region with larger fibrils (residual ligament) into and through a region with smaller fibrils (scar).

Animals↗

PubMed2000

Comparison of three methods of gluteal muscle attachment to an allograft/endoprosthetic composite in a canine model.

This study used radiography, gait analysis, gluteal muscle mass, mechanical testing, and qualitative histology to compare three methods of gluteal muscle attachment to an allograft/endoprosthetic composite of the proximal 25% of the femur in an in vivo canine model. The three methods of gluteal muscle attachment were identical to those used clinically in human patients for hip revision and proximal femoral limb salvage: the host gluteal tendon sutured to the allograft tendon (tendon group), the host greater trochanter with intact gluteal tendons secured to the allograft with a cable-grip system (grip group), and periosteally vascularized proximal femoral bone onlay with intact tendons wrapped around the allograft (wrap group). On the basis of radiographs taken every 2 months, the tendon group had more graft fractures than did the grip or wrap group. Radiographic union of the graft-host bone junction occurred more rapidly and there was less graft resorption in the wrap group than in the other two groups. In all dogs, peak vertical ground-reaction forces in the treated limb decreased immediately after surgery and then slowly increased over the length of the study. The dogs in the wrap group regained normal weight-bearing on the treated limb more quickly than did those in the other groups. The constructs in the tendon group were weaker and less stiff immediately after surgery than were those in the other groups or in intact controls. Histologic analysis confirmed that the wrap technique resulted in complete union of the host bone-allograft junction more often than did the other techniques. The wrap method had the best functional outcome after 9 months when an allograft/endoprosthetic composite was used during total hip arthroplasty in this canine model.

Animals↗

PubMed2000

The mechanism of joint capsule thermal modification in an in-vitro sheep model.

The purpose of this study was to understand the mechanism responsible for joint capsule shrinkage after nonablative laser application in an in-vitro sheep model. Femoropatellar joint capsular tissue specimens harvested from 20 adult sheep were treated with one of three power settings of a holmium:yttrium-aluminum-garnet laser or served as a control. Laser treatment significantly shortened the tissue and decreased tissue stiffness in all three laser groups, whereas failure strength was not altered significantly by laser treatment. Transmission electron microscopic examination showed swollen collagen fibrils and loss of membrane integrity of fibroblasts. A thermometric study revealed nonablative laser energy caused tissue temperature to rise in the range of 64 degrees C to 100 degrees C. Electrophoresis after trypsin digestion of the tissue revealed significant loss of distinct alpha bands of Type I collagen in laser treated samples, whereas alpha bands were present in laser treated tissue without trypsin digestion. The results of this study support the concept that the primary mechanism responsible for the effect of nonablative laser energy is thermal denaturation of collagen in joint capsular tissue associated with unwinding of the triple helical structure of the collagen molecule.

Analysis of Variance↗

PubMed2000

The biologic response to laser thermal modification in an in vivo sheep model.

The purpose of this study was to evaluate the effect of nonablative laser energy on mechanical, histologic, ultrastructural, and biochemical properties of joint capsular tissue in an in vivo sheep model. Femoropatellar joint capsule was treated with the holmium:yttrium-aluminum-garnet laser via an arthroscope, and tissues were harvested immediately after surgery, or at 3, 7, 14, 30, 60, 90, and 180 days after surgery (n = 8/group). Laser treatment caused significant decreases in tissue stiffness from 0 to 7 days after surgery, then stiffness gradually increased after 14 days. Tissue strength was lowest 3 days after laser treatment. Histologic examination revealed immediate collagen hyalinization and cell necrosis, followed by active cellular response characterized by extensive fibroblast migration and capillary sprouting. Tissue appeared to be normal histologically 60 days after surgery; however, collagen fibrils remained uniformly small. This study showed an active tissue response secondary to thermal modification with concomitant recovery of mechanical properties by 30 days after surgery. Whether the shrinkage or joint stability was maintained with time remains to be evaluated. To clarify the advantages and disadvantages of this technique, a carefully controlled clinical trial with long term followup should be performed.

Animals↗

PubMed2000

Effects of monopolar radiofrequency energy on ovine joint capsular mechanical properties.

Radiofrequency energy may provide a relatively noninvasive method to stabilize joints with excessive laxity by thermally shrinking redundant joint capsular tissue. The authors determined the percentage of shrinkage associated with five radiofrequency treatment temperatures and evaluated the effect of this energy on the structural properties of joint capsular tissue in vitro. First, 36 adult sheep femoropatellar joint capsular specimens were treated with one of five treatment temperatures (n = 6 per group) or served as a control to determine tissue shrinkage. An additional 24 specimens were treated with three temperatures that resulted in different shrinkage: 45 degrees C, 65 degrees C, and 85 degrees C. Tissue stiffness, relaxation, and failure strength were determined for each specimen (n = 6 per group). Tissue shrinkage was correlated significantly with treatment temperature. There was a significant decrease in tensile stiffness in the 65 degrees C and 85 degrees C treatment groups. There were no significant differences between stress relaxation before treatment and after treatment. Relaxation properties after treatment were not different from each other or from control values either normalized to pretreatment values or expressed as raw data. Failure strength was not affected significantly at any temperature.

Animals↗

PubMed2000

Muscle forces and spinal loads at C4/5 level during isometric voluntary efforts.

PURPOSE: The goal of this study was to determine neck muscle forces and spinal loads that result from isometric muscle contractions. METHODS: Electromyographic (EMG) activity of the neck musculature and a three-dimensional biomechanical model of the neck were used. The model was EMG-based and estimated muscle forces and spinal loads at the C4/5 level. EMG signals were collected from eight sites at the C4/5 level of the neck using Ag-AgCl surface electrodes from 10 adult male subjects. The subjects performed isometric contractions gradually developing to maximum efforts in flexion, extension, left lateral bending, and right lateral bending. RESULTS: During maximum voluntary contraction (MVC) trials most muscles generated high levels of EMG signal during cervical rotation. The posterior surface of the neck (trapezius) was the only electrode site at which maximum activity EMG consistently occurred by the same method (rotation) in all subjects. Variations in the EMG patterns were observed in different experiments that produced overall neck moments of equal magnitudes. With these data the model computed variations in load distribution among the agonist muscles. Consistent also with EMG distributions, the model also computed co-contractions of antagonist muscles. The average (+/- SD) magnitudes of peak moments were 28.3 (+/- 3.3) Nm in extension, 17.7 (+/- 3.1) Nm in flexion, 16.9 (+/- 2.8) Nm in left lateral bending, and 17.0 (+/- 2.9) Nm in right lateral bending. The model predicted C4/5 joint compressive forces during peak moments were 1372 (+/- 140) N in extension, 1654 (+/- 308) N in flexion, 956 (+/- 169) N in left lateral bending, and 1065 (+/- 207) N in right lateral bending. CONCLUSIONS: Results suggest that higher C4/5 joint loads than previously reported are possible during maximum isometric muscle contractions.

Adult↗

PubMed2000

Transient and cyclic responses of strain-generated potential in rabbit patellar tendon are frequency and pH dependent.

The goal of this study was to expand understanding of strain-generated potential (SGP) in ligamentous or tendinous tissues. Most SGP studies in the past have focused on cartilage or bone. Herein, rabbit patellar tendon (PT) was used as a model. Each patellar tendon had two Ag/AgCl electrodes inserted at axial positions of 1/4 and 1/2 from patellar to tibial insertions. Each specimen was electrically isolated, gripped in a servohydraulic test system, and then subjected to a short session of uniaxial haversine tension (2.5 percent maximum strain) at a frequency of 0.5, 1.0, 2.0, or 5.0 Hz. A cyclic (sinusoidal) electrical potential superimposed upon a larger transient (exponentially asymptotic) potential was consistently observed. Upon termination of loading, the cyclic SGP ended, and the shifted baseline of the SGP exponentially decayed and asymptotically returned to a residual potential which over all specimens was not different than the original potential. The transient and cyclic SGPs were frequency dependent (P < 0.001, P = 0.06, respectively). To our knowledge, this transient portion of the SGP, although theoretically predicted by Suh (1996, Biorheology, 33, pp. 289-304) and Chen (1996, Ph.D. thesis, University of Wisconsin-Madison) has not been observed in other experiments using different protocols. Additional PTs were dehydrated and the rehydrated in solution at different pH levels. The magnitude of SGPs increased in basic solution (pH 9.5) but diminished in pH 4.7 buffer. This pH dependency suggests that electrokinetics is the dominant mechanism for the transient and cyclic responses of the SGPs, although this study does not provide direct evidence.

Animals↗

PubMed1999

A probabilistic rule of mixtures for elastic moduli.

A novel approach is developed for mathematically modeling the variability observed in experimentally determined elastic moduli of longitudinally oriented fibrous tissues such as ligaments and tendons. The elastic modulus of these tissues is modeled with a rule of mixtures (ROM) where each parameter (fibril and matrix moduli and fibril volume fraction) is assumed to be an independent random variable. A joint density function formed from the independent densities results in a probabilistic ROM (pROM). This pROM is used to generate a distribution of moduli which agrees well with moduli determined from tests of rabbit medial collateral ligaments (Woo and Ohland, 1994, Unpublished experimental data as gift). Minimizing the error between the pROM and experimental distributions resulted in an integrated error of 9% for a constrained set of independent distribution parameters derived from the literature. This pROM thus incorporates microstructural observations (fibril and matrix moduli and fibril volume fraction) to partially explain the experimentally observed variability in a macroscopic property (tissue modulus).

Animals↗

PubMed1999

Interrelation of creep and relaxation: a modeling approach for ligaments.

Experimental data (Thornton et al., 1997) show that relaxation proceeds more rapidly (a greater slope on a log-log scale) than creep in ligament, a fact not explained by linear viscoelasticity. An interrelation between creep and relaxation is therefore developed for ligaments based on a single-integral nonlinear superposition model. This interrelation differs from the convolution relation obtained by Laplace transforms for linear materials. We demonstrate via continuum concepts of nonlinear viscoelasticity that such a difference in rate between creep and relaxation phenomenologically occurs when the nonlinearity is of a strain-stiffening type, i.e., the stress-strain curve is concave up as observed in ligament. We also show that it is inconsistent to assume a Fung-type constitutive law (Fung, 1972) for both creep and relaxation. Using the published data of Thornton et al. (1997), the nonlinear interrelation developed herein predicts creep behavior from relaxation data well (R > or = 0.998). Although data are limited and the causal mechanisms associated with viscoelastic tissue behavior are complex, continuum concepts demonstrated here appear capable of interrelating creep and relaxation with fidelity.

Elasticity↗

PubMed1998

In vitro and in vivo study on the effect of autogenous cancellous bone and intramedullary polymethylmethacrylate on allograft construct strength.

An in vitro study was performed to compare the effects of augmenting interlocking nails of one of two diameters (5 or 6 mm) with intramedullary polymethylmethacrylate. Subsequently, an in vivo study was performed to compare the effects of augmenting the interlocking nail with five combinations of intramedullary polymethylmethacrylate and autogenous cancellous bone applied to the periosteal surface or within the medullary canal. Dogs were killed 6 months after the procedure for biomechanical evaluation of the femora in axial compression, mediolateral and craniocaudal bending, and torsion. Results from the in vitro study at the proximal osteotomy indicated the 6-mm interlocking nail with intramedullary polymethylmethacrylate had greater stiffness than the 5-mm interlocking nail without it (p < 0.05). At the distal osteotomy, regardless of the diameter of the interlocking nail, the addition of intramedullary polymethylmethacrylate increased stiffness (p < 0.05). Results from the in vivo study indicated greater global construct stiffness with an interlocking nail alone, an interlocking nail augmented with intramedullary polymethylmethacrylate and cancellous bone at the periosteal surface, and an interlocking nail augmented with cancellous bone within the medullary canal and at the periosteal surface (p < 0.05). At the osteotomy level, the interlocking nail augmented with intramedullary polymethylmethacrylate and cancellous bone at the periosteal surface had greater stiffness than did an interlocking nail alone or an interlocking nail augmented with either intramedullary polymethylmethacrylate, cancellous bone within the medullary canal, or cancellous bone at the periosteal surface (p < 0.05) but produced the same results as an interlocking nail augmented with cancellous bone within the medullary canal and at the periosteal surface. The results suggest that augmenting interlocking nail fixation with intramedullary polymethylmethacrylate by itself offers no advantage but that a combination of intramedullary polymethylmethacrylate and cancellous bone at the periosteal surface improves structural properties at 6 months.

Animals↗

PubMed1998

Evaluation of a new method to create a standardized muscle stretch injury.

Herein we describe a new test system to produce a standardized partial muscle-tendon junction (MTJ) stretch injury. In anesthetized rabbits the tibialis anterior (TA) muscle-tendon unit is unilaterally shortened using a custom designed clamp roller system. An angular displacement (average velocity of 450 degrees x s[-1]) is applied about the foot to plantarflex the ankle 90 degrees while the lower extremity is fixed. During ankle rotation the TA muscle is tetanically stimulated to generate an eccentric stretch injury at the MTJ. Forty-eight hours after injury, isometric torque deficit (injured/sham) was measured. Two groups of animals (N = 6 in each group) were tested with the only difference between the two groups being the initial tendon shortening. In Group 1 (tendon shortening = 1.2 cm. N = 6) the torque deficit was 36.7+/-5.9% (mean+/-SD). In Group 2 (tendon shortening = 1.5 cm. N = 6) the torque deficit was 58.7+/-7.4% (mean+/-SD). No order effect was suggested by the data (P = 0.6062), but the difference in torque deficit between the two groups was highly significant (P = 0.0001). For all tests in which the tendon was temporarily shortened before muscle stimulation and stretch (N = 12) there was a visible hematoma at the MTJ similar to the injury that is common in athletic injuries. Histological evaluation 48 h after injury revealed both fiber tearing and inflammation at the MTJ. In addition, there was focal fiber damage in the muscle belly for both groups. The damage and inflammatory process, however, were more severe in the group with greater initial tendon shortening.

Analysis of Variance↗

PubMed1998

A device for measuring relative angular displacement.

A simple, inexpensive, and accurate way to measure relative segmental rotations resulting from torsional loadings locally is described. To measure these rotations, we fabricated a planar spatial linkage (open-loop kinematic chain) requiring only one rotational displacement transducer. This paper describes this device, defines its kinematics, and examines its accuracy.

Biophysics↗

PubMed1998

Cortical bone responses to 2G hypergravity in growing rats.

BACKGROUND: Rat cortical bone adaptation to chronic hypergravity (2G) was studied using young growing male Wistar rats (60 d). METHODS: Animals (10 rats) were subjected to chronic hypergravity (14 d) in order to understand the plastic nature of bone under a constant hypergravity stress using a special rodent habitat that was attached to a 12-ft. radius centrifuge. Also, an equal number of stationary controls were housed in a rodent vivarium containing identical cages that were used for centrifugation. After 14 d of centrifugation, femur bones were excised and prepared for morphological and biochemical measurements. RESULTS: Results showed that 2G had significantly shortened the femurs (3%) and reduced the cortical bone area (13%). In particular, hypergravity induced significant reductions in the thicknesses of cortical bone at the anterior (13%) and medial regions (15%) of the mid-diaphysis. However, femoral bone density, collagen and calcium concentrations were unaltered. The content of mature, stable bone collagen cross-links hydroxylysylpyridinoline (HP), lysylpyridinoline (LP), were significantly greater in bones from centrifuged animals. CONCLUSION: Our findings suggest that short term exposure to 2G does not enhance bone formation or induce changes in cortical bone composition, or alter specific gravity. These data also suggest that bone maturation as reflected by collagen cross-linking is upregulated. However, it is undetermined at this time whether the enhanced content of mature bone collagen in the centrifuged rats is a result of either an increased rate of cross-linking or reduction in the degradation of "older collagen."

Adaptation, Physiological↗