Biomedical subjects

L A Setton

Publications and source records attributed to L A Setton.

35 records · Page 2Linked to original sources

PubMed1999

Biomechanical factors in tissue engineered meniscal repair.

Damage to the meniscus after trauma or injury is associated with detrimental changes in joint function that can lead to pain, disability, and degenerative joint changes. Recently, tissue engineering strategies for meniscal repair have been suggested including using biocompatible grafts as a substrate for regeneration, and cellular supplementation to promote remodeling and healing. Little is known, however, about the contributions of these novel repair strategies to restoration of normal meniscal function. Biomechanical factors play a role in the design and synthesis of tissue engineered biomaterials and bioreactors, and also are important for evaluating the efficacy of these new strategies for restoring normal meniscal function. In this report, an overview is presented of biomechanical factors that are critical to meniscal function followed by a review of biomechanical considerations for the design and evaluation of tissue engineered strategies for meniscal repair. Recommendations for future study of biomechanical factors in tissue engineered meniscal repair also are provided.

Animals↗

PubMed1998

Degeneration affects the anisotropic and nonlinear behaviors of human anulus fibrosus in compression.

Axial and radial specimens of non-degenerate and degenerate human anulus fibrosus (AF) were tested in confined compression to test the hypothesis that degeneration significantly affects the compressive properties of AF. Due to the highly oriented structure of AF, a secondary objective was to investigate anisotropic behaviors of AF in compression. Uniaxial swelling and stress relaxation experiments were performed on site-matched samples of anulus from the anterior outer region of L2-3 intervertebral discs. The experimental stress-relaxation behavior was modeled using the finite deformation biphasic theory and a finite-difference approximation scheme. Significant effects of degeneration but not orientation were detected for the reference stress offset, sigma(offset), and parameters describing the compressive stiffness (i.e. reference aggregate modulus, H(A0), and nonlinear stiffening coefficient, beta). Average values were 0.13+/-0.06 and 0.05+/-0.05 MPa for sigma(offset), 0.56+/-0.21 and 1.10+/-0.53 MPa for H(A0) and 2.13+/-1.48 and 0.44+/-0.61 for beta for all normal and degenerate specimens, respectively. No significant effect of degeneration or orientation were detected for either of the parameters describing the strain-dependent permeability (i.e. reference permeability, k0 and strain-dependent permeability coefficient, M) with average values for all specimens of 0.20+/-0.10 x 10(-15) m4/N-s and 1.18+/-1.30 for k0 and M, respectively. The loss of sigma(offset) was compensated with an elastic stiffening and change in the shape of the equilibrium stress-strain curve with H(A0) for degenerate tissues almost twice that of normal tissues and beta less than one sixth. The increase in reference elastic modulus with degeneration is likely related to an increase in tissue density resulting from the loss of water content. The significant effects of degeneration reported in this study suggested a shift in load carriage from fluid pressurization and swelling pressure to deformation of the solid matrix of the AF. The results also suggest that the highly organized and layered network of the anulus fibrosus, which gives rise to significant anisotropic effects in tension, does not play a major role in contributing to the magnitude of compressive stiffness or the mechanisms of fluid flow of the anulus in the confined compression configuration.

Adolescent↗

PubMed1998

Swelling and curling behaviors of articular cartilage.

A new experimental method was developed to quantify parameters of swelling-induced shape change in articular cartilage. Full-thickness strips of cartilage were studied in free-swelling tests and the swelling-induced stretch, curvature, and areal change were measured. In general, swelling-induced stretch and curvature were found to increase in cartilage with decreasing ion concentration, reflecting an increasing tendency to swell and "curl" at higher swelling pressures. An exception was observed at the articular surface, which was inextensible for all ionic conditions. The swelling-induced residual strain at physiological ionic conditions was estimated from the swelling-induced stretch and found to be tensile and from 3-15 percent. Parameters of swelling were found to vary with sample orientation, reflecting a role for matrix anisotropy in controlling the swelling-induced residual strains. In addition, the surface zone was found to be a structurally important element, which greatly limits swelling of the entire cartilage layer. The findings of this study provide the first quantitative measures of swelling-induced residual strain in cartilage ex situ, and may be readily adapted to studies of cartilage swelling in situ.

Animals↗

PubMed1997

Alterations in the mechanical behavior of the human lumbar nucleus pulposus with degeneration and aging.

This study tested the hypothesis that changes in the morphology and composition of the nucleus pulposus with age and degeneration have associated changes in its mechanical properties. A torsional shear experiment was used to determine viscoelastic shear properties of cylindrical samples of human nucleus pulposus with large ranges of grades of morphological degeneration (normal to severely degenerated) and ages (range: 16-88 years; average: 57 +/- 21.5 years). Viscoelastic shear properties were determined from stress-relaxation and dynamic sinusoidal tests. A linear viscoelastic law with a variable-amplitude relaxation spectrum was used to model experimental behaviors of nucleus pulposus specimens. A statistically significant increase in the instantaneous and dynamic shear moduli was found with increasing age and grade of degeneration; the values for moduli ranged from 5.0 to 60 kPa. A significant decrease in tan delta was also detected; the values ranged from 0.43 to 0.33, indicating a decreased capacity for the nucleus pulposus to dissipate energy. The dynamic modulus and tan delta were also significantly affected by frequency. It was generally concluded that the nucleus pulposus undergoes a transition from "fluid-like" behavior to more "solid-like" behavior with aging and degeneration.

Adolescent↗

PubMed1997

The viscoelastic behavior of the non-degenerate human lumbar nucleus pulposus in shear.

The viscoelastic behavior of the nucleus pulposus was determined in shear under transient and dynamic conditions and was modeled using a linear viscoelastic model with a variable amplitude relaxation spectrum. During stress-relaxation tests, the shear stress of the nucleus pulposus relaxed nearly to zero indicative of the fluid nature of the tissue. Under dynamic conditions, however, the nucleus pulposus exhibited predominantly 'solid-like' behavior with values for dynamic modulus (magnitude of G*) ranging from 7 to 20 kPa and loss angle (delta) ranging from 23 to 30 degrees over the range of angular frequencies tested (1-100 rad s-1). This frequency-sensitive viscoelastic behavior is likely to be related to the highly polydisperse populations of nucleus pulposus molecular constituents. The stress-relaxation behavior, which was not linear on a semi-log plot (in the range t1 << t << t2), required a variable amplitude relaxation spectrum capable of describing this frequency sensitive behavior. The stress-relaxation behavior was well described by this linear viscoelastic model with variable amplitude relaxation spectrum; however, the dynamic moduli were underpredicted by the model which may be related to non-linearities in the material behavior.

Elasticity↗

PubMed1997

Mechanical behavior and biochemical composition of canine knee cartilage following periods of joint disuse and disuse with remobilization.

The mechanical behavior and biochemical composition of articular cartilage were studied in an experimental model of joint disuse, in which the canine knee was immobilized in a sling at 90 degrees of flexion. Articular cartilage from the surface zone of the femur was tested in an isometric tensile test and full-thickness cartilage on the tibial plateau was tested in a compressive indentation test. Water, proteoglycan and collagen contents were measured in site-matched samples. Site-specific increases in the tensile moduli (approximately 88% above control values in distal femoral groove) were observed in cartilage after 8 weeks of joint disuse, and after 3 weeks of remobilization following either 4 (approximately 140%, distal and proximal femoral groove) or 8 weeks (approximately 140%, distal femoral groove) of joint disuse. In contrast, the compressive properties of cartilage determined in the indentation test exhibited no change from control values with joint disuse or disuse followed by remobilization. Water contents increased at some sites on the tibia after 8 weeks of joint disuse (approximately 6% of tissue wet weight, posterior site), but not in the surface zone tissue of the femur. Proteoglycan/collagen and cartilage thickness were not found to change with disuse or disuse followed by remobilization. Reduced values for the ratio of proteoglycan:water were observed in the surface zone tissue of the femur (approximately 23%, distal femoral groove) and in the full-thickness tissue of the tibia (approximately 21%, anterior and posterior sites) after periods of joint disuse. In this study, the measured material properties suggest that the articular surface remains intact following periods of disuse or disuse with remobilization. This finding suggests one important difference between this model of joint disuse and other experimental models in which cartilage changes are both progressive and degenerative, such as surgically-induced joint instability.

Animals↗

PubMed1996

Is the nucleus pulposus a solid or a fluid? Mechanical behaviors of the nucleus pulposus of the human intervertebral disc.

STUDY DESIGN: A new technique to measure the viscoelastic behavior of the nucleus pulposus in shear was used to assess its solid and fluid characteristics. OBJECTIVES: To review existing knowledge on mechanical behaviors of the nucleus pulposus, and to develop a new technique to study the viscoelastic behaviors of isolated nucleus pulposus samples in torsional (pure) shear under transient and dynamic conditions. SUMMARY OF BACKGROUND DATA: Numerous studies have investigated the swelling behavior of the nucleus and found the swelling pressure to range approximately 0.05-3 MPa, depending on loading conditions. Very few studies, however, have investigated the load-deformational behaviors of the nucleus pulposus. METHODS: Thirteen nondegenerate samples of nucleus pulposus were harvested from lumbar discs and tested in torsional shear under transient and dynamic test conditions. A linear viscoelastic law with variable amplitude relaxation and dynamic frequency sweep experiments. The coefficients of the viscoelastic law were determined from the stress relaxation experiments, whereas the dynamic shear modulus and phase shift angle were determined from the frequency sweep. RESULTS: The nucleus exhibits significant viscoelastic effects in shear. Under transient conditions, the stress relaxed to values near zero, which is indicative of the "fluid-like" behaviors of the nucleus. Under dynamic conditions, however, the material parameters for the nucleus, magnitude of the complex modulus (7-21 kPa), and phase angle (23-31 degrees) were more characteristic of a viscoelastic solid. The authors' proposed stress-strain law exhibited excellent agreement with the viscoelastic data. CONCLUSIONS: In response to shear deformations, the nucleus pulposus exhibited significant viscoelastic effects, characteristic of a fluid and a solid. Whether the nucleus pulposus behaves more as a fluid or a solid in vivo depends on the rate of loading.

Biomechanical Phenomena↗

PubMed1996

Tensile properties of nondegenerate human lumbar anulus fibrosus.

STUDY DESIGN: The in vitro tensile behavior of multiple-layer samples of anulus fibrosus were investigated from nondegenerate intervertebral discs. OBJECTIVES: To quantify the intrinsic tensile behavior of nondegenerate anulus fibrosus and the variations with position and age in the intervertebral disc. SUMMARY OF BACKGROUND DATA: Tension is an important loading mode in the anulus fibrosus. The tensile behavior of single- and multiple-layer samples of anulus fibrosus has been shown to vary with specimen orientation, position in the disc, and environmental conditions. Little is known of the changes in these site-specific tensile properties of the anulus with aging or degeneration of the intervertebral disc. METHODS: Multiple-layer specimens of anulus fibrosus were harvested with an orientation parallel to the circumference of the disc. Constant strain rate and uniaxial tensile tests were performed in 0.15 mol/l NaCl at slow strain rates to measure the intrinsic properties of the collagen-proteoglycan matrix of the anulus fibrosus. The tensile modulus, failure stress, failure strain, and strain energy density were determined. Statistical analyses were done to evaluate regional and age-related differences in these properties. RESULTS: Significant radial and circumferential variations in the intrinsic tensile properties of anular samples were detected. The anterior anulus fibrosus had larger values for tensile moduli and failure stresses than the posterolateral anulus. Also, the outer regions of the anulus had greater moduli and failure stresses and lower failure strains than the inner regions. Strain energy density did not vary significantly with region. Significant, but very weak, correlations were detected between tensile properties and age of the intervertebral disc. CONCLUSIONS: The observed variations in tensile behavior of multiple-layer anulus samples indicate that larger variations in tensile modulus and failure properties occur with radial position in the disc than from anterior to posterolateral regions. This pattern is likely related to site-specific variations in the tensile properties of the single-layer samples of anulus fibrosus lamellae and the organization of successive lamellae and their interactions. The results of the present study suggest that factors other than age, such as compositional and structural variations in the disc, are the most important determinants of tensile behavior of the anulus fibrosus.

Adult↗

PubMed1995

Degeneration and aging affect the tensile behavior of human lumbar anulus fibrosus.

STUDY DESIGN: Samples of human lumbar (L3-L4) anulus fibrosus from four different anatomic sites (anterior outer, posterolateral inner), ranging from normal to severely degenerate, were studied in uniaxial tension and measured for water content. OBJECTIVES: To evaluate the effects of aging and degeneration on the tensile properties and hydration of the anulus fibrosus in a site-specific manner. The relationship between hydration and parameters of the tensile behavior were investigated. SUMMARY OF BACKGROUND DATA: Degeneration and aging have been shown to be related to dramatic changes in the composition and structure of the anulus fibrosus. The associated changes in the tensile, compressive, and shear properties of the anulus fibrosus have not been documented. Numerical studies using finite element models have attempted to simulate the degenerative process by incorporating estimated mechanical properties meant to represent the degenerate anulus fibrosus. Their results present findings that suggest that altered material properties of the anulus fibrosus affect the mechanics of the entire intervertebral disc. METHODS: Samples of human lumbar anulus fibrosus were classified by grade of degeneration based on a morphologic grading scheme. Multiple layer anulus specimens from four sites in the disc were tested in uniaxial tension under quasistatic conditions in a physiologic saline bath. The tensile modules, Poisson's ratio, failure stress and strain, the strain energy density to failure, and the corresponding hydration were determined for each sample. RESULTS: The Poisson's ratio, failure stress, and strain energy density of the anulus fibrosus were found to be affected significantly by degeneration, with some evidence of a sensitivity of the tensile modulus to grade of degeneration. All material properties were found to exhibit a significant and greater dependence on site within the disc than on degenerative grade. Weak correlations between aging and the Poisson's ratio and strain energy density were observed. Water content of anulus fibrosus tissue was not affected by degeneration or aging, although correlations with tensile properties were observed. CONCLUSIONS: The dramatic changes in morphology, composition, and structure that occur in anulus fibrosus with aging and degeneration are accompanied by specific variations in the tensile properties, which were generally small in magnitude. Position of the anulus fibrosus within the intervertebral disc, particularly in the radial direction, appeared to be the most important variable affecting anulus fibrosus tensile properties. This dependence on position did not change with either aging or degeneration. Results from the present study may be useful in future finite element models to assess how altered material properties of the anulus fibrosus during degeneration and aging may affect the mechanics of the entire intervertebral disc.

Adolescent↗

PubMed1995

Mechanical behavior of articular cartilage in shear is altered by transection of the anterior cruciate ligament.

The flow-independent viscoelastic and equilibrium behaviors of canine articular cartilage were examined with time after transection of the anterior cruciate ligament. The equilibrium, transient, and dynamic shear behaviors of cartilage were studied in biaxial compression-torsion testing at two time periods after transection of the anterior cruciate ligament and at two sites on the femoral condyle, in order to test for differences between sites of frequent and less frequent contact. Water content also was measured in cartilage at sites corresponding to the areas of mechanical testing. Transection of the anterior cruciate ligament produced significant decreases in all measured moduli of articular cartilage tested in equilibrium and dynamic shear and in equilibrium compression; the values for these moduli were 61, 56, and 77% of the control values, respectively, beginning at 6 weeks following transection of the anterior cruciate ligament. There was evidence of increased energy dissipation of cartilage in shear, with a 13 and 35% increase in tan delta at 6 and 12 weeks after transection of the anterior cruciate ligament, respectively. Changes in the viscoelastic relaxation function of cartilage in shear also were evident at 12 weeks after surgery. In all tissue, there was a significant increase in hydration of approximately 4% at 6 or 12 weeks after surgery. There was little difference between the material parameters for areas considered to be in frequent and less frequent contact, with the exception of hydration, which was greater for areas of less frequent contact.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1995

Cartilage metalloproteases in disuse atrophy.

A canine knee model of disuse atrophy produced by nonrigid fixation (sling) was characterized in respect to variables of proteoglycan size distribution, as well as biomechanical properties versus controls. Using this model, we found, in addition to the accepted dogma attributing changes to reduced protein synthesis by chondrocytes, that there is elevation of proteases and depression of tissue inhibitor of metalloproteases (TIMP) in atrophic knee cartilage. The findings are suggestive of cartilage remodelling reminiscent of bone remodelling in disuse atrophy reported by others. Whether the abnormal changes of protease-TIMP balance in knee cartilage can be retarded prophylactically by concurrent treatment with pentosan polysulfate and insulin like growth factor 1 remains uncertain.

Animals↗

PubMed1994

Compressive mechanical properties of the human anulus fibrosus and their relationship to biochemical composition.

To enhance understanding of the biomechanical role of the intervertebral disc, the compressive properties and biochemical composition of nondegenerate samples of anulus fibrosus were determined as a function of radial position, region, and level. Because of the large swelling propensity of this tissue, a method was developed to test excised specimens while maintaining their in situ geometry and hydration. Using an analysis based on linear biphasic theory, the compressive modulus, hydraulic permeability, and isometric swelling pressure of the anulus fibrosus were determined and correlated with the tissue composition. The findings indicate that the anulus fibrosus is inhomogeneous, with regional and radial variations in both material properties and biochemical composition. The results of this study suggest that both structural and compositional factors may determine the mechanical behavior.

Adult↗

PubMed1994

Mechanical properties of canine articular cartilage are significantly altered following transection of the anterior cruciate ligament.

The compressive, tensile, and swelling properties of articular cartilage were studied at two time periods following transection of the anterior cruciate ligament in the knee of greyhound dogs. An experimental protocol was designed to quantify the essential equilibrium and biphasic material properties of cartilage in tension, compression, and shear, as well as the parameters of isometric swelling behavior. All properties were measured at several sites to elicit differences between sites of frequent and less frequent contact. Hydration was determined at each site and was compared with the material properties of cartilage from corresponding sites. There were extensive changes in all compressive, tensile, and swelling properties of cartilage after transection of the anterior cruciate ligament. Twelve weeks after surgery, the intrinsic moduli were reduced significantly in compression (approximately 24% of control values), tension (approximately 64%), and shear (approximately 24%), and the hydraulic permeability was elevated significantly (approximately 48%). Significant increases in hydration (approximately 9%) also were observed, as well as a strong correlation of hydration with hydraulic permeability. The pattern of these changes was not found to differ with site in the joint, but significant differences were observed in the magnitude of change for cartilage from the femoral groove and the femoral condyle. The pattern and extent of changes in the material properties following transection of the anterior cruciate ligament indicate that altered loading of the joint severely compromises the overall mechanical behavior of articular cartilage. The observed loss of matrix stiffness in compression, tension, and shear is associated with increases in the deformation of the solid matrix, a diminished ability to resist swelling, and the increase in hydration observed in this study. The increased swelling and elevated water content were related directly to the increase in hydraulic permeability; this suggests an associated loss of fluid pressurization as the load support mechanism in the degenerated cartilage. Without a successful mechanism for repair, damage to the solid matrix may progress and lead to further degenerative changes in the biochemistry, morphology, and mechanical behavior of articular cartilage.

Animals↗

PubMed1994

Centrifugal and biochemical comparison of proteoglycan aggregates from articular cartilage in experimental joint disuse and joint instability.

Two models involving altered joint loading were compared with regard to their effects on the biochemical composition and proteoglycan aggregate structure of articular cartilage. Disuse atrophy was created in greyhound dogs by nonrigid immobilization of the right knee in 90 degrees of flexion, and joint instability was created by transection of the anterior cruciate ligament. Similarities and differences between the two experimental groups at two different time periods were examined to investigate why joint instability induces progressive and irreversible changes to the articular cartilage, whereas joint disuse induces changes that may be reversible when the joint is remobilized. The following studies were performed on the cartilage from all experimental and control groups: (a) compositional analyses to determine water, uronate, and hydroxyproline contents; (b) high performance liquid chromatography for detection of hyaluronan and chondroitin sulfates; and (c) centrifugation analyses of nondissociatively extracted and purified proteoglycans to isolate and quantify the populations of monomers and slow and fast-sedimenting families of aggregates. In general, all cartilage was found to have a decreased ratio of proteoglycan to collagen after 4 weeks of disuse, and this ratio returned to control values at 8 weeks. In contrast, cartilage had an elevated ratio of proteoglycan to collagen as well as increased hydration at 12 weeks after transection of the anterior cruciate ligament. The most striking contrast between the two models was the finding of an approximately 80% decrease in the content of hyaluronan at both time periods after transection of the anterior cruciate ligament, with no evidence of a change after disuse. The results of centrifugation analyses indicated a significant decrease in the quantity of proteoglycan aggregates in both models. However, this decrease was associated primarily with a loss of slow-sedimenting aggregates after disuse and a loss of both slow and fast-sedimenting aggregates after transection of the anterior cruciate ligament. Furthermore, the population of fast-sedimenting aggregates was depleted to a greater extent than that of the slow-sedimenting aggregates. The preservation of fast-sedimenting aggregates as well as hyaluronan after periods of joint disuse but not joint instability suggests a possible mechanism for the reversibility of cartilage changes. Although the proteoglycan aggregates were depleted after disuse atrophy, it is possible that an aggregate-depleted matrix could recover when normal proteoglycan synthesis is resumed. In contrast, although synthesis may be maintained or elevated after transection of the anterior cruciate ligament, the matrix may not be repopulated with aggregates because there is an insufficient amount of hyaluronan.

Analysis of Variance↗

PubMed1993

Compressive properties of the cartilaginous end-plate of the baboon lumbar spine.

The viscoelastic behavior of the cartilaginous end-plate of the baboon (Papio anubis) was studied in an experiment on compressive creep. Data were analyzed with the biphasic poroviscoelastic constitutive theory to assess the relative contributions of flow-dependent and flow-independent viscoelastic mechanisms to the observed creep behavior. Material coefficients describing the equilibrium compressive behavior (HA) and both flow-independent (c, tau 1, and tau 2) and flow-dependent (k) viscoelastic effects were determined for the end-plate by the curve-fitting of the theoretical solution to the experimental creep data. Biochemical analyses were performed to test for potential relationships between material properties and composition which may give rise to the viscoelastic behavior of the end-plate. The results indicate that the cartilaginous end-plate has a hydraulic permeability of 14.3 x 10(-14) m4/N-s, which is associated with rapid transport and pressurization of the interstitial fluid in response to loading and an increased emphasis on flow-independent viscoelastic effects. Biochemical analyses for water, sulfated glycosaminoglycan content, and hydroxyproline indicate that the end-plate of the baboon is compositionally similar to the cartilaginous end-plate in humans. Interpretation of the mechanical and compositional data suggests that fluid pressurization in the cartilaginous end-plate may be important in the maintenance of a uniform stress distribution across the boundary between vertebral body and intervertebral disc.

Animals↗

PubMed1993

Altered structure-function relationships for articular cartilage in human osteoarthritis and an experimental canine model.

A review of the structure-function relationships for normal articular cartilage is provided. This provides the foundation for understanding the roles played by collagen, proteoglycan and water in determining the material properties of the tissue. A summary of biomechanical and compositional changes in human osteoarthritic cartilage is also presented. Finally, the results from our recent interdisciplinary study on an experimental osteoarthritis model is described, and new hypotheses are proposed on the initiating factors responsible for the increase of tissue hydration. At present, it appears that microstructural alterations, rather than compositional changes, of the collagen-proteoglycan solid matrix are responsible for the early increase of hydration and the deterioration of biomechanical properties of articular cartilage.

Animals↗

PubMedDate not supplied

The biphasic poroviscoelastic behavior of articular cartilage: role of the surface zone in governing the compressive behavior.

Surface fibrillation of articular cartilage is an early sign of degenerative changes in the development of osteoarthritis. To assess the influence of the surface zone on the viscoelastic properties of cartilage under compressive loading, we prepared osteochondral plugs from skeletally mature steers, with and without the surface zone of articular cartilage, for study in the confined compression creep experiment. The relative contributions of two viscoelastic mechanisms, i.e. a flow-independent mechanism [Hayes and Bodine, J. Biomechanics 11, 407-419 (1978)], and a flow-dependent mechanism [Mow et al. J. biomech. Engng 102, 73-84 (1980)], to the compressive creep response of these two types of specimens were determined using the biphasic poroviscoelastic theory proposed by Mak. [J. Biomechanics 20, 703-714 (1986)]. From the experimental results and the biphasic poroviscoelastic theory, we found that frictional drag associated with interstitial fluid flow and fluid pressurization are the dominant mechanisms of load support in the intact specimens, i.e. the flow-dependent mechanisms alone were sufficient to describe normal articular cartilage compressive creep behavior. For specimens with the surface removed, we found an increased creep rate which was derived from an increased tissue permeability, as well as significant changes in the flow-independent parameters of the viscoelastic solid matrix. permeability, as well as significant changes in the flow-independent parameters of the viscoelastic solid matrix. From these tissue properties and the biphasic poroviscoelastic theory, we determined that the flow-dependent mechanisms of load support, i.e. frictional drag and fluid pressurization, were greatly diminished in cartilage without the articular surface. Calculations based upon these material parameters show that for specimens with the surface zone removed, the cartilage solid matrix became more highly loaded during the early stages of creep. This suggests that an important function of the articular surface is to provide for a low fluid permeability, and thereby serve to restrict fluid exudation and increase interstitial fluid pressurization. Thus, it is likely that with increasing severity of damage to the articular surface, load support in cartilage under compression shifts from the flow-dependent modes of fluid drag and pressurization to increased solid matrix stress. This suggests that it is important to maintain the integrity of the articular surface in preserving normal compressive behavior of the tissue and normal load carriage in the joint.

Animals↗