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This study investigated the resurfacing of full-thickness articular defects in the adult rabbit medial femoral condyle using a rib perichondrial graft. The graft was secured to a 4-mm-diameter bone core removed from the femoral condyle. Two postoperative treatment protocols were studied: one group had ad libitum cage activity (CAGE; n = 95) and the other group received two weeks of passive motion (PM; n = 73; eight hours per day, five days per week) followed by cage activity. Animals in both groups were killed at six, 12, 26, and 52 weeks. Repair tissue resembling hyaline cartilage formed in a majority of animals at all time periods and in both postoperative treatment groups. The overall success rates in which repair tissue formed were 58% in the CAGE group and 56% in the PM group. However, over time, a maturation of the neocartilage into nearly normal hyaline articular cartilage was noted with the percentage of Type II collagen increasing from 55% at six weeks to 82% at one year. The complex shear modulus of the repair tissue for both groups became similar to normal cartilage with increased healing time. There was no statistical difference in shear moduli between the two treatment modalities. These results show that cartilage repair tissue derived from rib perichondrium could mature into hyaline articular cartilage over time and would not degrade by one year after repair.

We developed an animal model of stretch injury to nerve in order to study in vivo conduction changes as a function of nerve strain. In 24 rabbits, the tibial nerve was exposed and stretched by 0%, 6% or 12% of its length. The strain was maintained for one hour. Nerve conduction was monitored during the period of stretch and for a one-hour recovery period. At 6% strain, the amplitude of the action potential had decreased by 70% at one hour and returned to normal during the recovery period. At 12% strain, conduction was completely blocked by one hour, and showed minimal recovery. These findings have clinical implications in nerve repair, limb trauma, and limb lengthening.

The finite element method using the principle of virtual work was applied to the biphasic theory to establish a numerical routine for analyses of articular cartilage behavior. The matrix equations that resulted contained displacements of the solid matrix (mu) and true fluid pressure (p) as the unknown variables at the element nodes. Both small and large strain conditions were considered. The algorithms and computer code for the analysis of two-dimensional plane strain, plane stress, and axially symmetric cases were developed. The u-p finite element numerical procedure demonstrated excellent agreement with available closed-form and numerical solutions for the configurations of confined compression and unconfined compression under small strains, and for confined compression under large strains. The model was also used to examine the behavior of a repaired articular surface. The differences in material properties between the repair tissue and normal cartilage resulted in significant deformation gradients across the repair interface as well as increased fluid efflux from the tissue.

This study determines the change in tension after an acute nerve lengthening, as would occur in peripheral nerve repair. Stress relaxation, a viscoelastic property, was studied with the use of 24 rabbit tibial nerves. The nerves were divided into three groups and were stretched 6%, 9%, or 12% beyond their original resting length. The mean 1-hour stress relaxations for the three groups were 48%, 34%, and 34%, respectively. Nerve stress relaxation was significantly greater at 6% strain than at 9% or 12% strain. These biomechanical findings have important clinical implications for nerve-stretch injury and for nerve repair.

Recent studies have shown that the meniscus is highly anisotropic in tension and that its compressive creep behavior can be modeled using biphasic theory. In this study, an alternative approach is used, where viscoelastic shear properties of the meniscal fibrocartilage are measured to determine the anisotropy and inhomogeneity of this tissue with respect to specimen location and fiber orientation. Medial menisci were obtained from eight skeletally-mature horses. Nine test specimens were taken from the circumferential midsubstance of each meniscus, at three circumferential and three axial positions. The magnitude of the complex shear modulus and the phase angle were determined for each specimen from 100-800 Hz, in 100 Hz increments. Data were gathered shearing parallel and perpendicular to the circumferentially-oriented fibers. The magnitude of the shear modulus and the phase angle were both found to be frequency dependent, anisotropic, and inhomogeneous. The magnitude of the shear modulus increased with frequency, and was greatest in specimens from the posterior superior region, shearing parallel to the fibers. The phase angle decreased slightly with frequency and was lowest in specimens from the midsubstance of the anterior region, shearing perpendicular to the fibers. Our data demonstrated that collagen fibers substantially stiffen the meniscus in the direction of its fibers and that the solid matrix of the meniscus, like articular cartilage, behaves largely as an elastic material.

The response to a compressive load of a repaired cartilage surface, consisting of full-thickness repair tissue adjacent to normal cartilage, was predicted by the u-p finite element method and compared to that of a normal cartilage surface under the same loading conditions. By individually varying the aggregate modulus, permeability and Poisson's ratio for the repair tissue, analyses were performed to assess the contributions of each to the changes in mechanical behaviour. In comparing the repaired to normal surfaces, the presence of a softer repair tissue resulted in increased axial and decreased radial deformations at any given time point, while a repaired surface with an increased permeability compressed more easily due to the increased fluid flow and caused equilibrium to occur sooner. For smaller Poisson's ratio, the axial deformation was not different from normal if the aggregate modulus was the same as normal; however, the radial expansion was reduced as the repair tissue experienced a larger volume change. These results indicate that the presence of repair tissue in a joint surface can have a strong influence on the mechanical behaviour of the surface.

We have studied long-term (to 60 days) effects of 4 degrees C storage in culture media on the histologic, mechanical, and chemical properties of the cartilage from osteochondral shell allografts from the dog. The structural integrity of the cartilage matrix was intact up to 60 days of storage, for the mechanical properties represented by the aggregate modulus and apparent permeability remained normal. These data are supported by normal safranin-O staining as well as normal glycosaminoglycan content and total collagen concentration. However, chondrocyte viability, as assessed by 35SO4 uptake and hematoxylin and eosin preparations, decreased dramatically with time. We believe that the longer storage to 60 days is not indicated, unless conditions can be modified to maintain cell viability.

Peripheral nerves are often mechanically stretched in association with trauma to extremities, resulting in varying degrees of impairment of nerve function. However, little is known about the biomechanical properties of peripheral nerves and limits of stretching that the nerve may undergo before structural changes occur. Also, the injury pattern of nerves under stretching is poorly understood. In the present study, fresh rabbit tibial nerves (n = 18) were harvested. Nine nerves were stretched to failure in an INSTRON materials testing machine at a rate of 1 cm/min (strain rate of 0.5%/s). Load-deformation and stress-strain curves were determined. Histological examination by light microscopy of the stretched nerves as well as six normal control nerves and three clamped nonstretched control nerves was performed. The results show that the rabbit tibial nerves have an in situ strain of 11.0 +/- 1.5% and exhibit a nonlinear stress-strain relationship. After 20% strain, the curve becomes linear up to failure. The ultimate strain and tensile strength of the nerves were 38.5 +/- 2.0% and 11.7 +/- 0.7 MPa, respectively. At failure, the load dropped suddenly, but the specimens remained grossly intact. Histological analysis of the stretched nerves showed multiple ruptures of perineurial sheaths when compared to controls.

The determination of valid stress-strain relations for articular cartilage under finite deformation conditions is a prerequisite for constructing models for synovial joint lubrication. Under physiological conditions of high strain rates and/or high stresses in the joint, large strains occur in cartilage. A finite deformation theory valid for describing cartilage, as well as other soft hydrated connective tissues under large loads, has been developed. This theory is based on the choice of a specific Helmholtz energy function which satisfies the generalized Coleman-Noll (GCN0) condition and the Baker-Ericksen (B-E) inequalities established in finite elasticity theory. In addition, the finite deformation biphasic theory includes the effects of strain-dependent porosity and permeability. These nonlinear effects are essential for properly describing the biomechanical behavior of articular cartilage, even when strain rates are low and strains are infinitesimal. The finite deformation theory describes the large strain behavior of cartilage observed in one-dimensional confined compression experiments at equilibrium, and it reduces to the linear biphasic theory under infinitesimal strain and slow strain rate conditions. Using this theory, we have determined the material coefficients of both human and bovine articular cartilages under large strain conditions at equilibrium. The theory compares very well with experimental results.

This paper describes the flow patterns and related viscoelastic behaviors of articular cartilage under four different loading configurations: 1) confined compression, 2) unconfined compression, 3) translating parabolic surface traction of constant loading span over a cartilage layer, and 4) spreading parabolic surface traction of periodically varying span on a cartilage layer. Brief summaries of formulations and solutions of these problems are given and discussed. For the first two cases, emphasis is given to the differences in the flow mechanisms giving rise to the observed compressive viscoelastic behavior of the tissue. These two fundamental solutions can provide a basis for interpreting the results of the deformation and flow patterns obtained from more complicated loading configurations. The last two cases simulate two main geometric features of joint articulations. Fluid efflux patterns at the articulating surface depend on the kinematics of loading as well as the intrinsic material properties of the tissue: the moduli and permeability of the solid matrix. The predominant mechanism controlling the viscoelastic behavior of the tissue in compression is determined by the relative magnitude of the fluid drag and the intrinsic stiffness of the organic solid matrix.

Perichondrial autografts were used for the repair of large, full-thickness articular cartilage defects in the rabbit medial femoral condyle. The effects of duration of implantation and activity were studied by evaluating the neocartilage mechanically and morphologically. The complex shear moduli for the neocartilage were found to increase during the 26 weeks' observation time. Complete filling of the defect with neocartilaginous tissue was seen in a total of 24 successful experimental samples. It appeared that in the initial 6 weeks passive motion applied intermittently for 2 weeks enhanced the formation of quality neocartilage, i.e., the magnitude of the complex shear moduli was higher than those nontreated. However, these differences disappeared when longer time periods were considered.

Normal and stored articular cartilage from the medial tibial plateaus of mature canine knee joints were evaluated histologically and biomechanically. The medial plateaus from the right knee (control) were assessed fresh, while the left (stored) were preserved in culture media at 4 degrees C for 3, 7, 14, or 28 days and then evaluated. Biomechanically, confined compression tests were performed on all specimens to determine the aggregate modulus and apparent permeability of the articular cartilage. Histologically, Safranin O- and hematoxylin and eosin (H&E)-stained sections were evaluated. All stored cartilage specimens had an aggregate modulus on average lower than normal, but the differences were not significant (p greater than 0.10). The apparent permeability was on average higher than but also not significantly different from normal (p greater than 0.10). Time in storage (up to 28 days) did not have a significant effect on the biomechanical properties of stored cartilage normalized by control values (p greater than 0.50). Safranin O and H&E histological evaluation also showed no overall changes in cell appearance or staining of the stored cartilage when compared with control for the time periods studied.

Monomer and aggregated proteoglycans were prepared from pig laryngeal cartilage. Vascoelastic flow properties, comprising linear complex dynamic shear modulus, nonlinear steady-state shear-rate dependent viscosity, and primary normal stress difference, were measured in proteoglycan solutions containing varying proportions of aggregate (0-80%) and at different concentrations (10-50 mg/ml). Results were analyzed using the simple Oldroyd four-parameter nonlinear rate-type rheological equation. All solution properties were strongly dependent on proteoglycan concentration and on the proportion of aggregates present. Aggregation was found to have a great effect on the zero shear-rate viscosity at 50 mg/ml, which increased fivefold from 0-100% aggregate. The results showed that network formation in proteoglycan solutions increased with concentration from 10-50 mg/ml and also increased with aggregation. All proteoglycan solutions showed shear thinning, which was most marked with aggregated proteoglycan at high concentration (50 mg/ml), where the viscosity decreased tenfold from the zero shear-rate limit to the infinite shear-rate limit. The intermolecular interactions in the network were therefore increasingly disrupted by increasing shear rate, but repeated measurements showed that these were reversible changes and that testing did not induce disaggregation or degradation of proteoglycan. These rheological properties show that aggregation is likely to immobilize proteoglycan at high concentration within cartilage and to contribute to the material properties of the porous solid matrix of articular cartilage that are important for its load-bearing function.

Neocartilage regenerated from rib perichondrium autografts implanted into full thickness cartilage defects made in the femoral condyle of rabbit knees were evaluated for periods up to 1 yr. Two postoperative treatment effects were studied, one with ad lib. caged activity (CAGE) and the other with the operated knee placed on a continuous passive motion machine for 2 weeks (8 h day-1 for 5 days week-1) followed by caged activity (PM). Animals were sacrificed at 6, 12, 26 and 52 weeks after surgery. The neocartilage was evaluated histologically and biomechanically and compared with the contralateral unoperated side. Visually, the neocartilage appeared to have an appearance similar to that of surrounding cartilage at 52 weeks, with an excellent degree of confluence with the neighboring tissue. The newly grown tissues were morphologically similar to normal hyaline articular cartilage. The dynamic shear moduli for the neocartilage from both the CAGE and PM groups significantly increased with postoperative healing time (p less than 0.05). However, there was no statistical difference between the two treatment modalities (p greater than 0.10), indicating that the passive motion did not enhance the long-term repair of the cartilage defect. These results support our hypothesis that neocartilage regenerated from perichondrial autograft remains intact over time.

An optical rear projection device, coupled with a video dimensional analyzer (VDA) system, was developed for simultaneous measurement of tensile strains on opposite surfaces of tendons and ligaments. Stress-strain behaviors of the tissue based on the strains measured on either surface were determined and compared. Data obtained from canine femur-medial collateral ligament-tibia complexes (FMT) and isolated chicken flexor tendons revealed that the strains on opposite surfaces of these parallel fiber soft connective tissues were similar during a uniaxial tensile test.