The load sharing of an applied load at the articular surface between the solid and fluid phases of articular cartilage in diarthrodial joints was investigated. As a prerequisite, a radiographic and image analysis method was developed and applied in porcine knees for the measurement of articular cartilage thickness in articulated joints. The feasibility of the approach was assessed by implementing the method for the lateral femoral condyle from harvested limbs. Measurement of the undeformed cartilage thickness with this method was found to have a great degree of precision, with approximately a 1.0% (14μm) mean variation. Accuracy was also high when compared to an optical method for measuring the true cartilage thickness. An excellent linear correlation (r² >0.99) between the thickness determined optically and that obtained from the radiographic images was demonstrated. No significant differences were found between these two measures of cartilage thickness. This method, which minimizes disturbance to the structures of the knee to maintain its physiologic environment, was used to measure the changes in cartilage thickness as the tissue is deformed during joint loading. Information about the undeformed and deformed cartilage thicknesses was then combined with finite element models to determine load sharing between solid and fluid phases of cartilage, as explained in the following paragraphs.
In the experimental component of this effort, the in situ mechanical conditions of cartilage in the articulated knee were quantified during joint loading. Six porcine knees were subjected to a 445 N compressive load while cartilage deformations and contact pressures were measured. From roentgenograms, cartilage thickness before and during loading allowed the calculation of tissue deformation on the lateral femoral condyle at different times during the loading process. Contact pressures on the articular surface were measured with miniature fiberoptic pressure transducers. Results showed that the medial side of the lateral femoral condyle had higher contact pressures as well as deformations. To begin to correlate the pressures and resulting deformations, the intrinsic material properties of the cartilage on the lateral condyle were obtained from indentation tests. Data from four normal control specimens indicated that the aggregate modulus of the medial side was significantly higher than in other areas of the condyle. These experimental measures of the in situ mechanical conditions of articular cartilage was then combined with theoretical modeling to obtain valuable information about the relative contributions of the solid and fluid phases to supporting the applied load on the cartilage surface.
In the modeling component of this effort, the u-p finite element model was used to simulate the loading of six separate porcine knee joints and to predict surface deformations of the cartilage layer on the lateral femoral condyle. Representative geometry for the condyle, contact pressures and intrinsic material properties of the cartilage layer were supplied from experimental measures. The u-p finite element predictions for surface deformations of the cartilage layer were obtained for several load partitioning states between the solid and fluid phases of cartilage at the articular surface. These were then compared to actual surface deformations obtained experimentally. It appeared from the comparison that approximately 75% of the applied load was borne by the fluid phase at the articular surface under this loading regime. This was qualitatively in agreement with the hypothesis that an applied load to articular joints is partitioned at the surface to the two phases according to the surface area ratios of the solid and fluid phases. It appeared that the solid phase was shielded from the total applied stress on the articular surface by the fluid and could be a reason for the excellent durability of the tissue under the demanding conditions in a diarthrodial joint.
In the last phase of the study, the in situ mechanical environment of repair and normal cartilage in articulated porcine knees were compared during joint loading. Repair cartilage was grown in lateral femoral condyles of Yorkshire pigs by creating 5mm defects to bleeding bone in both lateral femoral condyles and allowing healing for 12 weeks. Normal and repair knees were subjected to a 445 N compressive load while cartilage deformations and contact pressures were measured. To begin to correlate the pressures and resulting deformations, the intrinsic material properties of normal and repair cartilage on the lateral condyle were obtained from indentation tests. Experimental measurements in conjunction with theoretical predictions using the u-p finite element model were used to determine the extent of load supported by the fluid phase of cartilage at the articular surface for both normal and repair tissues. Results showed that, for small defects, although some differences in applied pressure on the cartilage surface and deformations of the cartilage layer existed between the normal and repair knees, the differences in load sharing between the solid and fluid phases were insignificant between normal and repair tissues.
The u-p finite element model used in this study is based on the biphasic theory of cartilage, which assumes that the solid phase of cartilage is intrinsically elastic. If the inherent viscoelasticity of the solid phase is taken into consideration, the behavior of the tissue under compressive load can be modeled more accurately. The u-p finite element formulation for the equations of the biphasic theory with a viscoelastic solid matrix was performed. When implemented, this could serve as a more accurate tool to determine the extent of load sharing in articular cartilage in diarthrodial joints.
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