Factors influencing cartilage wear in an accelerated in vitro test: collagen fiber orientation, anatomic location, cartilage composition, and photo-chemical crosslinking

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Mechanical Engineering
Indiana University
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Articular cartilage (AC) is a strong but flexible connective tissue that covers and protects the end of the long bones. Although cartilage has excellent friction and wear properties that allow smooth joint function during daily activities, these properties are not fully understood. Many material properties of cartilage are anisotropic and vary with anatomic location and the composition of the tissue, but whether this is also true for cartilage friction and wear has not been previously determined. Furthermore, cartilage disease and injury are major health concerns that affect millions of people, but there are few available treatments to prevent the progression of cartilage degeneration. Collagen crosslinking may be a potential treatment to reduce cartilage wear and slow or prevent the progression of cartilage disease. The objectives of this thesis were to investigate the relationships between the friction/wear characteristics of cartilage and the orientation of the preferred fiber direction, the anatomic location of the tissue, the composition of the tissue, and exogenous photochemical crosslinking.

In the superficial zone, AC has preferential fiber direction which leads to anisotropic material behavior. Therefore, we hypothesized that AC will show anisotropic behavior between longitudinal and transverse direction in an accelerated, in vitro wear test on bovine cartilage in terms of friction and wear. This hypothesis was proven by the quantification of glycosaminoglycans released from the tissue during the wear test, which showed that more glycosaminoglycans were released when the wear direction was transverse to the direction of the fibers. However, the hydroxyproline released from the tissue during the wear test was not significantly different between the two directions, nor was the coefficient of friction.

The material properties of AC can also vary with anatomic location, perhaps due to differences in how the tissue is loaded in vivo. We hypothesized that cartilage from a higher load bearing site will give better wear resistance than cartilage from lower load bearing regions. However, no differences in friction or wear were observed between the different anatomic locations on the bovine femoral condyles. The concentration of collagen, glycosaminoglycans, cells and water in the tissue was also quantified, but no significant differences in tissue composition were found among the locations that were tested.

Although wear did not vary with anatomic location, variation in the wear measurements were relatively high. One potential source of variation is the composition of the cartilage. To determine whether cartilage composition influences friction and wear, a correlation analysis was conducted. An accelerated, in vitro wear test was conducted on cartilage from bovine femoral condyles, and the tissue adjacent to the wear test specimens was analyzed for collagen, glycosaminoglycan, cell, and water content. Because wear occurs on the cartilage surface, the superficial zone of the cartilage might play an important role in wear test. Therefore, composition of the adjacent cartilage was determined in both the superficial zone and the full thickness of the tissue. A significant negative correlation was found between wear and collagen content in the full thickness of the tissue, and between the initial coefficient of friction and the collagen content in the superficial zone. This correlation suggests that variation in the collagen content in the full thickness of the cartilage partially explains differences in amount of wear between specimens.

The wear resistance of cartilage can be improved with exogenous crosslinking agents, but the use of photochemical crosslinking to improve wear resistance is not well understood. Two photochemical crosslinking protocols were analyzed to improve the wear resistance of the cartilage by using chloro-aluminum phthalocyanine tetrasulfonic acid (CASPc) and 670nm laser light. The cartilage treated with the two crosslinking protocols had lower wear than the non-treated group without changing the friction properties of the cartilage.

Indiana University-Purdue University Indianapolis (IUPUI)
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