Small-angle HeNe laser light scatter and the compressive modulus of articular cartilage

Light scattering is a widely used technique for probing the microarchitecture and interactions of biological materials and solutions. In this paper, we describe the use of this method in the study of articular cartilage. The experiments presented utilize small-angle static scattering of HeNe laser light (632.8 nm) from 40 microns thick samples of cartilage taken from the superficial zone of baboon femoral condyle. The specimens were taken from a total of 26 sites in eight animals of various ages. In addition to measuring the dependence of the intensity of scattered light on scatter angle, we performed mechanical testing at the test sites using creep-indentation techniques. The results from the optical and mechanical experiments were compared, and a significant correlation was noted between the average scatter angle and the compressive aggregate modulus. In addition, it was noted that the cartilage of skeletally immature animals had a smaller aggregate modulus and scattered to a higher average angle than the cartilage of skeletally mature animals. A quantitative theory was developed to explain the relation, between mechanical and optical properties in terms of the degree of order in the spatial arrangement of the collagen fibers in cartilage.