February 23, 2010

Engineers Study Cartilage Mechanics to Ease Arthritis Pain

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The painful impact of arthritis caused by a lifetime of wear and tear eventually affects every human body. With no discernable cure, arthritis costs the American economy an estimated annual $128 billion, according to the National Arthritis Foundation. Prof. Itai Cohen, physics, has taken a different approach when it comes to understanding the damage to the soft tissue that covers our bone joints. At Jan. 19’s Mechanical and Engineering Colloquium, Cohen discussed the new technology his team is employing to gain greater insight to the causes.

The mechanical nature of cartilage is one not often studied and is important for understanding strain response. Cartilage is able to hold the type of strain that would undoubtedly cause fracture in many other types of tissue. Cohen addressed the normal assumptions that people have about cartilage, including its ability to act as a localized energy dissipater, a shock absorber.

In actuality, cartilage is a very stiff substance under rapid compression that does not have the capability to dissipate the force which causes its eventual deterioration. Cartilage is made of avascular cells, cells that lack blood vessels — which keep tissues robust but also makes it slow to heal. Cartilage cells vary from the surface to the bone. While previous research has focused on the average properties of cartilage and treated it as being a homogeneous tissue, Cohen’s research has focused on how the differences in structure affect the local tissue mechanics.

As Cohen said, “cartilage varies in structure from the surface to the bone. For example, the collagen fibers that surround the cells and give the tissue integrity vary in their orientation with depth. Near the surface they are oriented parallel to the surface while near the bone they are oriented perpendicular to the surface.”

Cohen uses three-dimensional images of cartilage tissue, taken from cows during applied shear strain. To do this, he and his team have developed instruments that load onto his confocal microscope and allow for deforming the cartilage.

By measuring the forces on the cartilage and the degree of local deformation in the tissue, the dissipative properties of cartilage can be mapped out. Under shear strain, Cohen finds that the tissue has a compliant region just below the surface.

“The cartilage tissue resembles a strange sandwich with a stiff slice of bread for the surface, a soft jelly like substance just below the surface and a stiff loaf of bread below the jelly… Under shear strain, cartilage acts as a spectacular shock absorber,” Cohen explained of the cartilage structure. “Coincidentally, this region is located where the collagen fibers change their orientation from being parallel to being perpendicular.”

Cohen and his students’ research has shown that cartilage under shear strain absorbs 99% of the imparted energy. Most imparted energy tissue under shear strain gets absorbed in the soft jelly-like substance. Cohen speculates that the collagen fiber orientation plays an integral role in determining the unique properties of this tissue.

The implications of Cohen’s research may alter the future of arthritis care. “We are in the dark ages where this is concerned,” he said, referring to current treatments for arthritis pain.

Current procedures like cartilage shaving and knee-replacement surgery have proved to be very short-term and ineffective solutions. However, Cohen and his team hope that their new insights into the mechanics of cartilage function will pave the way for the future of arthritis care, including the design of artificial cartilage.

Original Author: Tajwar Mazhar