Summer 2007

MRI is Key to Understanding Cartilage Health

Cartilage injury, repair and regrowth have long been mysterious processes. In part, this is because injured cartilage doesn't act like many other injured tissues; cartilage continues to decline in function well after trauma, and is very slow to heal.

For the most part, however, the imaging tools traditionally used have not supplied enough quantitative data to successfully monitor the health of cartilage tissue. New imaging techniques and technologies are coming in place to change that, however.

"With MRI and MRI spectroscopy, we have the ability to understand easily and quickly exactly what is going on" in the joint, says UCSF orthopaedic surgeon Benjamin Ma, M.D. His research has been conducted in collaboration with Sharmila Majumdar, Ph.D., Thomas Link, M.D., and Xiaojuan Li, Ph.D., of the MQIR group in the Department of Radiology.

Traditionally, cartilage is examined via an arthroscopic camera or an X-ray. Both have limitations. Under visual inspection via arthroscope, cartilage tears or gross trauma show up well, but tissue that is under stress or dying often does not look any different than healthy cartilage. X-rays can show the thickness of the cartilage, but also cannot show tissue under mechanical or physiological stress.

MRI shows tissues that are swollen and under repair, tissues that may look fine to the human eye. In addition, MRI can provide a dynamic picture of pressure points in the joint as it moves through a complete articulation.

Areas that may be under transient stress during movement can also be imaged with MRI. "We can put someone in a device that allows us to look at knee flexion in the MRI, which simulates squatting," Ma says.

Traditional MRI of the cartilage can be supplemented by MRI spectroscopy, which allows radiologists to look at the chemistry inside the cell.composition of the tissue. MRI spectroscopy allows physicians to image fats, metabolic products and other cellular markers in the celltissue, providing a much more accurate picture of the health of cartilage tissue. MRI spectroscopy requires a stronger magnet than is used in standard MRI, but most radiology facilities will have the requisite magnetic power in the coming years, Ma says.

By far the most important result of the rise of MRI use for cartilage imaging, Ma says, is the ability to get frequent, comparable data about the state of the tissue.

"Right now, we have a hard time getting quantifiable data that can be compared over time," Ma says. With MRI, physicians can get numerical scores for various aspects of tissue health and performance, he says.

"Once we are able to get reliable data, we can do many more concrete studies that we can't do now," Ma says. "We can understand how healthy cartilage behaves normally, what the baseline is, and we can monitor what happens after injury. The tissue may look fine at first, but it may really have sustained a significant injury."

Quantifiable data are also the lifeblood of research on cartilage repair, allowing researchers to frequently monitor how a repair is doing. "Physicians come up with new methods that we think are better than older techniques. But right now, we have to wait 10 years or so to see if people with the newer technique get arthritis at a lower rate than those getting the older operation," Ma says. "With imaging, we are often able to see very quickly which one is better."

This ability accelerates the development cycle for new surgical techniques, and also allows the comparison of surgical, pharmaceutical and physical therapies. "It really makes the whole field into a much more solid science," Ma says.

C. Benjamin Ma can be contacted at (415) 885-3810.

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