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"With this new information, we are now able to explore pharmaceutical options to directly target the stem cell population that is responsible for the development of articular cartilage and progression of osteoarthritis."

As it turns out, recent research suggests that articular cartilage—the type in our joints—does have some limited repair capacity.

Nicholas DiNubile, an American orthopedic surgeon who specializes in knee surgery, said it would be challenging to return to the tour after articular cartilage surgery.

An MRI revealed a tear of the labrum as well as injury to the articular cartilage of the shoulder.

Hunter had a non-surgical procedure on Monday to address articular wear and tear in his right knee.