When quarterback Tom Brady tore a knee ligament last year, the New England Patriots got a rude lesson in the limits of modern medicine: Repair-ing injured ligaments, tendons, and cartilage is difficult, much trickier than mending a broken bone. When tendons-essential connectors between muscle and bone-are severed, surgical attempts to anchor the tendon to bone often fail because the materials are so different. The problem is akin to joining a rope to a cement wall.
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