Rookie forward DeJuan Blair began his professional basketball career with the San Antonio Spurs last week and has so far averaged a very respectable 8.3 points and 8.3 rebounds per game. The former University of Pittsburgh star was initially projected as a high pick in the 2009 NBA draft, but he slid to the second round after a physical revealed something unsettling: The player has no anterior cruciate ligament (ACL) in either knee—the result of two high school surgeries. How can Blair play at a professional level without an ACL to call his own?

With his fingers crossed. When an athlete (or anyone, for that matter) lands after a jump, the force of that impact moves up the leg to the knee. This force is dissipated or opposed by muscles, tendons, cartilage, and ligaments like the ACL, which connects the femur (thigh bone) to the tibia (shin bone) and helps keep the knee stable by limiting twisting motions. Since Blair lacks ACLs, his quadriceps and hamstring muscles, and the remaining soft tissues in his knee, will need to pick up the slack, as it were, straining harder to stabilize and absorb shocks. So long as his quads and hams remain strong, it’s possible for Blair to play.

During his high-school surgeries, Blair’s doctors did not purposefully remove his ACLs—the going theory is that they tried to repair some partial damage to the ligaments and failed. Then, over time, Blair’s ACLs deteriorated to the point of disappearance. Since this was a gradual change, his muscles and other ligaments could have adapted slowly to the added strain. Without such an adjustment period—if his doctors had simply cut out the ligaments, for example—it’s likely he would have injured himself on the court quite rapidly. Blair is now thought to face a higher risk of damaging his knees (especially his meniscus) than athletes with repaired ACLs—and he may end up with osteoarthritis.

Another athlete with Blair’s deficiency wouldn’t necessarily be capable of operating at such a high level. It’s possible that Blair, without consciously intending to do so, has found a way to jump and land that’s less harsh on his knees. Or that his other ligaments are particularly resilient. Anatomy may also have something to do with his success. Knock-kneed types are more dependent on their ACLs than the bow-legged.

Blair is not the first professional athlete to play with a missing ACL. In 2008, San Diego Chargers quarterback Philip Rivers blew out his right knee but played with a completely nonfunctional ACL in a conference championship game against the New England Patriots. The Chargers lost, and Rivers underwent an extensive reconstructive operation. Pittsburgh Steelers wide receiver Hines Ward is missing the ACL in his left knee—the result of a bike accident during childhood. Like Blair, he didn’t realize the deficiency until he left college to go professional.

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Explainer thanks Walter R. Lowe of the University of Texas Health Science Center at Houston and Scott McLean of the University of Michigan School of Kinesiology.