The human knee is an unusually complex structure. The decision of whether to let an athlete play on an injured one is no less complex.

That’s the conclusion of medical experts watching from afar as the drama of Robert Griffin III’s injury in Sunday’s game with the Seattle Seahawks unfolds.

Previous injury can change the stresses put on the knee’s components, making them more vulnerable. Equally important, pain from a new injury — even a minor one — can change an athlete’s running, foot-planting and body-turning mechanics and subject a joint to further damage.

Whether to let an injured — or reinjured — athlete continue to play is a collaborative decision, made by trainers, physicians, coaches and the player, who all must consider many variables.

“The assessment whether the player can perform at a high level — and equally important protect themselves — requires judgment,” said Les Matthews, chief of orthopedics at Union Memorial Hospital in Baltimore and a longtime team physician. “Sometimes it’s very difficult. It’s a subjective call.”

Ligaments work to allow the knee to bend forward while limiting its ability to rotate and flex side-to-side. On the play in which Robert Griffin III was hurt, the lower half of his knee makes both of those movements to excess, suggesting an injury to the anterior cruciate ligament.

The seriousness of Griffin’s injury was immediately apparent to James C. Dreese, an orthopedic surgeon for the University of Maryland at College Park’s sports teams. He suspects the 22-year-old Redskins quarterback injured his right anterior cruciate ligament, and possibly tore it. Griffin tore his right ACL in college and had it surgically repaired.

While the knee’s main action is a forward bend, it has some ability to rotate and to flex side-to-side. The joint’s four main ligaments work to allow the first movement and limit the second two.

Specifically, the ACL keeps the bone of the bottom half of the knee joint (the tibia) from sliding too far forward and rotating too much inward in relation to the bone of the upper half of the joint (the femur). In the fall that took him out of the game, Griffin’s tibia makes both of those movements to excess.

“What you can see on that video is that it looks like the knee failed in both the hinge mechanism and the rotational mechanism,” Dreese said. “That would suggest an ACL injury, but the MRI [exam] will be definitive.”

When ACLs rupture, they usually tear in the middle rather than pulling off at one of the attachments to the bone, Dreese said. A successfully repaired and rehabilitated previous ACL rupture would not make the ligament more likely to rupture a second time, Matthews said.

Four weeks earlier, Griffin suffered a sprain to the lateral collateral ligament, which runs lengthwise on the outer side of the knee strapping the femur to the fibula. He missed one game and wore a knee brace when he returned to play.

That sprain wouldn’t specifically predispose a person to an ACL tear, the experts said. But residual pain can be a more general hazard.

“If they have to alter their running mechanism because their knee is not at 100 percent, that is potentially a situation where there is risk for further injury,” Dreese said.

The people deciding whether a player will return to play must answer two questions: Is it safe? Can the athlete perform? The player’s desires count, although not always.

“There are times when there’s no way someone is going to be sent back even if they want to be,” Matthews said. “The best example is concussion. If they don’t meet the criteria and pass the test there is no way they are going back on the field.”

Judging the fitness of an athlete with a mild joint injury, however, is especially difficult. The variables to be weighed include not only the severity of the earlier injury, the success of the rehabilitation, the outcome of tests of strength and agility and the amount of residual pain. Also important is what a player may be called upon to do. A sore knee may be acceptable for a center pushing straight ahead, but not a quarterback whose job includes cutting and running.

“I think it’s fair to say that many athletes return to play before there’s complete resolution of their symptoms,” Dreese said. “In football, it’s part of the mentality that if they have to put up with some pain then that’s part of the game.”

Nevertheless, both experts said that professional teams tend to err on the side of protecting their players.

“I have every belief that they would not have allowed him [Griffin] to go back to play if there was any feeling that he was at enhanced risk of reinjury or anything that would jeopardize his career,” Matthews said.

If Griffin’s right ACL turns out to be torn and need surgery again, the previous repair doesn’t rule out a good result.

“That makes it a more complex procedure, but not necessarily one that precludes return to play,” Matthews said. “There are countless athletes who’ve had revision of an ACL and who’ve returned to high level of competition.”