A knee injury sent shockwaves through the NFC West once again when Arizona Cardinals quarterback Carson Palmer suffered a torn ACL on Sunday following a seemingly innocuous plant of his left leg. He joins fellow NFC West quarterback Sam Bradford on the sidelines, who tore his ACL in the preseason.
Like most ACL tears in the NFL, Palmer went down without contact. Rather, an unfortunate perfect storm of positioning and weight distribution led to his injury.
A brief review of the knee's anatomy shows why.
The ACL, short for anterior cruciate ligament, runs from the femur to the tibia inside of the knee joint. It prevents the tibia from moving forward or over-rotating with respect to the femur. As such, it stabilizes the knee during pivoting, twisting and changing directions.
When a movement forces the tibia to rotate further than the ACL can withstand, it tears. Sometimes, that movement is quite subtle.
Palmer's was no exception.
At full speed, the former Heisman Trophy winner's injury, which occurs at the 1:05 mark in the above NFL.com highlight video, does not appear very impressive. Nonetheless, a few screenshots make clear what led to the serious injury.
It started with Palmer's foot.
When the Cardinals signal-caller steps forward, he plants his foot with his toes turned outward relative to the rest of his body. At the same time, his knee bends inward and, as a result, rests closer to his body's midline, a position known as "valgus stress."
Then, when Palmer sharply planted most of his weight on his awkwardly positioned leg, his knee absorbed that weight and twisted inward too much for his ACL to bear.
Those watching the game saw the apparent instability of Palmer's knee as he walked on the sidelines. At one point, his left leg appeared to buckle underneath him, an ominous but classic sign of an ACL injury.
The tear represents a heartbreaking blow to a veteran quarterback who was putting together one of his better seasons in recent memory despite a three-week absence due to an axillary nerve contusion.
Heading into Week 10, Palmer boasted 10 touchdown passes and just two interceptions. He also led the Cardinals to victory in all six of his starts this season—including Sunday's matchup against the Rams—a streak that included wins over the San Diego Chargers, Philadelphia Eagles and Dallas Cowboys.
His stellar season certainly wasn't lost on him, either. According to Hanzus, the circumstances of the injury overwhelmed Palmer following the game:
I'm not going to lie, I cried like a baby last night," he said. "And I'm not an emotional guy. I don't cry. The last time I cried like that was when I lost my friend and teammate Chris Henry back in '09."
"My phone's been non-stop with prayers," he added. "There's a lot of people that care for me and I'm very fortunate for that.
Now, the Cardinals must turn to backup quarterback Drew Stanton once more.
In four games this season, Stanton did not awe opposing defenses—his 2014 completion percentage sits at an unimpressive 49.5 percent—but his 2-1 record as a starter is certainly respectable. He also avoided turning the ball over even once during his time on the field this year.
In the meantime, Palmer can only watch.
ACL tear recoveries usually require the better part of a year. A very brief and unscientific review of recent NFL cases comes up with a typical timeline of nine to 10 months, though the range runs the gamut from seven months to well over a year.
Fortunately, nothing suggests Palmer can't return to form once again. His prior knowledge and experience with ACL rehab—he tore his left ACL in 2006 as part of a much more serious injury—will also serve him well.
Dr. Dave Siebert is a second-year resident physician at the University of Washington and a member of the Professional Football Writers of America. He plans to pursue fellowship training in Primary Care (non-operative) Sports Medicine.
Stats were obtained from NFL.com's official statistics.