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Dr Buck back again: 
Last week I focused on knee dislocations, a rare and particularly severe knee injury. This week I’ll cover Pivot Shift injuries, which are much more common.
A pivot-shift is a partial dislocation of the knee. It usually occurs with a non-contact injury and accounts for a large majority of ACL injuries.
While running, an athlete plants his/her foot and changes direction. The foot sticks, the upper leg rotates and with an intense enough stress, the ACL ruptures allowing the tibia to rotate forward on the femur. Immediately, the knee subluxation goes back into place and the knee joint returns to its normal alignment. This partial dislocation is called the “pivot shift” and is one of the most common ways an ACL is torn. Unfortunately, this injury is disturbingly frequent in sports activity, especially football, soccer, basketball, and gymnastics.
In the millisecond the knee is out of joint, the tibia and femur impinge violently, creating bone bruises. On MRI scan, there are bone bruises on the back on the tibia and mid portion of the femur (see MRI image). If we see this typical pattern of bone bruises on MRI, we will look carefully at the ACL to see if it is injured.
This what an MRI would like at the time of injury, the moment of the pivot shift:
An athlete will tell us that their joint “went in and out of place.”
Or
“My lower leg went one way and the knee the other.”
Or
“I felt a pop and a movement in the knee.”
or
“My knee dislocated.”
These are descriptions of a pivot shift injury. Often this occurs in a non-contact situation, and often during a movement the athlete has done thousands of times. As you know, ACL injuries are all too frequent.
I saw an athlete in a sports clinic a few weeks back who walked in with minimal swelling in his knee. His gait was normal. He was not using a brace or crutches. His pain was relatively mild. He described a pop in his knee and a movement at the time of his injury.
His exam appeared ok at first. The collateral ligaments were stable. His initial ACL test (the Lachman) was only mildly abnormal. I thought he was going to have a mild knee sprain until I performed the pivot shift test and his knee subluxed like it did during his injury. Thus there was high suspicion for a ruptured ACL. Unfortunately his ACL was found to be completely torn and he is scheduled for reconstructive surgery.
So now you probably know more than you want to know about the pivot shift injury to the knee.
