What the butcher may have been referring to is liver that was kashered after three days; in which case it could not be cooked. If mixed with other meat and then cooked or reheated, it would be a problem.
Look at a side view diagram of the knee. The femur meets the tibia at the lower end of the patella. That means the bone that is commonly held as the shok actually makes up the “knee”.
I am not a medical student, but the knee is the joint where the femur and tibia meet. It is covered by the kneecap-patella. Pain in the joint would not preclude the meeting of several other longer bones.
Actually, according to Wikipedia’s image, the femur ends behind the patella, at the bottom of it. That would bring the Shok down to the bottom of the patella(kneecap).