The knees, unlike the hips (which are a ball and socket joint with a large range of motion), are a hinge joint capable of two main movements: flexion and extension. When the knee is extended, no active rotation of this joint is possible. However, when in a flexed position a certain degree of rotation is possible.
The knee joint is comprised of the epiphyses of two long bones (the femur and tibia) in addition to the patella. The knee joint, unlike the hip joint, has poor boney stability because there is relatively little contact between the bones of the knee. This makes the knee more vulnerable to acute injuries involving ligaments, tendons and cartilage because they play a bigger role in holding this particular joint together.
The femur (thigh bone) also forms part of the hip joint. Accordingly, what happens at the hip joint can also have implications for the knee joint. For example, the femur will stop exterally rotating partway into a yoga posture (such as Lotus) when it makes contact with the bone of the hip socket. This bone on bone contact is known as compression and prevents any further progression in that particular direction, regardless of flexibility around the hip joint.
Unaware of the boney block, sometimes yoga students attempt to go deeper into the posture and will use their knees to bypass this resistance. For example, forcing the feet up to the opposite thighs in Lotus pose by laterally rotating the knees (turning them sideways).
Carli demonstrating the correct knee and foot position in Lotus (Padmasana). Carli is able to perform full Lotus because she can externally rotate her hips to 115 degrees due to flexibilty around her hip joint but also due to the bone structure of her thigh and hip bones.
However, the knee is extremely vulnerable to injury in this flexed and rotated position. Specifically, when forced into this angle in Lotus, the ends of the femur and tibia can squeeze the inner cartilage of the knee, damaging the meniscus and resulting in an painful knee injury.
Accordingly, forcing the knees out to the sides in Lotus, or other yoga asanas such as Baddha Konasana and Janu Sirsasana, places immense pressure on the intra-articular structures of the knee and is the source of most yoga related knee injuries which can include ligament strains, tendonitis, mal-tracking of the patella or, even worse, a meniscus (knee cartilage) tear.