The knee has poor bony stability. It is held together with ligaments and tendons, which help keep the bones lined up and help distribute the weight. The knee unfortunately does not have a lot of protection from trauma or stress. Therefore pressure or force can lead to osteoarthritis or wear and tear as well as injury.
To practice safely, always make sure the knee joint is acting as a hinge by having the knee turned to face the same direction as the foot. When the foot is turned outwards for a standing posture, protect the knee by drawing up the quads up gently as if the corners of your knees are forming a “smile”. This action lifts and stabilises the kneecap. When done correctly, you will not be able to move your kneecap side to side with your fingers. Another way to practice is to sit with your legs extended on the floor. Raise one leg an inch off the floor and you will be using the same muscles that lift and protect the kneecap. NEVER press the knee back to lock it as this hyperextends the joint rather than using or building strength in your legs.
Some more detail for those who wish to know…
The general principle to apply is that there is no such thing as “tightness in the knee”. It is a joint. Therefore if a desired posture cannot be achieved, it is probably due to a tightness in a muscle somewhere or simply down to body structure.
There are some positions in the ashtanga Vinyasa primary series which are potentially harmful for the knees if not practiced with care. Janu Sirsana C is particularly strenuous. Sukhasana (happy pose) itself can extremely unhappy on the knees for a beginner or somebody who has particularly stiff hips flexors. Our hips and knees are closely linked therefore stiff hips could result in the quads pulling on the knee and resulting in strain or over rotating a hinge joint which is not built for rotation. It is a good idea to circle the hips in their socket to release the hips instead of pushing the body and injuring the knee. Similarly, in pigeon pose tightness in the hips can lead to the quadriceps tightening up and pulling the front of the knee thus straining it.
Now for the science stuff…
In yoga asana the most common injury is collateral ligament damage, ligament strains or tendonitis (inflammation of the ligaments). In the extreme case, injury could result in a meniscus tear. The medial collateral ligament runs from the inner side of the femoral condyle to the inner side of the tibial condyle. It prevents the knee bending inwards. The lateral collateral ligament runs from the outer side of the femoral condyle to the head of the fibula and it prevents the knee from leaning outward. The medial ligament is fused to the medial meniscus, which makes the medial meniscus more vulnerable than the lateral meniscus, which is free. If the knee is forced into a position say into padmasana, the collateral ligaments could tear and with them the medial meniscus. As the meniscus does not have its own blood supply it is difficult to heal, unlike a torn muscle.
Cartilage injury is less common and cruciate ligament damage is least common in asana but common in skiing / football. I suffer from an ACL injury and have found that yoga has helped me to strengthen the quads to support the knee and keep my IT band mobilised which would otherwise tense up.
As well as injury, wear and tear of the knee can be caused by over time asanas such as Virabhadrasana 2 or parsvakonasana if the knee is tracking in. If the knee is not straight quadriceps pull the platella over rather than tracking btw the 2 condyles. Care also needs to be taken in yoga with locking the knee or hyperextending e.g. in trikonasana or prasarita padmottanasana.
Remember with yoga – knees are for life! And take ego out of the practice.
Reference article that I found useful
I also referred to Donna Farhi’s book – Yoga Mind, Body and Spirit