There are many variables in the type of Yoga training each of us might choose to practice and there are many ways to injure one's knee. Let's look at the safest way for a person to enter a class, without slowing down the recovery process or causing self-injury. Therefore, medical clearance in the form of your doctor's approval is imperative before considering asana (posture) practice. A conference explaining your surgery, with a certified Yoga instructor before you take a class, insures that you will get proper attention.
The type of physical Yoga suggested for recovery is a style that holds postures for a significant length of time. Therefore, flowing, vinyasa, lunar flows and sun salutations might not be such a good idea at first. You might be wise to attend Yoga schools that have classes such as: Restorative, Viniyoga, Therapeutic or Iyengar.
Returning to, or starting, Yoga sessions after an injury can be done, but keep in mind there might be some modifications that will be needed, especially at first. One of the most common injured areas of the body are the knees. Muscles and ligaments can tear due to injury or age and that often results in the need for surgical intervention. Many people fear they will be limited after such a surgery, but with modern medicine that is most often not the case. Some people might try to avoid surgery and live with the pain, but that often causes more limitations and can lead to more serious injury to the knee area.
Practicing asana after knee surgery (or any surgery) can be done in order to strengthen the area. Keep in mind that stiffness and some joint pain might occur, which will require you to listen to your body. One of the reasons that people do Yoga is to strengthen and tone their bodies in a low or no impact way. Doing high impact activities can make the problem worse, especially soon after surgery. Yoga can be used as the first step on the path to a full recovery. Starting with seated, supine, and prone, asanas that put no pressure on the joint is probably a smart idea. You can always work up to the standing poses, but it is extremely wise to keep a chair handy or practice near a wall that you can use for support.
Since Hatha Yoga is a total body exercise, it will help you to tone and increase flexibility in all areas. This is particularly important while recovering from surgery, as the body needs to get its strength back gradually and heal properly. Trying to do too much too early won't do anything except lengthen recovery time.
Advancements in medicine have greatly reduced the invasiveness of knee surgery, as well as recovery time. Now, many people are walking very shortly after surgery and are released for normal activity more quickly than ever. Yoga can help to keep that forward momentum going. It can also have other health benefits, which is why so many people do it and have been doing it for thousands of years.