It can’t hurt to put a blanket under the front of your knee when asking it to support the weight of the body. The patella, or kneecap, a small bone at the front of the knee joint, protects the knee to a degree but you don’t need to put the body on top of it during your yoga practice.
The patella is the largest of the sesamoid bones—roundish bones embedded within tendons. They are usually found in the hand, knee and foot where a tendon passes over a joint. The patella is embedded in the tendon of the quadriceps muscle which functions mostly as an extensor of the knee. The patella aids in the execution of this action because it increases the leverage that the quadriceps can exert on the femur, or thigh, bone.
The patella articulates with the femur, which has a groove that the kneecap slides within. The ends of the femur and the underside of the patella are covered with cartilage that helps the bones glide smoothly over each other.
There are no hard fast rules about putting blankets under the knee. I didn’t do it for the first ten years that I practiced, even with knee problems.
But there are plenty of students I see working on hands and knees, or doing lunges with tight hips and hamstrings, not to mention cores that could use a little help. I think these folks would all be well served by practicing with a blanket under the knees.