The knee is a hinge joint flexing easily and freely backward and extending itself in the forward position just opposite to the movements of the thigh and elbow joints. The knee joint is capable of only slight rotating motion when it is flexed,while the elbow rotates beautifully in a semicircle.
The limited movement of the knee joint is necessary because it is the main support of the entire body in the erect posture, having the body line of gravity running through the center of it.
The knee is the largest and most elaborately constructed joint in the body for the important reason just mentioned. The lower end (condyles) of the thigh bone (femur), the upper end of the large shinbone (tibia) and the kneecap (patella) are the bones entering into the formation of the joint.
The outer and smaller bone of the leg (fibula) is only indirectly connected with the joint, having the outer ligament of the joint partly attached to it.
The joint's most powerful ligament bands are attached on all four sides, besides the two inner sets of ligaments. Inside the joint, between the articulating bone surfaces, are two thick pads of semicircular cartilages (semilunar cartilages).
A strong fibrous capsule makes an all over covering for the joint.This capsule is lined on the inside with a smooth, secreting (serous) membrane. Extending from the serous membrane are little sacs filled with fluid (bursa) to soft-pad the joint.
The bursa in the front below the kneecap is large, and when pressure is exerted too long on this part, as in the case of workers standing on their knees, this bursa swells up with fluids (housemaid's knee)