The knee is, at its most basic, a hinge joint between the femur (thigh bone) and the tibia (shin bone). This weight bearing section of the joint is also known as the tibiofemoral joint. As well as bending and straightening it allows a small amount of rotation.
The knee cap (patella) sits in a grove on the femur and slides up and down the leg as well as being able to move sideways to a varying amount. We call this the patellofemoral joint. The knee cap or patella femoral joint can commonly causes pain in the front of the knee.
The purpose of the patella is to improve the efficiency of the quadriceps muscle group as it crosses the knee. The quadriceps is the large group of muscles in the front of our thigh that works to straighten our knee. These muscles are really important to everyday activities such as getting up from a chair, walking and going up and down stairs.
Movement between the bones is limited by ligaments. Ligaments are mainly injured when we have a fast or very forceful movement. Injury most often causes immediate swelling and instability.
Inside joints we also have cartilage, a softer substance that covers the ends of the bone to provide a slippery and protective surface. Inside our knee we also have two crescent shaped discs of cartilage known as menisci. One sits on the inside of the weight bearing knee joint – medial meniscus, and the other on the outside – the lateral meniscus. The menisci act as shock absorbers and also contribute to joint stability. The menisci can be injured, normally with rapid twisting movements, but also can show signs of aging including thinning and tears which is often associated with osteoarthritis.
A number of fluid sacks exist around the knee known as bursae. These are normal structures which may fill up with fluid and become more noticeable. An egg shaped swelling in the front of the knee cap may relate to a pre-patella bursitis and is most commonly caused by frequent kneeling. Similarly an enlarged fluid sack at the back of the knee is often referred to a Baker’s cyst.