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Patellofemoral arthritis

Patellofemoral arthritis affects the underside of the patella (kneecap) and the channel-like groove in the femur (thighbone) that the patella rests in. It causes pain in the front of your knee and can make it difficult to kneel, squat, and climb and descend stairs

Anatomy

The patella is a small bone located in front of your knee joint — where the thighbone (femur) and shinbone (tibia) meet. It protects your knee and connects the muscles in the front of your thigh to your tibia.

The patella rests in a groove on top of the femur called the trochlear groove. When you bend and straighten your knee, the patella moves back and forth inside this groove.

A slippery substance called articular cartilage covers the ends of the femur, trochlear groove, and the underside of the patella. Articular cartilage helps your bones glide smoothly against each other as you move your leg.

 
Kneecap function

(Left) The patella rests in a small groove at the end of the femur called the trochlear groove.  (Right) As you bend and straighten your knee, the patella slides up and down within the groove.

Reproduced and adapted from The Body Almanac. American Academy of Orthopaedic Surgeons, 2003.

Description

Patellofemoral arthritis occurs when the articular cartilage along the trochlear groove and on the underside of the patella wears down and becomes inflamed. When cartilage wears away, it becomes frayed and, when the wear is severe, the underlying bone may become exposed. Moving the bones along this rough surface may be painful.

 
Patellofemoral arthritis

Illustration shows patellofemoral arthritis. The patella (kneecap) has been removed to show damage to the cartilage on the underside.

 
X-rays of healthy knee and knee with patellofemoral arthritis

(Left) This x-ray shows a normal knee from the side. The arrows point to the normal amount of space between the bones. (Middle)  In this x-ray, the arrows point to narrowed joint space due to patellofemoral arthritis. (Right)  Here, the arrows point to bone spurs that have developed due to the arthritis.

Cause

Dysplasia

Dysplasia occurs when the patella does not fit properly in the trochlear groove of the femur. Because of this, when the knee moves, there are increased stresses on the cartilage. This begins to wear the cartilage down.

 
X-rays of knee dysplasia and deformed trochlear groove

X-ray taken from above the knee shows dysplasia that has developed into severe arthritis. There is no joint space remaining in the knee on the left. On the right, the trochlear groove has become deformed.
Courtesy of Ronald P. Grelsamer, MD

Kneecap Fracture

Patellar (kneecap) fractures often damage the articular cartilage that covers and protects the underside of the bone. Even though the broken bone heals, the joint surface may no longer be smooth. There is friction when the patella moves against the joint surface of the femur. Over time, this can lead to arthritis.

Symptoms

The main symptom of patellofemoral arthritis is pain. Because the patellofemoral joint is in front of the knee, you may have pain in this area. The pain can be present at rest or with no activity at all. Most of the time, however, it is brought on by activities that put pressure on the kneecap, such as kneeling, squatting, climbing and descending stairs, and getting up from a low chair.

In addition, you may experience a crackling sensation called crepitus when you move your knee. Crepitus is sometimes painful and can be loud enough for other people to hear. When the disease is advanced, your kneecap may get stuck or “catch” when you straighten your knee

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