The knee joint is made up of three bones – the thigh bone (femur), the shin bone (tibia), and the kneecap (patella). The joint between that back of the patella and front of the femur is known as the patello-femoral joint. There are two main conditions affecting the patello-femoral joint:
Tightening and over-stress of the joint can develop over time and lead to pain in the front of the knee
Instability or abnormal motion of the patella may develop due to injury or imbalance of structures that restrain the patella
The back of the patella has a ridge on it, known as the patellar ridge, and the front end of the thigh bone contains a groove called the trochlea. The ridge of the patella slides up and down in the trochlea as the knee flexes and extends. The shape of the ridge and groove helps to stabilize the patella during knee motion. In addition to the bony anatomy, the patella is also stabilized by ligaments. The main ligament stabilizer of the knee is called the medial patellofemoral ligament (MPFL), and there is also a lateral patellofemoral ligament on the opposite side. Finally, the patella is also stabilized by the dynamic action of the quadriceps muscle and tendon that is attached to the top of the patella and functions to extend or straighten the knee.
Tightening of the patello-femoral joint can occur in all age ranges and for a variety of reasons. It is one of the most common causes of pain in the knee. Over time, patients develop a relative tightening and stiffness of the quadriceps muscle, which pulls on the tendon attached to the kneecap and “binds” the kneecap down into the trochlear groove. In addition, one or more of the patello-femoral ligaments may become tight and stiff. Certain sports or work activities may cause increased stress to the patello-femoral joint. All of these changes have the ultimate effect of causing soreness and irritation in the patellofemoral cartilage. This condition is usually referred to by a variety of terms, such as patello-femoral syndrome, patello-femoral overload, anterior knee pain, and runner’s knee.
Patellar instability can occur following a traumatic episode, such as a sudden twisting injury to the knee. Patellar instability can also occur in the absence of a specific injury in a patient who has an anatomic predisposition to patellar instability. Factors that contribute to instability of the patella include weakness or atrophy of the inner portion of the quadriceps muscle, a shallow trochlear groove, generalized ligamentous laxity, and other abnormal anatomic configurations affecting the complex motion between the patella, femur and tibia.
Common mechanisms of injury include falls, sports injuries, or motor vehicle accidents. Injuries to the articular cartilage may also occur in association with knee ligament injuries, such as ACL tears or meniscus injuries. Small pieces of cartilage may occasionally break off and float around the knee as “loose bodies.” Occasionally, however, there is no clear history of a single injury. The condition may result from a series of minor injuries that have occurred over time.
Symptoms & Diagnosis
Symptoms of patello-femoral syndrome include pain in the front of the knee, which is generalized. There is usually no significant swelling or fluid accumulation in the joint. Patients may complain of clicking and popping in the joint. Pain is usually worse going up and down stairs, sitting for prolonged periods, and when the patient begins walking after sitting for a period of time. Physical examination shows evidence of tightness in the patello-femoral ligaments and relative tightness of the quadriceps muscles. X-rays are usually normal, but may show a slight tilt of the patella to one side or another indicating tightening of the patello-femoral ligaments. A MRI scan is usually not needed to make the diagnosis, but may show irritation in the trochlea cartilage and stress reactions in the patella and trochlear bones.
Symptoms of patellar instability depend on whether an injury occurred. A patellar dislocation leads to obvious deformity and pain. Patients may complain of recurrent sensations of instability. There may be some associated joint swelling and mechanical symptoms (such as catching, locking, or painful popping) may be present if there has been damage to the articular cartilage of the patello-femoral joint. Patients who have instability without a history of injury generally have more subtle symptoms, but will note achy pain, and often a “popping” sensation as the knee is straightened and bent due to shifting of the kneecap in the trochlea. Physical examination may show apprehension and fear of impending patellar dislocation with the knee in certain positions. X-rays may show incongruence of the patello-femoral joint with some tilting of the patella in the groove. A MRI is usually ordered in cases of persistent pain and swelling after injury to rule out damage to articular cartilage.
Treatment of patello-femoral syndrome is always initially non-operative and focuses on limiting irritation in the joint and relieving the tightness in surrounding tissues. Rest, icing, and avoidance of aggravating activities, as well as anti-inflammatory medicine is prescribed. In addition, physical therapy exercises that are done on a twice-daily basis at home are helpful in alleviating tightness in the quadriceps and patello-femoral ligaments. If non-operative treatment fails to provide lasting benefit, arthroscopic surgery may be recommended to release tight patello-femoral ligaments in a procedure called “lateral patellar release.”
Treatment of patellar instability depends on the nature of the instability (dislocation versus subluxation), the number of instability episodes, the presence of predisposing factors to patellar instability, the existence of associated injuries in the knee, and the response to previous treatments.
For patients with first-time dislocations, non-operative treatment is recommended initially consisting of short-term immobilization, bracing, and physical therapy. Instability may be recurrent in a certain number of cases despite appropriate initial non-operative treatment. For patients with painful, recurrent instability that affects overall function who have failed non-operative treatment, surgery may be recommended. The nature of the procedure depends on certain factors causing the instability and the quality of soft-tissue restraints to the patella (i.e. – MPFL). It may require a combination of bony and soft-tissue procedures to restore stable patello-femoral motion.
For more information, visit http://orthoinfo.aaos.org/topic.cfm?topic=A00382.