Patellofemoral joint complaints are one of the most common musculoskeletal complaints in all age groups. Complaints vary from anterior knee pain to peripatellar knee pain to retropatellar knee pain. [1, 2, 3, 4, 5, 6, 7] Nonspecific complaints may include global or generalized knee pain, joint line pain, or posterior knee pain. Often, there is a paucity of objective findings despite subjective complaints. The problem may vary from one of short duration to one of a recurrent or chronic nature.
The etiology of patellofemoral joint syndrome is multifactorial and results from a combination of intrinsic and extrinsic factors. [1, 2, 3, 4, 5, 6, 7, 8] Treatment is often conservative in nature.  Because of the variable nature of the complaints and an often lack of objective identifiable pathologic cause of patellofemoral joint complaints, this condition can be difficult to evaluate, diagnose, and treat, which may cause great frustration for the physician and patient alike. 
For excellent patient education resources, visit eMedicineHealth's First Aid and Injuries Center and Osteoporosis Center. Also, see eMedicineHealth's patient education articles Knee Pain and Knee Injury.
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Patellofemoral joint syndrome may affect as many as 25% of all athletes.
The patellofemoral joint is composed of the articulation of the patella with the femoral condyles of the femur. The patella has a configuration of a triangle with its apex directed inferiorly. Superiorly, it articulates with the trochlea, the distal articulating surface of the femur.
The patella is the largest sesamoid bone in the body and protects the knee from direct trauma. Localized within the quadriceps tendon, the patella also acts as a fulcrum for extension of the quadriceps.
Medial movement of the patella is controlled by the vastus medialis oblique (VMO) muscle. Lateral tracking is guided by both the vastus lateralis and the iliotibial band. Patellar motion is further constrained by the patellofemoral ligament, the patellotibial ligament, and the retinaculum.
The patella is engaged with the trochlea at 20-30 º of knee flexion. At 90 º, the patella contacts the lateral and medial femoral facets within the condylar fossa. At 130-135 º of knee flexion, the medial facets of the patella contact the articulating surface of the femoral condyles. In knee extension, the patella abuts the suprapatellar fat pad.
The patella lies within the quadriceps tendon and thereby increases the mechanical advantage of the quadriceps mechanism. Not only does the patella increase the force of knee extension by 50%, but it also provides stability to the patellar tendon and minimizes the forces placed on the femoral condyles.
Tracking of the patella begins with the lower patellar border lying in contact with the suprapatellar fat pad when the knee is fully extended. With knee flexion, the patella moves proximally with a lateral shift, which is limited in excursion by the lateral retinaculum. As the knee continues to flex, the tibia internally rotates and the patella moves upward. The amount of force placed on the patellofemoral joint increases with increasing knee flexion. On the other hand, knee hyperflexion increases patellofemoral stress, as does extreme extension.
The vector force placed on the patella may be affected by the Q-angle.  The Q-angle is a line created from the anterior superior iliac spine (ASIS) to the mid patella, which intersects with a line from the mid patella to the tibial tubercle when the knee is in full extension. An average Q-angle for a male is 14 º, whereas that for a female is 17 º. Q-angles larger than average can indicate abnormal patellar tracking.
Other factors that may affect the vector force on the patella include the following:
Hyperpronation of the foot
Atrophy of the VMO muscle
A tight lateral retinaculum
Patella position (patella alta/baja or subluxation)
Inflexibility of the quadriceps, hamstring, iliotibial, and calf muscle-tendon units
General ligamentous laxity
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