Patellofemoral Pain Syndrome (PFPS) is one of the most common causes of anterior knee pain, particularly in adolescents and young active adults. Often referred to as “Runner’s Knee,” it is a diagnosis of exclusion characterized by pain behind or around the patella. Unlike intra-articular pathologies like meniscal tears, PFPS is primarily a biomechanical issue of patellar tracking. For the board-certified physician, success depends on moving beyond the knee to assess hip stability and implementing a “quad-sparing” rehabilitation program.

1. Functional Anatomy and the Q-Angle

The patella is the largest sesamoid bone in the body, functioning as a fulcrum to increase the mechanical advantage of the quadriceps.

• The Trochlear Groove: The patella sits within the femoral trochlea. Stability is provided by the bony geometry, the medial patellofemoral ligament (MPFL), and the balance between the Vastus Lateralis and the Vastus Medialis Obliquus (VMO).
• The Q-Angle: A critical high-yield board concept. The Quadriceps Angle (Q-angle) is formed by a line from the ASIS to the mid-patella and a line from the mid-patella to the tibial tubercle.
  • Normal: ~13° for men and ~18° for women.
  • Increased Q-Angle: Leads to a lateral “bowstringing” force on the patella, increasing the pressure on the lateral facet of the trochlea and predisposed the patient to PFPS.

2. Clinical Presentation: The “Theater Sign”

Patients with PFPS describe an insidious onset of diffuse anterior knee pain.

• The Theater Sign: Pain that is exacerbated by prolonged sitting with the knees flexed (e.g., at a movie theater or on a long flight). This position increases the compressive force of the patella against the femur.
• Aggravating Factors: Pain with stair climbing (especially descending), squatting, and running.
• Crepitus: Patients often report a “grinding” sensation, which represents chondromalacia patellae—the softening and breakdown of the articular cartilage on the underside of the kneecap.

3. Physical Examination and Provocative Testing

• Patellar Compression (Clarke’s) Test: The clinician applies downward pressure on the superior pole of the patella while the patient contracts the quadriceps. Pain or the inability to complete the contraction suggests PFPS (though this test has a high false-positive rate).
• Patellar Tilt and Glide: Assessing the mobility of the patella. A “tight” lateral retinaculum will limit medial glide.
• J-Sign: As the knee moves from flexion to extension, the patella tracks laterally at the end-range, forming a “J” shape. This indicates significant maltracking.

4. Patellar Tendinopathy (Jumper’s Knee)

While PFPS involves the joint surface, Patellar Tendinopathy involves the tendon itself, typically at the inferior pole of the patella.

• Pathophysiology: Like epicondylitis, this is a degenerative “angiofibroblastic” process rather than acute inflammation. It is common in sports involving explosive jumping (basketball, volleyball).
• Physical Exam: Tenderness is strictly localized to the patellar tendon. Pain is reproduced with resisted knee extension or a “decline squat.”

5. Differential Diagnosis: Osgood-Schlatter and Sinding-Larsen-Johansson

In the pediatric and adolescent population, the clinician must rule out apophysitis:

• Osgood-Schlatter Disease: Traction apophysitis at the tibial tubercle.
• Sinding-Larsen-Johansson Syndrome: Traction apophysitis at the inferior pole of the patella.

6. The “Hip-Knee” Connection

“The knee is a slave to the hip.” PFPS is rarely just a knee problem.

• Valgus Collapse: Weakness in the hip abductors (gluteus medius) and external rotators causes the femur to internally rotate and aduct during weight-bearing (valgus). This functionally increases the Q-angle and pulls the trochlea underneath the patella.
• Rehabilitation Focus: * Proximal Strengthening: Strengthening the gluteus medius and core is often more effective for PFPS than knee-focused exercises.
  • VMO Retraining: Traditional “leg extensions” can exacerbate patellofemoral pain. Instead, “quad sets” or “straight leg raises” with the hip externally rotated are used to isolate the VMO.
  • Foot Mechanics: Over-pronation leads to internal tibial rotation; a simple orthotic can sometimes “reset” the tracking from the bottom up.

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High-Yield Board “Fast Facts”

• Theater Sign: Anterior knee pain with prolonged sitting; classic for PFPS.
• Q-Angle: Higher in females; increased angle leads to lateral patellar subluxation.
• Vastus Medialis Obliquus (VMO): The primary medial dynamic stabilizer of the patella.
• Chondromalacia Patellae: The pathologic softening of patellar cartilage.
• Descending Stairs: More painful than ascending in PFPS due to increased eccentric load.