What joint or ligament is likely affected in a 3-year consistent workout individual experiencing knee pain, particularly on the right side, during knee flexion?

Likely Diagnosis: Patellofemoral Pain Syndrome

Based on your presentation of anterior knee pain during knee flexion (especially when lowering down during workouts), the most likely affected structure is the patellofemoral joint—specifically involving the articular cartilage of the patella and femoral trochlea, with potential involvement of the medial patellofemoral ligament (MPFL) and surrounding retinacular tissues. 1, 2, 3

Key Anatomical Structures Involved

Primary Structure: Patellofemoral Joint

• The patella (kneecap) articulates with the femoral trochlea (groove on the front of the thighbone) 3, 4
• Pain during knee flexion, particularly during squatting or lowering movements, is 91% sensitive for patellofemoral pain 2
• This joint bears the highest loads during activities requiring knee flexion under load, such as squats, lunges, or descending stairs 3, 4

Secondary Structures That May Be Affected

Medial Patellofemoral Ligament (MPFL):

• Connects the medial edge of the patella to the femoral medial epicondyle 5
• Serves as the primary passive restraint preventing lateral patellar displacement 5
• Most vulnerable at 20 degrees of knee flexion and contributes most to stability in the extended knee 5

Vastus Medialis Obliquus (VMO) Muscle:

• The inner quadriceps muscle that stabilizes the patella 3
• Weakness or imbalance between VMO and vastus lateralis (outer quadriceps) leads to patellar maltracking 3
• Hypotrophy and reduced VMO activity are commonly found in anterior knee pain patients 3

Clinical Reasoning for Your Specific Case

Why Patellofemoral Pain is Most Likely:

• Age and activity profile: Patellofemoral pain typically affects physically active individuals younger than 40 years with a lifetime prevalence of approximately 25% 2
• Pain pattern: Anterior knee pain during knee flexion (lowering movements) is the hallmark presentation 2, 3
• Sudden onset: After 3 years of consistent training, sudden onset suggests either acute overload or development of muscle imbalance 3

What to Rule Out:

While less likely given your presentation, consider:

• Meniscal tear: Would typically present with joint line tenderness, mechanical symptoms (locking/catching), and pain with twisting rather than pure flexion 2, 6
• Early osteoarthritis: More common in patients over 45 years with morning stiffness and weight-bearing pain 2
• Patellar tendinopathy: Would show point tenderness specifically over the patellar tendon insertion 1

Diagnostic Approach

Clinical Examination Findings to Expect:

• Positive squat test: Anterior knee pain reproduction during single-leg squat 2, 4
• Patellar compression test: Pain with direct patellar compression against the femoral trochlea 4
• Patellar tracking assessment: Observe for lateral patellar deviation during knee extension 3, 4
• VMO assessment: Check for visible muscle atrophy or weakness compared to the unaffected side 3

Imaging Strategy:

• Initial imaging is NOT routinely indicated for typical patellofemoral pain in young, active individuals 7, 2
• Weight-bearing radiographs (AP, lateral, and Merchant views) should be obtained first IF imaging is pursued, to rule out structural abnormalities or early osteoarthritis 7, 1
• MRI without contrast is reserved for cases where diagnosis is unclear, symptoms persist despite conservative treatment, or to rule out other pathology 1, 8

Important Clinical Pitfalls to Avoid

Do not assume imaging is necessary: The diagnosis of patellofemoral pain is primarily clinical, and imaging often reveals incidental findings that don't correlate with symptoms 8, 2

Do not attribute pain to incidental MRI findings: If MRI is performed, meniscal tears and cartilage changes may be present but asymptomatic—bone marrow lesions and synovitis correlate better with actual pain 8

Do not neglect hip muscle assessment: Hip muscle weakness, particularly hip abductors and external rotators, contributes significantly to patellofemoral pain through altered lower extremity mechanics 3

Visual Anatomy Reference

The patellofemoral joint consists of:

• Patella: Triangular bone embedded in the quadriceps tendon
• Femoral trochlea: V-shaped groove on the anterior distal femur
• MPFL: Thin ligamentous band on the medial (inner) side connecting patella to femur
• Retinacular tissues: Fibrous tissues surrounding the patella that provide additional stability

The pain occurs when the patella doesn't track properly in the femoral groove during knee flexion, causing increased pressure on specific areas of the articular cartilage, particularly during loaded flexion activities like squats or descending movements 3, 5, 4.