What is the most likely cause of knee pain in a young, otherwise healthy adult with no significant past medical history?

Common Causes of Knee Pain in Young Adults

Patellofemoral pain syndrome is the most likely cause of knee pain in young, otherwise healthy adults, affecting approximately 25% of this population over their lifetime, particularly those who are physically active and under age 40. 1, 2

Most Common Diagnoses by Age and Activity Level

Primary Causes in Young Adults (<40 years)

• Patellofemoral pain syndrome is the leading diagnosis in physically active young adults under 40 years, with anterior knee pain during squatting being 91% sensitive (though only 50% specific) for this condition 1, 2
• Patellar tracking problems (including patellar subluxation) occur more frequently in teenage girls and young women 3
• Knee extensor mechanism problems such as patellar tendonitis and tibial apophysitis (Osgood-Schlatter lesion) are more common in teenage boys and young men 3
• Meniscal tears from acute trauma (twisting injuries) affect approximately 12% of adults and should be suspected when there is joint line tenderness (83% sensitive and 83% specific) or a positive McMurray test (61% sensitive, 84% specific) 1

Critical Red Flags to Exclude First

Before attributing knee pain to common musculoskeletal causes, you must rule out serious pathology:

• Referred pain from hip pathology, particularly slipped capital femoral epiphesis in adolescents and young adults, can present as knee pain rather than hip pain 4, 3
• Septic arthritis can develop at any age and presents with acute onset, fever, severe pain at rest, and inability to bear weight 5
• Stress fractures present with insidious onset, night pain, focal tenderness, and inability to bear weight in the context of repetitive loading 4, 5
• Tumors manifest with insidious onset, night pain, constitutional symptoms, and progressive worsening unrelated to activity 4, 5

Diagnostic Algorithm

Step 1: Identify Pain Location and Pattern

• Anterior knee pain during squatting or stairs suggests patellofemoral pain syndrome, especially in active individuals under 40 years 1, 2
• Joint line tenderness with mechanical symptoms (locking, catching) after twisting injury suggests acute meniscal tear 1
• Medial knee pain with activity in active patients suggests overuse injuries like pes anserine bursitis 3
• Knee pain in adolescent without clear knee pathology mandates hip examination to exclude slipped capital femoral epiphysis 4, 3

Step 2: Physical Examination Maneuvers

• Squat test: Anterior knee pain during squatting is 91% sensitive for patellofemoral pain syndrome 1
• McMurray test: Concurrent knee rotation and extension is 61% sensitive and 84% specific for meniscal tears 1
• Joint line palpation: Tenderness is 83% sensitive and 83% specific for meniscal tears 1
• Hip range of motion and FADIR test: Must be performed to exclude hip-related pain presenting as knee pain 4, 5
• Lumbar spine screening: Mandatory in all cases as competing source of referred pain 4, 6

Step 3: Imaging Strategy

• Radiographs are NOT recommended for young adults with typical patellofemoral pain syndrome or suspected overuse injuries without red flags 1
• Plain radiographs ARE indicated when suspecting fracture, tumor, or if hip pathology needs exclusion (AP pelvis view) 4, 5
• MRI is reserved for cases where intraarticular pathology (meniscal tear, ligament injury) is suspected and would change management, or when hip pathology requires further characterization 4, 1

First-Line Treatment Approach

For Patellofemoral Pain Syndrome (Most Common)

• Hip and knee strengthening exercises combined with foot orthoses or patellar taping are the recommended first-line treatment 1
• Surgery has no indication for patellofemoral pain syndrome 1
• Education and self-management programs should be provided to empower patients 1

For Meniscal Tears

• Conservative management with exercise therapy for 4-6 weeks is appropriate for most meniscal tears, even in the presence of mechanical symptoms 1
• Surgery is only required for severe traumatic bucket-handle tears with displaced meniscal tissue 1
• Degenerative meniscal tears (more common in patients ≥40 years) should receive exercise therapy as first-line treatment, with surgery not indicated even with mechanical symptoms 1

Common Pitfalls to Avoid

• Do not assume knee pain originates from the knee: Hip pathology, particularly in adolescents and young adults, commonly presents as knee pain without hip symptoms 4, 3
• Do not order routine radiographs: Young adults with typical patellofemoral pain syndrome do not require imaging unless red flags are present 1
• Do not rush to surgery for meniscal tears: Conservative management with exercise therapy achieves good results in most cases, and surgery for degenerative tears does not improve outcomes 1
• Do not overlook inflammatory causes: While less common in young adults, septic arthritis can occur at any age and requires immediate recognition 5, 3
• Do not ignore post-traumatic hemarthrosis: Knee injuries with hemarthrosis carry 25-50% risk of developing post-traumatic osteoarthritis, suggesting need for anti-inflammatory strategies beyond mechanical treatment 7