Why Does the Knee Hurt? Common Etiologies of Knee Pain
Osteoarthritis is the most common cause of chronic knee pain, particularly affecting the knee as the most frequently impacted lower extremity joint 1. However, knee pain encompasses a broad spectrum of etiologies that vary significantly by age, mechanism of injury, and anatomical location.
Primary Etiologies by Age and Presentation
In Adults ≥45 Years Old
- Osteoarthritis dominates this age group, presenting with activity-related joint pain and less than 30 minutes of morning stiffness (95% sensitivity, 69% specificity) 2
- Degenerative meniscal tears occur in approximately 12% of adults, often coexisting with osteoarthritis rather than as isolated pathology 2
- Subchondral insufficiency fractures (previously termed spontaneous osteonecrosis) most commonly affect the medial femoral condyle in middle-aged to elderly females 1
In Adults <40 Years Old
- Patellofemoral pain syndrome is the leading cause, with a lifetime prevalence of approximately 25%, characterized by anterior knee pain during squatting (91% sensitivity, 50% specificity) 2
- Acute traumatic meniscal tears from twisting injuries 2
- Overuse injuries including tendonitis, bursitis, and iliotibial band friction syndrome 3
In Young Athletes
- Idiopathic anterior knee pain (patellofemoral pain syndrome) is most common 4
- Osgood-Schlatter disease and Sinding-Larsen-Johanssen syndrome from apophyseal overuse 4
- Juvenile osteochondritis dissecans (JOCD) 4
Intra-articular vs Extra-articular Sources
Intra-articular Pathology
- Meniscal tears (traumatic or degenerative): Diagnosed with McMurray test (61% sensitivity, 84% specificity) and joint line tenderness (83% sensitivity, 83% specificity) 2
- Articular cartilage damage: Chondromalacia of femoral condyle found in 57.6% of arthroscopic evaluations 5
- Synovitis and joint effusion: Associated with pain in osteoarthritis patients, readily visible on MRI 1
- Bone marrow lesions (BMLs): New or increasing BMLs correlate with increased knee pain, especially in males or those with family history of osteoarthritis 1
- Loose bodies and osteochondritis dissecans 1
- Anterior cruciate ligament injuries: Found in 8% of patients with knee pain 5
Extra-articular Pathology
- Patellofemoral disorders: Including patellar tendinopathy, fat pad impingement syndromes, Hoffa's disease, and lateral patellofemoral friction syndrome 1
- Bursitis: Deep infrapatellar bursitis 1
- Tendinitis: "Pes anserinus" tendinitis and other periarticular tendon inflammation 5
- Plica syndrome and adhesive capsulitis 1
Critical Referred Pain Sources
Do not overlook referred pain from the hip or lumbar spine, especially when knee radiographs are unremarkable 1. Hip pathology and lumbar spine pathology must be considered and evaluated with appropriate imaging of those regions when clinical suspicion exists.
Diagnostic Approach Algorithm
Step 1: Clinical Assessment
- Age: Determines likelihood of osteoarthritis (≥45 years) vs patellofemoral pain (<40 years)
- Mechanism: Acute trauma suggests ligamentous/meniscal injury; gradual onset suggests overuse or degenerative pathology
- Pain location: Anterior suggests patellofemoral pathology; medial/lateral joint line suggests meniscal or compartmental osteoarthritis
- Mechanical symptoms: Locking/catching suggests meniscal tear or loose bodies
- Morning stiffness duration: <30 minutes suggests osteoarthritis
Step 2: Physical Examination Maneuvers
- McMurray test for meniscal tears
- Joint line palpation for meniscal pathology
- Squat test for patellofemoral pain
- Effusion assessment for intra-articular pathology
Step 3: Imaging Strategy
Radiography is the mandatory initial imaging study for chronic knee pain 1. This should include at minimum: one frontal projection (AP, Rosenberg, or tunnel view), tangential patellar view, and lateral view 1.
MRI without IV contrast is the next appropriate study when radiographs are normal or show only joint effusion but pain persists 1. MRI is more sensitive than radiography and accurately depicts effusions, synovitis, meniscal tears, cartilage abnormalities, subchondral cysts, and bone marrow lesions 1.
Important Caveat
Approximately 20% of patients with chronic knee pain undergo MRI without recent radiographs within the prior year—this is premature and unnecessary 1. Always obtain radiographs first unless there is a specific contraindication.
Age-Specific Considerations
In patients 45-55 years old, meniscal tears are equally likely in painful and asymptomatic knees, and in those over 70 years, the majority have asymptomatic meniscal tears 1. This means MRI findings of meniscal tears in older patients may be incidental and not the actual pain source—correlation with clinical examination is essential.