| Mechanical (acute) |
| Collateral ligament sprain or rupture (MCL, LCL)3–7 | Medial or lateral pain | Pain with applied force |
| Injury from valgus (MCL) or varus (LCL) force | Asymmetric gapping or laxity |
| Associated internal derangements |
| Cruciate ligament sprain or rupture (ACL, PCL)3–6,8–13 | ACL Sudden pivoting injury Audible pop Instability Effusion in 1 to 2 hours
| ACL Lachman test Anterior drawer test Pivot shift test Loss of hyperextension
|
| PCL | PCL Posterior “sag” sign “Quad activation” Posterior drawer test
|
| Medial plica syndrome3–7 | Acute (or chronic) medial pain | Tender mobile tissue band along medial joint line |
| Overuse; onset of new activities |
| May report mechanical symptoms (e.g., catching, clicking) |
| Meniscal tear 3,5,6,9–17 | Male; age > 40 years | Thessaly test |
| Cutting or twisting injury while bearing weight | McMurray test |
| Effusion in 24 to 48 hours | Joint-line tenderness |
| Locking or giving way | Loss of extension (locked) |
| Patellar subluxation or dislocation3–5,8 | Anterior pain | Apprehension |
| Children or adolescents | Laxity |
| History of subluxation | Effusion |
| Mechanical (chronic) |
| Distal patellar apophysitis (Sinding-Larsen-Johannson syndrome)8,18 | Adolescents (10 to 13 years of age) | Tenderness of inferior pole of patella |
| Repetitive running, jumping, or squatting | Local soft tissue swelling |
| Decreased flexibility of quadriceps and hamstrings on affected side |
| Iliotibial band syndrome3–5,7 | Lateral knee pain | Poor hamstring flexibility |
| Repetitive flexion | Pain along entirety of iliotibial band |
| Runners, cyclists |
| Meniscal derangement or tear5,6,9–12,14–17,19 | Overuse | Thessaly test |
| Medial or lateral pain | McMurray test |
| Advanced osteoarthritis |
| Osteoarthritis1,3–5,20–22 | Diffuse pain | Chronic bony deformity |
| Stiffness when initiating movement | Leg asymmetry |
| Exacerbated by bearing weight | Appreciable crepitus |
| Age > 50 years | |
| Absence of trauma | |
| Inflammatory signs | |
| Pain worse at end of day | |
| Patellofemoral pain syndrome (chondromalacia patellae)3,18,23–25 | Anterior pain | Patellar tilt test |
| Runners, cyclists | Inhibition “shrug” test |
| “J” sign (abnormal tracking) |
| Poor vastus medialis oblique tone |
| Patellar grind |
| Pes anserine bursitis3–7,18 | Medial (or anteromedial) knee pain | Tender nodule overlying anteromedial proximal tibia |
| Overuse |
| Quadriceps or patellar tendinopathy (jumper's knee)3–5,8,18,26 | Anterior pain | Pain specific to the quadriceps or patellar tendon |
| Athletes |
| Overuse and repetitive stress |
| Tibial apophysitis (Osgood-Schlatter disease)4,8,18,26 | Adolescents; associated with growth spurt | Tenderness at tibial tubercle |
| Anterior pain; atraumatic |
| Inflammatory (noninfectious) |
| Crystal-induced arthropathy (gout or pseudogout)3,5,6,9,11,15,27 | Acute, atraumatic, monoarticular pain | Limited flexion/extension |
| Fever is possible | Possible effusion and erythema |
| Older adults (> 60 years) | Arthrocentesis demonstrating crystals on microscopy |
| Risk factors for gout: male or postmenopausal female, high intake of purine-rich foods, critical illness, specific medications | Gout: negative birefringence |
| Risk factors for pseudogout: hyperparathyroidism, hemochromatosis, hypomagnesemia, hypophosphatemia, osteoarthritis | Pseudogout: positive birefringence |
| Inflammatory (infectious) |
| Septic joint5,6,9,11,15 | Acute/subacute | Limited flexion/extension |
| Systemic symptoms | Effusion and erythema |
| Joint swelling, pain, erythema, warmth, and joint immobility | Arthrocentesis with Gram stain and culture |
| Elevated white blood cell count, erythrocyte sedimentation rate, and C-reactive protein |
