1. When confronted with a painful knee, you’re probably looking at one of three fundamental causes: injury, joint sepsis or osteoarthritis (OA).

2. But there are other possible causes to consider: rheumatoid arthritis, reactive arthritis, crystal arthropathies (gout or pseudogout), bursitis, or pain referred from hip or spine. In kids 11-15, especially in active boys, a likely cause of knee pain is Osgood-Schlatter disease, a commonly self-limited problem related to growth spurts. This demographic also suffers patellar tendonitis (jumper’s knee). Anterior pain on exercise in healthy young adults, especially women, is often chondromalacia patella (runner’s knee).

3. History and close physical examination are key. The first question is: was there an injury, trauma, sudden twinge or giving way, or did the arthropathy develop over time? If there’s swelling, did it develop gradually, or immediately after suspected injury?

4. Swelling within 6 hours of injury indicates hemarthrosis, and requires immediate referral and imaging, as there could be a fracture within the joint. MRI is best for all knee imaging, as it can spot soft tissue pathology, and will also reveal OA earlier than plain x-ray. If taking plain x-rays, get three angles: anterior-posterior, lateral and skyline.

5. Palpate the entire joint, to pinpoint damage. Tenderness along the joint line (medial or lateral) often signals a meniscal tear, or OA degeneration in the compartment on that side. Compare range of motion to the other knee, keeping your hand on the patella to feel for crepitus. A sciatic stretch test may uncover referred pain. If it’s not a recent injury, look for muscle wasting, a sure sign of underlying pathology. Difficulty with the straight leg raise may point to patellar tendonitis, or a quadriceps tear.

6. If there’s effusion without trauma, aspirate the joint. This should relieve some pain as well as giving you a handle on diagnosis. Check aspirate for leukocyte count (> 50,000 cells/mm³ with over 85% neutrophils likely indicates septic arthritis), gram stain (not very sensitive) and culture, plus polarized-light microscopy to look for crystals.

7. Evidence of inflammatory arthropathy (i.e. synovitis, which will often cause fullness above the patella), should prompt tests for ESR, urate, CRP, rheumatoid factor and HLA B27. But never assume in an RA patient that swollen knee is just a flare-up. In fact RA is a major risk factor for septic arthritis.

8. If the injury occurred during vigorous activity, like contact sports, consider first the anterior cruciate ligament. If it was standing up, or descending stairs, or a twisting injury,first suspect a torn meniscus. ACL injuries often cause a popping noise. Pain on squatting often signals meniscal injury. Knee locking can signify either.

9. Septic arthritis can develop fast, or slowly. Don’t exclude it just because the patient is afebrile (> 20% of cases are). Don’t exclude it just because WBC is normal (33% of cases are) or because the joint has full range of motion. Aspirate culture is usually definitive, but may be false negative if the patient is on antibiotics. About one case in 7 is polyarticular.

10. Knee injury often resolves with rest and NSAIDs. Topical NSAIDs like diclofenac are making a comeback, matching oral drugs for efficacy and beating them for tolerability. Unless there’s evidence on imaging of major tear or fracture requiring surgery, or OA degeneration requiring replacement, the next step should be physiotherapy. If that fails, refer to an orthopedic surgeon. Of course, some GPs may need to refer just to get an MRI in the first place.