This a good opportunity to review the approach to an acute monoarthritis and to osteoarthritis.
Let's start with some Canadian content. Here's a recent review published in the CMAJ looking at the management of a patient with an acute monoarthritis.
The NEJM published a very thorough review article on OA of the knee in 2006.
There are many causes of knee pain outside of a monoarthritis or osteoarthritis, many of which we do not see very often as internists (i.e. orthopedic problems in younger patients).
To summarize, here are some key points on evaluating a patient with acute knee pain:
1. Determine if there is a recent history of trauma. This raises the pre-test probability of a fracture, hemarthrosis, meniscal tear or ligamentous injury.
2. Decide whether this is an intra-articular, peri-articular or referred process (there is significant overlap in some of these signs/symptoms):
a) Intra-articular: pain localized to the knee without radiation, knee effusion, mechanical symptoms (clicking, locking, giving way), abnormal structural physical exam, reduced range of motion.b) Peri-articular: focal area of pain or abnormalities on exam, pain only during parts of the range of motion, +/- mechanical symptoms.
c) Referred pain: vague location of pain, back or hip pain, pain not exacerbated by knee movement, lack of an effusion, normal knee exam.
3. If the pain is articular, you need to decide if it is inflammatory (i.e. arthritis - pain, redness, swelling, warmth) or non-inflammatory (structural). Any acute effusion should be drained and analyzed.
4. Remember if an effusion is present -> rule out INFECTION. Missing a septic joint can have catastrophic consequences for the joint, and very quickly.
5. Arthrocentesis fluid should be sent for cell count and differential, gram stain, C&S, microscopy for crystals at a minimum.
6. Imaging: Start with x-rays. If there is any history of trauma, lack of localizing symptoms to the knee or a question of pain origin, image the ipsilateral joints above and below the knee.
7. Consider MRI as the next imaging test of choice after x-ray if no diagnosis can be found or if a soft tissue etiology is being considered.
8. An effusion can be present in an acute exacerbation of osteoarthritis.
