Diagrams for all the joints...
Traditionally, synovial fluid with a WBC count > 50,000/mm3 with a polymorphonuclear (PMN) cell count > 90% have been associated with infectious arthritis. However, in culture-proven septic arthritis, the WBC count only reached this level in 50 – 75% of cases. A synovial fluid WBC count >100,000/mm3 has been shown to be more specific than the traditional 50,000/mm3 and carries a LR of 28. Synovial lactate, glucose, and protein measurements have not been shown to be helpful in the evaluation of septic arthritis, though studies are currently underway evaluating synovial lactate.
Suspect gonococcal if sexually active young adults; infection will typically have a prodromal phase of migratory arthritis and tenosynovitis before symptoms settle in one or more joints.
This is a retrospective review of ED patients who received an ultrasound-guided or ultrasound-assisted arthrocentesis performed in the ED over a 6-year period by an EM resident physician.
The purpose of performing arthrocentesis of the knee or any other joint is twofold: therapeutic procedure to drain large effusions, hemarthroses and/or instill steroids or anesthetics; and to diagnose crystal arthropathies or septic arthritis. Arthrocentesis is contraindicated in patients with cellulitis overlying the site of needle entry. Suspected bacteremia is a relative contraindication. The largest synovial cavity in the body resides within the knee joint. The knee may be tapped 1cm medial or lateral to the superior third of the patella and is directed toward the intracondylar notch.
Common tips to help improve your technique with knee aspiration.
Pocus can reliably confirm the presence of a knee joint effusion.
Trick of the Trade: Use the angiocatheter for knee arthrocentesis.
Joint pain is a particularly common presentation to the ED, and while history and physical exam can often assist in differentiating the etiologies, confirmation of several pathologies requires a diagnostic arthrocentesis.
Using landmarks and going in “blind” is the original method for arthrocentesis; however, the availability of point-of-care ultrasound (POCUS) in the emergency department now provides a useful tool to assist with this procedure. Ultrasound has been reported to visualize as little as 2 mL of synovial fluid and has been shown to increase success rates in the emergency department.
The goal is to avoid the Dorsalis pedal artery, the peroneal nerve and the tendon of the Extensor Hallucis Longus (EHL). It is recommended to use an anterolateral approach where the joint line can be found between the lateral edge of the EDL and the medial edge of the lateral malleolus.
While external landmarks can be easily identifiable, penetration of the joint space can be difficult without direct visualization. As an adjunct to landmarks, point of care ultrasound (POCUS) can be used to assist in ankle arthrocentesis.
If you think you truly are in the joint space, but you aren’t getting any fluid back, consider the “backflow technique”: inject a small amount of saline, and see if you are able to aspirate it back. If so, you are in the right place
Palpate Lister’s tubercle (dorsal head of distal radius) .
Find the anatomic snuffbox and locate the extensor pollicis longus (EPL).
Sterile field, apply a wheal of lidocaine lateral to EPL and above Lister’s tubercle and direct a 22-gauge needle in this direction and pull back.
sWBC > 50,000 - essentially rules IN septic arthritis,
sWBC < 50,000 - unhelpful for clinical diagnosis,
Gram Stain: useful when positive, unhelpful when negative,
Culture: not available when we need it in the ED,
Lactate >10 - essentially rules IN septic arthritis.
Due to the necessity for diagnosis there exist no absolute contraindications to arthrocentesis.
Arthrocentesis is required procedure in majority of patients with monoarthritis and is mandatory if an infection is suspected.