The knee is a hinge joint and susceptible to injury from trauma, inflammation, infection, and degenerative changes. A knee effusion may result from acute or chronic conditions. Causes range from trauma or overuse to systemic disease. Understanding the basic anatomy and pathophysiology of knee effusions is important to make an accurate diagnosis. However, the most important information in making the diagnosis will be to correlate findings with a thorough history and physical exam.
The knee contains sac-like structures containing synovial fluid, called bursae, which are located between the skin and bony prominences. Fluid accumulation in the intra-articular space of a joint is called an effusion. Small, asymptomatic effusions can occur in healthy individuals. Larger joint effusions indicate intra-articular pathology. In the knee joint, trauma, overuse, infections, systemic causes, or changes in osteoarthritis may cause the effusion. These large effusions can also cause popliteal cysts. Acute swelling can be the result of a hemarthrosis from an injury to a ligament or fracture.
The knee is more frequently injured than other joints because it is part of a weight-bearing limb, and it does not have the stability brought by the joint congruity of the hip and ankle. The lifetime prevalence of knee swelling has been reported as high 27%. It may occur at any age in children, but most commonly occurs in infants and teenagers.
Any condition that may cause knee pathology can initially present as a knee effusion from a broad differential diagnosis. A knee effusion can be caused by an acute or chronic condition which may be inflammatory or non-inflammatory. The most common diagnoses in the primary care setting are osteoarthritis, trauma, and gout.
The knee joint is comprised of synovial fluid, which is an ultrafiltrate of blood plasma and includes hyaluronic acid, glycoproteins, lubricin, proteinases, and collagenases. Inflamed synovium contains large clefts, which then allows molecules of almost any size to pass through its membrane. Synovial fluid also contains proteins that are identical to plasma proteins. The hyaluronic acid in the synovial fluid provides friction and lubrication to the knee joint. Synovial fluid has antibacterial properties that help maintain a sterile environment to the joint.
The evaluation of an acutely swollen knee must begin with a very thorough history. Important questions to ask include mechanism of injury, duration, acuity of onset, aggravating symptoms, or if swelling occurred without trauma. A knee effusion with a history of recent injury may suggest a derangement such as a ligament or meniscal tear, while an atraumatic effusion would have a higher suspicion for septic arthritis. A history of previous surgery should be determined in every patient who presents with knee swelling. Systemic symptoms of inflammation or infection should be investigated as well. Patients commonly complain of swelling and stiffness with decreased range of motion.
Red flags include fever, non-weight bearing, loss of distal pulses, loss of sensation distal to the knee. These red flags typically need immediate evaluation.
A knee joint effusion will demonstrate swelling around the patella and distend of the suprapatellar space. Patients may have a restricted range of motion along with pain with ambulation. The exam should include observation of gait, palpation of the external knee, range of motion, joint line tenderness, McMurray tests, Thessaly test, duck walk, patellar tendon stability (ACL, PCL, valgus, varus), and a patellofemoral joint test (compression test). Both the ballottement test and bulge test are done to look for knee effusion. The ballottement test is done by pressing upward on the medial aspect of the knee 2 to 3 times, then tapping the lateral patella to see if it floats outward due to effusion. Always compare the exam with the unaffected knee. In septic arthritis, the following symptoms are the only ones to occur in more than 50% of patients: joint pain, a history of joint swelling, and fever.
In patients presenting with an acutely swollen knee, initially plain, weight-bearing radiographs in 2 planes should be ordered to look for a fracture, in case of trauma. A fabella, a sesamoid bone located inside the gastrocnemius may be seen on an x-ray. It is a radio-opaque marker for the posterior border of a knee's synovium. The fabella sign or displacement of the fabella is seen with a synovial effusion and popliteal mass. The same lateral knee radiograph may show an increased opacity and widening of the suprapatellar bursa, which should be assessed if the fabella sign is seen. Another reason for plain radiographs is to detect erosive disease found in rheumatoid arthritis (RA), or joint space narrowing found both in osteoarthritis and RA.
An ultrasound of the effusion can help assess a complicated effusion from a simple effusion and can also be used to performed arthrocentesis. A Saline load test may be utilized to determine if a wound near a joint communicates with the joint. In the knee, 155 mL of saline is needed to reach 95% sensitivity.
Arthrocentesis and subsequent synovial fluid analysis should be done in all cases of unexplained knee effusion. The aspirated fluid should be analyzed for cell counts, Gram stain, cultures, and crystal analysis. Hemarthrosis is commonly caused by joint trauma. Fat droplets (detected by polarized microscopy) also indicate an articular fracture. Other clotting disorders like hemophilia can cause hemarthrosis in the absence of trauma.
The synovial fluid aspirate should be analyzed for:
Non-Inflammatory Synovial Fluid
Inflammatory Synovial Fluid
Poor outcomes may occur in septic arthritis if the patient has any of the following:
Risks of delaying treatment beyond 24 to 48 hours include a permanent limitation in joint function and subchondral bone loss. Bacterial invasion can lead to permanent damage to articular cartilage. An infection in a prosthetic joint may result from a local infection that is left untreated and has spread to the prosthesis via blood flow to the area.
Managing knee effusions requires an interprofessional team of healthcare professionals that includes a nurse, laboratory technologists, and a number of physicians in different specialties. Without proper management, the morbidity and mortality may be increased from an undiagnosed septic knee effusion. The moment the triage nurse sees an acute knee effusion, the emergency department physician must be notified to determine whether there is a high suspicion for septic arthritis.
The best outcome for a patient can be achieved with a coordinated therapy plan between multi-specialty and interprofessional teams. This includes the moment the patient is in triage in the ER, to the diagnosis of the septic knee, to orthopedic management, and finally post-hospitalization treatment. Essential in timely diagnosis is the diagnostic laboratory. Consultation with the pharmacist and infectious disease physician about the choice and administration of antibiotics will increase efficacy and patient safety. If the patient requires any resources for home, the social worker and case manager should be notified. Finally, due to knee pain and decreased mobility, a physical therapist must be consulted for early ambulation.
Using this shared decision-making model, communication among providers is a key element for the best result. This interprofessional approach must use evidence-based medicine and a unique, integrated care pathway. The best prognosis and outcomes depend on the early recognition of potential complications. (Level III)
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