Arthrocentesis is performed when an infected joint is suspected. It is considered the gold standard in determining whether infection is the cause of a painful knee arthroplasty. In infected total knee replacement, Barrack et al15 report arthrocentesis sensitivity of 75%, specificity of 96%, and accuracy of 90%. They found false positive results to be less frequent than in arthrocentesis of hip replacements, however false negative results to be more common. This was attributed to the frequent use of antibiotics in patients with painful total knee replacements, and they recommend discontinuing antibiotics and waiting at least two weeks before aspiration is performed.
Provocative anesthetic joint injections is another indication for arthrocentesis. Referred pain from the ipsilateral hip can result in knee pain symptomology. Older patients may have difficulty localizing pain and confuse hip with knee pain, being unsure of the actual source. In this situation, the hip or knee can be injected with bupivicaine. Relief of symptoms after joint injection indicates that particular joint is the source of pain.
Arthrocentesis is usually performed without image guidance since the knee joint is easily accessible using physical examination landmarks. Occasionally, these landmarks may be difficult to palpate due to morbid obesity or severe soft-tissue swelling. In these instances, fluroroscopic or ultrasound-guided arthrocentesis can be performed. Digital subtraction arthrography is performed after aspiration to document intraarticular harvest of fluid and to evaluate for synovial cyst or sinus-track formation.
The surgical scar should be examined for areas of dehiscence or leakage. If present, markers should be placed near these areas. The suprapatellar bursa is wrapped with an elastic bandage to force fluid into the articular portion of the joint. A patellofemoral or anterior approach is used. When using an anterior approach, the needle is advanced to the anterior femoral component near its junction with the polyethylene tray.
Aspirated fluid is sent for gram stain, aerobic and anaerobic cultures, and cell count and differential. Other cultures such as fungal and AFB may be sent as well.
Dry taps occur for the following reasons:
- Dry joint
- Large periarticular bursa acting as sump for fluid
- Sinus track allowing for continuous drainage of joint
In these instances, a wash is performed with non-bacteriostatic normal saline. Washed fluid is sent for the usual labs. Cell count and differential, however, is not performed on joint washes.
After aspiration or wash, contrast is injected slowly into the joint and digitally subtracted images are obtained in the anterior projection. Some of this contrast is aspirated and sent for aerobic and anaerobic cultures labeled knee wash. The needle is then withdrawn and additional spot films may be obtained in various projections.
Aspiration-arthrogram performed with fluoroscopy in patient with bilateral lower extremity swelling and lymphadema secondary to chronic liver disease. The patient had no palpable landmarks to guide clinician in needle placement. The needle was placed in the lateral compartment and only minimal fluid was aspirated. Digitally subtracted spot film obtained early during injection demonstrates pes anserine bursa (arrow) and linear sinus track (arrow head) acting as sumps for knee fluid. AP and lateral overhead films post injection demonstrate a large pes anserine bursa, popliteal cyst, and synovial cyst anterior to the tibia.