To avoid puncture of tendons, blood vessels, and nerves, the clinician performing the procedure should be familiar with the anatomy of the specific joint. The risk of such injuries can be minimized by using the extensor surface of the joint for needle insertion while keeping the joint in minimal flexion.
Arthrocentesis (synovial fluid aspiration) of the knee can be performed either diagnostically (for identification of the etiology of acute arthritis) or therapeutically (for pain relief, drainage of effusion, or injection of medications).[1, 2]
Although this approach is not covered in this article, it seems that ultrasound-guided arthrocentesis and injection of the knee are superior to arthrocentesis and injection guided by anatomic landmarks and palpation, resulting in significantly less procedural pain, improved arthrocentesis success, greater synovial fluid yield, more complete joint decompression, and improved clinical outcomes.
Indications for diagnostic knee arthrocentesis include the following:
Evaluation of monoarticular arthritis
- Evaluation of suspected septic arthritis
- Evaluation of joint effusion
- Identification of intra-articular fracture
- Identification of crystal arthropathy
Indications for therapeutic knee arthrocentesis include the following:
Relief of pain by aspirating effusion or hemarthrosis
- Injection of medications (eg, corticosteroids, antibiotics, or anesthetics)
- Drainage of septic effusion
There are no absolute contraindications for knee arthrocentesis. Relative contraindications include the following:
Cellulitis overlying the joint – If arthrocentesis is performed, the patient should be admitted for the administration of intravenous (IV) antibiotics, even if the synovial fluid is not suggestive of infectious arthritis
- Skin lesion or dermatitis overlying the joint
- Known bacteremia
- Adjacent osteomyelitis
- Uncontrolled coagulopathy
- Joint prosthesis – Preferably, a joint prosthesis is tapped by an orthopedist
The materials required for knee arthrocentesis include the following:
Sterile gloves and drapes
- Gauze pads (5), 4 × 4 in.
- Skin preparatory solution
- Lidocaine 1%
- Syringes, 5 mL, 20 mL, 30 mL, 60 mL
- Needles, 18 or 20 gauge and 25 or 27 gauge
- Patients who are morbidly obese might require a 21-gauge spinal needle for arthrocentesis
- Specimen tubes
Patients who are anxious, in severe pain, or unable to cooperate with the procedure (most commonly children) might require procedural sedation and/or analgesia.
Local anesthesia is always warranted. After skin preparation, draping, and identification of the needle insertion site, use a 25- or 27-gauge needle to inject 2-5 mL of local anesthetic (eg, lidocaine 1%) into the subcutaneous tissue (see the image below). (See Local Anesthetic Agents, Infiltrative Administration.) Deep injections that might enter the joint space are not recommended, because they may alter the synovial fluid analysis results.
Infiltration of local anesthetic via medial parapatellar approach (left knee).
After obtaining informed consent, place the patient supine on a gurney. Place a rolled towel below the patient’s knee. A small but randomized study demonstrated that more joint fluid was aspirated from patients in the supine position than from patients in the sitting position.
The clinician performing the procedure should be familiar with the anatomy of the specific joint and cognizant of the relevant landmarks in order to avoid puncture of tendons, blood vessels, and nerves (see the images below).
Anterior view of right knee.
Anatomic landmarks for knee arthrocentesis.
Aspiration of Synovial Fluid From Knee
Knee arthrocentesis may be done via a parapatellar approach (which is generally preferred),[5, 6] , a suprapatellar approach, or an infrapatellar approach. Once the insertion site is chosen, prepare the skin with sterile solution, allow drying, and then drape.
Using a sterile technique, attach the 18- or 20-gauge needle to the 20-mL syringe, and pull the plunger in order to break resistance. Because the knee may hold up to 70 mL of fluid, using a larger (60-mL) syringe is advisable in certain cases; accordingly, an extra syringe should be available for use if necessary.
Stretch the skin over the insertion site, and insert the needle briskly into the joint space while gently aspirating until synovial fluid enters the syringe (in an adult of average size, this usually occurs at 1-2 cm). Relaxation of the quadriceps muscle facilitates insertion of the needle. Placement of a towel under the popliteal region to flex the knee to 15-20° may facilitate entry by opening up the joint space.
For the parapatellar approach , identify the midpoint of either the medial or the lateral border of the patella. Insert an 18-gauge needle 3-4 mm below the midpoint of either the medial or the lateral border of the patella (see the image below). Direct the needle perpendicular to the long axis of the femur and toward the intercondylar notch of the femur.
Left-knee aspiration via medial parapatellar approach.
For the suprapatellar approach, identify the midpoint of either the superomedial or the superolateral border of the patella. Insert an 18-gauge needle through the midpoint of either set of superior borders. Direct the needle toward the intercondylar notch of the femur. With this approach, the needle enters the suprapatellar bursa. Remember that in 10% of the population, the suprapatellar bursa does not communicate with the knee joint.
For the infrapatellar approach, position the patient sitting upright with the knee bent at 90° over the edge of the bed. Identify either side of the inferior border of the patella and the patellar tendon. Insert an 18-gauge needle 5 mm below the inferior border of the patella and just lateral to the edge of the patellar tendon. Be careful not to go through the patellar tendon while inserting the needle.
If a bone is encountered during needle insertion, pull the needle back, verify the anatomic landmarks, and advance the needle in a corrected direction.
If fluid stops flowing into the syringe, attempt to “milk” the suprapatellar region by applying gentle pressure to the region.
If removal of more fluid is desired, a hemostat can be used to secure the needle in place while the syringe is replaced with a new one.
Once aspiration is complete, the needle is removed and a bandage applied (see the image below).
Application of bandage after left-knee aspiration.
Analysis of Synovial Fluid
The aspirated synovial fluid is then analyzed (see Table 1 below).[7, 8]
Table 1. Characteristics of Synovial Fluid on Analysis (Open Table in a new window)
|Appearance||WBCs, cells/µL||PMN cells||Glucose concentration, mg/dL||Protein concentration, g/dL|
|PMN = polymorphonuclear; WBC = white blood cell.|
Synovial fluid characteristics associated with septic joint effusion are as follows:
Total white blood cell (WBC) count higher than 25,000/µL (sensitivity, 77%; specificity, 73%; positive likelihood ratio [LR +], 29; negative likelihood ratio [LR –], 0.71)
- Total WBC count higher than 50,000/µL (sensitivity, 62%; specificity, 92%; LR +, 7.7; LR –, 0.42) 
- Total WBC count higher than 100,000/µL (sensitivity, 22%; specificity, 99%; LR +, 2.9; LR –, 0.32)
- PMN cell proportion 0.9 or higher (sensitivity, 73%; specificity, 79%; LR +, 3.4; LR –, 0.34)
- Synovial glucose or serum glucose concentration lower than 0.5 (sensitivity, 51%; specificity, 85%; LR +, 3.4; LR –, 0.58)
- Protein concentration higher than 3 g/dL (sensitivity, 48%; specificity, 46%; LR +, 0.9; LR – -, 1.1)
- Lactic dehydrogenase (LDH) concentration higher than 250 U/L (sensitivity, 100%; specificity, 51%; LR +, 1.9; LR –, 0.1)
Improper needle placement, a small amount of effusion, or mechanical obstruction of the needle against cartilage or thickened synovium can cause a dry tap.
Potential damage to cartilage can be avoided by understanding the joint anatomy and avoiding further advancement of the needle into the joint space once synovial effusion enters the syringe.
Most hemarthroses are small and self-limited and can be managed with observation alone. Hemarthrosis in coagulopathic patients requires correction of the coagulopathy in consultation with a hematologist.
With proper skin cleansing and the use of aseptic technique, the risk of introducing infection into a sterile joint can be reduced to less than 1:10,000.
When arthrocentesis is performed through infected skin for the diagnosis of a potentially septic joint, intravenous antibiotics should be administered immediately after the procedure, and the patient should be admitted for continuation of the antibiotics.
The goals of pharmacotherapy are to reduce morbidity and prevent complications.
Local Anesthetics, Amides
Local anesthetics block the initiation and conduction of nerve impulses.
Lidocaine is an amide local anesthetic used in 1-2% concentration. The 1% preparation contains 10 mg of lidocaine for each 1 mL of solution; the 2% preparation contains 20 mg of lidocaine for each 1 mL of solution. Lidocaine inhibits depolarization of type C sensory neurons by blocking sodium channels.
To improve local anesthetic injection, cool the skin with ethyl chloride before injection. Use a 25- or 27-gauge needle to inject 2-5 mL of local anesthetic (eg, lidocaine 1%) into the subcutaneous tissue. Make sure the solution is at body temperature. Infiltrate very slowly to minimize the pain. The time from administration to onset of action is 2-5 minutes, and the effect lasts for 1.5-2 hours.
Buffering the solution helps reduce the pain of local lidocaine injection. Sodium bicarbonate can be added to injectable lidocaine vials (1 part bicarbonate to 9 parts lidocaine) to produce buffered lidocaine. The shelf-life of buffered lidocaine is approximately 1 week at room temperature. All vials should be marked “buffered,” labeled with the time and date, and signed by the person who created the buffered mixture.
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