Knee joint aspiration and injection are performed to aid in the diagnosis and treatment of knee joint diseases. The knee joint is the most common and the easiest joint for the physician to aspirate. One approach involves the insertion of a needle 1 cm above and 1 cm lateral to the superior lateral aspect of the patella at a 45-degree angle. Once the needle has been inserted 1 to 1½ inches, aspiration aided by local compression is performed. Local corticosteroid injections can provide significant relief and often ameliorate acute exacerbations of knee osteoarthritis associated with significant effusions. Among the indications for arthrocentesis are crystal-induced arthropathy, hemarthrosis, unexplained joint effusion, and symptomatic relief of a large effusion. Contraindications include bacteremia, inaccessible joints, joint prosthesis, and overlying infection in the soft tissue. Large effusions can recur and may require repeat aspiration. Anti-inflammatory medications may prove beneficial in reducing joint inflammation and fluid accumulations.
Arthrocentesis is a procedure performed to aspirate synovial fluid from a joint cavity. It has both diagnostic and therapeutic uses. While arthrocentesis is deemed to be a minor surgical procedure, there is always the potential to injure blood vessels, nerves, and tendons. Thus, the procedure should only be performed by clinicians with extensive knowledge of the anatomy of joints. To minimize the risk of injury, the extensor surface of the joint should be in extension with minimal flexion.[rx][rx]
Anatomy and Physiology
Anatomical considerations include:
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Surgeons can access the knee medially or laterally to the patella, superior or inferior to the patella.
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The patient should extend their knee with the quadriceps muscle relaxed at 20 degrees flexion.
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The physician should place their thumb on the patella and push it aside as they insert the needle.
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During the suprapatellar approach, they should place it 1 cm lateral and 1 cm superior.
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Upon identifying the midpoint of the patella, they should insert a needle either 1 cm lateral or medial.
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They should aim the needle posterior to the patella and up toward the joint space, then squeeze or “milk” both sides of the joint space to facilitate aspiration.
Indications of Knee Joint Aspiration
- Therapeutic – Large and painful joint effusions can be aspirated for pain relief. Also, it can be used for intra-articular injection of certain medications (primarily steroids). Evaluate response to treatment.[rx][rx][rx]
- Diagnostic – Synovial fluid analysis can provide physicians with an unequivocal etiology of acute arthritis, specifically to differentiate between septic arthritis and an inflammatory cause of bloody mono-arthritis.
The procedure can be done in the clinic or in the minor operating room.
Contraindications of Knee Joint Aspiration
No absolute contraindications. Relative contraindications include overlying cellulitis (potential intra-articular seeding of bacteria), coagulopathy/bleeding disorders, a joint prosthesis (preferably performed by orthopedic surgery), acute fracture, adjacent osteomyelitis, and uncooperative patient.
- Bacteremia
- Clinician unfamiliar with anatomy of or approach to the joint
- Inaccessible joints
- Joint prosthesis
- Overlying infection in the soft tissues
- Severe coagulopathy
- Severe overlying dermatitis
- Uncooperative patient
Equipment
Skin cleansing solution (betadine, chlorhexidine, etc.), skin marking pen, sterile gloves, sterile gauze, 1% lidocaine, 10 cc syringe, 30 cc to 60 cc syringe, 18 g and 27 g needle, specimen tubes (cell count, gram stain, and culture, and crystal analysis).
Personnel
A skilled clinician can usually perform the procedure without an assistant. If the patient is anxious or exceptional circumstances apply, an assistant will make it much easier to perform the procedure.
Preparation
Patients who are anxious, in pain, or unable to cooperate with the procedure might require assistant and procedural sedation. It is a strong recommendation to utilize some type of local anesthesia. For most surgeons, lidocaine 1% will work to numb the skin. It is important to avoid deep injections with the local anesthetic because there is a risk that it may alter the synovial fluid analysis.
Technique
The clinician or medical care professional must obtain informed consent. A timeout should take place to confirm the patient and correct the joint. The most crucial step is having the patient lay in a comfortable position with the affected knee fully extended or flexed at 15 to 20 degrees with a towel roll under the knee. This position helps to facilitate procedure success by ensuring quadriceps muscle relaxation. The clinician should then locate the patella. The clinician may mark it with a marking pen.
The knee is the largest synovial cavity in the body and is easily accessible from either the medial or lateral aspect, and superior, inferior at the midpoint of the patella. Sterilize area and drape in a typical sterile fashion.
Choose the approach, then use a small syringe and small-bore needle, draw up lidocaine and anesthetize superficial skin and then deeper tissue in the projected trajectory of joint aspiration to anesthetize the track.
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For the midpoint approach, insert 18 g needle with 30 cc to 60 cc syringe one cm lateral or medial to the patella, directing the needle posterior and horizontal toward the intercondylar notch of the femur.
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Make sure to pull back on the syringe while inserting and stop once you aspirate synovial fluid.
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Attempt to aspirate as much fluid as possible.
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The superior approach is performed one cm superior and one cm medial or lateral to the patella, directing towards the intercondylar notch of the femur.
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The infrapatellar approach requires the patient to be sitting upright, with the knee flexed at 90 degrees. Needle insertion is five mm below the inferior border of the patella while also directing posterior to the patellar tendon, making this a less desirable approach.
Milking or compressing the joint can help facilitate the aspiration of fluid. Transfer fluid to specimen tubes. Remove the needle from the joint and place bandage over the insertion site.
Procedure Description
- The patient is supine on the table with the knee extended (some physicians prefer that the knee be bent to 90 degrees). Some physicians prefer the medial approach for smaller effusions, but the lateral approach will be discussed here. The knee is examined to determine the amount of joint fluid present and to check for overlying cellulitis or coexisting pathology in the joint or surrounding tissues.
- The superior lateral aspect of the patella is palpated. The skin is marked with a pen, one fingerbreadth above and one fingerbreadth lateral to this site. This location provides the most direct access to the synovium.
- The skin is washed with povidone-iodine solution. The physician should be gloved, although there is no consensus as to whether sterile gloves must be used. A 21-gauge, 1-inch needle is attached to a 5- to a 20-mL syringe, depending on the anticipated amount of fluid present for removal.
- The needle is inserted through stretched skin. Some physicians administer lidocaine (Xylocaine) into the skin, but stretching the pain fibers in the skin with the nondominant hand can also reduce needle-insertion discomfort. The needle is directed at a 45-degree angle distally and 45 degrees into the knee, tilted below the patella.
- Once the needle has been inserted 1 to ¼ inches, aspiration is performed, and the syringe should fill with fluid. Using the non-dominant hand to compress the opposite side of the joint or the patella may aid in arthrocentesis.
- Once the syringe has filled, a hemostat can be placed on the hub of the needle. With the needle stabilized with the hemostat, the syringe can be disconnected and the fluid sent for studies. Care should be taken not to touch the needle tip against the joint surfaces when removing the syringe. A syringe filled with corticosteroid medication can then be attached to the needle.
- For injection, use betamethasone (Celestone, 6 mg per mL), 1 mL, mixed with 3 to 5 mL of 1 percent lidocaine. Alternately, methylprednisolone (Depo-Medrol, 40 mg per mL), 1 mL, mixed with 3 to 5 mL of 1 percent lidocaine can be used. After the injection of the medication, the needle and syringe are withdrawn.
- The skin is cleansed, and a bandage is applied over the needle puncture site. The patient is warned to avoid forceful activity on the joint while it is anesthetized.
Knee synovial fluid analyzed
The synovial fluid aspirate should be analyzed for[rx]:
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Complete blood count (CBC) with differential (white blood cell [WBC], polymorphonuclear leukocytes)
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Crystal examination of synovial fluid
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Culture and Gram staining of synovial fluid
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Viscosity (RA: expect decreased viscosity and poor mucin clot formation)
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Glucose
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Low level of synovial-fluid glucose is suggestive of an infected joint, but low glucose levels are present in only about 50% of patients with septic joints and can also occur in rheumatoid arthritis
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Fasting glucose levels are usually reduced to less than half of the simultaneously obtained blood levels
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The presence of crystals cannot exclude septic arthritis with certainty.[8] Septic arthritis occurs concurrently with gout or pseudogout in less than 5% of cases.
Septic Arthritis
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Joint fluid appears cloudy or purulent
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Cell count with WBC greater than 50,000 is considered diagnostic for septic arthritis. However, lower counts may still indicate infection (not sensitive)[rx]
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Prosthetic joint with WBC greater than 1100 is considered septic
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Gram stains only identify infective organism one-third of the time
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Glucose less than 50% of serum level
Non-Inflammatory Synovial Fluid
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Contains less than 60 to 180 cells per mL, most of which should be mononuclear
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Synovial fluid is considered to be non-inflammatory if it contains less than 2000 cells/mL, but most samples of synovial fluids from pts with osteoarthritis contain less than 500 cells per ml.
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The most common cause of non-inflammatory effusions of the knee (synovial fluid white blood cell count less than 2000 cells/mcL) is osteoarthritis; other causes include osteonecrosis, Charcot arthropathy, sarcoidosis, amyloidosis, hypothyroidism, and acromegaly. Inflammatory arthritis (synovial fluid white blood cell greater than 2000 cells/mL) can be caused by infection, autoimmune disease, and crystal-induced arthritis. Aspiration of dark brown serosanguinous fluid should raise the possibility of pigmented villonodular synovitis.
Inflammatory Synovial Fluid
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Greater than 2000 leukocytes/mL
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Traumatic: Less than 5000 (w/RBCs)
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Toxic Synovitis: 5000 to 15,000 and less than 25% polymorphs
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Acute Rheumatic Fever: 10,000 to 15,000 and 50% polymorphs
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JRA 15,000 to 80,000 and 75% polymorphs
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Greater 50,000 leukocytes/mL;
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Although other diseases including trauma, may produce WBC cells in joint fluid, levels greater than 50,000/mm3 are usually due to infectious arthritis.
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Usually causes most intense synovial fluid leukocytosis, w/ 50,000 to 200,000 cells/mL and usually over 90% PMNs
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Lower leukocyte counts are more common early in course of bacterial arthritis and in patients with disseminated gonococcal infection
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Non-infectious conditions such as gout, pseudogout, acute rheumatic fever, reactive arthritis, and RA can cause a markedly inflammatory synovial effusion. Finding of greater than 90% PMNs despite relatively low total leukocyte count should prompt concern about infection or crystal-induced disease. However, the presence of crystals cannot exclude septic arthritis with certainty[rx]
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Septic arthritis 80,000 to 200,000 and greater than 75% polymorphs
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In synovial fluid WBC count and percentage of polymorphonuclear cells from arthrocentesis are the most powerful predictors for septic arthritis. The LR is increased as the synovial fluid WBC count increased.
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For counts greater than 50,000/microL (LR, 7.7; 95% CI, 5.7-11.0) and for counts greater than 100,000/microL (LR, 28.0; 95% CI, 12.0-66.0). On the same synovial fluid sample, a polymorphonuclear cell count of at least 90% suggests septic arthritis with an LR of 3.4 (95% CI, 2.8-4.2), while a PMN cell count of less than 90% lowers the likelihood (LR, 0.34; 95% CI, 0.25-0.47)[rx]
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Arthrocentesis is performed for the identification of the etiology or pain relief, injection of medications, or effusion drainage. To avoid complications, the clinician should be familiar with the anatomy. The risk of complications can be minimized by using an appropriate technique.
Crystal analysis is usually an important result of a successful knee aspiration. Most commonly, the clinician differentiates gout (negatively birefringent urate crystals), pseudogout (weakly positive birefringent crystals), and an infectious process.[rx]
Synovial fluid associated with septic or infectious joint effusion is as follows:
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Total white blood cell count higher than 25,000/microliter (approximately 75% sensitive, 75% specific)
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Total white blood cell count higher than 50,000/microliter (approximately 60% sensitive, approximately 90% specific)
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Total white blood count higher than 100,000/microliter (approximately 20% sensitive, approximately 99% specific)
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PMN cell proportion 0.9 or higher (approximately 75% specific, 80% sensitive)
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Lactic dehydrogenase (LDH) concentration higher than 250 U/L (approximately 100% sensitive, 50% specific)
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Synovial glucose or serum glucose concentration lower than 0.5 (approximately 50% sensitive, 85% specific)
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Protein concentration higher than 3 g/dL (approximately 50% sensitive, 50% specific).
Complications
Local trauma, including damage to nearby structures, pain, infection, and reaccumulation of effusion.[rx][rx][rx]
If the needle placement is poor or if the synovium is thickened, it may result in a dry tap.
- Hemarthrosis – can occur if a large needle damages a blood vessel when performing multiple attempts. In most cases, the hemarthrosis presents within a few hours after the procedure. It is often associated with pain, stiffness, and swelling of the joint. The majority of hemarthrosis is self-limited and resolves within a few weeks. If the patient has a coagulopathy, it may need to be corrected, and consultation with a hematologist may be helpful.
- The Patient Complains of Severe Pain During the Procedure – Severe pain during the procedure usually results from the needle coming into contact with the highly innervated cartilaginous surfaces. The needle can be redirected or withdrawn when pain is encountered. The slow, steady movement of the needle during insertion can prevent damage to the cartilage surface from the needle bevel.
- The Patient’s Effusion Was Sterile – But Became Infected After the Joint Injection. Introduction of infection into a joint is a rare event, occurring in less than 0.01 percent of injections; however, the infection can develop when the needle is introduced into the joint through an area of cellulitis. Severe dermatitis or soft tissue infection overlying a joint is a contraindication for arthrocentesis. Some physicians advocate that steroid injection should not be performed before excluding joint infection.
- A recognized complication of steroid injections – to joints is the postinjection flare. The flare reaction represents an increase in joint pain occurring in 1 to 2 percent of persons. The steroid crystals can induce inflammatory synovitis that usually begins about six to 12 hours after the injection. The postinjection flare can present with swelling, tenderness, and warmth over the joint that persists for hours or days. If the patient takes anti-inflammatory medications immediately after the injection, they may reduce or abort this reaction. Aspiration should be performed to rule out joint sepsis if symptoms persist beyond two to three days.
- The most serious complication – of repeated injections is joint instability from the development of osteonecrosis of juxta-articular bone and weakened capsular ligaments. Although this complication occurs in less than 1 percent of patients, it is recommended that injections be performed no more frequently than every six to eight weeks, and no more than three times per year in weight-bearing joints.
- Large effusions – from the knee can rapidly re-accumulate. Some physicians advocate placing an elastic wrap around the knee immediately after large effusion drainage.
- A major disadvantage to intra-articular corticosteroid injections – is the short duration of action. The average duration of benefit maybe only two to three weeks; however, a small percentage of patients with osteoarthritis may have sustained relief after one or two injections.
References
