Arthrocentesis in Dogs

Arthrocentesis, commonly known as a “joint tap,” is a diagnostic procedure in which a needle is inserted into a joint to withdraw a sample of synovial fluid for analysis. It is a valuable tool in veterinary medicine for diagnosing various conditions causing joint pain, lameness, and swelling in dogs.

Indications for Arthrocentesis in Dogs:

The primary reasons for performing an arthrocentesis include:

Unexplained Lameness: Especially if multiple joints are affected (polyarthropathy) or if lameness is accompanied by systemic signs like fever.
Joint Swelling (Effusion): To determine the nature of the fluid (inflammatory, hemorrhagic, non-inflammatory).
Joint Pain: When pain is localized to a specific joint and the cause is not obvious on radiographs.
Rule Out Septic Arthritis (Joint Infection): This is a critical indication, as septic arthritis requires aggressive and immediate treatment.
Rule Out Immune-Mediated Polyarthritis (IMPA): A common cause of sterile joint inflammation, often affecting multiple joints.
Rule Out Neoplasia (Joint Cancer): Although rare, abnormal cells can sometimes be detected.
Monitoring Response to Therapy: Less common, but can be used to assess the resolution of inflammation or infection.
Fever of Unknown Origin (FUO): If lameness is also present, it can help identify a source of inflammation or infection.

Contraindications:

Skin Infection over the Joint: Risk of introducing bacteria into the joint.
Severe Coagulopathy: Increased risk of hemorrhage within the joint.
Fracture or Severe Joint Instability: Risk of further damage.

Procedure Overview:

Arthrocentesis requires a sterile technique and often sedation or general anesthesia to ensure patient comfort and proper positioning.

Patient Preparation:
Sedation/Anesthesia: Most dogs require sedation or light general anesthesia to remain still, minimize stress, and allow for a precise, sterile procedure. Local anesthetic is also used at the injection site.
Positioning: The dog is positioned to expose the joint(s) to be aspirated. Different joints require specific positioning.
Aseptic Technique: The area over the joint is clipped of hair, thoroughly scrubbed with an antiseptic solution (e.g., chlorhexidine or povidone-iodine) multiple times, and often draped with sterile towels. The veterinarian wears sterile gloves.

Locating the Joint:
The veterinarian will meticulously palpate the joint to identify anatomical landmarks that guide needle placement. Knowledge of joint anatomy is crucial. Common joints aspirated include the carpus (wrist), elbow, shoulder, stifle (knee), hock (ankle), and hip.

Needle Insertion:
A small-gauge (22 or 25 gauge) needle, often 1 to 1.5 inches long (depending on the joint and size of the dog), is carefully inserted into the joint space.
The needle is advanced until joint fluid is appreciated or until the needle “pops” through the joint capsule.

Fluid Aspiration:
Once the needle is in the joint space, a small syringe (typically 1 or 3 ml) is attached, and gentle negative pressure is applied to aspirate synovial fluid. Only a small amount (often just a few drops to 0.5-1.0 ml) is needed for diagnostic analysis.
It’s common to perform arthrocentesis on multiple joints, especially when immune-mediated polyarthritis is suspected.

Post-Procedure:
The needle is withdrawn, and gentle pressure is applied to the site for a few minutes to prevent bleeding.
The dog is monitored during recovery from sedation.

Sample Handling and Analysis:

Synovial fluid is typically collected into different tubes for various analyses:

EDTA Tube (Purple Top): For total nucleated cell count (TNCC) and differential cell count (cytology). EDTA prevents clotting.
Plain Tube (Red Top or Serum Separator): For total protein, mucin clot test, and sometimes culture if inadequate volume for a dedicated culture tube.
Culture Tube/Media: If septic arthritis is suspected, a sterile sample is submitted for bacterial culture and sensitivity testing.
Direct Smears: A drop of fluid can be directly smeared onto microscope slides for immediate microscopic examination.

Analysis of Synovial Fluid:

Gross Examination:
Volume: Small volume is normal.
Color: Normal is clear to pale yellow. Turbidity, redness (hemorrhage), or a deeper yellow can indicate disease.
Clarity: Normal is clear. Turbidity suggests inflammation or infection.
Viscosity: Normal fluid is highly viscous and forms a “string” when dropped, due to hyaluronic acid. Decreased viscosity indicates inflammation.

Cytology (Microscopic Examination):
Total Nucleated Cell Count (TNCC): Normal typically less than 3,000 cells/µL. Increased counts indicate inflammation.
Differential Cell Count:
Normal: Predominantly mononuclear cells (lymphocytes, monocytes, macrophages) with very few neutrophils (<5-10%).
Inflammation: Increased neutrophils. If neutrophils are over 12-15% of the total cells, it’s considered inflammatory.
Septic Arthritis: Neutrophils often >90%, with evidence of intracellular bacteria (phagocytized by neutrophils).
Non-Septic/Immune-Mediated: Neutrophils often >50-70%, but no bacteria are seen.
Other Cells: Eosinophils (rare), plasma cells, mast cells, or neoplastic cells can occasionally be found.

Total Protein:
Normal is low (<2.5 g/dL). Increased protein suggests inflammation, as proteins leak from blood into the joint space.

Mucin Clot Test:
Assesses the integrity of hyaluronic acid. Normal fluid forms a tight, ropy clot in acetic acid. A poor or friable clot indicates inflammation.

Culture and Sensitivity:
If infection is suspected based on cytology, the fluid is cultured to identify the specific bacteria and determine which antibiotics will be most effective.

Interpretation of Results:

Normal Fluid: Clear, pale yellow, highly viscous, low TNCC (<3000 cells/µL), predominantly mononuclear cells, low protein, good mucin clot.
Inflammatory Joint Disease:
Non-septic (e.g., Immune-Mediated Polyarthritis): Increased TNCC (often >5,000-100,000 cells/µL), high percentage of non-degenerate neutrophils (>50%), decreased viscosity, increased protein, poor mucin clot. No bacteria seen or cultured.
Septic Arthritis: Markedly increased TNCC (often >50,000-200,000 cells/µL), very high percentage of degenerate neutrophils (>90%), often with intracellular bacteria visible. Positive bacterial culture is definitive.
Degenerative Joint Disease (Osteoarthritis): Usually non-inflammatory or mildly inflammatory. Mildly increased TNCC (<5,000 cells/µL), mostly mononuclear cells, normal or mildly decreased viscosity.
Hemorrhagic Fluid: Reddish or bloody appearance. Can be due to a traumatic tap (needle hitting a blood vessel) or actual bleeding into the joint (e.g., coagulopathy, tumor).

Potential Complications:

Complications are rare but can include:

Pain: Transient pain or discomfort at the injection site.
Hemorrhage/Hematoma: Minor bleeding or bruising, usually self-limiting.
Iatrogenic Infection: Introduction of bacteria into the joint, resulting in septic arthritis. This is the most serious complication and highlights the absolute necessity of strict aseptic technique.
Damage to Articular Cartilage or Meniscus: Rare with proper technique.
Failure to Obtain Fluid (“Dry Tap”): Can occur if the joint is difficult to access, there’s minimal effusion, or the needle is not precisely in the joint space.

Arthrocentesis is a safe and highly informative diagnostic procedure that plays a crucial role in managing orthopedic and systemic diseases in dogs.

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