Knee Joint Aspiration

Anatomy of anterolateral aspect of right knee.

Indications

General arthrocentesis indications

• Suspicion of septic arthritis
• Suspicion of crystal arthropathy
• Evaluation of therapeutic response for septic arthritis
• Unexplained arthritis with synovial effusion
• Evaluation of joint capsule integrity if overlying laceration
• Relative: therapeutic (decrease intra-articular pressure, injection of anaesthetics/steroids)

Contraindications

General arthrocentesis contraindications

• No absolute contraindications for diagnostic arthrocentesis
• Do not inject steroids into a joint that you suspect is already infected
• Relative Contraindications:
  • Overlying cellulitis
  • Coagulopathy
  • Joint prosthesis (refer to ortho)

Equipment Needed

General arthrocentesis equipment

• Betadine or Chlorhexadine
• Sterile gloves/drape
• Sterile gauze
• Lidocaine
• Syringes
  • Small syringe (6-12cc) for injection of local anesthetic
  • Large syringe (one 60cc or two 30cc) for aspiration
• Needles
  • 18 gauge: knee
  • 20 gauge: most other joints
  • 25 gauge: MTP joints
  • 27 gauge for anaesthetic injection
• Collection tubes (red top and purple for crystal analysis)
• Culture bottles
• Consider utilizing U/S to assess for effusion

Procedure

General Setup

• Prep area with betadine or chlorhexadine using circular motion moving away from joint x 3
• Drape joint in sterile fashion
• Inject lidocaine with 25-30ga needle superficially and then into deeper tissues
• Insert 18ga needle (for larger joints) into joint space while pulling back on syringe
• Stop once you aspirate fluid; aspirate as much fluid as possible
  • Send: cell count, culture, gram stain, crystal analysis

Specific Approach

• Can be entered medially or laterally to the patella, superior or inferior to patella
• Fully extend knee and ensure quadriceps muscle is relaxed (optionally bump with 20 deg of flexion)
  • Place your thumb on the patella and slide it over as you enter with needle
  • For Suprapatellar Approach 1 cm lateral and 1 cm superior
• Identify midpoint of patella; insert needle either 1 cm lateral or medial
• Direct needle posterior to patella and horizontally toward the joint space
• Compression or "milking" applied to both sides of joint space may facilitate aspiration

Evaluation

Arthrocentesis of synoval fluid

Synovium	Normal	Noninflammatory	Inflammatory	Septic
Clarity	Transparent	Transparent	Cloudy	Cloudy
Color	Clear	Yellow	Yellow	Yellow
WBC	<200	<200-2000	200-50,000	>1,100 (prosthetic joint) >25,000; LR=2.9 >50,000; LR=7.7 >100,000; LR=28
PMN	<25%	<25%	>50%	>64% (prosthetic joint) >90%
Culture	Neg	Neg	Neg	>50% positive
Lactate	<5.6 mmol/L	<5.6 mmol/L	<5.6 mmol/L	>5.6 mmol/L
LDH	<250	<250	<250	>250
Crystals	None	None	Multiple or none	None

• Viscosity of synovial fluid may actually be decreased in inflammatory or infectious etiologies, as hyaluronic acid concentrations decrease
• The presence of crystals does not rule out septic arthritis; however, the diagnosis is highly unlikely with synovial WBC < 50,000

Complications

General arthrocentesis complications

• Pain
• Infection
• Re-accumulation of effusion
• Damage to tendons, nerves, or blood vessels

External Links

• http://www.youtube.com/watch?v=fZ2dcZhoGP8 Arthrocentesis
• http://emprocedures.com/arthrocentesis/introduction.htm