What is the mechanism of action for knee steroid injections?

Mechanism of Action for Knee Steroid Injections

Intra-articular corticosteroid injections work primarily by reducing inflammation through inhibition of inflammatory mediators, providing short-term pain relief and improved function in knee osteoarthritis. 1

Anti-inflammatory Effects

• Direct anti-inflammatory action: Corticosteroids suppress the inflammatory cascade by:

  • Inhibiting phospholipase A2, reducing production of arachidonic acid and subsequent inflammatory mediators
  • Decreasing production of pro-inflammatory cytokines (IL-1, TNF-α)
  • Reducing prostaglandin and leukotriene synthesis
  • Stabilizing lysosomal membranes to prevent release of destructive enzymes

• Synovial effects: Corticosteroids reduce synovial inflammation, which is particularly beneficial in cases with effusion 1

  • Decreases synovial hyperplasia
  • Reduces synovial fluid volume in effusions
  • Diminishes synovial vascularity and capillary permeability

Pain Reduction Mechanism

• Direct pain modulation: Corticosteroids reduce pain through:
  • Decreasing sensitization of nociceptive nerve endings in the joint
  • Reducing pressure within the joint capsule by decreasing effusion
  • Inhibiting substance P and other pain mediators

Clinical Efficacy Timeline

• Onset and duration: Evidence shows that intra-articular corticosteroids provide:
  • Rapid onset of action (within days)
  • Peak effect at 1-4 weeks
  • Short-lived benefit typically lasting 1-4 weeks 1, 2
  • Limited long-term efficacy beyond 24 weeks 1

Dosing Considerations

• Recent evidence suggests that lower doses (10mg triamcinolone acetonide) may be non-inferior to higher doses (40mg) for pain relief, potentially reducing cartilage volume loss concerns 3

Limitations and Caveats

• Temporary effect: Corticosteroids provide symptomatic relief but do not modify disease progression 1

• Potential risks with repeated use:

  • Potential cartilage volume reduction with frequent injections 1
  • Possible increased risk of requiring knee arthroplasty with repeated injections 4
  • Concerns about accelerating osteoarthritis progression with multiple injections

• Accuracy considerations: Up to 20% of knee injections may not achieve accurate intra-articular placement, affecting efficacy 5

Clinical Application

• Most appropriate for acute flares of knee pain, especially when accompanied by effusion 1, 2
• Should be limited in frequency (generally no more than 3-4 injections per year in the same joint) 2
• Consider avoiding in the 3 months preceding joint replacement surgery 1

While corticosteroid injections provide effective short-term relief for knee osteoarthritis pain, their mechanism focuses on symptom management rather than disease modification, making them most appropriate for acute pain flares rather than long-term management.