How is a steroid (corticosteroid) knee injection absorbed systemically?

Systemic Absorption of Corticosteroid Knee Injections

Intra-articular corticosteroid knee injections are systemically absorbed primarily through diffusion from the joint space into the systemic circulation, with peak absorption typically occurring within 24-48 hours post-injection and effects potentially lasting 1-4 weeks. 1, 2

Mechanism of Systemic Absorption

• Corticosteroids injected into the knee joint are absorbed into the bloodstream through the synovial membrane's rich vascular network
• After injection, the medication remains primarily in the joint space but gradually diffuses into surrounding tissues and blood vessels 1
• The rate and extent of systemic absorption depends on several factors:
  • Type of corticosteroid preparation (solubility)
  • Dose administered
  • Joint size (knee being a large joint allows for significant absorption)
  • Presence of inflammation (increased vascularity enhances absorption)
  • Individual patient factors 2, 3

Pharmacokinetics of Systemic Absorption

• Triamcinolone acetonide (commonly used in knee injections) shows measurable systemic effects within 24 hours of injection 1
• Peak systemic levels typically occur at 24-48 hours post-injection 2
• Duration of systemic effects varies:
  • Adrenal suppression may last 1-4 weeks, sometimes longer 2
  • Duration correlates with the extended therapeutic action of the drug 1
• Recovery to baseline cortisol levels depends on:
  • Type of corticosteroid used (methylprednisolone vs. others)
  • Dose administered
  • Number of joints injected simultaneously 2, 3

Measurable Systemic Effects

1. HPA Axis Suppression:

   • Serum cortisol decreases within hours of injection
   • Nadir typically occurs at 24-48 hours post-injection
   • Recovery to baseline takes 1-4 weeks, sometimes longer 2, 4
   • Blunted cortisol response to ACTH stimulation may occur, particularly with certain preparations 2

2. Metabolic Effects:

   • Transient hyperglycemia in diabetic patients
   • Blood glucose peaks around 300 mg/dL
   • Effect typically lasts several days 2, 3

3. Anti-inflammatory Effects:

   • Reduction in inflammatory markers (CRP, ESR)
   • Begins within days of injection
   • May persist for months
   • Immediate decrease in inflammatory cytokines (within hours) 2

4. Other Potential Systemic Effects:

   • Sodium retention and potassium loss
   • Increased risk of infection
   • Bone density reduction with repeated injections
   • Cushing syndrome with frequent injections 3, 5

Clinical Implications

• The American College of Rheumatology recommends intra-articular corticosteroid injections for acute exacerbation of knee pain, especially with effusion 6
• EULAR guidelines indicate that diabetic patients should be informed about the risk of transient hyperglycemia following intra-articular glucocorticoid injections 6
• Patients should monitor glucose levels particularly from the first to third day post-injection 6
• Systemic absorption should be considered when calculating total glucocorticoid exposure, especially in patients receiving multiple injections or concurrent oral steroids 3, 5
• Special caution is warranted in high-risk populations:
  • Postmenopausal women (osteoporosis risk)
  • Diabetic patients (glycemic control)
  • Patients scheduled for surgery
  • Patients with cardiovascular or renal conditions 1, 3

Minimizing Systemic Effects

• Use the lowest effective dose of corticosteroid 1
• Consider joint rest for 24 hours following injection (though complete immobilization is discouraged) 6
• Space injections appropriately (generally no more than 3-4 per year in the same joint) 7
• Monitor for systemic effects in high-risk patients 3

Understanding the systemic absorption of intra-articular corticosteroids is essential for appropriate patient counseling and risk assessment, particularly in patients with comorbidities that may be affected by systemic corticosteroid exposure.