How should I prepare a single 50 mL peri‑articular injectate containing preservative‑free 0.25% bupivacaine and dexamethasone for an older adult undergoing primary total knee arthroplasty with no hepatic failure, no active thrombo‑embolism, and no known drug allergies?

Peri-articular Injectate Preparation for Total Knee Arthroplasty

For a single 50 mL peri-articular injection in an older adult undergoing primary TKA, prepare: 20 mL of preservative-free 0.25% bupivacaine (50 mg) + 8 mg dexamethasone + 30 mL preservative-free normal saline, administered as a single posteromedial capsular injection after implantation, with an additional 10 mL of 0.25% bupivacaine (25 mg) reserved for incisional infiltration before wound closure.

Rationale and Evidence-Based Approach

Bupivacaine Concentration and Volume

The 0.25% concentration is optimal for peri-articular infiltration in TKA because it provides effective sensory blockade without significant motor impairment 1. The FDA labeling confirms that 0.25% bupivacaine produces incomplete motor block, which is appropriate when muscle relaxation is not required and allows for early mobilization 1.

Total bupivacaine dose breakdown:

• 50 mg in the 50 mL peri-articular injection (20 mL of 0.25% bupivacaine)
• 25 mg for incisional infiltration (10 mL of 0.25% bupivacaine)
• Total: 75 mg bupivacaine - well below the maximum single dose of 175 mg without epinephrine 1

Recent evidence demonstrates that a single 50 mL posteromedial capsular injection provides broad distribution across the entire posterior knee capsule, covering approximately 80% of the capsular width without risking neurovascular injury 2. This is superior to multi-site injection techniques and eliminates the risk of popliteal vessel or nerve injury associated with posterolateral injections.

Dexamethasone Addition

Add 8 mg dexamethasone intravenously OR into the peri-articular injection. The 2021 PROSPECT guidelines for hip arthroplasty (transferable to knee) strongly recommend intravenous dexamethasone 8-10 mg for its analgesic and anti-emetic effects 3.

Recent high-quality evidence shows that:

• Single-dose intravenous dexamethasone (8 mg) provides superior analgesia and anti-emetic effects compared to divided dosing 4
• Peri-articular dexamethasone produces equivalent analgesic effects to intravenous administration in the first 24 hours without causing significant blood sugar elevation or wound complications 4
• Dexamethasone as an adjunct to bupivacaine significantly reduces morphine consumption and pain scores at 24-48 hours postoperatively 5

Clinical decision point: If the patient receives intraoperative IV dexamethasone for systemic effects, you may omit it from the peri-articular injection. If not given IV, include 8 mg dexamethasone in the 50 mL injectate.

Injection Technique

Timing and location are critical:

• Administer the 50 mL mixture as a single posteromedial capsular injection after implantation 6
• Reserve 10 mL of 0.25% bupivacaine for incisional infiltration before wound closure 6

This two-stage approach (peri-articular after implantation + incisional before closure) demonstrates superior efficacy compared to single-stage injection after implantation alone 6.

Preparation Steps

1. Draw up 20 mL of preservative-free 0.25% bupivacaine (50 mg total)
2. Add 8 mg dexamethasone (if not given IV)
3. Dilute to 50 mL total volume with preservative-free normal saline (add 30 mL saline, or 28 mL if dexamethasone volume is 2 mL)
4. Label clearly: "Peri-articular injection - 50 mL - Bupivacaine 50 mg + Dexamethasone 8 mg"
5. Prepare separately: 10 mL of 0.25% bupivacaine for incisional use

Important Caveats

Contraindications to avoid:

• Do NOT use for intra-articular infusion via catheter - this is associated with chondrolysis and is not FDA-approved 1
• Do NOT add epinephrine in older adults - increased cardiovascular risk without clear benefit in this population
• Ensure preservative-free formulations only - methylparaben preservatives can cause neurotoxicity with peri-articular use

Dosing considerations for older adults:

• The 75 mg total bupivacaine dose is conservative and appropriate for elderly patients 1
• Maximum safe dose is 175 mg without epinephrine, but doses should be reduced for elderly and debilitated patients 1
• Monitor for 3 hours post-injection for signs of local anesthetic systemic toxicity

Multimodal Analgesia Context

This peri-articular injection should be part of a comprehensive multimodal regimen including:

• Pre-operative or intra-operative paracetamol and NSAIDs/COX-2 inhibitors 3
• Consideration of adductor canal block if additional analgesia needed 7
• Opioids reserved as rescue analgesics only 3

The combination of peri-articular multimodal drug injection with systemic analgesics provides superior pain control compared to femoral nerve blocks while preserving quadriceps function and facilitating early mobilization 6.