Femoral, adductor canal and popliteal blocks
Stem
A 67-year-old woman, 74 kg, is having a primary total knee arthroplasty under spinal anesthesia in an enhanced-recovery program. The surgeon wants her walking with physical therapy on the afternoon of surgery and home the next day. She has osteoarthritis, hypertension and mild cognitive impairment, and she fell at home two months ago.
Question
What peripheral nerve block would you add for this knee replacement, and why that one?
Ideal answer
HeadlineAn adductor canal block, not femoral: analgesia without quadriceps weakness, because she must walk today and has already fallen once.
- Ultrasound-guided adductor canal block, targeting the saphenous nerve. It runs deep to sartorius with the femoral artery.1
- Not femoral, which weakens the quadriceps. It covers the anterior and medial thigh down to and including the knee;2 in a randomized trial in foot and ankle surgery, the canal block preserved strength better with no difference in pain.3
- Surgeon periarticular infiltration and multimodal analgesia, because the canal block misses the posterior capsule: scheduled acetaminophen, an NSAID if her kidneys allow, opioids only for breakthrough.
- Long-acting local within her weight-based maximum, needle tip scanned throughout, aspirated increments, for duration and to catch intravascular injection early.
- Physical therapy tests her quadriceps before she stands; she walks with assistance and a fall-prevention plan.
Breaking down the answer
- Femoral block when strength does not matter — femur, patella, quadriceps tendon and knee surgery, and hip fracture analgesia, with 10–15 mL lateral to the artery under fascia iliaca.2
- Proximal spread can still weaken — the saphenous nerve is a terminal branch of the femoral nerve, so blocking it spares motor function;1 higher volumes that spread proximally can still weaken the quadriceps, which is why strength is checked before walking.
- The trial evidence — the randomized comparison was in foot and ankle surgery, adductor canal against femoral block;3 the strength-sparing principle is what carries over to the knee.
- Popliteal sciatic for foot and ankle — the sciatic nerve divides into tibial and common peroneal nerves 5–10 cm above the crease, and 15–20 mL around the nerve within its sheath covers foot, ankle and Achilles surgery.4
- Remember the saphenous gap — the medial leg and foot belong to the saphenous nerve, so ankle surgery needs a saphenous block added to the popliteal block.4
Follow-up question
Six hours later the spinal has fully regressed, but when the therapist stands her up her knee buckles and she cannot do a straight-leg raise. Sensation over the anterior thigh is reduced. What do you do?
Follow-up answer
HeadlineQuadriceps weakness beyond what a saphenous block explains, most likely proximal spread to the femoral nerve. I stop mobilizing her and start fall precautions.
She gets a knee immobilizer, and I explain to her and the surgeon that the weakness should resolve as the block wears off. I examine both legs to confirm it is one-sided and confined to femoral territory; bilateral weakness or back pain after a spinal sends me to an urgent neuraxial evaluation instead. I adjust her analgesia so she is comfortable without sedation and re-examine her strength every few hours. If it does not recover on the expected timeline, I arrange neurology review and document the deficit. Discharge waits until she walks safely.
Pitfalls that cost points
- Assuming new weakness after a spinal is always the peripheral block.
- Saying the adductor canal block is purely sensory and then not testing strength before ambulation.
Rapid-fire
Q: Where is the femoral nerve relative to the femoral artery? A: Lateral to it and covered by the fascia iliaca.2
Q: Why can a femoral catheter dislodge? A: The inguinal area is mobile and the nerve is shallow.2
Q: Where is the popliteal vein relative to the artery? A: Just superficial (posterior) to it; the tibial nerve lies superficial and lateral to the vein.4
Q: What block would you offer an elderly patient with a hip fracture in the emergency department? A: A fascia iliaca block with 30 to 40 mL; it targets the femoral and lateral femoral cutaneous nerves and reduces pain and opioid use.5
Numbers to know
| Item | Value |
|---|---|
| Femoral block volume | 10–15 mL2 |
| Popliteal block volume | 15–20 mL4 |
| Sciatic division | 5–10 cm above the popliteal crease4 |
| Popliteal artery depth | About 3–4 cm4 |
| Fascia iliaca volume | 30–40 mL5 |
Links
- Femoral, adductor canal, popliteal sciatic, Local anesthetic systemic toxicity, PCA and multimodal analgesia, Spinal anesthesia: dosing, spread, hypotension
- Atlas: Femoral, adductor canal, popliteal sciatic
- ASRA Checklist for Treatment of Local Anesthetic Systemic Toxicity (2020)
Sources
- 1.Ultrasound-Guided Nerve Blocks of the Lower Limb (saphenous nerve section) · NYSORA · nysora.com
- 2.Ultrasound-Guided Femoral Nerve Block · NYSORA · nysora.com
- 3.Adductor canal block versus femoral nerve block combined with sciatic nerve block as an anesthetic technique for hindfoot and ankle surgery: a prospective, randomized noninferiority trial · Medicine (Baltimore), 2016 · pmc.ncbi.nlm.nih.gov60 patients; quadriceps strength higher with adductor canal block at 30 min and 2 h; pain noninferior.
- 4.Ultrasound-Guided Popliteal Sciatic Block · NYSORA · nysora.com
- 5.Fascia Iliaca Compartment Block: An Update (Anaesthesia Tutorial of the Week 489) · WFSA, 2023 · resources.wfsahq.org
Facts on this page were checked against these sources on Oct 15, 2026. Guidelines change; confirm doses against current institutional protocols.