Femoral, adductor canal, popliteal sciatic
Stem
At the ultrasound station, the standardized patient lies supine. The scenario card describes a 68-year-old woman, 74 kg, scheduled for total knee arthroplasty, who wants to walk with physical therapy on the day of surgery. The examiner will ask for one of the outline's three lower-extremity block targets, in a task of up to 2 minutes 30 seconds.
Question
Which block do you choose for her knee, and how do you find and identify whichever target I name: the femoral nerve, the saphenous nerve in the adductor canal or the sciatic nerve in the popliteal fossa?
Ideal answer
HeadlineAdductor canal for her knee: analgesia while sparing most quadriceps strength, so she mobilizes the same day.
- Linear 6–15 MHz, curvilinear for a deep sciatic. Linear for femoral and adductor canal; curvilinear for a deep popliteal sciatic in a large leg.
- Femoral: artery first at the inguinal crease. Nerve lateral, a flat hyperechoic triangle deep to the fascia iliaca, on the iliopsoas.
- Adductor canal: anteromedial mid-thigh. Where the medial borders of sartorius and adductor longus meet: femoral artery under the sartorius, saphenous nerve beside it.1
- Popliteal: tibial nerve at the crease, then proximal. Leg lifted or turned; tibial superficial to the popliteal vessels, traced up to the common peroneal junction.
- Freeze, name, state in-plane and the tip target each time, because that endpoint is scored.
Watch for: spread in the right compartment.
Breaking down the answer
- Three skills — the outline's three blocks2 teach a nerve beside an artery under a fascia, a sensory nerve in a muscular canal, and a deep nerve traced to its division. I show orientation, two structures and the tip target;3 the trade-off is graded.
| Block | Probe position | Identifying features | Covers | Trade-off |
|---|---|---|---|---|
| Femoral | Transverse, inguinal crease | Nerve lateral to the artery, under fascia iliaca, on iliopsoas; NAV lateral to medial | Anterior thigh, knee, femur, medial leg via saphenous | Quadriceps weakness, fall risk |
| Adductor canal | Transverse, anteromedial mid-thigh | Femoral artery deep to sartorius, saphenous nerve lateral then anterior to it, vastus medialis lateral, adductors posterior1 | Knee (partial), medial leg | Less quadriceps weakness; incomplete posterior knee |
| Popliteal sciatic | Transverse at the crease, then proximal | Tibial superficial and lateral to vessels; common peroneal joins from lateral; bifurcation ~5–10 cm above the crease | Below the knee except the medial strip | Slow onset if outside the paraneural sheath |
- Femoral under the fascia iliaca — the nerve is deep to it, the artery under the fascia lata, a different compartment; local anesthetic above the fascia iliaca fails however close it looks.
- Adductor canal over femoral — for knee arthroplasty it targets the purely sensory saphenous nerve and sensory branches of the nerve to vastus medialis, sparing most motor branches,1 so physical therapy starts on the day of surgery and fall risk drops.
- True canal, not femoral triangle — the canal begins at the apex of the femoral triangle, where the medial border of the sartorius crosses the medial border of the adductor longus, and ends at the adductor hiatus;1 higher, spread reaches motor branches and weakens the quadriceps.
- Popliteal at the bifurcation — tibial and common peroneal nerves share a paraneural sheath at the division; inside it, at or just above the split, 20–30 mL gives fast, complete onset.
- "See-saw" sign — dorsiflexion and plantarflexion move tibial and peroneal nerves in opposite directions on screen, confirming them in a poor image.
Follow-up question
The examiner asks for the popliteal sciatic. The vessels are visible, but the space beside them looks uniformly gray and you cannot find the nerve. What do you do?
Follow-up answer
HeadlineAn image problem, not an absent nerve; I say what I see.
More depth, then a caudal tilt to bring the nerve perpendicular to the beam, because anisotropy hides it at the wrong angle. She flexes the foot so I see the nerve move. Failing that, I trace up from the crease, where the tibial nerve is superficial to the popliteal vein, to the common peroneal junction. Short of time, I name the vessels and where the nerve should sit.
Pitfalls that cost points
- Injecting low at the crease: only the tibial nerve is blocked, and the peroneal side stays awake.
- Forgetting fall precautions after a femoral block.
Rapid-fire
Q: Why is the femoral nerve hyperechoic and flat? A: A peripheral nerve rich in connective tissue, compressed between the fascia iliaca and the iliopsoas.
Q: Why does an adductor canal block spare most quadriceps strength? A: It is distal to most motor branches of the femoral nerve; the saphenous nerve is purely sensory.1
Q: Why add a popliteal block to an adductor canal block for knee surgery? A: Sciatic branches supply the posterior capsule; an IPACK injection between the popliteal artery and the capsule is a foot-sparing alternative.
Q: Why does the sciatic bifurcate at a variable level? A: The division can occur anywhere from the pelvis to the popliteal fossa, so I trace rather than assume.
Q: After a femoral block for knee arthroplasty, the patient falls trying to stand. What do you do? A: Examine for injury, explain the predictable quadriceps weakness, knee immobilizer and fall precautions until it resolves; adductor canal was the better choice for early mobilization.
Numbers to know
| Item | Value |
|---|---|
| Femoral block volume | 15–20 mL |
| Adductor canal volume | 10–20 mL lateral to the artery1 |
| Popliteal sciatic volume | 20–30 mL |
| Sciatic bifurcation | ~5–10 cm above the crease |
| Ropivacaine maximum | ~3 mg/kg |
| Probe | Linear 6–15 MHz; curvilinear 2–5 MHz for deep sciatic |
Links
- Femoral, adductor canal and popliteal blocks, Local anesthetic systemic toxicity, Bone cement, tourniquets and hip fracture, The ultrasound application station: format and tasks
- ASRA Checklist for Treatment of Local Anesthetic Systemic Toxicity (2020); ASRA Practice Advisory on Neurologic Complications in Regional Anesthesia (2015)
Sources
- 1.Adductor Canal Block · OpenAnesthesia, 2025 · openanesthesia.org
- 2.APPLIED Exam Objective Structured Clinical Examination (OSCE) Content Outline · ABA · theaba.org
- 3.OSCE Example Scenarios (all stations) · ABA, 2023 · theaba.orgNerve block task template: probe orientation, two structures, needle-tip location for local anesthetic.
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.