Femoral, adductor canal, popliteal sciatic
Why examiners ask this
Lower-extremity blocks are the backbone of knee, ankle and hip analgesia, and the three in the outline (femoral, adductor canal, popliteal sciatic)1 teach three skills: a nerve beside an artery under a fascia, a sensory nerve inside a muscular canal, and a deep nerve that must be traced to its division. The examiner asks me to find each target on a standardized patient, state the probe orientation, name two structures, and show where the needle tip should deposit local anesthetic.2 It tests application of knowledge and judgment, especially the trade-off between analgesia and quadriceps weakness.
The 60-second answer
I use a linear probe at 6–15 MHz for the femoral and adductor canal and switch to a curvilinear probe for a deep popliteal sciatic in a large leg. For the femoral block I scan transverse at the inguinal crease, find the femoral artery, and identify the nerve lateral to it as a flat hyperechoic triangle deep to the fascia iliaca and on top of the iliopsoas. For the adductor canal I move to the anteromedial mid-thigh, where the medial borders of the sartorius and adductor longus meet, find the femoral artery under the sartorius, and identify the saphenous nerve beside it.3 For the popliteal sciatic I scan at the popliteal crease, find the tibial nerve superficial to the popliteal vessels, then slide proximally until the tibial and common peroneal nerves join. In each case I name the structures and state an in-plane approach with the target.
Decision points and the "why"
| Block | Probe position | Identifying features | Covers | Trade-off |
|---|---|---|---|---|
| Femoral | Transverse at the 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 posterior3 | Knee (partial) and 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 | Leg below the knee except the medial strip | Slow onset if outside the paraneural sheath |
- Femoral needle under the fascia iliaca — the nerve lies deep to it and the artery lies under the fascia lata, in a different compartment. Local anesthetic above the fascia iliaca fails no matter how close it looks.
- Adductor canal over femoral for knee arthroplasty — it preserves most quadriceps strength because it targets the purely sensory saphenous nerve and the sensory branches of the nerve to vastus medialis, sparing most motor branches.3 That lets the patient work with physical therapy on the day of surgery and lowers fall risk.
- True canal versus 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.3 Injecting higher, in the femoral triangle, spreads to motor branches and weakens the quadriceps.
- Popliteal injection at the bifurcation — the tibial and common peroneal nerves share a paraneural sheath at the division. Injecting inside it, at or just above the split, gives fast and complete onset with 20–30 mL.
- Find the sciatic with the "see-saw" sign — dorsiflexion and plantarflexion of the foot make the tibial and peroneal nerves move in opposite directions on screen, which confirms them when the image is poor.
The complication they will inject
On the popliteal task I cannot find the nerve. The vessels are visible but the space beside them looks uniformly gray. I say what I see, then fix it: I ask for more depth, tilt the probe caudally to bring the nerve perpendicular to the beam, because anisotropy makes it disappear at the wrong angle, and ask the patient to flex the foot to see the nerve move. If needed, I trace up from the crease, where the tibial nerve is superficial to the popliteal vein. The postoperative variant is the patient after knee arthroplasty with a femoral block who falls trying to stand. I examine for injury, explain that the block causes predictable quadriceps weakness, enforce a knee immobilizer and fall precautions until it resolves, and say that an adductor canal block was the better choice for early mobilization.
Pitfalls that cost points
- Injecting the femoral above the fascia iliaca.
- Calling the femoral triangle injection an adductor canal block.
- Injecting the sciatic far below the bifurcation and blocking only the tibial nerve.
- Forgetting fall precautions after a femoral block.
- Using a linear probe set too shallow for a deep sciatic in an obese thigh.
Rapid-why
Q: Why is the femoral nerve hyperechoic and flat? A: It is a peripheral nerve with abundant connective tissue lying 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.3
Q: Why add a popliteal block to an adductor canal block for knee surgery? A: The posterior knee capsule is innervated by sciatic branches; an IPACK injection between the popliteal artery and the capsule is an alternative that spares the foot.
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.
Numbers to know
| Item | Value |
|---|---|
| Femoral block volume | 15–20 mL |
| Adductor canal volume | 10–20 mL lateral to the artery3 |
| 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 (coming), Local anesthetic systemic toxicity, Bone cement, tourniquets and hip fracture (coming), 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.APPLIED Exam Objective Structured Clinical Examination (OSCE) Content Outline · ABA · theaba.org
- 2.OSCE Example Scenarios (all stations) · ABA, 2023 · theaba.orgNerve block task template: probe orientation, two structures, needle-tip location for local anesthetic.
- 3.Adductor Canal Block · OpenAnesthesia, 2025 · openanesthesia.org
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.