Lower Limb & Gait
Peroneal vs tibial, the gluteal nerves, and reading a gait
Lumbosacral plexus overview
Lumbar plexus (L1–L4). Gives the iliohypogastric, ilioinguinal, genitofemoral, lateral femoral cutaneous, femoral (L2–L4), and obturator (L2–L4) nerves. The small sensory branches matter clinically: the iliohypogastric and ilioinguinal nerves can be caught in lower abdominal incisions or hernia repair, and compression of the lateral femoral cutaneous nerve under the inguinal ligament (tight belts, obesity, pregnancy) causes meralgia paresthetica, a purely sensory burning of the lateral thigh.
Sacral plexus (L4–S4). Gives the sciatic (L4–S3), superior gluteal (L4–S1), inferior gluteal (L5–S2), and pudendal (S2–S4) nerves. The pudendal nerve supplies perineal sensation and the external sphincters and can be blocked near the ischial spine for labor analgesia.
Hip and thigh: femoral, obturator, and gluteal nerves
| Nerve (roots) | Muscles and action | Classic injury | Deficit |
|---|---|---|---|
| Femoral (L2–L4) | Iliopsoas (hip flexion), quadriceps (knee extension) | Pelvic fracture; iatrogenic in hernia or hip surgery | Weak knee extension, reduced patellar reflex, numb anterior thigh and medial leg (saphenous) |
| Obturator (L2–L4) | Adductor longus, brevis, magnus; gracilis | Anterior hip dislocation, pelvic surgery | Weak thigh adduction, numb medial thigh |
| Superior gluteal (L4–S1) | Gluteus medius and minimus, tensor fasciae latae (hip abduction, pelvic stability) | Misplaced intramuscular injection, hip surgery | Trendelenburg sign |
| Inferior gluteal (L5–S2) | Gluteus maximus (main hip extensor) | Posterior hip dislocation | Trouble rising from a chair, climbing stairs, jumping; no Trendelenburg sign |
Trendelenburg sign. The hip abductors of the stance leg hold the pelvis level while the other foot is off the ground. When they fail, the pelvis sags toward the unsupported side:
- Superior gluteal nerve lesion (L4–S1)Gluteus medius and minimus are weak.
- Stand on the affected legThe abductors cannot stabilize the pelvis.
- Contralateral hip drops"Sound side sags": a right-sided lesion drops the LEFT hip when standing on the right leg.
Gait. A unilateral lesion makes the patient lurch the trunk over the weak side with each step to keep the pelvis level; bilateral abductor weakness produces a waddling gait.
Gluteal injection safety: the superior gluteal nerve lies under the superomedial buttock and the sciatic nerve under the inferomedial buttock. Give injections in the upper-outer (superolateral) quadrant, or better the anterolateral hip, to spare both.
Sciatic, common peroneal, and tibial nerves
Sciatic nerve (L4–S3). Usually leaves the pelvis below the piriformis, supplies the hamstrings, and divides into the tibial and common peroneal nerves above the knee. It is injured by posterior hip dislocation or a misplaced gluteal injection; piriformis syndrome can compress it, and herniated lower lumbar discs (such as L5–S1) produce sciatica.
| Feature | Common peroneal (fibular) | Tibial |
|---|---|---|
| Actions | PED: Peroneal Everts and Dorsiflexes | TIP: Tibial Inverts and Plantarflexes |
| Classic injury | Fibular neck: leg crossing, tight cast, lateral knee trauma; the most commonly injured nerve of the lower limb | Knee dislocation (posterior), tarsal tunnel |
| Sign | Foot drop, high-steppage gait; foot rests plantarflexed and inverted | Cannot stand on tiptoes |
| Sensory loss | Dorsum of foot (and lateral leg) | Sole of foot |
Peroneal branches. The common peroneal nerve wraps around the fibular neck, where it is superficial and easily compressed, then splits in two. The deep peroneal nerve runs in the anterior compartment, dorsiflexes the foot via tibialis anterior, and supplies sensation to the first web space; anterior compartment syndrome threatens it. The superficial peroneal nerve runs in the lateral compartment, everts the foot via peroneus longus and brevis, and supplies most of the dorsum of the foot.
Tibial nerve. Runs in the posterior compartment behind the knee and passes behind the medial malleolus under the flexor retinaculum (tarsal tunnel). Tarsal tunnel syndrome causes medial ankle pain and numbness of the sole.
Localizing foot drop
Definition. Foot drop is weak dorsiflexion; the patient lifts the knee high and slaps the foot down. Common peroneal palsy at the fibular neck is the classic cause, but L5 radiculopathy and sciatic or lumbosacral plexus lesions can look similar.
| Finding | Common peroneal palsy | L5 radiculopathy |
|---|---|---|
| Dorsiflexion | Weak | Weak |
| Eversion | Weak | Weak |
| Inversion (tibialis posterior) | Spared (tibial nerve) | Weak |
| Hip abduction | Spared | Often weak (gluteus medius) |
| Back pain, radiating leg pain | Absent | Common |
Knee and ankle injuries
Unhappy triad. A blow to the lateral knee of a planted leg applies valgus stress and tears the ACL, MCL, and a meniscus. The medial meniscus is attached to the MCL, which is why it is classically included, although lateral meniscal tears are common with acute ACL injury.
- ACL (lateral femoral condyle to anterior tibia): resists anterior tibial translation; positive anterior drawer and Lachman tests
- PCL (medial femoral condyle to posterior tibia): positive posterior drawer
- MCL: laxity on valgus stress; LCL: laxity on varus stress
Ankle sprains. Most sprains are inversion injuries of the plantarflexed foot that tear the lateral ligaments, beginning with the anterior talofibular ligament (ATFL), the weakest and most commonly torn; the calcaneofibular ligament goes next. Eversion sprains are less common and injure the strong medial deltoid ligament, which may avulse the medial malleolus rather than tear.