Back of Thigh: Facts to Remember and Clinicoanatomical Problems

Topic Overview

Facts to Remember

• The sciatic nerve is the thickest nerve in the body.

• The “sleeping foot” is a temporary condition due to compression of the sciatic nerve during prolonged sitting.

• Hamstrings act as flexors of the knee and weak extensors of the hip.

• Adductor magnus is a hybrid muscle—partly supplied by the obturator nerve and partly by the tibial component of the sciatic nerve.

• The thin artery accompanying the sciatic nerve represents a remnant of the axial artery of the lower limb.

Clinicoanatomical Problem

A 26-year-old woman presents with severe pain in the back of her right thigh and leg.

• Question 1: Which nerve is likely involved, and what is its root value?
  → The sciatic nerve, with root values L4 to S3, is affected.

• Question 2: What is the Straight Leg Raising (SLR) Test, and how is it interpreted?
  → In this test, the patient lies supine, and the affected leg is raised with the knee extended.
  → Pain radiating down the posterior thigh and leg when the leg is raised between 30°–70° indicates sciatic nerve irritation, commonly due to lumbar intervertebral disc herniation compressing L5 or S1 nerve roots.

• Question 3: Why is this pain called sciatica?
  → Because it radiates along the distribution of the sciatic nerve, from the gluteal region to the foot.

• Question 4: What are common causes of sciatica?
  → Herniated disc, piriformis syndrome, nerve compression, or pelvic tumor impinging on the sciatic nerve.

Clinicoanatomical Problems — Back of Thigh

1. Hamstring Strain (“Pulled Hamstring”)
A 22-year-old sprinter suddenly feels a sharp pain at the back of his thigh during a 100-meter sprint.

• Anatomical Basis: Hamstring muscles (especially biceps femoris long head) are overstretched when hip is flexed and knee extended.

• Common Site: At or near the ischial tuberosity where tendons originate.

• Clinical Signs: Pain, bruising, tenderness, and difficulty in extending the hip or flexing the knee.

• Rehabilitation: Rest, ice, compression, and physiotherapy; severe cases may need surgical repair.

2. Avulsion Fracture of the Ischial Tuberosity
A gymnast complains of pain in the lower buttock after forceful kicking.

• Cause: Violent contraction of hamstrings detaches the ischial tuberosity apophysis (in adolescents).

• Finding: Pain on resisted knee flexion and tenderness over the ischial region.

• Mechanism: Sudden hip flexion with knee extension (e.g., hurdling).

3. Semimembranosus Bursitis
An elderly patient presents with swelling in the upper medial part of the popliteal fossa.

• Cause: Inflammation of bursa between semimembranosus tendon and medial head of gastrocnemius.

• Clinical Note: Swelling becomes prominent on extending the knee — can mimic a Baker’s cyst.

4. Baker’s (Popliteal) Cyst due to Hamstring Dysfunction
Chronic knee arthritis causes fluid to collect in the semimembranosus bursa.

• Pathophysiology: Communication between the knee joint and the bursa allows synovial fluid to herniate posteriorly.

• Relevance: Patients may report tightness or lump in the popliteal fossa when extending the knee.

5. Entrapment of Sciatic Nerve — Piriformis Syndrome
A patient complains of deep buttock pain radiating down the thigh after prolonged sitting.

• Mechanism: Compression or irritation of the sciatic nerve by the piriformis muscle as it passes below or through it.

• Symptoms: Radiating pain, numbness, tingling, or weakness in posterior thigh and leg.

• Diagnosis: Pain worsens on hip internal rotation or flexion (Fair test).

6. Sciatic Nerve Injury due to Faulty Intramuscular Injection
A nurse gives a gluteal IM injection in the lower medial quadrant, leading to foot weakness.

• Mechanism: Injury to sciatic nerve branches in gluteal region.

• Clinical Features: Loss of knee flexion (hamstrings), paralysis below the knee, and foot drop.

• Prevention: Injections should be given in upper outer quadrant of gluteal region.

7. Compression Neuropathy — “Wallet Sciatica”
A man who keeps a thick wallet in his back pocket reports numbness in the posterior thigh and calf.

• Cause: Prolonged pressure on the sciatic nerve while sitting.

• Finding: Local tenderness over the nerve exit point below piriformis.

• Solution: Removing the wallet or sitting posture modification.

8. Posterior Thigh Pain from Lumbar Disc Herniation
A 40-year-old office worker complains of pain radiating from buttock to foot.

• Mechanism: Herniation of L4–L5 or L5–S1 disc compressing sciatic nerve roots.

• Clinical Test: Straight leg raising test positive.

• Finding: Decreased ankle reflex, sensory loss on lateral leg and foot.

9. Sciatic Nerve Entrapment During Hamstring Repair Surgery
A surgeon operating near ischial tuberosity inadvertently stretches the sciatic nerve.

• Result: Postoperative weakness in knee flexion and dorsiflexion.

• Lesson: The sciatic nerve runs midway between the ischial tuberosity and greater trochanter — should be avoided in posterior incisions.

10. Referred Pain from the Hamstrings to the Knee Joint
Patients with hamstring strain may complain of knee discomfort.

• Explanation: The tibial part of sciatic nerve supplies both hamstrings and the knee joint, producing referred pain along shared sensory pathways.

11. Hamstring Weakness in Stroke or Nerve Lesion
A patient with upper motor neuron lesion exhibits difficulty rising from sitting or climbing stairs.

• Reason: Weakness of hip extensors (hamstrings) and gluteus maximus.

• Compensation: Uses hands to push up from seated position.

12. Fibrotic Myopathy of Biceps Femoris (Rider’s Strain)
Seen in horse riders or cyclists due to repeated minor trauma.

• Finding: Hard tender band in posterior thigh.

• Treatment: Rest and physiotherapy.