## Clinical Presentation Analysis The patient presents with classic features of L5 nerve root compression: - **Sensory distribution:** Dorsum of foot and lateral leg = L5 dermatome - **Motor deficit:** Ankle dorsiflexion weakness = L5 myotome (tibialis anterior innervation) - **Pain pattern:** Radiates from lower back to lateral leg and foot ## Why L4–L5 Herniation? **Key Point:** When a disc herniates, it compresses the nerve root that exits *below* that disc level. An L4–L5 disc herniation compresses the L5 nerve root as it exits the L4–L5 foramen. **High-Yield:** L5 nerve root compression is the **most common radiculopathy** in clinical practice, accounting for ~45% of all lumbar radiculopathies. L4–L5 is the most frequent site of herniation because: 1. Maximum mobility and stress concentration at lumbosacral junction 2. Highest intradiscal pressure at this level 3. Posterior longitudinal ligament is weakest here ## Dermatome and Myotome Correlation | Nerve Root | Dermatome | Motor (Myotome) | Reflex | |---|---|---|---| | **L5** | Dorsum of foot, lateral leg, lateral thigh | Ankle dorsiflexion (tibialis anterior), hip abduction (gluteus medius) | None specific | | L4 | Medial leg, medial foot, anterior thigh | Knee extension (quadriceps), ankle dorsiflexion | Patellar | | S1 | Sole of foot, posterior leg, lateral foot | Ankle plantarflexion, hip extension | Achilles | **Clinical Pearl:** The L5 dermatome does NOT include the sole of the foot (that is S1); this distinction helps differentiate L5 from S1 radiculopathy. ## Epidemiology **High-Yield:** Lumbar disc herniation frequency by level: - L4–L5: ~45% (most common) - L5–S1: ~40% - L3–L4: ~10% - L2–L3: ~5% [cite:Robbins 10e Ch 28]
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