A 52-year-old man from Delhi presents with inability to invert the foot and loss of sensation over the medial border of the sole. Which nerve injury best explains these findings, and what is the key distinguishing feature from a superficial peroneal nerve injury?

Explanation

## Tibial vs Superficial Peroneal Nerve Injury: Key Discriminator ### Clinical Presentation Analysis **Key Point:** The patient has: - **Loss of foot inversion** → tibialis posterior is paralyzed - **Sensory loss over medial border of the sole** → medial plantar nerve (branch of tibial nerve) is affected These findings are pathognomonic for **tibial nerve injury**. ### Comparative Anatomy and Function | Nerve | Motor Innervation | Sensory Distribution | Key Loss with Injury | |-------|-------------------|----------------------|----------------------| | **Tibial** | Posterior compartment: gastrocnemius, soleus, tibialis posterior, FDL, FHL | Medial + lateral plantar (sole) | Plantarflexion + **Inversion** | | **Superficial Peroneal (Fibular)** | Lateral compartment: fibularis longus & brevis | Dorsum of foot, lateral leg | **Eversion only** | | **Deep Peroneal (Fibular)** | Anterior compartment: tibialis anterior, EDL, EHL | First web space | Dorsiflexion only | ### Why Preserved Eversion Distinguishes Tibial from Superficial Peroneal Injury **High-Yield:** The single best discriminating feature between tibial nerve injury and superficial peroneal nerve injury is **preserved eversion**: 1. **Tibial nerve injury** → Eversion is **PRESERVED** (fibularis longus & brevis, innervated by superficial peroneal nerve, are unaffected) 2. **Superficial peroneal nerve injury** → Eversion is **LOST** (fibularis longus & brevis are paralyzed) **Clinical Pearl:** In tibial nerve injury, the patient loses inversion (tibialis posterior paralyzed) and plantarflexion (gastrocnemius/soleus paralyzed), but can still evert the foot normally. In superficial peroneal nerve injury, the patient loses eversion but retains inversion and plantarflexion. Therefore, **preserved eversion** is the key feature that distinguishes tibial nerve injury from superficial peroneal nerve injury. ### Why Option B (Loss of Plantarflexion) Is Incorrect as the "Key Distinguishing Feature" While tibial nerve injury does cause loss of plantarflexion, this is not the **best distinguishing feature** from superficial peroneal nerve injury in the context of this question. The question asks what distinguishes the two injuries from each other. Superficial peroneal nerve injury does not affect plantarflexion (posterior compartment is intact), so plantarflexion loss is indeed unique to tibial injury — however, the more clinically precise and anatomically direct distinguishing feature is **preserved eversion** in tibial injury versus **lost eversion** in superficial peroneal injury. This directly contrasts the two nerves' motor territories (lateral compartment evertors vs. posterior compartment invertors/plantarflexors). ### Anatomical Basis ```mermaid flowchart TD A[Sciatic Nerve]:::outcome --> B{Bifurcation at popliteal fossa}:::decision B -->|Medial| C[Tibial Nerve]:::action B -->|Lateral| D[Common Peroneal Nerve]:::action C --> C1[Posterior compartment muscles]:::action C --> C2[Plantarflexion + Inversion LOST]:::outcome C --> C3[Eversion PRESERVED]:::outcome D --> D1[Superficial Peroneal]:::action D1 --> D2[Lateral compartment: Eversion LOST]:::outcome D --> D3[Deep Peroneal]:::action D3 --> D4[Anterior compartment: Dorsiflexion LOST]:::outcome ``` **Key Point:** Preserved eversion is the hallmark distinguishing feature of tibial nerve injury from superficial peroneal nerve injury, because the evertors (fibularis longus and brevis) are innervated by the superficial peroneal nerve and remain intact in tibial nerve injury. [cite: Clinically Oriented Anatomy, Moore et al., 8th edition, Chapter 6; Gray's Anatomy for Students, 3rd edition]