In Sunderland's classification of nerve injuries, the distinction between Grade III and Grade IV injury is:

• A Grade III — endoneurial tubes disrupted, perineurium intact; Grade IV — perineurium also disrupted, epineurium intact ✓
• B Grade III — endoneurium intact; Grade IV — perineurium disrupted
• C Grade III — axonal disruption, myelin intact; Grade IV — myelin and axon both disrupted
• D Grade III — Wallerian degeneration distally only; Grade IV — retrograde degeneration to the cell body

Explanation

Sunderland's 5-grade classification: Grade I (neuropraxia) — conduction block, no axonal injury; Grade II (axonotmesis) — axonal disruption, endoneurial tube intact — good recovery; Grade III — endoneurial tubes disrupted but perineurium intact — incomplete recovery due to axonal misrouting within fascicle; Grade IV — perineurium also disrupted, epineurium intact — poor spontaneous recovery, usually needs surgery; Grade V (neurotmesis) — complete nerve transection. Seddon's classification maps: neuropraxia = Grade I, axonotmesis = Grades II–IV, neurotmesis = Grade V.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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