Peripheral nerves like those found in the foot can be injured by many mechanisms--blunt force to the nerve, chronic low-grade pressure from weight bearing or surrounding tissue swelling, lacerations, or excessive stretching, for example.

The nerve injury is most commonly classified in one of two ways. 

Sutherland Classification of Nerve Injury

• First Degree: Injury with only local changes to the nerve sheath (myelin).

• Second Degree: Incomplete injury to the nerve axons (the functional unit of the nerve).  Nerve itself is still intact.  

• Third Degree:  Severe axonal injury with scar tissue.  Nerve itself may be injured, but is still intact.  

• Fourth Degree: Complete disruption of axon.  Nerve itself may be severely injured, but is still intact.

• Fifth Degree:  Complete transection of the nerve.

Seddon's Classification of Nerve Injury 

This probably the most well-known classification system.  

• Neuropraxia  --an injury to the nerve covering (called the myelin sheath), but not the nerve, by trauma or compression, which causes blockage of the nerve signals.  Larger nerves covered by greater amounts of myelin are most susceptible to this injury.  Reflexes, muscle function, vibratory and two-point discrimination are typically lost, while pain, temperature and autonomic function (sweating and circulatory regulation controlled by nerves) is typically preserved.  Repair may take days to months and healing is usually perfect as only the sheath need be repaired. This corresponds to Sutherland's 1st Degree injury.

• Axonotmesis  --involves disruption of the nerve itself, but the surrounding and supportive nerve myelin sheath is not affected.  Causes include long or severe periods of compression, pulling or loss of blood flow to the nerve.  All nerve types may be affected.  Nerve fibre healing occurs at a rate of 1mm per day from the point of injury.  Recovery is usually good, though the further from the spine the injury occurs, the better the prognosis.  This corresponds to a 2nd or 3rd degree injury in Sutherland's system.   

• Neurotmesis  --a disruption of both the nerve fibre and the supportive myelin sheath. This injury may be seen with severe trauma such as lacerations, gunshot wounds, open bone fractures, punctures, and exposure to toxins.   Healing is usually poor without surgical repair. This corresponds to a 4th or 5th degree injury in Sutherland's system.