Nerve and tendon lacerations (i.e. deep cuts) of the foot and ankle region are a relatively common injury. Treating it appropriately is important to maintain long-term sensation and motor functions of the injured foot and limb.

The Anatomy of the Foot

Five nerves supply sensation and motor function to the foot:

  • The Saphenous nerve supplies sensation to the inner or medial aspect of the ankle. 
  • The multiple branches of the superficial peroneal nerve run down the knee anterior aspect or front of the leg. They supply sensation to the majority of the dorsum (top) of the foot. 
  • The deep peroneal nerve runs down the anterior aspect of the leg as well. It supplies sensation to the web space in between the big toe and second toe. A small branch of the deep peroneal nerve also supplies motor power to the small extensor digitorum brevis muscle on top of the foot. 
  • The sural nerve runs down the back of the leg, around the lateral (outer) aspect of the ankle. It supplies sensation to the lateral portion of the foot and fifth toe. 
  • The posterior tibial nerve runs down the back (posterior) aspect of the leg, around the medial (inner) side of the ankle. It then splits into the medial plantar nerve and lateral plantar nerve before running around to the bottom (plantar) aspect of the foot. The medial plantar nerve supplies sensation to the big toe, second and third toe and motor function to the small intrinsic muscles of the big toe. The lateral plantar nerve supplies sensation to the fourth and fifth toe, as well as motor function to the small intrinsic muscles of the fifth toe. 

Each of these nerves is composed of various sensory and/or motor nerve fibers, and are covered by an outer protective tissue layer noted as perineurium.

The Symptoms

Following a nerve laceration, the distal portion (end towards the toes) of the nerve begins to degenerate in a process called Wallerian degeneration. The nerve ends may also retract from each other in cases of a complete laceration. Scar tissue then forms at both ends. 

In motor nerves, the motor endplate (the attachment of the nerve to the muscle) may also degenerate after a period of time. 

All of these factors can work against the complete return of nerve function after repair. 

The Types of Lacerations

There are many types of lacerations that can contribute to the treatment needs of a patient:

  • Dirty, jagged, ripping injuries (such as a chainsaw-related laceration) will damage a far bigger segment of the nerve then a clean sharp laceration from glass or a sharp knife. 
  • A tearing or avulsion injury (in which a segment of a nerve is torn) damages a far bigger section then a simple laceration. 
  • The condition of the tissue around the nerve (nerve bed) and any contamination of the area also affects nerve healing. 
  • Finally, the patient's age and overall health are important. Several studies have shown that nerve repair results over the age of 50 do not compare to nerve repairs of younger adults. Additionally, smoking and nicotine impair microcirculation which affects the results of nerve repair, as do malnutrition and alcoholism.

The Exam(s), & Evaluation

  • Treatment of nerve lacerations in the foot requires a detailed, complete assessment of nerve function and overall foot function.
  • Additionally, cases of partial sensation loss may require sensory mapping with two-point discrimination (a tool to determine the amount of sensation that remains) in order to determine whether any protective sensation is still present in the foot. 
  • Electrodiagnostic studies may also be required in chronic cases of lost sensation to determine exactly which muscles are affected by a partial nerve laceration.

These, other diagnostic studies (which are prescribed as needed), and a very detailed examination the injured foot will all help to provide a complete picture of the functional loss caused by any foot-based nerve laceration.

The original level of injury is also important for doctors to consider. For example, lacerations that injure small and/or mixed nerves (i.e. or both sensory and motor nerve branches) will be more difficult to treat. 

Ultimately, all assessments must be coupled with a decision on how much functional loss can be well tolerated in order to create a plan to restore as much function as possible. For example, numbness can sometimes be tolerated on the inner aspect of several toes or on top of the foot. It usually cannot be tolerated on the bottom of the foot due to the risk of burns or injury. However, numbness is better tolerated than pain and hypersensitivity. Additionally, partial weakness, motor loss or lack of coordination are better tolerated in some patients than others (ex. pianist, athlete).

The Treatment

Early/Immediate Care

Early treatment of lacerations usually consists of:

  • Cleaning or debriding the surrounding area of laceration.
  • Repairing any other structures that were damaged.
  • Repairing the nerves directly with microsurgical techniques. 

Early treatment typically takes place either immediately after an injury or approximately 2 weeks later. In cases treated later, the wounds have been cleaned and the skin repaired by the emergency room. The results in such cases are approximately the same.

In this surgery listed above, an operating microscope - or loupe magnification – is used to trim the nerve ends carefully and repair the nerve with several sutures that are smaller than a human hair. If a large segment of nerve has been damaged and needs to be removed, or is actually missing, a nerve graft (section of matching nerve taken from another donor area) may be used to bridge the gap between the nerve ends. 

A different, newer surgical technique involves using a neural tube - a small tube of collagen that surrounds both ends of the nerve repair. This allows the nerve to heal down the tube, bridging the gap. 

Both surgical techniques can allow function to return when there has been a missing segment of nerve or damaged nerve. The results are not as good as a direct, primary repair of the nerve ends.

Delayed/Long-Term Care

For delayed repairs of the nerve laceration:

  • The ends of both nerves must be found in the scar tissue of an already healed wound.
  • The nerves must be “freed up” and the ends must be trimmed back to the viable sections of the nerve. 
  • Often the nerve ends have retracted, and cannot be reapproximated. In these cases, a nerve graft or nerve tube are often utilized to complete treatment. 
  • In cases where a partial nerve laceration has healed (and is producing a painful neuroma and painful sensation known as dysesthesia), there is still partial nerve function. To address this situation, a decision is made at the time of surgery whether to completely divide the nerve; to take out the section of scar tissue and repair it; and/or to simply take out a wedge of diseased nerve tissue and repair that, leaving the more normal appearing nerve intact. 
  • Finally, in chronic cases with chronic weakness or deformity (and where nerve repair is not felt to be possible), other surgical procedures (such as a microvascular nerve transfer, tendon transfers or joint fusions) are utilized to restore function. 

As you can see, each nerve laceration case and patient requirements are individual and unique, requiring a different approach.

All nerve repair surgeries are usually performed under general anesthesia to avoid any patient motion, which is important while operating under magnification. Afterward, the patient is often placed in a protective boot to avoid any tension on the repaired nerve. There is no weight-bearing allowed on the foot at that time. 

The actual nerve repair site typically takes 4 to 6 weeks to heal. Mobilization of the foot is then begun to regain motion. Return to work and normal activities are usually allowed after the nerve repair site is healed and function and strength are within acceptable limits.

Approximately 2 weeks after nerve repair surgery, the nerve starts the slow process of regeneration at about the rate of an inch a month. This process is often faster in younger, healthier patients. Regeneration is also faster in primary nerve repairs than with nerve grafts.

It ultimately may be months before the return of sensation is noted. Return of muscle function often takes longer. Additionally, it is important to remember that despite our best efforts and advanced microsurgical technique, all nerve function does not return in every patient.