Q: Our 16-year-old son has been practicing knife throwing. Although he has always been very careful with this activity, he ended up slicing a nerve near the elbow. The nerve was not completely severed so we are watching and waiting before considering surgery. How is this wait-and-see approach decided on and how long does it last?
A: Here are three main nerves near the elbow that can be affected: the median nerve, the ulnar nerve, and the radial nerve. Whether it's a sharp, high-energy injury (e.g., knife, saw blade, bullet) or a low-velocity injury (e.g., fracture, traction, crush), the rule of 18 guides treatment.
The rule of 18 says that motor recovery won't happen past 18 inches from the nerve injury after 18 months have passed. This is because nerves regenerate at about one inch per month. And motor endplates degenerate 18 months after nerve damage.
The motor endplate is the highly-excitable region of muscle fiber responsible for the start of action potentials (firing signals) across the muscle's surface, ultimately causing the muscle to contract. Without a nerve signal, the motor endplate remains silent. And over time, that silence translates into a breakdown of the endplate. If that happens, permanent silence occurs and loss of motor recovery.
Surgeons monitor patients carefully week-by-week and month-by-month for any signs of nerve regeneration and motor recovery. They use pinch and grip strength and sensory testing to measure change. A special test called Semmes-Weinstein monofilament exam measures the patient's ability to feel two points of touch on the skin. If no change or improvement is seen, then special neurodiagnostic tests can be done.
This is where the rule of 18 comes in handy. Rate of recovery is matched against the time of injury to determine whether surgery to repair or reconstruct the damaged nerve is needed. For example, the rule of 18 tells the surgeon that the farthest recovery will occur is 18 inches from the injury. Any loss of sensation or motor function further away than that will require a tendon or nerve transfer.
For every month surgery is delayed, one inch of restored motor function may be lost. In other words, say the repair is done six months after the injury. Motor recovery is possible up to 12 inches from the level of the injury.
When making the decision when to do surgery, the surgeon also takes into account the type of injury, the patient's age, and the nerve(s) involved. Your son's young age and the fact that he is probably still growing is in his favor. When your son goes in for his next appointment, go with him and ask the surgeon this question. He or she will be able to give you a more definitive answer based on recovery so far.
Reference: Andrew R. Tyser and Kenneth R. Means, Jr, MD. Nerve Injuries About the Elbow: Treatment Options. In Current Orthopaedic Practice. January/February 2012. Vol. 23. No. 1. Pp. 29-33.