Treatment of Nerve Injuries Follows the "Rule of 18"Nerve injuries around the elbow are the focus of this review article. There are three main nerves 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.
Bone fractures of the forearm are most likely to cut into the radial nerve. Sometimes surgery to repair the fracture is actually the cause of the nerve injury (e.g., a pin or screw used to hold the bone fragments together pierce the nerve).
The ulnar nerve is close to the surface of the skin and most likely to be damaged due to direct injury. This is the nerve that causes pain when you hit your "funny bone" (the bony bump of the elbow closest to the body). Damage to this nerve often requires a procedure called nerve transposition. The surgeon has to move the nerve away from the bone where it is being bumped or compressed.
Thumb function depends on the median nerve. Surgery to perform a nerve or tendon transfer is advised if the median nerve has been damaged high up by the elbow or if the injury is severe enough to compromise thumb movement. The median nerve also provides sensation to most of the hand so full recovery is not complete until 18 months at the earliest. Continued sensory recovery can take two years or more after nerve repair.
Any one of these nerves can be sewn back together if it is damaged by a clean laceration (cut) through the nerve. This is called an end-to-end repair. Nerve grafting is more likely when there has been a crush injury to the nerve. The area crushed is removed and a piece of donor nerve is used to replace the crushed portion.
Nerve transfers are used when it's clear that the sensory and/or motor function to the hand isn't going to be restored. Certain extra branches of nerves can be separated and divided to be used in place of the lost nerve without losing motor function at the site of harvest.
Some surgeons suggest early surgical treatment is better. Instead of waiting to see what kind of recovery occurs spontaneously, the surgeon performs a tendon transfer to improve hand function. Maintaining hand movement during the period between injury and recovery may yield better results compared with the wait-and-see (sometimes until it is too late) approach.
The authors conclude by saying that planning treatment for nerve injuries can be a complex and challenging process. The surgeon must take into consideration which nerve was injured, the location and severity of the lesion, the patient's age, and the rule of 18. They agree that tendon or nerve transfers should be done sooner than later if the surgeon's assessment is that recovery will be significantly delayed or not occur at all.
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.
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