In deciding when to repair a peripheral nerve injury , the neurosurgeon must define:

(1) when the time for useful recovery by spontaneous regeneration has passed

(2) the elapsed time when a nerve repair has little to offer.

24-month rule: when the duration of total muscle denervation exceeds 24 months (“24 month rule”), most muscles are subject to relatively severe time limitations for the return of useful function. This is less likely to be so for large bulky muscles, such as biceps and gastrocnemius–soleus, than for smaller muscles, such as those of the forearm and hand. An exception to this guideline is the facial muscles which, although relatively small, may benefit from late reinnervation by facial nerve repair or neurotization procedures.

Other exceptions to the “24-month rule” may occur in a few lesions that have maintained some nerve fiber continuity. If some fibers traverse the lesion, even though their number is insufficient to produce useful function distally, they may promote distal stump architecture preservation. Very late repair after resection of the lesion in continuity can occasionally produce function ¹⁾