Outcome measures of peripheral nerve regeneration.

Animal models of nerve compression, crush, and transection injuries of peripheral nerves have been subject to extensive study in order to understand the mechanisms of injury and axon regeneration and to investigate methods to promote axon regeneration and improve functional outcomes following nerve injury. Six outcome measures of regenerative success including axon and neuron counts, muscle and motor unit contractile forces, and behavior are reviewed in the context of nerve injury types, crush, transection and nerve repair by direct coaptation, or transection and repair via a nerve graft or conduit. The measures are evaluated for sciatic, tibial, common peroneal, femoral, single nerve branches such as the soleus nerve, and facial nerves. Their validity is discussed in the context of study objectives and the nerve branch. The case is made that outcome measures of axon counts and muscle contractile forces may be valid during the early phases of axon regeneration when regenerating sprouts emerge asynchronously from the proximal nerve stump and regenerate towards their denervated targets. However, care must be taken especially when experimental interventions differentially affect how many neurons regenerate axons and the number of axons per neuron that sprout from the proximal nerve stumps. Examples of erroneous conclusions are given to illustrate the need for researchers to ensure that the appropriate outcome measures are used in the evaluation of the success of peripheral nerve regeneration.