Characterization of tests of functional recovery after median and ulnar nerve injury and repair in the rat forelimb

The majority of human peripheral nerve injuries occur in the upper limb but the majority of studies in the rat are performed in the hindlimb. The upper and lower limbs differ in dexterity and control by supraspinal systems, so an upper limb model is a better representation of the common form of human injury. The purpose of this study was to further develop a rat model involving lesions of the median and ulnar nerves. To produce different degrees of misdirection of axons following nerve repair, we studied nerve crush, cut and repair of the two nerves, and cut and repair with crossover. Assessment of functional recovery was performed using a battery of motor and sensory tests: the staircase test, which assesses skilled forepaw reaching; grip strength meter, which assesses grip strength; pawprint analysis, which assesses toe spread and print length; horizontal ladder, which assesses forepaw placement during skilled locomotion; modified Randall-Selitto device and electronic von Frey probes, which assess fine touch; and cold probes, which assess temperature sensation. All tests revealed deficits in forepaw function after nerve injury except the print length and modified Randall-Selitto device. The time course of functional recovery was observed over 15 weeks. The final degree of functional recovery achieved was related to the misdirection of axon regeneration. The tests that most clearly revealed the effects of axon misdirection on function were the skilled paw reaching and grip strength tests. The lesion model and functional tests that we have developed will be useful in testing therapeutic strategies for treating the consequences of inaccurate axon regeneration following peripheral nerve injury in humans.