Dental nerve regeneration in rats. II. Autoradiographic studies of nerve regeneration to molar pulp and dentin

The autoradiographic technique was used to analyze the degeneration and regeneration of sensory nerves to rat molars and gingiva following cut or crush injury to the right inferior alveolar nerve. At 2 days after nerve injury there was almost complete denervation of the first molar, partial denervation of the second molar, and minimal effect on the innervation to the third molar and gingiva. The degree of sensory deficit and recovery for these same rats had been previously determined. Reinnervation of the first molar was analyzed in terms of axon number and location, intensity of axon labeling, and type of nerve injury. At 6 days, neither the cut injury nor crush injury rats had any reinnervation of their first molars. By 7 days, 3 of 4 rats had axons reinnervating first molars; in those teeth there was approximately one-fourth of the normal number of axons in the pulp, and very few axons in the dentin. These rats still had as large a molar sensory deficit as the 7 day rat and 6 day rats that had no reinnervation. By 3 weeks there were one-half to three-fourths of the normal axon numbers in the pulp, one-fourth to one-half of the normal axon numbers in dentin; and sensitivity was at least half-recovered. By 6 weeks, numbers of axons in the pulp and dentin were either normal or slightly less than normal; axons had grown back into dentin to the same depth as in normal teeth; and complete recovery of sensitivity had occurred. The regenerating axons had greater than normal labeling intensity at 1 week and 3 weeks in all rats. Those with the crush nerve injury had somewhat greater numbers of reinnervating axons at 1 week and 3 weeks than the cut injury rats. A structure-function comparison for the molars showed that return of sensitivity correlated with reinnervation of both pulp and dentin.