The Effect of Fractional Radiation on Dentinogenesis in the Rat

¹ University of Pittsburgh School of Dentistry, Pittsburgh, Pennsylvania

Numerous investigations have studied the effect of single acute doses of X-radiation on tooth development. This study attempts to clarify the conflicting evidence concerning disturbances in dentinogenesis in the few studies that employed fractional irradiation.

Thirty Long-Evans rats were selected for six experimental groups using a random group design. The radiation was administered in five fractional doses that were equally distributed over a 5-day period. The total air-dosage ranged from 300 r to 1,500 r, and the source of radiation was an X-ray therapy unit. The radiation beam was directed to the top of the animal's head while the body was shielded from radiation by collimation of the primary beam and a protective lead screening apparatus. Irradiations were initiated on the 9th day postpartum and continued through the 13th day.

The cellular response to fractional irradiation in odontoblasts of the maxillary incisors and maxillary second and third molars was recorded during different stages of tooth development. The criteria used for the measurement of disturbances in dentinogenesis included the formation of osteo-dentin and/or the presence of a dentinal niche.

The analyses of the data collected, using the chi-square test and Fisher's Exact Probability Test, revealed that there was a significant difference in the occurrence of dentinal disturbances at various levels of radiation dosage in both the incisors and the molars. This precluded the possibility of chance accounting for these differences in reaction to X-radiation.

The minimum osteo-dentinogenic dose in the maxillary incisors at the 60 per cent level of occurrence was found to be 600 r (air-dose). This represents a dose lower than reported in previous investigations. The minimum osteo-dentinogenic dose in the third molars (predentin- or non-dentin-producing odontoblasts) at the 50 per cent level of occurrence was found to be 1,500 r. There was no such effect recorded in any of the maxillary second molars (mature or dentin-producing odontoblasts).

It was concluded from the results of this experimentation that a lower minimum osteo-dentinogenic dose range existed than was formerly believed and that this lower fractionated dose may become increasingly important in future years. It was also concluded that odontoblastic age or stage of odontoblastic differentiation was definitely a factor in determining the reaction of these cells to fractional irradiation. The young, differentiating, but still non-dentin-producing odontoblasts appeared to be the most radiosensitive cells of the developing tooth germ.