Malocclusion in aging Wistar rats

Abstract

Insufficient wear of rodent incisors often results in malocclusion and rapid tooth elongation. This condition may go undetected for a prolonged time and have a negative effect on affected animals' welfare. Dental overgrowth can lead to undernutrition due to chronic difficulty in feeding and may cause trauma to the surrounding tissues, potentially ultimately resulting in death. Here we describe the increased incidence of malocclusion observed during a longitudinal study of the normal growth and aging of Wistar rats. Histologic examination of the temporomandibular joint of affected animals did not reveal any inflammatory or degenerative changes. Because no environmental factor could be implicated in the condition, we considered that aging or genetic factors were responsible for its appearance. We conclude that special attention should be given to the potential appearance of malocclusion during long-term rodent studies, because its incidence may adversely affect the animals' health and general wellbeing.

Figure 1.

Graph of the 13 cases of malocclusion in the present rat study indicating the animal's age (in weeks) when the condition was diagnosed. Squares, female rats; diamonds, male rats.

Figure 2.

(A) Representative rat (no. 16) with class II malocclusion (incisor overbite). The lower incisors deviate to the right side of the animal (subcategory 1). The white arrow indicates the lower right incisor. (B) Corresponding frontal radiograph of the head of rat 16, showing the deviation of the mandibular incisors to the right side of the animal. The white arrow indicates the lower right incisor. Note the misalignment of the mandible.

Figure 3.

(A) Representative rat (no. 21) with class II malocclusion (incisor overbite). The lower left incisor deviates to the left side of the animal, resulting in widening between the mandibular incisors (subcategory 2). The white arrow indicates the lower left incisor. (B) Corresponding radiograph of rat 21. The white arrow indicates the lower left incisor. Note that the jaws are well aligned.

Figure 4.

This mandibular condyle of an affected animal (rat 21) is normal in appearance. Hematoxylin and eosin stain; magnification, ×4.

Figure 5.

Higher magnification (×10) of the mandibular condyle of rat 21, showing normal component cartilage layers. Hematoxylin and eosin stain.