Prognathism

Prognathism refers to a condition wherein the lower jaw (mandible) is longer than the upper jaw. Mandible is the largest and strongest bone of the face (muzzle) constituting the lower jaw. It supports the lower teeth. All brachychephalic (short-nosed) breeds have a prognathic occlusion pattern (pugs, boxers and bulldogs). The brachycephalic head type is characteristic of many breeds, including the Pekingese, pug, boxer and Boston terrier. The bulldog is perhaps the most easily recognized of the brachycephalic breeds. The standards for brachycephalic heads all allow or require prognathism.

One of the mild types of prognathism, level bite, refers to a condition when by the incisors meet cusp-tip to cusp-tip rather than having the uppers overlap the lowers (scissor bite). Due to the abnormal incisor cusp-tip contact, all incisors will experience abnormal crown wear. Individual incisors will also experience a greater frequency of fracture than incisors with a scissor occlusion.

Brachygnathism

Brachygnathism refers to a condition in dogs in which an overbite, or overshot upper jaw exists. Both brachygnathism and prognathism are inheritable traits, passed from one generation to another. Although not life-threatening, these anatomic maladies can interfere with normal biting action and eating and can predispose to dental and jaw problems in affected dogs. As a result, dogs suffering from a distinct overbite or underbite (unless normal for the breed) should be neutered to prevent propagation of these undesirable traits.

Treatment of Prognathism and Brachygnathism

Orthodontic correction of brachygnathism and prognathism can be utilized in select cases for medical purposes.

References:
1. Uc Davis Book of Dogs. Mordecai Siegal, Jeffrey E. Barlough, University of California, Davis, University of California, Davis School of Veterinary Medicine
2. The Complete Home Veterinary Guide. Chris C. Pinney