Overshot and Undershot Jaw in Dogs

Occlusion refers to the contact and alignment of the teeth in the upper and lower jaws. In normal occlusion, the lower incisors are overlapped by the upper incisors, called a scissor bite, the lower canines are equidistant between the upper canine teeth and the third lower incisor, and the premolars point to the spaces between the upper jaw teeth. Malocclusions occur when there are variations from the classic scissor bite, and can be caused by a misalignment of the jaws or teeth.

Overshot and undershot jaws are types of misalignments of the mandible, or lower jaw, and the maxilla, or upper jaw. Overshot refers to an upper jaw that is longer than the lower jaw, while undershot is when the lower jaw is longer. These misalignments, or malocclusions, can cause trauma, discomfort, and problems with eating in affected dogs.

If your dog has an overshot or undershot jaw, he may have trouble closing his mouth. The slightly open mouth has sometimes been called salmon jaw, as it can resemble the open mouth of a fish. The longer upper jaw associated with overshot jaw can cause a parrot beak-like appearance. Besides looking unusual, a misaligned jaw can cause trauma and discomfort to various parts of the mouth as the misaligned teeth come into contact with tissues that would not be touched in normal occlusion. Signs your dog may have a misaligned upper or lower jaw include:

• Overbite 
• Protruding lower jaw
• Abnormal jaw growth 
• Incisors that meet edge to edge
• Inability to close mouth
• Slightly open mouth 
• Difficulty chewing food
• Preference for certain kinds of foods, such as bigger pieces
• Inability to keep food in mouth while chewing
• Salivation 
• Rubbing at face
• Oral pain
• Hard palate trauma
• Defects in the hard palate 
• Lip or soft tissue trauma
• Oral ulcers
• Tooth wear
• Exposure of tooth pulp
• Tooth loss

Types

Overshot and undershot jaws are two types of malocclusions seen in dogs. Some defining features of each include:

Overshot

This is a Class II malocclusion that is also called mandibular brachygnathism, mandibular distoclusion, or an overbite. This type of misalignment is characterized by a shorter lower jaw and a longer upper jaw, which causes the lower canine teeth to strike the palate or upper canines. All dogs are born with an overshot jaw that allows them to nurse as new puppies. As the puppies grow, the lower jaw grows to catch up to the upper jaw. If the jaw does not have that growth spurt, it can cause displacement of the canines, which can then also inhibit the mandible growth.

Undershot

This is a Class III malocclusion that is also referred to as mandibular prognathism, maxillary brachygnathism, mandibular mesioclusion, or an underbite. This malocclusion is characterized by a shorter upper jaw and a longer lower jaw, resulting in lower teeth that are in front of the upper teeth. While this condition is 'normal' for some breeds, such as Bulldogs, in many breeds it is unusual. An undershot jaw occurs when the lower jaw grows faster than normal and becomes longer than the upper jaw, and is usually evident around 8 weeks of age in puppies. This misalignment can cause soft tissue trauma, such as to the lips. When the incisors meet instead of fitting next to each other, it is called a level bite. When the malocclusion causes the lower incisors to be placed in front of the upper incisors, it is called a reverse scissors bite.

The cause of overshot and undershot jaws in dogs relate to the increased or decreased rate of growth of the upper and lower jaws in relation to one another. This can occur due to a:

• Genetic disorder
• Trauma
• Systemic infection
• Nutritional disorder
• Endocrine disorder
• Abnormal setting of puppy teeth
• Early or late loss of puppy teeth

If you have noticed that your pup’s jaw seems abnormally shaped, or that he is having a hard time chewing or keeping food in his mouth, you may need to take them in to your veterinarian. After a quick physical exam, your vet may have to sedate your dog in order to perform a thorough oral exam and take x-rays or a CT scan. This will assess your dog’s skull type and teeth location in relation to the teeth on the opposite jaw.

Often, the placement of the upper and lower incisors in relation to one another can determine what type of malocclusion your dog has. Your vet will note any areas of trauma due to teeth striking those areas, and any cysts, tumors, abscesses, or remaining puppy teeth that may be present.

Treatment of a jaw misalignment will depend on the severity of the condition. If your dog has a misalignment, but can still bite and chew food without problems, no treatment may be needed. If the misalignment is caught early in a puppy’s life, it may only be temporary and may correct itself over time.

However, there are times when intervention may be needed. If your puppy’s teeth are stopping the normal growth of his jaws, then surgery to remove those puppy teeth may be performed. This may allow the jaws to continue to grow.

For older dogs who are experiencing pain and trauma due to misaligned jaws and teeth, oral surgery is generally performed to extract teeth that are causing trauma, to move teeth so that they fit, or to create space for a misaligned tooth to occupy. Other therapies include crown reductions or braces, though these are more specialised.

Overshot and undershot jaws in dogs are often genetic defects that cannot be controlled. Mild to moderate cases don't often need treatment and don't tend to cause issues. In more severe cases, the earlier the intervention, the better the outcome for your dog. While the condition can be severe if the misalignment causes your dog to be unable to eat, in many cases, orthodontic intervention can lessen the pain and trauma associated with malocclusion and help your dog to comfortably live with the condition. Recovery will depend on how early the condition was treated and how mild or severe it is, but is generally fair.

If your dog is genetically programmed to have an overshot or undershot jaw, intervention can help, but will not slow or stop the abnormal growth of either jaw. Prevent jaw misalignments in puppies by not breeding dogs who have overshot or undershot jaws.