What Is Craniomandibular Osteopathy?
Craniomandibular osteopathy (CMO), often called “lion jaw”, is a non-cancerous disease in which extra bone builds up on the lower jaw (mandible) and parts of the skull, especially near the jaw joint. It affects growing puppies, typically between 3 and 8 months of age, and makes opening the mouth and chewing painful.
The good news is that CMO is usually self-limiting: the abnormal bone growth tends to slow and partly recede once the puppy stops growing, at around 12 months of age.
A genetic terrier disease
In West Highland White Terriers and some other terriers, CMO is linked to an inherited gene variant, and DNA tests are available. It is one of several growth-related bone diseases in puppies, alongside panosteitis.
Breeds at Risk
- West Highland White Terriers (most common)
- Scottish Terriers and Cairn Terriers
- Boston Terriers
- Occasionally larger breeds such as Labrador Retrievers, Great Danes and Dobermans
Signs to Watch For
- Pain when opening the mouth, yelping when yawning or chewing
- Reluctance to eat, dropping food, or weight loss
- Drooling
- Firm, symmetrical swelling along the lower jaw
- Intermittent fever, often in cycles
- In severe cases, being unable to open the mouth more than a little
Diagnosis
A vet will feel the jaw and check how far your puppy can open its mouth. Skull X-rays showing thickened, irregular bone on the jaw in a young terrier are usually enough to diagnose CMO. A CT scan or bone biopsy may be used to rule out infection or tumors in atypical cases, and DNA testing can confirm the genetic form in Westies and related breeds.
Treatment
There is no way to stop the bone growth directly, so treatment focuses on comfort and nutrition until the disease settles:
- Pain relief: anti-inflammatory drugs and other pain medication, especially during flare-ups
- Soft food: canned food, gruel or soaked kibble that needs little chewing
- Nutrition support: in severe cases where the mouth barely opens, a temporary feeding tube
- Surgery: rarely, if bone fuses the jaw joint permanently
Outlook
Most puppies improve once they finish growing and go on to live normal lives, sometimes with mildly reduced jaw movement. The outlook is more guarded when bone has built up around the jaw joint itself, since that can permanently limit how wide the mouth opens.
Prevention
Because CMO is inherited, the main prevention is responsible breeding: affected dogs and known carriers should not be bred. If you are getting a Westie, Scottie or Cairn puppy, ask the breeder about DNA testing of the parents. For other causes of puppy bone pain, see panosteitis and why dogs limp.
Frequently Asked Questions
Is lion jaw cancer?
No. Craniomandibular osteopathy is a benign overgrowth of bone, not a tumor, and it does not spread.
Will my puppy outgrow craniomandibular osteopathy?
Usually. Bone growth typically stops around a year of age and the extra bone often shrinks, though some dogs keep a degree of jaw stiffness.
Can adult dogs get CMO?
New cases in adult dogs are very rare. It is a disease of growing puppies.
If your puppy cries when eating or yawning, book a check at a veterinary clinic near you.