Craniomandibular osteopathy (CMO) is an idiopathic, non-neoplastic, proliferative disease of the cranial bones in the dog. The disease has commonly been referred to as Westie disease because of the overrepresentation of West Highland white terriers, or as lion jaw because of the exuberant new bone formation that typically affects the mandibles.1 Although identified primarily in terrier breeds, CMO has been reported in other breeds.2,3 It is typically self- limiting and is known to affect skull bones, including the occipital bones, tympanic bullae, mandibles, and temporomandibular joints. Although CMO is typically a bilateral disease, asymmetric lesions are often seen.4,5 The age of onset is typically between 3 to 8 months; however, clinical signs tend to resolve by 1 year of age.3 There is no known sex predilection. The cause is unknown; however, an autosomal recessive inheritance is suspected,1 and other causes such as bacterial and viral infections have been implicated.1 

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