What began as an astute observation by a very sharp dental technician has resulted in discovery that the mandibular symphysis
of the cat is very easily and frequently separated.
The mandible is comprised of two halves joined together on the midline at the mandibular symphysis. Unlike dogs and humans,
this joint is not ossified. Rather, there is a fibrocartilagenous plate between the two halves of the mandible. This plate
is composed of a layer of fibrous connective tissue and cartilage that is radiolucent. This means that when the mandibular
symphysis is radiographed, there is a "space" between the two bony halves.
After observing several cats with this abnormality, my technicians begin to incorporate a mandibular symphyseal examination
as part of all dental procedures. During a 12 month period, 417 cats underwent dental therapy; 67 (16%) of them had mandibular
separation. Of those 67 cats, 69% were 10 years of age or more, and 21 (31%) were less than 10 years old. Thus, there appears
to be a propensity for this disease in older cats.
The two halves of the mandible are moved in opposite vertical directions easily demonstrating joint laxity. It is notable
that this condition is rarely detectable in awake cats, but it is easily diagnosed with the cat under anesthesia.
When queried about clinical signs, some owners report that none are observed. However, we frequently hear two things: 1) The
cat is making awkward movements of the mandible during chewing ("jawing"). 2) The cat that eats dry food has small particles
of food drop from the mouth as it is chewing. Occasionally, following repair, owners report that their cats eat significantly
better; however, this is a retrospective observation.
Initially, there was concern that renal secondary hyperparathyroidism might be involved in the etiology; however, blood panels
of the affected and non-affected cats were not significantly different in regard to renal or electrolyte (calcium and phosphorus)
levels. If renal disease was detected, it was almost always with creatinine values less than 3.5 and with normal total calcium
and phosphorus levels.
Because of the significantly higher incidence in cats over 10 years of age, our working etiologic explanation is simply years
of chewing on hard objects; dry cat food is a likely object. However, some of the older cats with the disease are primarily
or exclusively canned cat food eaters.