Mandibular separation: an epidemic of significance (Proceedings)

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.

Anatomy

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.

Incidence

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.

Diagnosis

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.

Clinical signs

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.

Etiology

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.

Treatment

Four methods of repair have been used over a three year period. Three involve the use of a stainless steel cerclage wire. The fourth involves the use of an intermandibular screw.

The first stabilization method employed was the use of two 30 gauge stainless steel wires. They were placed simultaneously as one piece of wire. After passage around the mandible using a large suture needle in the center of a 12 inch piece of stainless steel wire, the wire was cut so the needle could be removed. The ends of one wire were twisted together caudolateral to the right canine tooth to the instability was removed. The ends of the other wire were twisted together in a similar fashion behind the left canine tooth. For the sake of consistency, the wires were twisted in a clockwise direction. The ends were cut so about1 mm remained, and it was bent so that the ends were buried within the gingiva and not palpable. This was to prevent tissue irritation after the cat awakened. After using this method for several months, the rate of healing was nearly zero so we theorized that the wires were of insufficient gauge to keep the jaw stable. That conclusion led to a change in wire placement technique.

The second stabilization method employed a 24 gauge stainless steel wire. Placement began with a 1 cm incision on the ventral midline about the level of the lower canine teeth. An 18 gauge hypodermic needle was inserted through the incision and passed as close to the bone as possible until it exited the lip immediately caudal to the left lower canine tooth. The wired was passed through it. With both ends protruding, the needle was removed through the skin incision. It was passed again through the skin incision near the bone until it exited the lip immediately caudal to the right lower canine tooth. The wire was passed through the needle beginning near the canine tooth until it exited the needle as it passed through the skin incision. The needle was withdrawn. The two ends of the wire were twisted together in a clockwise direction on the ventral midline until symphyseal stability was achieved. The ends were cut so about 1-2 mm remained and directed dorsally. The skin incision was closed with surgical glue.

There were three problems with this technique. First, after two months there was no better stability than the first method. Second, with the ends of the wire buried, it was not possible to tighten the wire without making a new skin incision. Third, several cats developed draining tracts due to the ends of the wire causing constant irritation to the subcutaneous tissue.

The third technique was a combination of the first two. A single 24 gauge wire was placed using the first technique. The ends were twisted together in a clockwise direction caudal and lateral to the right lower canine tooth. This is the technique currently employed on most cases.

The fourth technique utilized a 2 mm diameter 18 mm long titanium screw. A skin incision was made over the lateral aspect of the left mandible to the level of the periosteum. The periosteum was removed to expose the bone. A transverse hole was drilled immediately caudal to the root of the left lower canine tooth and through the left side of the mandible. A slightly smaller hole was drilled through the right side of the mandible immediately caudal to the right lower canine tooth so the lag screw principle could be employed. The screw was tightened until the mandibular symphysis was stable. The skin and subcutis were closed using 5-0 Vicryl.

Results

An examination of each cat was requested for two months post-op. Very few cats healed sufficiently to permit removal of the stabilization device. With each of the three wiring techniques, almost all cats had return of some instability at two months. This was likely due to stretching of the wire. Our current protocol is to tighten the wire and leave it in place long-term. Although only two cats to date received screw placement, they, too, were unstable at two months. It was not deemed appropriate to tighten the screws because both were eating so much better than before screw placement that the owners considered the treatment successful.

Although we have only rarely achieved stabilization so the wire could be removed, the cats are clinically better with the wire in place than prior to its placement. Our current approach of long-term placement seems pragmatically successful.

Technique improvement

The only way to truly achieve long-term stabilization would be to open the joint surgically, remove the cartilaginous and fibrous tissue, and place a stabilization device long enough for ossification of the joint to occur. However, to date, this approach does not seem warranted.