What’s your diagnosis? TMJ? OMJL?

This is not likely a disease you learned about in veterinary school, but regular readers will have an advantage for figuring out this one

Waffles, a one-year-old Shih Tzu, who presented with inability to close the mouth. Photos courtesy John Lewis
Figure 1: Waffles, a one-year-old Shih Tzu, who presented with inability to close the mouth.
Photos courtesy John Lewis

I’ve got another zebra diagnosis for you. If you’ve been a regular reader of this column over the past five years, you’ll probably figure this one out. Hint: This is not likely a disease you learned about in veterinary school.

We recently had two very similar cases, one in a dog and one in a cat. Both patients presented with inability to close the mouth, and due to the infrequency of the disease, provided a diagnostic challenge for their veterinarians. Seems like a good time for a refresher on the topic.

Everybody loves Waffles

Waffles, a one-year-old neutered male Shih Tzu, presented to an emergency/specialty hospital for acute inability to close his mouth (Figure 1). He was pawing at his mouth incessantly and showed reluctance and difficulty eating. Similar episodes had occurred in the past, but they resolved without intervention. On presentation to an emergency/specialty hospital, physical examination showed the mouth was unable to close, being approximately 2 cm open with no contact of the teeth on either side. There was crepitus on attempts to open and close the mouth. Oral manipulation done to obtain skull radiographs resulted in Waffles’ ability to suddenly be able to close his mouth.

CT scan was then performed while the mouth was able to close, and revealed subluxation of the left condylar process in the mandibular fossa of the temporal bone. A loose tape muzzle was placed to maintain reduction while allowing some ability to open the mouth for eating/drinking. Unfortunately, Waffles was able to remove his tape muzzle and his jaw locked in the open position again. He then visited our practice, and when he was presented, he was in the locked position. As soon as I palpated his face, my eyes lit up and I had the diagnosis. There was a palpable swelling ventrolateral to the left zygomatic arch, which was most easily felt when Waffles tried to close the mouth.

Luna eclipsed

Shortly after we met Waffles, a four-year-old longhaired cat name Luna was presented with a similar problem. Luna had a dentistry procedure one month prior at her primary care veterinarians’ practice. Two weeks after the procedure, she had an episode of inability to close the mouth. In addition to this, Luna’s primary care veterinarians noted her symphysis showed significant laxity, so they planned to place a symphyseal wire the next day. On the overnight prior to the wire’s placement, Luna’s jaw locking resolved and occlusion was normal. The symphyseal wire was placed—no more locking episodes were seen for a month until another locking episode was noted and Luna returned to her primary care veterinarians. Here’s what was noted in the medical record: “Mouth wide open without being able to close, with left mandibular dental arch positioned more ventrally [than the right] and no contact between maxillary and mandibular teeth. Protuberance palpable at the ventral aspect of the left zygomatic arch.” This wonderfully descriptive physical examination report helped the rDVM to formulate a diagnosis, which I agreed with, and allowed me to know which side was currently affected. What’s your guess?

Differential diagnoses

When a dog or cat suddenly loses its ability to close the mouth, there are only a handful of differentials:

1) Temporomandibular joint (TMJ) luxation

2) TMJ fracture or caudal mandible fracture

3) Open-mouth jaw locking (OMJL)

4) Acquired dental malocclusion (often due to luxation of a periodontally diseased tooth)

5) Trigeminal neuropathy (a.k.a. mandibular neurapraxia or trigeminal neuritis)

If no history of trauma exists and no signs of head trauma are seen on examination, TMJ luxation and TMJ/caudal mandibular fractures are unlikely causes. Caudal mandibular fractures and TMJ fractures may result in a shift of the mandible and inability to close the mouth. A fracture of the caudal mandible usually results in a shift of the lower jaw to the side of the fracture, since the portions of the mandible rostral and caudal to the fracture site will overlap upon themselves. In contrast, traumatic luxation of the TMJ in the most common direction (rostrodorsal) results in a shift of the mandible to the side opposite the luxation. Trigeminal neuropathy may occur secondary to trauma, neoplasia, or infectious causes. Unlike the other four differentials on our list, this disease presents with a “dropped jaw” that can be manually closed by the clinician, but the patient cannot keep its mouth closed without support. Acquired dental malocclusions are one of the most common reasons for inability to close the mouth. They occur either due to severe tooth extrusion and/or severe periodontal disease, most commonly seen with the canine teeth or palatal luxation of the maxillary fourth premolar tooth, resulting in tooth-to-tooth contact. Periodontal disease and the loss of tooth attachment structures can cause luxation of teeth from their sockets, resulting in inability to close the mouth.

So what’s the diagnosis?

Figure 2: A portion of Waffles’ zygomatic arch and coronoid process has been removed to prevent locking. If locking was able to be induced on the contralateral side, bilateral surgery would have been necessary.
Figure 2: A portion of Waffles’ zygomatic arch and coronoid process has been removed to prevent locking. If locking was able to be induced on the contralateral side, bilateral surgery would have been necessary.

Unlike TMJ luxation or mandibular fractures, open-mouth jaw locking causes inability to close the mouth while the maxillary and mandibular teeth are not in contact. In cases of OMJL, the mouth is wide open, and the mandible often has a subtle ventrolateral shift to the side where the coronoid process is locking on the ventral aspect of the zygomatic arch. The most common cause of OMJL is laxity of the temporomandibular joints due to congenital TMJ dysplasia. This condition results in increased lateral movement of the mandibular condyles, which in turn, allows the coronoid process to get stuck on the ventral surface of the ipsilateral zygomatic arch. Other possible causes of OMJL exist, though they are less common. Excessive symphyseal laxity can also allow for excessive lateral movement of the coronoid processes, but this is usually in conjunction with TMJ laxity. Medial compression of the zygomatic arch due to prior trauma or congenital reasons, bony tumor, and bony callus of the ramus or zygomatic arch may result in locking and inability to close the mouth after opening the mouth wide.

Short-term treatment

Acute cases of open-mouth jaw locking are relieved by opening the patient’s mouth as wide as possible, and moving the mandible to midline from the side to which it is shifted. This may require sedation, but in many cases, it is a very quick procedure that relieves the lock in the awake patient. However, this is a temporary fix. The next time the patient yawns or opens its mouth very wide, the jaw may lock again. A tape muzzle may be placed to restrict the patient from opening the mouth too wide.

Long-term treatment

Since TMJ dysplasia often occurs bilaterally in affected patients, intermittent bilateral locking may be possible; therefore, both sides must be assessed for locking when the patient is placed under anesthesia for definitive treatment. Unilateral surgery is performed if locking is achieved on only one side. Bilateral surgery should ideally be done during the same anesthesia if locking is found to occur on both sides. A variety of surgeries have been advocated for treatment of OMJL, but I prefer the combination of full-thickness zygomatic arch resection and coronoid process reduction (Figure 2), with the surgery ideally done in the locking position to allow for intraoperative assessment of the relationship between the two bones. Even when zygomectomy and coronoidectomy are done bilaterally, postoperative function and cosmesis are excellent. Postoperative complications are rare.

John Lewis, VMD, FAVD, DAVDC, practices dentistry and oral surgery at Veterinary Dentistry Specialists and is the founder of Silo Academy Education Center, both located in Chadds Ford, Pa.

Recommended reading

  • Constantaras ME, Charlier CJ. Maxillofacial injuries and diseases that cause an open mouth in cats. J Vet Dent. 2014; 31: 168-175.
  • Lewis JR. bit.ly/2MvRMOJ. Veterinary Practice News, March 2015.
  • Lewis JR. bit.ly/2AGTfyZ. Veterinary Practice News, January 2015.
  • Reiter AM. Symphysiotomy, symphysiectomy, and intermandibular arthrodesis in a cat with open-mouth jaw locking-case report and literature review. J Vet Dent. 2004;21(3):147-158.
  • Soukup JW, Snyder CJ, Gengler WR. Computed tomography and partial coronoidectomy for open-mouth jaw locking in two cats. J Vet Dent. 2009;26(4):226-233.
  • Nutt AE, Anderson T, Gracis M, Doran I, Warren-Smith C, Langley-Hobbs SJ. Open-mouth jaw locking in cats: a literature review and use of CT in three cases. J Feline Med Surg. 2018;20(12):1180-1191.
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