Canis ISSN: 2398-2942

Jaw: open-mouth locking

Synonym(s): Temporomandibular joint dysplasia; TMJ dysplasia

Contributor(s): Alexander M Reiter

Introduction

  • Cause: temporomandibular joint (TMJ) dysplasia (shallow mandibular fossa, incongruent mandibular condyle, hypoplastic retroarticular process) resulting in TMJ laxity; however, there are dogs with obvious TMJ dysplasia (eg Cavalier King Charles Spaniels) that do not usually develop open-mouth jaw locking. Vice versa, many dog breeds without TMJ dysplasia can be affected by open-mouth jaw locking.
  • Head shape (brachycephalism).
  • Long coronoid processes.
  • Flattened zygomatic arches.
  • Obliquely arranged mandibular fossae and condylar processes.
  • Signs: mouth locked wide open due to displacement of the coronoid process of the mandible ventrolateral to the zygomatic arch.
  • Diagnosis: clinical signs (often pathognomonic) and diagnostic imaging features (radiography, computed tomography).
  • Treatment: partial zygomectomy, partial coronoidectomy, or a combination of both procedures (may need to be done bilaterally).
  • Prognosis: excellent.

Pathogenesis

Etiology

  • Temporomandibular joint dysplasia: rare congenital or acquired malformation manifesting as shallow mandibular fossae, underdeveloped/misshapen retroarticular processes, flattened (incongruent) mandibular condyles.
  • Laxity of the mandibular symphysis, head shape (brachycephalism), long coronoid processes, flattened zygomatic arches, obliquely arranged mandibular fossae and condylar processes.
  • Open-mouth jaw locking: medial pulling of the mandible upon contraction of the pterygoid muscles at maximal mouth opening and lateral flaring of the coronoid process (for example immediately after yawning) resulting in locking of the coronoid process ventrolateral to the zygomatic arch; mouth is locked wide open without contact between maxillary and mandibular teeth.

Predisposing factors

General

  • Temporomandibular joint dysplasia.
  • Temporomandibular joint laxity.
  • Mandibular symphyseal laxity.
  • Flattening of the zygomatic arches.

Specific

  • Trauma to temporomandibular joint or adjacent tissues.
  • After unilateral total mandibulectomy Hemimandibulectomy.
  • Canine tooth extrusion with abnormal contact between maxillary and mandibular canine teeth and subsequent levering forces resulting in increased temporomandibular joint and mandibular symphyseal laxity.

Pathophysiology

  • Medial pulling of the mandible upon contraction of the pterygoid muscles at maximal mouth opening and lateral flaring of the coronoid process (for example immediately after yawning) resulting in locking of the coronoid process ventrolateral to the zygomatic arch; mouth is locked wide open without contact between maxillary and mandibular teeth.

Timecourse

  • One or more episodes (once a month to several times a day, ranging from a few seconds or minutes to many hours or days) of open-mouth jaw locking after yawning, grooming, playing, eating, or vocalizing.
  • Spontaneous correction sometimes associated with an audible ‘click’.

Epidemiology

  • Rare.
  • Usually adult dogs.
  • No gender predisposition.

Diagnosis

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Treatment

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Prevention

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Outcomes

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Reiter A M (2004) Symphysiotomy, symphysiectomy and intermandibular arthrodesis in a cat with open-mouth jaw locking - case report and literature review. Journal of Veterinary Dentistry 21 (3), 147-158 PubMed.


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