Shear mouth in Horses (Equis) | Vetlexicon
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Shear mouth

ISSN 2398-2977

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Introduction

  • Cause: jaw and dental conformation, lack of fiber in diet and increased concentrates, lack of routine dental care, dental pain.
  • Signs: dysmastication (quidding), loss of weight or poor condition, abnormal bitting behavior, soft tissue damage and ulceration, particularly of the buccal mucosa.
  • Diagnosis: overgrowths of the buccal aspects of the maxillary teeth and, to a lesser extent, the lingual aspect of the mandibular teeth on full oral examination. Extra- and intraoral palpation. Dental mirror or oroscope will identify various degrees of mucosal trauma and the extent of the overgrowths Dental examination. Increased molar table angles (angle of occlusion) to an extent where lateral excursion of the jaw is reduced/locked.
  • Treatment: removal of overgrowths – full dental equilibration over staged dental reductions to slowly return the angle of occlusion to normal molar table angles of 12-18 degrees.
  • Prognosis: guarded to fair.

Presenting signs

  • Pain on eating with dropping of food from the mouth (quidding) Dysmastication.
  • In chronic cases, loss of weight or condition.
  • Bitting or head-carriage problems.
  • Muscle atrophy.

Acute presentation

  • Severe oral pain and dysmastication.

Geographic incidence

  • May be more common in countries where there is limited access to a fiber diet.

Age predisposition

  • Middle-aged to older horse.

Cost considerations

  • Costs of treatment which may be repetitive and involve extractions/diastema, etc.
  • Routine dental care.

Pathogenesis

Predisposing factors

General

  • The maxillary arcade of cheek teeth is 23-30% wider than the mandibular arcade at any point (anisognathism). 
  • An anatomic survey has reported the range of the normal occlusal angle for the cheek teeth to be much more variable than was originally thought. Individual horses may be predisposed to this condition due to their jaw and dental conformation.
  • Although this condition is often thought to be more common in stabled horses, recent reports have confirmed free-living horses have as many enamel points as those that are stabled.

Specific

Pathophysiology

  • As the horse masticates, the mandible is lowered and is then raised in a dorsal movement, during which it circumducts from the buccal to lingual aspects.
  • The mandibular teeth occlude in an axial direction and a grinding action occurs as their occlusal surfaces shear over each other. This is known as the “power-stroke”.
  • Due to the anisognathism in the equine mouth, only one molar arcade is in occlusion during each masticatory stroke.
  • The temporomandibular joint permits free lateral movement and a degree of rostro-caudal movement. As the horse lowers its head to graze, the mandible drops and moves rostrally, and at this point normal occlusion would be expected.
  • Pathology that limits lateral jaw movement can result in a gradual increase in the table angle of occlusion. Over time this may lead to dysmastication and further uneven wear on the occlusal surfaces. Horses eating concentrated or pellet feeds have a reduced grinding action compared to forage feeds (such as grass or hay) Resulting in a shorter lateral to midline stride of mandibular to maxillary occlusion.
  • Overgrowths on the lateral buccal cingulae of the maxillary arcades can become particularly sharp leading to ulceration to the oral mucosa of the cheeks and elsewhere with pain on mastication.
  • Further development of the overgrowths will worsen the occlusal angles to 45-50 degrees when the condition is termed shear mouth. In this situation, the lateral overgrowths stop any lateral masticatory movement, only a scissor-like action is possible, and this exacerbates the problem further.
  • Shear mouth can lead to widespread periodontal disease Teeth: periodontal disease and wave mouth.

Diagnosis

Presenting problems

  • Pain on eating with dropping of food from the mouth (quidding) Dysmastication.
  • In chronic cases, loss of weight or condition.
  • Bitting or head-carriage problems

Client history

  • Loss of weight.
  • Dropping of food from mouth (quidding) Dysmastication.
  • Equitation problems.

Clinical signs

  • Dysmastication with dropping of partially masticated food from the mouth while eating (quidding) Dysmastication.
  • Pouching of food in cheeks of mouth.
  • Loss of weight or poor condition Weight loss: overview.
  • Abnormal bitting behavior.
  • Headshaking during work Behavior: headshaking.
  • Soft tissue damage and ulceration particularly of the buccal mucosa.
  • Overgrowths of the buccal aspects of the maxillary teeth and, to a lesser extent, the lingual aspect of the mandibular teeth. 
  • 45/50 degree molar table angles.

Diagnostic investigation

  • Extra- and intraoral palpation at a full routine dental examination Dental examination.
  • Examination with a dental mirror Dental examination or oroscope will identify various degrees of mucosal trauma and the extent of the overgrowths.

Confirmation of diagnosis

Discriminatory diagnostic features

  • Clinical signs.
  • Routine dental examination.

Definitive diagnostic features

  • Oroscope and/or dental mirror examination.

Differential diagnosis

  • Other causes of dysmastication and oral pain.
  • Other causes of oral mucosal damage.

Treatment

Standard treatment

  • Removal of overgrowths: staged correction of table angles with care to protect the secondary dentine.
  • This must be done gradually, particularly in severe cases. Gradual reductions, with several treatments over a long period (up to 6 months), may be necessary.
  • A whole variety of instrumentation is available for this, ranging from hand-held teeth rasps with different shaped heads, handles and blades (chipped or solid tungsten carbide), to electrically-operated rasps. These may be mains or battery driven with diamond-coated burrs and they are highly effective at removing molar overgrowths.
  • Mechanically operated burrs can easily overheat cheek teeth, and so they should always be used with water-cooling.
  • The patient should always be sedated Anesthesia: standing chemical restraint to allow the reduction to be visually controlled and thereby avoid iatrogenic damage to teeth or soft tissues. 

Monitoring

  • Healing of soft tissue injuries as sharp points are removed.
  • Regular routine oral examination every 6 months, after treatment has finished, is essential. This will limit the amount of sharp edges that require removal. 

Subsequent management

Treatment

  • Regular oral examination and preventive dental care.

Monitoring

  • 6 monthly routine dental examination.

Prevention

Control

  • Adjust diet to increase fiber content and decrease concentrate level in diet.

Outcomes

Prognosis

  • Guarded for large sharp points, and particularly if there is additional tooth pathology present.

Expected response to treatment

  • Oral pain should diminish rapidly once the sharp points are removed and the soft tissue injuries heal.

Reasons for treatment failure

  • Failure to remove the sharp points adequately and address other dental problems.
  • Failure to address factors that have caused formation of sharp points.

Further Reading

Publications

Refereed Papers

  • Recent references from PubMed and VetMedResource.
  • Casey M (2013) A new understanding of oral and dental pathology of the equine cheek teeth. Vet Clin Equine 29 (2), 301-324 PubMed.
  • Tremaine H (2013) Advances in the treatment of diseased equine cheek teeth. Vet Clin Equine 29 (2), 441-465 PubMed.
  • Brown S L, Arkins S, Shaw D J et al (2008) Occlusal angles of cheek teeth in normal horses and horses with dental disease. Vet Rec 162 (25), 807-810 PubMed.
  • Carmalt J L & Allen A (2008) The relationship between cheek tooth occlusal morphology, apparent digestibility, and ingesta particle size reduction in horses. JAVMA 23 (3), 452-455 PubMed.
  • Dixon P M (2000) Removal of equine dental overgrowths. Equine Vet Educ 12 (2), 68-81.

Other sources of information

  • Easley J, Dixon P M & Schumacher J (2011) Equine Dentistry. 3rd edn. Saunders Elsevier, USA.