Trismus in a Dog

Kevin is a 13kg 16-month cross male neutered Staffy, with a cheeky swagger He was acquired from the pound at 6 months of age with no prior medical history. From the outset, the owners noted that he could not carry objects larger than a golf ball. Over the next few months he lost his ability to eat larger dog biscuits and carry balls of any size. He would tire rapidly when playing in the park. Trismus (reduced ability to open the mouth) was diagnosed by Dr Graeme Wilson at Plympton Veterinary Clinic, and he was referred for investigation.

Physical examination was unremarkable, other than trismus and moderate temporalis muscle atrophy. His maximum mouth opening was 5mm between incisors (Figure 1). There was no pain on attempts to open the mouth. The primary differential diagnosis was masticatory muscle myositis. Other differentials were caudal mandibular, TMJ or zygomatic fracture, TMJ dysplasia.

Kevin was anaesthetised for skull radiographs to exclude bony abnormalities of the TMJ or mandible, with additional plans to perform temporal muscle biopsy and draw blood for 2M antibody ELISA in order to confirm masticatory muscle myositis. His anaesthesia plan was complicated by the inability to easily intubate his trachea. He was premedicated with acepromazine and methadone, with micro-dose induction and top-up dosing of propafol to maintain a light plane of anaesthesia while radiographs were taken. Oxygen was supplied by mask, and emergency tracheostomy instruments were close at hand.

Lateral, dorsal and oblique skull radiographs (Figure 2) revealed a hypertrophic non union of a right zygomatic arch fracture, with bony remodelling of mandibular ramus and TMJ. At this point it would have been fantastic to have 3 dimensional images of the skull to accurately plan surgery, however the owners declined CT.

Kevin returned the following week for surgery. Anaesthesia was induced and a temporary tracheostomy was placed (Figure 3). After skin incision over the zygomatic arch, subperiosteal elevation of soft tissue was followed by ostectomy of the caudal 1/2 of the arch. Soft callus extended from the fracture site at the caudal zygomatic arch, enveloping the mandibular ramus and TMJ.

Interestingly this mass of tissue was all medial to the zygomatic arch and could not be palpated on the original physical examination. Careful dissection was required to avoid regional neurovascular structures such as facial nerve and auriculotemporal.

Disarticulation of the TMJ, elevation of masseter and temporalis muscle insertions was followed by and ostectomy of the mandibular ramus and condyloid process (Figure 4), thus removing the restriction on mouth opening. An immediate oral opening of 5cm was achieved. Moderate dental calculus was present due to inability to chew for a protracted period. Soft tissue closure was routine.

Kevin resumed eating within hours of anaesthetic recovery and was discharged the following day. The owners were warned that some teeth clicking caused by lateral mandibular drift may occur. They were asked to challenge Kevin with foods and toys that encouraged chewing and wide mouth opening.

At 14 days post surgery (Figure 5) Kevin was bright and alert, able to carry a tennis ball and chew bones. No teeth clicking had been recognised by the owners. The removal of the caudal zygomatic arch and mandible ramus has left a facial depression which is very acceptable.


  • Mild subcutaneous emphysema originating from the temporary tracheostomy site formed in the 12 hours after surgery and had resolved by day 14.
  • Kevin is now able to chew household items and has destroyed the owners Playstation controllers.