Equine case report: Zygomatic arch resection and coronoidectomy in a 2-year-old Thoroughbred filly

A 2-year-old Thoroughbred filly presented for a facial deformity that occurred from a traumatic injury as a foal. A Computed Tomography examination revealed that the coronoid process of the right vertical ramus (mandible) fused with the zygomatic arch creating a pseudoankylosis. The boney callus limited the horse’s mobility of the jaw and her ability to prehend food, graze and masticate. Additionally, a severe skull asymmetry was present which created a “shearing” malocclusion involving the left maxillary and mandibular 2nd and 3rd deciduous premolars.
The horse was placed under general anesthesia. An initial vertical incision was created over the zygomatic arch in alignment with median aspect of the long axis of the coronoid process. Once the zygomatic arch was isolated a sagittal saw was used to create a 2.5 cm segment of the arch directly over the coronoid process. A Hall’s surgical drill with an oval carbide bur was then used to create an osteotomy of the segment and section through the callus down to the coronoid. The segment and callus were removed to expose the coronoid process. Once isolated a 3.5 cm long section of the coronoid process was sectioned using the surgery drill with a long bur and guarded shank. Care was taken to leave a thin rim of bone along the medial aspect of the coronoid to avoid trauma to soft tissue, vascular and nerve structures. Using gentle pressure with an osteotome the final portion of the coronoid was severed.
An additional incision was created just dorsal to the coronoid process. A Periosteal elevator was used along the axial and lateral aspect of the sectioned coronoid process to remove soft tissue attachments so that the segment could be elevated and removed through the dorsal incision. Both incisions were closed and increased mobility of the jaw was noted.
A dental speculum was placed. The left maxillary and mandibular deciduous 2nd and 3rd premolars (606, 607, 706 and 707) were extracted due to severe malocclusion. Slight odontoplasty was performed on the left maxillary and mandibular deciduous 4th premolars and permanent 1st molars (608, 708, 209 and 309).

A one-month follow-up surgical and oral examination was performed. Both surgery sites had healed. The range of jaw motion improved, and the dental speculum could be opened an additional ~20 mm. The left maxillary and mandibular permanent 2nd and 3rd premolars were partially erupting.