A 5-year-old neutered male English Bulldog was evaluated for an intermittent inability to open its jaw. One week previously, the owner had observed that the dog was pushing food around its bowl with its nose; the following day, the dog appeared to have difficulty opening its jaw to catch or pick up a tossed treat. The owner took the dog to the referring veterinarian, who manipulated the jaw and detected a click. The dog regained function of its jaw, and the veterinarian administered dexamethasonea (0.22 mg/kg [0.1 mg/lb], SC). One week later, the dog was referred for evaluation when it was again unable to catch treats and the referring veterinarian was unable to restore function via manipulation. No history of trauma was reported. Physical examination revealed that the jaw could not be manually opened > 2 cm; however, no pain response was elicited from the dog when its jaw was manipulated, and there was no evidence of a lateral shift of the mandible, muscular asymmetry, or active otitis externa.
Differential diagnoses included atypical TMJ luxation, masticatory muscle myositis, and neoplasia. In preparation for induction of anesthesia for diagnostic imaging, serum biochemical analysesb were performed, revealing a mild elevation in alkaline phosphatase activity (218 U/L; reference range, 20 to 150 U/L). The dog was medicated with hydromorphonec (0.04 mg/kg [0.018 mg/lb], IV), acepromazined (0.02 mg/kg [0.009 mg/lb], IV), atropinee (0.008 mg/kg [0.0036 mg/lb], IV), and metoclopramidef (0.1 mg/kg [0.045 mg/lb], SC) 30 minutes prior to induction of anesthesia. Preparations for temporary tracheostomy were made; however, before anesthesia was induced, the jaw was manipulated and normal function was regained. Anesthesia was induced with propofolg (6 mg/kg [2.73 mg/lb], IV, to effect). Tracheal intubation was routine and did not cause jaw locking. Anesthesia was maintained with 1.5% isoflurane and 1.5 L of oxygen/min in a circle rebreathing system. The dog received 539 mL (10 mL/kg/h [4.54 mL/lb/h]) of lactated Ringer's solutionh and cefazolini (22 mg/kg [10 mg/lb], IV, q 90 min) during surgery. The dog was monitored throughout anesthesia with a lead-II ECG, capnograph, pulse oximeter, and sphygnomanometer. Body temperature was maintained with a recirculating warm water heating pad during the intraoperative period and with a forced-air warming systemj after surgery. No important physiologic abnormalities were detected during anesthesia; mild hypotension was managed with increased rate of administration of IV fluids (15 mL/kg/h [6.8 mL/lb/h]). Computed tomographyk of the head (120 kV [peak], 60 mA, and 3-mm slice thickness) was performed with the dog positioned in sternal recumbency. The first series of images was acquired with the TMJ extended (open mouth), followed by a neutrally positioned series (closed mouth) prior to and after IV administration of iodinated contrast medium.l The TMJs were apparently normal. No evidence of a mass or contrast agent–enhanced lesion was identified.
Fluoroscopicm examination of the zygomatic arches was performed to evaluate movement of the TMJs. With the patient in dorsal recumbency, porous tape was used to fix the position of the maxilla and calvarium. A segment of porous tape was also secured around the rostral aspect of the mandible to allow for distant manipulation of the TMJs during the evaluation. Fluoroscopy revealed that when the mandible was extended ventrally and slightly to the right, the rostrodorsal aspect of the coronoid process of the left mandibular ramus appeared to interfere with the orbital (medial) surface of the left zygomatic bone at the level of the frontal process (Figure 1). This interference precluded full extension of both TMJs; however, slight leftward displacement during attempted extension of the left TMJ allowed for full and repeatable extension of the joint without any interference. Closer examination revealed a mild asymmetry of the size of the frontal processes of the zygomatic bones; the process of the left zygomatic bone appeared to be larger than the process of the right zygomatic bone (Figure 2). It was concluded that the inability to open the jaw was a result of dynamic interference of the rostrodorsal aspect of the coronoid process of the left mandibular ramus with the medial surface of the frontal process of the left zygomatic bone or the left orbital ligament. No evidence of interference was detected between the coronoid process of the right mandibular ramus and the right zygomatic bone. The articulations of the right and left TMJs were normal.
Static frame ventrodorsal fluoroscopic image of the skull of a dog with closed-mouth jaw locking of the left side of the mandible. With ventral to slight rightward ventral extension of the mandible, the rostrodorsal aspect of the coronoid process of the left mandibular ramus appeared to interfere along the orbital (medial) surface of the left zygomatic bone at the level of the frontal process (arrow). With left ventrolateral extension of the mandible, interference between the left vertical ramus and frontal process was avoided.
Citation: Journal of the American Veterinary Medical Association 233, 5; 10.2460/javma.233.5.748
Static frame ventrodorsal fluoroscopic image of the skull of a dog with closed-mouth jaw locking of the left side of the mandible. With ventral to slight rightward ventral extension of the mandible, the rostrodorsal aspect of the coronoid process of the left mandibular ramus appeared to interfere along the orbital (medial) surface of the left zygomatic bone at the level of the frontal process (arrow). With left ventrolateral extension of the mandible, interference between the left vertical ramus and frontal process was avoided.
Citation: Journal of the American Veterinary Medical Association 233, 5; 10.2460/javma.233.5.748
Static frame ventrodorsal fluoroscopic image of the skull of a dog with closed-mouth jaw locking of the left side of the mandible. With ventral to slight rightward ventral extension of the mandible, the rostrodorsal aspect of the coronoid process of the left mandibular ramus appeared to interfere along the orbital (medial) surface of the left zygomatic bone at the level of the frontal process (arrow). With left ventrolateral extension of the mandible, interference between the left vertical ramus and frontal process was avoided.
Citation: Journal of the American Veterinary Medical Association 233, 5; 10.2460/javma.233.5.748
Static frame ventrodorsal fluoroscopic image of the skull of a dog with closed-mouth jaw locking of the left side of the mandible. Notice an asymmetry of the size of the frontal processes of the zygomatic bones, with the process of the left zygomatic bone (arrow) appearing larger than the process of the right zygomatic bone.
Citation: Journal of the American Veterinary Medical Association 233, 5; 10.2460/javma.233.5.748
Static frame ventrodorsal fluoroscopic image of the skull of a dog with closed-mouth jaw locking of the left side of the mandible. Notice an asymmetry of the size of the frontal processes of the zygomatic bones, with the process of the left zygomatic bone (arrow) appearing larger than the process of the right zygomatic bone.
Citation: Journal of the American Veterinary Medical Association 233, 5; 10.2460/javma.233.5.748
Static frame ventrodorsal fluoroscopic image of the skull of a dog with closed-mouth jaw locking of the left side of the mandible. Notice an asymmetry of the size of the frontal processes of the zygomatic bones, with the process of the left zygomatic bone (arrow) appearing larger than the process of the right zygomatic bone.
Citation: Journal of the American Veterinary Medical Association 233, 5; 10.2460/javma.233.5.748
On the basis of the results of fluoroscopy, excision of a portion of the left mandibular ramus, including the coronoid process, was recommended. The dog was prepared for surgery and positioned in right lateral recumbency. An approximately 8-cm skin incision was made over the left zygomatic arch. The underlying subcutaneous tissues and platysma muscle were incised along the same line, and care was taken to identify and preserve the dorsal and ventral buccal branches of the facial nerve and the parotid gland and duct. A periosteal incision was made along the left zygomatic arch to allow elevation of the masseter muscle from the mandibular ramus. Once exposed, the dorsal aspect of the mandibular ramus was excised to the level of the ventral aspect of the zygomatic arch with an oscillating saw. Subsequent manipulation of the jaw prior to closure could not reproduce the locking. The superficial layer of the masseter muscle was closed with 2-0 polydioxanone suturen in an interrupted cruciate pattern. The platysma muscle and subcutaneous tissues were closed with 3-0 poliglecaprone 25 sutureo in a simple continuous pattern. The skin was closed with 3-0 nylon suturep in an interrupted cruciate pattern.
The dog recovered from surgery. Subsequently, it received hydromorphonec (0.06 mg/kg [0.027 mg/lb], IV, q 6 h for 24 hours) and cefazolini (22 mg/kg, IV, q 8 h for 16 hours). The dog was able to eat soft food comfortably within 6 hours after surgery. Discharge instructions included feeding soft food only and restricting access to chew toys for 4 weeks to prevent possible discomfort for the dog. Treatment with tramadolq (3 mg/kg [1.36 mg/lb], PO, q 8 h as needed for pain), carprofenr (2 mg/kg [0.9 mg/lb], PO, q 12 h for 5 days) and cephalexins (20 mg/kg [9.1 mg/lb], PO, q 8 h for 10 days) was initiated. Sutures were removed 13 days after surgery. At that time, the incision had healed but remained mildly swollen and the dog was able to open its mouth fully. At a follow-up examination 6 months after surgery, the owners reported that they had not detected any additional episodes of jaw locking in their dog but that they heard a click or rubbing sound when the dog yawned. During physical examination, the dog allowed full extension of the jaw; however, crepitus was detected when the jaw was manipulated through a typical range of motion.
Discussion
The TMJ is a condylar joint that allows a wide range of motion; however, the joint capsule and lateral ligament should provide stability to prevent inappropriate movement of the jaw, such as that associated with openmouth jaw locking.1 In an animal with a typical dysplastic TMJ, the coronoid process of the mandibular ramus can be displaced lateral to the zygomatic arch causing an inability to close the jaw. Generally, the locking can be relieved when the jaw is opened wider, which allows for the coronoid process to return to the correct anatomic position. Findings in animals with open-mouth jaw locking secondary to TMJ dysplasia include a shallow mandibular fossa and lateral ligament laxity.2 To the authors' knowledge, the underlying cause of TMJ dysplasia is unclear but may be attributable to rapid growth in chondrodystrophic breeds, prognathism, and laxity of the mandibular symphysis.3–5 Temporomandibular joint dysplasia in several breeds has been described, but the Irish Setter4,6 and Basset Hound3,7 are overrepresented in the literature. Other breeds in which TMJ dysplasia has been detected include the American Cocker Spaniel,8 Cavalier King Charles Spaniel,5 Boxer,9 Dalmatian,10 Golden Retriever,9 Labrador Retriever,9 and Persian cat.11–14 Other reported causes of open-mouth jaw locking include trauma,15,16 oral foreign bodies,17 trigeminal neuritis,17 and callus formation following fracture.14,18
In contrast to jaw locking in the open position, the jaw locked in the closed position in the dog of this report. Differential diagnoses for inability to open the mouth include TMJ ankylosis from trauma or neoplasia, masticatory muscle myositis, extension of ear inflammation attributable to perforation of the bulla or aural canal, or craniomandibular osteopathy.17 To our knowledge, dynamic interference of the coronoid process of the left mandibular ramus with the medial aspect of the frontal process of the left zygomatic bone or the left orbital ligament has not been reported in the veterinary literature. Possible causes of this atypical jaw locking included congenital or developmental abnormal conformation of the zygomatic arch with overstretching of the lateral ligament and laxity of the TMJ, selective masticatory myopathy, and soft tissue dysplasia.19
Computed tomography was chosen as the first diagnostic procedure because of its excellent ability to model the bony components of the TMJ.20,21 The addition of contrast agent also helped rule out any neoplastic, inflammatory, or infectious processes, particularly in the retrobulbar space. In the dog of this report, CT was not able to reveal the cause of the closed-mouth jaw locking and fluoroscopy was required. Had the jaw locking not been relieved prior to performance of the CT scan, fluoroscopy may not have been needed; however, because of the dynamic interference discovered via fluoroscopy, real-time manipulation of the mandible was likely necessary to determine the cause of the closed-mouth jaw locking.
Surgical treatments of animals with open-mouth jaw locking include complete or partial zygomatic arch resection, partial excision of the coronoid process of the ramus, and unilateral or bilateral mandibular condylectomy.3,9,19,22–24 Partial or complete resection of the midzygomatic arch has been described as a surgical treatment for dogs with jaw locking.4 In the dog of this report, the site of zygomatic arch interference was the orbital (medial) surface of the zygomatic bone at the level of the frontal process. Because the interference was located rostrally in this English Bulldog, there was a concern that corrective surgery could result in orbital or facial deformity. Additionally, there was concern that resection of the zygomatic arch could cause disruption of the orbital ligament, which is an important consideration in a brachycephalic breed in which the orbital ligament forms a large proportion of the orbital margin.25 As a result of concerns associated with zygomatic arch resection, partial excision of the coronoid process of the ramus was performed. The crepitus detected during range of motion of the TMJ at the 6-month followup examination did not appear to cause jaw locking or discomfort; therefore, we do not recommend modification of the surgical procedure used.
We suspect that lateral TMJ ligament stretching in the dog of this report contributed to the laxity needed for movement of the left side of the mandible, and mild zygomatic arch asymmetry allowed for the dynamic interference of the coronoid process of the left mandibular ramus with the medial aspect of the frontal process of the left zygomatic bone or the left orbital ligament. Surgical excision of the coronoid process of the mandibular ramus appeared to provide a successful outcome in this dog.
ABBREVIATIONS
| CT | Computed tomography |
| TMJ | Temporomandibular joint |
Dexamethasone SP, Phoenix Pharmaceuticals Inc, St Joseph, Mo.
VetScan point-of-care analyzer, Abaxis Inc, Sunnyvale, Calif.
Hydromorphone, Baxter, Deerfield, Ill.
Acepromazine, Vetmedica, St Joseph, Mo.
Atropine, IVX Animal Health Inc, St Joseph, Mo.
Metoclopramide, Sicor Pharmaceuticals, Irvine, Calif.
Propoflo, Abbott Laboratories Inc, North Chicago, Ill.
Lactated Ringer's solution, Baxter, Deefield, Ill.
Cefazolin, Watson Lab Inc, Corona, Calif.
Bair Hugger, Arizant Healthcare, Eden Prairie, Minn.
GE Sytec SRi, Milwaukee, Wis.
Isovue, Bracco Diagnostics Inc, Princeton, NJ.
VET High Frequency Systems, Sedecal USA Inc, Arlington Heights, Ill.
PDS, Ethicon, Sommerville, NJ.
Monocryl, Ethicon, Sommerville, NJ.
Ethilon, Ethicon, Sommerville, NJ.
Tramadol, Caraco Pharmaceutical Laboratories Ltd, Detroit, Mich.
Rimadyl, Pfizer Animal Health, Exton, Pa.
Cephalexin, Ranbaxy Pharmaceuticals Inc, Jacksonville, Fla.
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