Dogs can have a locked jaw when they open the mouth widely. This should not be confused with a condition in which the patient cannot close the jaw due to paralysis of the trigeminal nerve. Lock jaw can be caused by dislocation of the temporomandibular joint (jaw joints). It can also be caused by laxity of the TMJ such that a portion of the jaw called the coronoid process gets hooked onto the zygomatic arch.
The mandible or lower jaw is connected to the skull by the temporomandibular joint (TMJ). The coronoid process is an extension of the mandible is located behind the zygomatic arch. The temporalis muscle attaches on the coronoid process and is functions to close the jaw. The body of the mandible contains the teeth. The zygomatic arch is a bridge of bone that connects the upper jaw to the back of the skull. This portion of bone also forms a portion of the eye socket. The masseter muscle is attached on the zygomatic arch and the back portion of the mandible and works together with the temporalis muscle to shut the mouth.
Pathophysiology of Lock Jaw
If there is laxity of the TMJ caused by a congenital malformation, from trauma or from chronically carrying heavy objects with the mouth, the mandible shifts to one side or the other and the coronoid process gets stuck on the zygomatic arch. As the muscles tighten or spasm due to pain caused by the impingement, the jaw gets locked in an open position. As the dog relaxes, opens the mouth wider and then shifts the position of the mandible in normal alignment, the jaw becomes unlocked.
Dogs that experience lock jaw may yawn and then have the jaw lock open. After a few moments, they typically can unlock the jaw. If the patient cannot close the mouth after a brief period of time, then another condition such as a dislocated TMJ should be suspected. Another cause of inability to close the mouth may be due to a foreign body impacted over the molar teeth. Finally, trigeminal nerve paralysis (motor division) may cause the inability of the mouth to close voluntarily by the dog, but can be easily manually closed by the pet owner or veterinarian.
Here is an example of a dog that has lock jaw:
The primary differential diagnoses for a dog that has lock jaw includes subluxation of the TMJ with mandibulozygomatic impingement, full luxation of the TMJ, tumor, foreign body on the molars and trigeminal nerve paralysis. Further characterizing the locking of the jaw as intermittent with the dog being normal between episodes narrows the list down to either intermittent mandibulozygomatic impingement or intermittent luxation of the TMJ. To confirm the cause of the problem, the veterinarian should observe if the mandible shifts to one side when the jaw locks. If so, then likely that same side is experiencing the impingement. With the patient sedated, the jaw should be opened and closed and then opened with simultaneous shifting of the mandible to one side and then the other side to see if the jaw will lock open.
3D CT scan with the mandible in the locked position will confirm the cause of the problem. Below is an example of a dog that has lock jaw. Take note that the coronoid process is jammed beneath the left zygomatic arch and the mandible is shifted to the left and somewhat tilted.