By John R. Lewis, VMD, DAVDC, Fellow, AVDC OMFS

Do you know the pencil trick? It will be a handy addition to your knowledge base, whether you are involved in emergency medicine, general practice, or dentistry/oral surgery specialty practice. In this month's column, it will prove helpful when treating temporomandibular joint (TMJ) dislocations.

TMJ dislocations, also known as TMJ luxations, occur most commonly due to significant trauma and are more common in cats than in dogs. Dislocation is a term referring to a separation of two bones that meet within a joint.

In the case of the temporomandibular joint, the two bones that meet are the condylar process of the mandible and the mandibular fossa of the temporal bone. The term "luxation" is used synonymously with "dislocation." However, subluxation is a condition where the condyle translates rostrally, but returns to the fossa on its own.

Identifying TMJ dislocations

TMJ dislocations almost always occur due to a rostrodorsal displacement of the condylar process. Clinically, this results in a protrusion of the mandible, with a shift of the mandible to the side opposite the TMJ dislocation. The teeth contact each other, resulting in inability to fully close the mouth (Figure 1).

Radiographically, TMJ dislocation is easy to identify on a three-dimensional reconstruction of a CT scan, but even if your practice does not have CT capability, the diagnosis can be made on radiographs of the TMJ either with conventional skull films or extraoral dental films of the TMJ with a size 4 phosphor plate. The radiographic appearance on a dorsoventral radiograph shows an increased space between the mandibular fossa and the caudal aspect of the condylar process (Figure 2). Performing a CT scan is helpful to assess for possible mandibular fractures/condylar fractures that might be too subtle to see on a radiograph.

The pencil trick

We often will perform a closed reduction of a TMJ dislocation with a common household item: a pencil. Not just any pencil, but rather a good old-fashioned wooden pencil with a hexagonal cross-section. It should be wood so it will not fracture teeth when used as a fulcrum. It should be a hexagonal cross-section so the pencil can be spun if leverage is needed to encourage the condylar process to relocate into its fossa.

Here's the technique: place the patient in sternal recumbency under general anesthesia. Place the pencil on the side of the dislocation between the maxillary cheek teeth (third or fourth premolar) and the mandibular cheek teeth (fourth premolar or first molar). The pencil does not span across to the opposite mandible (Figure 3).

Close the mouth at the front while holding the pencil level between the teeth. If the condylar process does not reduce, twist the pencil in a counterclockwise direction on the right and a clockwise direction on the left when looking at the pencil's cross-section from outside the mouth. The slightest bit of extra leverage may allow for the condylar process to reduce into the fossa.

It will occasionally be obvious when the condylar process reduces back into the fossa, but sometimes the return to its normal location is subtle. If the condylar process returns to the fossa, the shift of the mandible and protrusion of the mandible will resolve. A repeat radiograph or CT scan can be utilized to confirm position of the condylar process. Once confirmed, move the mandible laterally in both directions to see if the condylar process stays within the fossa or if it repeatedly luxates. If the TMJ repeatedly luxates, it may be advisable to perform a temporary maxillomandibular fixation to keep proper occlusion while the torn ligaments heal.¹

Other diagnoses

TMJ dislocation is not the only possible diagnosis that results in inability to close the mouth, and it is important to distinguish between it and other differentials, since the pencil trick will not fix other problems and may make them worse.

When a cat presents with inability to close its mouth, there are a variety of possible causes. Something as simple as a luxated tooth can result in a malocclusion and may prevent the ability to close the mouth. Teeth can become luxated due to trauma, such as falls from heights, motor vehicle trauma and fights with other animals. We often also see luxated teeth due to advanced periodontal disease.

Another reason for a sudden inability to close the mouth is a condition called open-mouth jaw locking (OMJL). This occurs most commonly in cats due to laxity/subluxation of the temporomandibular joint, which allows for the coronoid process of the mandibular ramus to get caught on the ventral aspect of the zygomatic arch. Due to TMJ laxity, something as simple as yawning might result in an episode of OMJL. The main clinical sign of OMJL that differentiates it from TMJ dislocation is with OMJL, there is no tooth-to-tooth contact. The mouth is locked in a much more wide-open position. (To learn more about treatment of OMJL, see my previous article on the topic.)

Another reason for an inability to close the mouth would be a mandibular fracture. With a mandibular fracture, the mandible shifts toward the side of the fracture due to collapse of the fragments on top of each other. In contrast, a TMJ dislocation causes a shift to the side opposite the dislocation. Diagnosis of caudal mandibular fractures/fractures of the condylar process can be easily made with CT scan. Radiographs may miss subtle fractures.

Use of the pencil trick for either OMJL or a mandibular fracture will not help anything and may hurt the situation. However, when a TMJ dislocation is the confirmed diagnosis, pull out that pencil to feel like a hero!

John Lewis, VMD, DAVDC, FF-OMFS practices and teaches at Veterinary Dentistry Specialists and Silo Academy Education Center, both located in Chadds Ford, Pa.

References

1. Goodman AE, Carmichael DT. Modified Labial Button Technique for Maintaining Occlusion After Caudal Mandibular Fracture/Temporomandibular Joint Luxation in the Cat. J Vet Dent. 2016;33(1):47-52. https://pubmed.ncbi.nlm.nih.gov/27487655