2013 Radiology Exam - Musculoskeletal Case 2

History

  • Feline, domestic, male neutered, approximately 2 years old.
  • Found outside vocalising.
  • Brought in by a good Samaritan

Observation

  • Rostrodorsal displacement of the left mandibular condyloid process relative to the mandibular fossa and retroarticular process of the temporal bone. This causes rightward displacement of the mandible and malocclusion, seen on the DV projection, and overlap of the condyloid process and mandibular fossa, best seen on the left ventral oblique.
  • Widening of the mandibular symphysis.
  • Submandibular swelling, more prominent on the left.
  • Fracture of the crown of the left maxillary canine.

Synthesis

  • Traumatic luxation of the left temporomandibular joint and mandibular symphyseal fracture causing malocclusion and traumatic submandibular swelling (hematoma, edema).

Case management

  • Potentially, no further case management from an imaging standpoint and reduction of the luxation.
  • Other projections are beneficial to further evaluate TMJ anatomy: open-mouth oblique and open-mouth rostrocaudal projections. However, care must be used not to further the patient’s injury and the ability to open the mouth is often limited.
  • CT would be beneficial to rule out further traumatic lesions (i.e. fractures) that might not be seen on the skull radiographs, although none were present in this case.

Traumatic TMJ luxation with symphyseal fracture is a fairly common traumatic skull injury in cats. One of the keys to identifying TMJ luxation is using the DV projection to evaluate mandibular asymmetry and rostral displacement of the coronoid process relative to the mandibular fossa and retroarticular process. In this case, the left condyloid process is seen displaced rostral to the rostral edge of the zygomatic process of the left temporal bone. Good knowledge of specific anatomical structures is required and comparison of the right and left sides is helpful. Several candidates did not appear to be familiar or comfortable with normal anatomical terminology of this area.

Most candidates identified the mandibular asymmetry and accompanying malocclusion, either on the DV or oblique projections. However, it was frequently attributed to unilateral or bilateral mandibular fractures as opposed to TMJ luxation. Overriding of the left mandibular fossa and angular process of the left hemimandible was incorrectly interpreted as a fracture line. On the right lateral and right ventral oblique projections, the normal right TM joint space was incorrectly interpreted as a fracture line. In situations where a right mandibular fracture was identified, candidates often did not notice that soft tissue swelling was asymmetric and greater on the left, indicative of a left-sided lesion.

Alternatively, the mandibular asymmetry visible on the DV projection was attributed to poor patient positioning, although the calvarium and maxilla could be used to assess that the patient was indeed straight. The positioning of the vertical and horizontal rami of the left hemimandible differed from that of the right hemimandible because of the displacement and outward rotation caused by the luxation and symphyseal fracture. This also lead to the molars and premolars appearing different between the left and right sides and several candidates incorrectly diagnosing missing molars/premolars.

Many candidates did not identify the symphyseal and maxillary canine crown fractures, although these were more minor findings.

For a really nice review of radiographic anatomy of the TMJ: Schwarz et all, VRUS 2002, 43(2); 85-97