When Should I Order an MRI?

The Piper MRI protocol is designed to offer valuable information to the restorative dentist, the specialist and the patient. MR imaging allows for the assessment of TM joint anatomy in patients who present with an increased likelihood of structural changes on the TM joints. Patients who have an increased likelihood of structural changes on the TM joint typically present with malocclusion or pain or both. In most cases, the malocclusion will be the first clinical manifestation of structural changes in the TM joint. Like other joints in the body, pain lags behind structural alteration.

In assessing the occlusion, the dentist has to decide what condylar position will be used to evaluate the bite. The two logical choices would be a fully-seated condylar position (CR if the TM joint is structurally intact) or maximum intercuspation. While maximum intercuspation is where many patients will function, there is a great deal of diagnostic information that can be gained from evaluating the occlusion in a fully-seated condylar position. When assessing the occlusion in a fully seated condylar position, it is possible to evaluate the anterior tooth relationship in both a horizontal and a vertical perspective. This allows for a determination of how close the anterior teeth are to providing anterior guidance in excursive movements. Assuming normal growth and development, most patients will present with the canines relatively close to contacting and providing anterior guidance.

If patients present with the canines significantly uncoupled (greater than 2mm), a possible cause may be the loss of vertical dimension in the TM joint. Structural alterations in the TM joint usually begin with a soft tissue breakdown that can lead to a change in how the anterior teeth contact. Understanding how the anterior teeth relate to vertical dimension changes in the TM joint gives the dentist an insight into patients who present with structural changes in the TM joint which do not present with a pain component. This becomes increasingly important in growing patients since structural changes in the TM joint can occur commonly in growing patients without a pain component.

When assessing the occlusion in maximum intercuspation, the muscles can position the mandible forward to allow for anterior tooth contact, and the ability to screen for structural changes in the TM joint is significantly diminished. Evaluating the occlusion at the tooth level allows the teeth to hide the skeletal defect. Evaluating the occlusion at a skeletal level (a fully-seated condylar position) offers a more accurate screening of the TM joints.

There are two assumptions when evaluating the occlusion in a fully-seated condylar position. The first is that tooth position has not been changed through orthodontic, restorative or orthognathic treatment. If tooth position has been altered, the clinician must take this into consideration when evaluating the occlusion.

The second is that the teeth are in a normal position from a three-dimensional perspective. If, for example, anterior teeth are retroclined and do not have a normal axial inclination, it would be necessary to imagine the teeth with a normal tooth inclination when evaluating the occlusion. The same would be true with supraerupted teeth as well.

Assessing the occlusion in a fully-seated condylar position also allows for an evaluation of pain through load testing. While load testing provides valuable diagnostic information, it is important to understand what information can be gained from load testing. One of the most important things that can be learned from load testing is whether the joint tissue can be compressed (similar to how it would be compressed in function) without pain. If light load testing produces pain through tissue compression, it may be prudent to assess TM joint condition since normal tissues can be compressed with significant forces without pain. The confusion in load testing occurs because some structurally-altered TM joints can be loaded without pain. While positive light loading usually implies structural alterations to the TM joint, negative load testing does not correlate to joint stability.

When dentists ask when patients should get an MRI, the usual answer is when a loss of TM joint dimension is suspected due to anterior tooth uncoupling or when TM joints are painful during light compression of the tissues. Like other dental conditions, the patient has to understand the problem and the benefits MR imaging will provide before they will obtain an MRI. Understanding the structure of the TM joints will help the dentist and the patient make treatment decisions that offer the patient the best chance for predictable treatment.

Dr. Jim McKee, Spear Resident Faculty