Centrically Related Cephalometrics

Articulated study models often reveal greater discrepancies in malocclusions than those noted clinically.1 Cephalometric radiographs taken in the habitual position of maximum intercuspation nearly always mask jaw relation discrepancies--particularly such vertical discrepancies as are seen in Class II open bite cases.

The alert reader can often spot enlarged superior joint spaces in published photographs of cephalometric studies. To us, these indicate that the mandible has fulcrumed around the second molars, or whatever most posterior teeth are in heavy centric occlusion. Roth calls this a "second molar protected occlusion", with a fulcrum in the second molar area around which the mandible rotates counterclockwise in closing.2,3

According to Guichet, the masticatory musculature usually brings the mandible downward to avoid posterior prematurities.4 Bell says that in such instances the thicker, lateral portion of the TMJ articular disc is pulled anteromedially to stabilize the joint, because vertical stability is the primary responsibility of the disc.5

A Simple Solution

A temporary jaw positioning splint, made from silicon impression material, can help provide accurate, centrically related study models and lateral cephalograms.

To make the splint, guide the mandible gently into the hinge position. Roll the material into a cylinder, which can be tapered at the ends if desired. Mold the cylinder over the lower eight anterior teeth.

Supporting the angles of the mandible from behind, close the jaw gently to first tooth contact. Take care not to allow eccentric deflection of the mandible. Close the lips to accelerate setting, then remove and trim the positioning splint.

If x-rays are taken at another location, as is the case in our practice, instruct the patient on proper placement of the positioning splint for radiography. The splint should be transported in a retainer case.

Discussion

Figure 1 shows a case in which a marked open bite required stabilization for accurate cephalometric radiography. Figure 2 demonstrates the splint.

Figure 3 compares cephalometric tracings from another case to show the difference between habitual occlusion and centric relation--in this case, a 3mm difference in incisor position. Failure to diagnose in centric relation can contribute to undesired results and prolonged treatment time. In addition, it should be recognized that articulation errors of 2-3mm are possible in published cephalometric growth studies, many of which discriminate between annual changes of as little as lmm.

Skeptics may argue that, in cases of anterior disc displacement, the jaw relation is not ideal anyway because of posterior condylar displacement, so why record a faulty position? Even if the condyle is not centered, failure to record the jaw relation--and not use this knowledge in treatment planning--is like planning a building without a proper survey.

This procedure adds no more than three minutes to a diagnostic records appointment. The more accurate the cephalometric procedure, the better informed the diagnosis will be.