Drs. Sameshima and Sinclair reported upon "the mother of all root resorption studies," which was undertaken at the USC Dept. of Orthodontics. The study included 1,000 patients from six private practices in Southern California. Researchers analyzed periapical radiographs before and after fixed edgewise orthodontic treatment. As expected, the maxillary incisors demonstrated the greatest degree of resorption.

On average, 10% of the root length of the maxillary laterals is lost during orthodontic treatment. Six percent of these teeth had more than 4 mm of root length resorbed with 26% having lost more than 2mm. A second observation was that teeth with unusual root morphology are more prone to resorption.

Dilacerated and thin tapering roots experienced more resorption than blunted roots. Shorter teeth had less root resorption than longer teeth. Teeth with incompletely formed apices seem to be protected from resorption and adult mandibular teeth seem more disposed to resorb than their younger counterparts.

Males seemed to be more likely to have roots resorb than females but the differences were not large. Asian patients were less likely to have root resorption of the maxillary incisors while Hispanic patients were subject to more resorption. Horizontal movement of the incisors was directly correlated with resorption, as was duration of treatment time.

Slot size and wire type were not correlated with resorption. Extraction cases demonstrated a higher degree of resorption than non-extraction cases. There were large differences between practices in the number of cases with resorption but this may be due in part to the ethnicity of their patient populations.

Based on these results, caution should be exercised in adult patients with unusually shaped maxillary incisors with a large overjet and an extended treatment time. Dr. Sameshima recommends an expanded informed consent in cases where the risk factors favor resorption. He also feels that in these cases, progress films at six-month intervals are warranted.

When more than 2mm occurs on a progress, if film treatment is stopped for four to six months with passive wires, it can then be resumed with good success. Resorption stops when appliances are removed but can continue if a positioner is used for retention. Any occlusal interferences should be equilibrated to minimize resorption.

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