Crown-root ratio as an indicator for assessing orthodontic treatment-induced external apical root resorption

External apical root resorption (EARR) is a common sequel of orthodontic treatment. Yet, there is no simple, universal and reliable method to quantify this. This retrospective study assessed EARR by comparing crown-root ratios calculated before and after fixed orthodontic treatment as determined from digital orthopantomograms, using software.

Pre- and post-treatment digital orthopantomograms of 75 patients (37 males, 38 females) aged 15-30 years, who underwent fixed orthodontic mechanotherapy with a nonextraction protocol for 12-18 months, were selected. Crown and root lengths of specific maxillary and mandibular teeth were measured on these 150 digital orthopantomograms (OPG) using modified Lind method after calibration on the WebCeph software (version 1.5.0, Premium, Assemble Circle Corp, Seongnam-si, Gyeonggi-do, South Korea) and crown-root ratios were calculated. Statistical analysis was done using the 't' test.

The post-treatment crown-root ratios were significantly higher than pre-treatment ratios for all teeth in both males and females (p < 0.05). EARR assessed from crown-root ratios was highest for maxillary lateral incisors followed by maxillary central incisors, with least values observed for mandibular canines. No significant gender differences were noticed for crown-root ratios among all cases. Maxillary teeth experienced significantly more EARR compared to mandibular teeth, though this was not statistically significant for the second premolars.

Crown-root ratio is a useful means for evaluating EARR, as it is not affected by the changes in magnification. The study proved that this method may serve as a valuable adjunctive tool for clinicians to ascertain an important iatrogenic effect, usually encountered and often overlooked during and after orthodontic treatment.