This study evaluated,over a 4-month study period,the amount of apical root resorption occurring in maxillary central incisors following their retraction when employing either micro-implant or J-hook headgear anchorage.The prospective randomised clinical trial was conducted in Orthodontic Clinic,College of Stomatology,China from 2008-2009.Subjects are patients requiring fixed appliances on waiting list (n=20).In female Han Chinese patients aged from 16-26 years,standardized periapical radiographs from 10 randomly assigned patients with maxillary protrusions comprising the micro-implant group,and from 10 similar patients comprising the J-hook headgear group,were assessed for maxillary central incisor apical root resorption.Measurements before and after orthodontic therapy were also obtained from lateral cephalometric radiographs to calculate incisor horizontal retraction and vertical intrusion distances.Estimated retraction force vectors were calculated in horizontal and vertical directions for both treatment groups.Data analysis employed t-tests and the Pearson correlation test,with α=0.05 for statistical significance.The results showed that when compared with the J-hook group,significantly more apical root resorption shortening of the maxillary central incisors was observed in the micro-implant group (1.27 mm difference,95% CI=0.70-1.84,P<0.001),which was associated with a significantly larger retraction distance (P=0.004) and a smaller vertical force component (P<0.0001).We are led to conclude that continuous activation of the nickel-titanium coil springs used in the micro-implant group resulted in significantly more apical root resorption shortening and maxillary central incisor retraction than when intermittent J-hook retraction was employed.The employment of continuous duration orthodontic forces presents a risk for increased apical root resorption that requires careful radiographic monitoring. This study evaluated, over a 4-month study period, the amount of apical root resorption occurring in maxillary central incisors following their retraction when employing either micro-implant or J-hook headgear anchorage. The prospective randomized clinical trial was conducted in Orthodontic Clinic, College of Stomatology, China from 2008-2009. Subjects are patients requiring fixed appliances on waiting list (n = 20) .In female Han Chinese patients aged from 16-26 years, standardized periapical radiographs from 10 randomly assigned patients with maxillary pins comprising the micro-implant group, and from 10 similar individuals comprising the J-hook headgear group, were assessed for maxillary central incisor apical root resorption. Measurement before and after orthodontic therapy were also obtained from lateral cephalometric radiographs to calculate incisor horizontal retraction and vertical intrusion distances .Estimated retraction force vectors were calculated in horizontal and vertical directi ons for both treatment groups. Data analysis employed t-tests and the Pearson correlation test, with α = 0.05 for statistical significance. The results showed that when compared with the J-hook group, significantly more apical root resorption shortening of the maxillary central incisors was was observed in the micro-implant group (1.27 mm difference, 95% CI = 0.70-1.84, P <0.001), which was associated with a significantly larger retraction distance (P = 0.004) and a smaller vertical force component We are led to conclude that continuous activation of the nickel-titanium coil springs used in the micro-implant group resulted in significantly more apical root resorption shortening and maxillary central incisor retraction than when intermittent J-hook retraction was employed. The employment of continuous duration orthodontic forces presents a risk for increased apical root resorption that requires careful radiographic monitoring.