Relapse during the retention phase of orthodontic treatment remains a considerable problem, occurring in roughly 70–90% of cases. This study investigated the potential of blood cockle shells as a natural source for producing carbonated hydroxyapatite (CHA) combined with chitosan (CS), and examined its influence on orthodontic relapse in rats. Eighteen male Wistar rats were randomly assigned into two groups: a CHA–CS group and a control group (CG). A constant orthodontic force of 35 cN was applied for 7 days, followed by a passive phase. During this period, animals in the CHA–CS group received daily applications of CHA–CS hydrogel synthesized from blood cockle shell material. Afterward, the orthodontic devices were removed to allow relapse. The mesial tip distance was measured using a digital caliper on days 1, 5, and 7 post-debonding. Histological analysis with hematoxylin–eosin (HE) staining was used to quantify osteoblasts, osteoclasts, and fibroblasts. Data were evaluated with a t-test. On day 7, the relapse distance in the CHA–CS group was significantly less than in the CG. Microscopic evaluation revealed increased osteoblast and fibroblast counts and reduced osteoclast activity during relapse (p < 0.05). The findings suggest that CHA–CS derived from blood cockle shells can help minimize orthodontic relapse by enhancing bone and connective tissue formation while suppressing bone resorption.
