Orthodontic tooth movement triggers complex and coordinated remodeling within the periodontal ligament (PDL) and adjacent alveolar bone. In certain clinical situations, such as cleft lip, alveolus, and palate cases, teeth must be guided into surgically augmented bone defects. Although the role of the PDL in tooth displacement is recognized, its response under these conditions is not completely clarified. This study evaluated the PDL after 23 weeks of tooth movement into critical-sized augmented defects. In four beagle dogs, second molars were orthodontically moved into critical-sized defects filled with bovine xenograft or nanocrystalline hydroxyapatite, with autogenous bone as the control. After 23 weeks, histological samples were examined under the microscope, and PDL dimensions were measured. Statistical analysis was conducted using the Wilcoxon–Mann–Whitney test. Across all graft materials, the PDL was consistently wider on the tension side than on the compression side (p ≤ 0.05), for both mesial and distal roots. By 23 weeks, PDL remodeling had stabilized, producing a wider ligament on the tension side, in contrast to the initial response observed shortly after force application.
