Effects of early primary molar loss on immature permanent molar mechanics: a finite element comparison of transpalatal arch and nance appliances

To compare the biomechanical effectiveness of the TPA and Nance appliance in controlling displacement and stress distribution of an immature maxillary first permanent molar under various clinically relevant mesial edentulous conditions. A three-dimensional maxillary model was reconstructed from cone-beam computed tomography data and used to generate ten distinct scenarios representing unilateral, bilateral, and combined primary molar deficiencies, both with and without space maintainers. Appliances were digitally designed and adapted in ANSYS SpaceClaim, and finite element simulations were performed under a 70-N occlusal load to quantify tooth displacement and Von Mises stress values. Across all ten simulated scenarios, premature loss of primary molars resulted in increased mesial tipping, occlusal displacement, and higher stress concentration within the periodontal ligament (PDL). The greatest crown displacement was observed in Model 8 (unilateral loss of primary first and second molars), reaching 0.002999 mm along the Y-axis, whereas bilateral second primary molar loss (Model 5) produced the highest vertical displacement (0.007329 mm along the Z-axis). In general, stress intensity and distribution area increased as mesial support decreased, particularly in bilateral deficiency models without appliance reinforcement. The highest Von Mises stress values were observed in models without space maintainers, while both TPA and Nance applications reduced stress transmission to the PDL. Among the appliances, the Nance appliance demonstrated a more pronounced reduction in displacement and stress concentration compared to the TPA. Both space maintainers helped preserve molar position following premature primary molar loss, with the Nance appliance exhibiting superior biomechanical performance. The TPA remained a viable alternative in cases where acrylic intolerance or hygiene limitations contraindicated Nance use. The evaluation of ten different clinical deficiency patterns under identical modeling conditions provides comprehensive biomechanical insights to support individualized space-maintenance decisions.