A root-extended overlay design and finite element analysis for permanent molars with a 4-wall defect in adolescents

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PURPOSE: To develop a newly designed root-extended overlay (REO) for adolescents' permanent mandibular first molars (M1Ms) with a 4-wall defect and to analyze the stress distribution of REOs using finite element analysis (FEA). METHODS: A post-endodontic M1M was 3D reconstructed from Cone-Beam Computed Tomography, and the model was then modified to simulate a 4-wall defect located 2 mm above the enamel-dentinal junction. Subsequently, REOs with 0-2 mm distal root extensions were designed using Ti-6Al-4V (3D printed), lithium disilicate (IPS e.max Press), and composite resin. In FEA, vertical (400 N) and lateral (225 N) loads were applied to evaluate the maximum von Mises stress (mVMS) and principal stress. RESULTS: Ti-6Al-4V, lithium disilicate, and composite resin exhibited similar stress distribution. Under vertical loading, the 2-mm group showed the highest mVMS in the restorations. Additionally, the 2-mm group exhibited the minimal tensile stress in the alveolar bone. The 2-mm group with Ti-6Al-4V minimized tensile stress in the cement layer and tooth structure under lateral loading. CONCLUSIONS: The REO design was feasible for M1Ms with a 4-wall defect. The stress distribution of 3D printed Ti-6Al-4V was comparable to that of lithium disilicate. A 2-mm root extension exhibited optimal stress distribution across the restoration, cement layer, tooth structure, and periodontal tissues. This optimized stress distribution may reduce root fracture, debonding, and periodontal overload, supporting long-term preservation of mandibular first molars during occlusal and craniofacial development.

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