Thermo-mechanical loading of innovative "Mini-skirt" preparation design versus conventional full-coverage maxillary central incisor preparation design: an in vitro study.

Abstract

Background: Conventional full-coverage anterior crown preparations for zirconia often require substantial enamel and dentine reduction and may concentrate palatal-cervical stresses, risking marginal discrepancy and compromised seal. A conservative "Mini-skirt" (MS) design that positions the palatal finish line coronally (above the cingulum) could preserve tooth structure and enhance adhesive support, but its thermo-mechanical performance under thermal cycling and fatigue loading remains unclear.

Purpose: To evaluate whether a conservative MS anterior preparation, with a palatal finish line above the cingulum, improves the thermo-mechanical behavior of generation-5 zirconia crowns compared with a conventional full-coverage (CF) design.

Methods: Sixty extracted maxillary central incisors were randomly assigned to MS or CF groups (n = 30). Monolithic 4Y zirconia crowns were CAD/CAM-manufactured, air-abraded and adhesively luted. Specimens were thermocycled 10,000 times between 5 °C and 55 °C, then obliquely loaded at 45° for 1.7 × 10⁶ cycles (50 N, 2 Hz); survivors underwent static fracture testing (0.5 mm·min⁻¹). Marginal gaps were quantified from silicone replicas; failures were classified Grades I-IV (restorable = I-III). Finite-element models (~ 1.3 M elements) reproduced boundary conditions to map stress.

Results: The MS preparation endured thermo-mechanical ageing markedly better than CF: 93.3% (28/30) versus 53.3% (16/30) of crowns completed the fatigue protocol (χ² = 9.01, p = 0.003). Post-fatigue, median fracture load increased from 860 N (IQR 780-930 N) in CF to 1,140 N (1,030-1,260 N) in MS (p < 0.05). Mean marginal gap contracted from 112 ± 21 μm to 67 ± 14 μm (40% reduction, p < 0.001). Restorable fractures predominated in MS (83%) but were minority in CF (37%); catastrophic Grade IV failures fell from 63% to 17% (χ² = 13.6, p < 0.001). Finite-element analysis corroborated the laboratory data, showing a 40% drop in peak palatal-cervical von Mises stress (415 to 250 MPa) and parallel reductions in tensile dentine stress and cement-shear hotspots.

Conclusions: Within the limitations of this in vitro study, the MS preparation showed favorable biomechanical performance under simulated thermo-mechanical loading. These findings provide a preclinical rationale for further investigation of the MS design, although long-term clinical studies are required before definitive clinical recommendations can be made.