Effect of connector dimensions and span length on fracture resistance of high-translucency 4Y-TZP: An in vitro and finite element study

STATEMENT OF PROBLEM High-translucency 4-mol% yttria-stabilized tetragonal zirconia polycrystal (4Y-TZP) has been increasingly used for implant-supported prostheses because of its superior esthetics, yet connector size recommendations have been derived from studies of stronger conventional 3-mol% Y-TZP. Whether these guidelines remain valid for materials with 30 to 40% lower flexural strength is unknown. PURPOSE The purpose of this in vitro and finite element study was to evaluate the effect of connector cross-sectional dimensions and span length on the fracture resistance of high-translucency monolithic 4Y-TZP and to characterize stress distribution and concentration at the connector. MATERIAL AND METHODS Bar-shaped specimens incorporating 4 connector regions were milled from presintered high-translucency 4Y-TZP (KATANA Zirconia HTML PLUS A2). Four connector configurations were tested: 3×3 mm, 4×3 mm, 4×4 mm (elliptical cross-sections), and 5×4 mm (full cross-section control), with cross-sectional areas of 7.1, 9.4, 12.6, and 20.0 mm². The specimens were tested at 2 span lengths (22 and 42 mm) by 3-point bend testing (n=6 per group). Fracture load (N) was recorded, and flexural stress at the connector (MPa) was calculated from the local bending moment and the section modulus appropriate to each cross-sectional geometry. Three-dimensional finite element models determined maximum principal stress (σ₁) and stress concentration factors. Data were analyzed with 2-way analysis of variance (ANOVA), the Tukey honestly significant difference (HSD) test, and the Pearson correlation (α=.05). RESULTS Connector configuration (P<.001), span length (P<.001), and their interaction (P<.001) significantly affected fracture load. Mean ±standard deviation fracture load ranged from 208 ±14 N (3×3 mm, 42-mm span) to 1806 ±142 N (5×4 mm, 22-mm span) and correlated strongly with connector area (r≥.979, P<.001). Load gain per mm² of connector area was approximately twice as large for the 22-mm span (98.3 N per mm²) as for the 42-mm span (52.3 N per mm²). When load was converted to flexural stress at the connector, only connector configuration remained significant (P<.001); span length (P=.816) and the interaction (P=.162) were not significant. Connector stress converged into a narrow range (533 to 745 MPa) within published 4Y-TZP flexural strength values. Finite element analysis confirmed stress concentration at the connector base, with σ₁ spanning a factor of only 1.5 (613 to 939 MPa) despite an 8.7-fold range in fracture loads. Under equal applied load, stress concentration factors ranged from 1.67 to 3.92 at the 22-mm span and from 2.62 to 6.42 at the 42-mm span. CONCLUSIONS Reducing connector area disproportionately decreased fracture load, with greater reduction at longer spans. Connector stress values fell within a narrower range than fracture loads, suggesting that load differences are driven predominantly by geometry.