Lesion Morphology Modification Is Unnecessary for Non-carious Cervical Lesion Restorations: A Comparison of Stiff and Flowable Composites

This study investigates the impact of non-carious cervical lesion (NCCL) morphology - horizontal-oval-round (HOR) and horizontal-oval-wedge (HOW) - and 2 different restoratives - stiff composite (Filtek-Z350XT, Z) vs flowable composite (Filtek-Bulk-Fill Flowable, F) - on fracture risk and stress distribution in human teeth. Forty-five extracted sound maxillary first premolars were allocated into 3 groups (n = 15): sound (S), nonrestored HOR (R), and nonrestored HOW (W). Standardized NCCLs were prepared, and fracture strength was measured using a universal testing machine. One-way ANOVA and posthoc tests (P < .05) were used for statistical analysis. Validated 3D-FEA models were used to simulate von Mises stress under an average occlusal load (54.64 N) across 7 models: S, R, W, and restored counterparts using Z and F composites (RZ, RF, WZ, WF). Fracture strength was significantly lower in R (509.2 MPa) and W (520.6 MPa) compared with sound teeth (821.7 MPa; P < .001), with no difference between R and W (P > .05). In nonrestored NCCLs, stress concentrated at the lesion base (13-15 MPa), threefold greater than that in sound teeth. Restoration redistributed stress to 3 to 5 MPa, relocating peak stress to the palatal furcation. Bulk-fill flowable composite reduced interfacial stress (2.3-2.8 MPa) though slightly increased lesion-base stress, while stiff composite showed the inverse (3.0-3.5 MPa). NCCLs compromise tooth strength irrespective of morphology. Restoration improves stress distribution to near-physiological levels without lesion morphology modification. Clinically, this supports conservative restoration, with flowable composites offering biomechanical advantages, though their long-term wear resistance requires further validation.