Evaluation of the correlation between degree of conversion and shear bond strength in four contemporary orthodontic adhesives: an in vitro study

Background The success of fixed orthodontic treatment is critically dependent on the mechanical integrity of the bracket-enamel bond. Shear Bond Strength (SBS) and Degree of Conversion (DC) are two fundamental properties governing the performance of orthodontic adhesives. This study aimed to evaluate the relationship between DC and SBS in four contemporary orthodontic adhesives, including conventional and no-primer systems. Methods Sixty sound human premolars were randomly allocated into four groups (n = 15). Metal brackets were bonded using one of four adhesives: Transbond XT (conventional), Brace Paste (conventional), Orthocem (no-primer), or GC Orthoconnect (no-primer). Then specimens were stored for 24 hours and subjected to 3,000 thermal cycles. SBS was measured using a universal testing machine. The DC of residual adhesive was determined using Fourier-Transform Infrared (FTIR) Spectroscopy on a subset of ten specimens per group. The Adhesive Remnant Index (ARI) was assessed using stereomicroscopy. Data were analyzed using ANOVA, Tukey’s HSD test, Pearson's correlation, and Fisher's exact test (p < 0.05). Results Statistically significant differences were observed among the groups for both mean SBS (p < 0.001) and mean DC (p = 0.007). GC Orthoconnect exhibited the highest SBS (23.66 ± 5.77 MPa), while Orthocem showed the lowest (10.95 ± 3.31 MPa). Conversely, Orthocem demonstrated the highest DC (79.0 ± 13.8%), whereas Transbond XT had the lowest (58.7 ± 9.9%). Pearson's correlation test revealed no statistically significant correlation between SBS and DC for any of the tested adhesives (p > 0.05). ARI score distributions also differed significantly among the groups (p < 0.001). Conclusions All four adhesives provided clinically acceptable bond strength, and the adhesive remnant was greater in the 3-step groups. However, a higher degree of polymerization did not lead to superior shear bond strength. This finding indicates that the clinical performance of orthodontic adhesives is governed by a complex interplay of various factors beyond the bulk property of DC. Furthermore, composites with a two-step system can be a good clinical alternative for orthodontic treatments due to their adequate bond strength.