Comparative Analysis of Streptococcus mutans and Lactobacillus spp. Colonization on Stainless Steel versus Zirconia Crowns in Root Canal-Treated Teeth.

INTRODUCTION: Root canal-treated teeth are often restored using crowns to maintain function and protect against damage. The choice of crown material, such as stainless steel or zirconia, may influence microbial colonization and potentially affect restoration longevity. Bacteria commonly found in the oral environment, such as Streptococcus mutans and Lactobacillus spp., may adhere to crown surfaces and form biofilms. Differences in surface properties such as roughness and hydrophilicity may affect bacterial adhesion and biofilm formation. This study aimed to quantify and compare the microbial loads of S. mutans and Lactobacillus spp. on stainless steel and zirconia crowns in RCT-treated teeth and to characterize the species diversity within biofilms formed on these surfaces. MATERIALS AND METHODS: This prospective, non-randomized, clinical study enrolled 20 patients requiring crowns on root canal-treated lower first molars using a split-mouth design. The patients received stainless steel (3M ESPE AG, Seefeld, Germany) and zirconia (VITA Zahnfabrik, Bad Säckingen, Germany) crowns based on clinical indications and preferences. Oral prophylaxis was performed using ultrasonic scalers (E.M.S. Electro Medical Systems S.A., Nyon, Switzerland), and crowns were placed within 14 days of RCT by using an endodontic system (Dentsply Sirona Inc., Charlotte, North Carolina, United States). Impressions were made with light body polyvinyl siloxane putty (Elite HD+; Zhermack SpA, Badia Polesine, Italy), and marginal fit was verified clinically. Microbial samples were collected at baseline, one month, and six months using sterile paper points, and the gingival and plaque indices were recorded. Quantitative polymerase chain reaction (qPCR) using a LightCycler 480 System (Roche Diagnostics Corporation, Indianapolis, Indiana, United States) was used to quantify S. mutans and Lactobacillus spp. Data were statistically analyzed. RESULTS: Both crown types showed significant increases in plaque index, probing depth, S. mutans, and Lactobacillus spp. over time (p = 0.001). Stainless steel crowns exhibited significantly greater increases in all parameters than zirconia crowns (p < 0.05), with the largest difference in Lactobacillus spp. colonization. CONCLUSION: Zirconia crowns demonstrated lower microbial colonization than stainless steel crowns, suggesting greater biocompatibility for root canal-treated teeth. These findings support zirconia as the preferred material for minimizing microbial-related complications in restorative dentistry.