Frictional Behaviour and Biomechanical Performance of Polycarbonate Orthodontic Brackets: A Comprehensive Critical Review

Polycarbonate brackets were among the earliest aesthetic alternatives to stainless steel fixed appliances, and reinforced polycarbonate designs remain in clinical use for patients who prefer less conspicuous treatment. Their biomechanical credibility has been questioned since their introduction because polycarbonate is a comparatively soft, viscoelastic thermoplastic whose slot geometry, surface condition and stiffness determine how efficiently an archwire can slide and how faithfully third-order torque can be transmitted. This critical narrative review examines the laboratory, simulation and limited clinical evidence on the frictional behaviour and biomechanical performance of polycarbonate orthodontic brackets, with attention to reinforcement strategy, slot design, ligation, ageing and test methodology. Literature was identified through structured searches of biomedical, multidisciplinary and regional scholarly indexes, supplemented by citation and related-article searching, and was appraised qualitatively for material characterisation, control of test conditions, analytical adequacy and clinical transferability. The synthesis indicates that frictional rankings of polycarbonate brackets relative to stainless steel and ceramic brackets are inconsistent, largely because test configurations, ligation forces, archwire combinations and product compositions differ, and because compliant slots can deform rather than bind. The evidence on torque is more coherent: unreinforced and particle- or fibre-reinforced polycarbonate slots deform, lose torque and creep under clinically relevant moments, whereas stainless steel slot inserts consistently improve torque stability without reliably reducing resistance to sliding. Bond strength to polycarbonate bases is generally lower than to metal or ceramic bases but appears adequate for many adhesive combinations, while intraoral ageing, water sorption, hydrolytic degradation and discolouration modify surface and bulk properties over time. Major uncertainties concern the clinical significance of laboratory friction values, the long-term dimensional stability of polymer slots in service, inconsistent identification of bracket polymers and the absence of controlled clinical trials linking bracket material to treatment efficiency. The available evidence supports metal-slot reinforcement as the minimum requirement for polycarbonate brackets intended for torque-dependent mechanics and calls for standardised, clinically anchored testing before newer polymer and additively manufactured brackets are regarded as equivalent alternatives.