Evaluation of the biocompatibility of NiTi dental wires: A comparison of laboratory experiments and clinical conditions

Investigation of the Dissolution–Reformation Cycle of the Passive Oxide Layer on NiTi Orthodontic Archwires

“…The Nickel-Titanium (NiTi) shape memory alloys (SMAs) have been employed in biomedical applications at an increasing rate within the last decades owing to their superior thermo-mechanical properties [1][2][3][4][5][6][7][8][9][10]. Specifically, the NiTi alloys can assume two distinctly different shapes by means of solid-to-solid phase transformation between austenitic and martensitic phases upon temperature change or loading [2,5,[8][9][10][11], opening the venue for their utility in various dental, cardiovascular and orthopedic applications [2,7].…”

“…One of the well-known applications is the orthodontic archwire, which benefits from the pseudoelastic property of NiTi [2,5,7,9]: austenite-to-martensite phase transformation upon loading allows for attaining a state of constant stress for a relatively wide strain range [2,7,9,11], which eliminates the need for frequent readjustments of the orthodontic arch wire to keep the stress at an optimum level for simultaneous tooth movement and periodontal tissue regeneration despite the changing strain on the wires [9]. Shape memory gloves which are used for the physiotherapy of partially atrophied muscles benefit from another important property, namely the two-way shape memory effect, such that the natural motion of a hand is mimicked with the wires placed in the regions over the fingers which contract and stretch by heating and cooling [5,12].…”

“…. Even though the NiTi alloy has been employed as a material of choice in various biomedical applications, biocompatibility related issues have constituted a significant concern and continue to come under focus in various studies [1][2][3][4][5][6][7][8][9][10]13]. In particular, when NiTi alloys are implanted in the body they are attacked by the aggressive ions of the body fluids, such as chlorides, hydrocarbonates or sulfates [14].…”

“…Recently, evidence has been forwarded demonstrating that the Ni ion release is dictated by several parameters, including the body location where the NiTi implant comes into contact with tissue, surface morphology of the implant material, and the passive oxide layer on the alloy surface [8,9]. Among these parameters, the passive oxide layer plays a critical role since it influences the Ni ion release, which in turn affects the biocompatibility of the implant [2,6].…”

“…Thermodynamically, formation of titanium oxide is favored over that of nickel oxide since titanium oxidizes more easily, and particularly forms titanium dioxide, which is stoichiometrically the most favorable oxide on the alloy surface [8,9]. Indeed, the surface of NiTi implants is usually covered with a protective oxide layer during the manufacturing process to prevent ion release [2].…”