An Evidence-based Protocol for the Management of Orthodontic Patients Undergoing MRI Scans

Physical effects: radiofrequency-induced heating

Magnetic radiofrequency fields employed during MRI scans can induce electrical currents in metallic objects. The resultant thermal heating can cause localized tissue damage and lead to dysfunction of implanted medical devices.⁹

Studies have shown that pulpal health can be compromised at temperature increases of above 5.6°C within the pulp chamber. Alveolar bone necrosis can occur when there is a temperature rise above 7–10°C. The safety threshold for the periodontal ligament is a temperature rise above 10°C for greater than 1 minute.¹⁰

Hasegawa et al¹⁰ carried out an in vitro study on the effects of radiofrequency-induced heating on a full-arch fixed orthodontic appliance with brackets, molar bands and stainless steel wire and found that the temperature change was +2.61°C. Although this temperature increase would be insufficient to cause damage to the surrounding tissues, the authors postulated that orthodontic appliances may exhibit heating above the European industrial standard (CENELEC standard prEN45502-1) and concluded that the wire should be removed, or a spacer placed between the appliance and the oral mucosa duringMRI.¹⁰

Görgülü et al³ evaluated the effects on various combinations of brackets, archwires and ligatures of radiofrequency heating and magnetic interactions during 3-T MRI. They found that none of the groups exhibited excessive heating, with the highest temperature change being <3.04°C. They concluded that the temperature changes were within acceptable ranges and not high enough to affect the pulpal or mucosal health of the oral cavity. However, the authors also noted that nickel–titanium and stainless steel wires caused magnetic interactions that may pose a risk of movement in situ and recommended their removal before MRI.²

Mechanical effects: magnetically induced displacement

A well-known and potentially catastrophic effect of MRI is the mechanical displacement of objects containing ferromagnetic and other magnetic materials. The strong static magnetic field of the scanner can exert an attractive or a torque force on objects within or near the field. This is known as the ‘projectile effect’ and can pose significant risks to patients. There can be movement of ferromagnetic materials, which can cause discomfort of injury to patients.⁹ Studies have been carried out to evaluate the mechanical displacement effects of fixed orthodontic appliances and have had mixed conclusions. The general consensus however, appears to be that fixed orthodontic components are safe to use in MRI scanners as long as they are securely attached.^1,8,11,12

Görgülü et al³ studied the effects of fixed orthodontic appliances on magnetic interactions during 3-T MRI and found that, while the average deflection angle for stainless steel brackets was 13°, the deflection angles for nickel–titanium archwires and stainless steel archwires were 71° and 62°, respectively. A deflection angle of <45° indicates that the deflection force is less than the force of gravity, and that the device does not pose any risk during MRI. The torque force exhibited by the brackets was also found to be mild. Nickel–titanium wire exhibited moderate torque and stainless steel wire exhibited strong torque. The authors concluded that, while brackets do not present a risk during 3-T MRI and can be considered ‘MR safe’, nickel–titanium and stainless steel wires pose a potential risk. They went on to advise that the wires be removed prior to imaging for the patient's safety.²

Zachriat et al⁵ stated that acceleration and torsion have a negligible impact on attachments that are satisfactorily fixed. The authors cited a study by Sung et al¹³, who observed forces of 61–104N required for debonding in different bracket materials. They also cited Klocke et al¹⁴, who found that a complete orthodontic appliance consisting of stainless steel brackets, ligature wires and arch wires only experienced forces of 1.69N when exposed to a strong 3-T magnetic field.⁵

Artefact formation

An artefact may be defined as ‘a distortion in MR image signal intensity with no identifiable anatomical source in the imaging field’¹⁵ or, more simply, ‘pixels that do not faithfully represent the tissue components under study’.¹ Factors that can lead to artefact formation include patient movement, fluid flow and the presence of metallic objects, such as pacemakers, surgical clips, foreign bodies and dental materials.^16,17

Metal artefacts occur at the interfaces between soft tissue and soft tissue or ferromagnetic metal, where there are differing magnetic susceptibilities. There is a distortion of the external magnetic field by local magnetic fields that can lead to image distortion and signal loss.¹⁷ The size or shape of an image can be distorted, the extent and severity of which being dependent on the metallic object's size, shape, location, orientation, its distance from the anatomical region being imaged, the nature of the alloy, and the MRI sequence and sequence parameters used.^{1,4,7,11,18,19}

Dental alloys are a major source of metal artefacts. Certain metallic components of fixed orthodontic appliances have been shown to produce significant artefacts that can render MRI scans difficult or impossible to interpret.^3,20Table 1 summarizes the evidence on the effects of archwires, brackets and bonded retainers on MRI scans.