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The concept of interproximal enamel reduction (also referred to as IPR or stripping) as a means of providing space to alleviate a crowded dentition was first introduced in orthodontics as early as the 1960s. The pioneer in the introduction of this approach was John Sheridan, a renowned clinician in orthodontics. His primary contributions included the introduction of the clear Essix retainer, a prelude to today’s aligners. He was also an advocate of interproximal air rotor stripping, which he used primarily for adult patients as a method to gain dental arch space to avoid extractions.
The IPR technique has also been applied in fixed appliance treatment, in order to address a Bolton discrepancy, correct black triangles and increase stability of the final outcome. The significance of IPR in clear aligner treatment is more important throughout the treatment compared with fixed appliances. During the levelling phase in fixed appliance treatment, the round nickel–titanium wires allow free labial movement of teeth; thus, small collisions on marginal ridges do not tend to block the movement. On the contrary, with clear aligner therapy, the plastic aligner material covers the labial surface of the teeth completely, and the teeth cannot spontaneously move to a larger arch perimeter; because of this effect, any marginal ridge collision that may occur can influence the aligner’s performance. Maintaining incisor inclination is better controlled in aligner treatment, and aligners have an advantage in this aspect. Interproximal reduction can be an attractive alternative to tooth extraction in patients with mild to moderate crowding because it allows the transverse arch dimensions and anterior tooth inclinations to be maintained. With the increasing popularity of aligner movement in recent years, as more adult patients are seeking aesthetic non-extraction orthodontic treatment, it appears that interproximal reduction has become a more common practice in the orthodontic office.
Nowadays, most aligner digital set-up software shows the exact staging of the movement and the exact point where IPR must be applied, as well as the required amount of IPR. In this article, we will discuss the steps of an IPR approach in aligner treatment, covering proper treatment planning, clinical tips on IPR application and proper finishing techniques. ClinCheck software (Align Technology) will be used as an example of how to handle IPR in treatment planning, followed by a description of the clinical procedure using a vibratory oscillating device.
ClinCheck software can suggest the location of IPR based on the clinician’s treatment criteria, that is, the final labial position of the teeth. The software can be automatically adjusted to detect collisions and apply the necessary amount of IPR if required, an amount that the clinician can also manually adjust. An important principle is that with aligner treatment the clinician can specify the exact envelope of movement, setting a labial or buccal limit of the movement and avoiding any unwanted expansion or proclination of the teeth. Based on this principle, the software will calculate the amount of IPR for the specific arch width. Using the 3D control interface with ClinCheck , the clinician can see real-time changes and view occlusal contacts continuously during modifications.
There is also the possibility of modification of the stage at which IPR will be performed, and this is entirely controlled by the clinician. Performing the IPR with contacts as occlusally parallel as possible is paramount to avoid irregularities. Rotations and misaligned teeth can result in irregular enamel reduction and non ideal final interproximal contacts (Figs. 1 & 2).
How much IPR is considered safe?
The amount of IPR that is to be performed in a specific contact must be carefully considered and relative to the enamel thickness of the teeth. Using an IPR gauge is essential (Fig. 3). For example, since enamel thickness in mandibular incisors is on average 0.8 mm, IPR in the range of 0.2–0.5 mm per contact (if performed to 0.10–0.25 mm per tooth surface) should not be harmful for the tooth. The clinician also must consider that the distal surface of the tooth has slightly thicker enamel; therefore, focusing on that surface between two contacts is ideal. I personally tend to limit IPR on a first ClinCheck treatment plan to a value of 0.3 mm, if possible, as usually we may need some extra tenths of a millimetre on a refinement set-up. As a personal rule of thumb, I do not exceed 0.5 mm per contact. ClinCheck software may be set to alert the clinician when a non conventional IPR value is entered and to add all IPR that has been performed in previous treatments.
When to apply IPR
Many clinicians who are using various aligner systems chose to perform all IPR in the beginning of the treatment despite the fact that it may be needed at different stages in the treatment. I strongly disagree with that, as it may cause unnecessary enamel reduction, irregularities on the marginal ridges when they are not parallel and reduced treatment efficiency. I follow the exact staging that I planned on the software, deviating to a maximum of one to two aligners before the stage of planned IPR and never after.
Checking interproximal contacts chairside
Any interproximal blockage in the neighbouring areas (not where IPR will be performed) may also cause alignment problems. All contacts must be frequently checked with dental floss (Fig. 4), and if needed they can be cleared with a thin manual IPR strip without significantly reducing tooth substance. This is recommended after the first few weeks of aligner treatment, as contacts tend to become looser after that period. Areas of severe crowding must be checked regularly for heavy interproximal contacts.
Keeping the symmetry on the tooth surface
Although IPR, when done within reasonable values, does not alter tooth size noticeably to the naked eye, the clinician has a digital tool to help him or her achieve symmetry when performing this procedure. ClinCheck software provides the Bolton analysis table with measurements for all tooth sizes (Fig. 5). Using this table, the clinician can chose to apply more of the IPR on the larger tooth surface when comparing the two same opposing teeth. I would like the future software evolution to incorporate a feature on the 3D model to guide the clinician towards the surface that needs more reduction (perhaps a highlighted tooth surface on the virtual model). As Align’s protocols are regularly enhanced with new features, I have communicated this wish to Align’s development team and hope to see it incorporated soon. The latest software development is the live update feature, which enables real-time viewing of the changes and staging pattern without having to send the modifications back to the technician. This saves a considerable amount of time for the clinician and allows faster approval of the modified treatment plan (Fig. 6).
IPR on anterior teethAs a rule, for a contact of 0.2 mm or less, I use manual stripping files, as mechanical stripping can create more space than needed. For anything above that value, I prefer a manual vibratory handpiece, the Oscident handpiece (Fig. 7). I have observed that oscillating diamond strips work more efficiently compared with the manual IPR system and that they result in a more refined enamel surface. We use a set of diamond strips ranging from extra fine to coarse (Fig. 8). The stripping is always performed with water spray, as this helps create a smoother surface on the enamel (Figs. 9 & 10). The handpiece also comes in a version with a light, which I find very helpful in performing detail work. The use of surgical loupes is highly recommended, as this helps visualise the detail in altering tooth morphology and the detection of any irregularities that may require finishing (Fig. 11).
IPR on posterior teeth
It is my personal preference to avoid stripping in the premolar and molar areas unless really necessary. I do not agree with massive posterior IPR for the sake of creating a Class I occlusion. Furthermore, in these areas, there is a tendency for food entrapment, and this creates a potential risk for caries if IPR is excessive. Additionally, premolars and molars have point interproximal contacts and flattening their proximal surfaces could result in periodontal problems. If the clinician needs to perform IPR in the posterior areas, he or she should avoid flat stripping files, as it is not possible to create a point contact using them. If there is need for IPR in the premolar or molar region and the desired value is 0.3 mm or more, the best way to achieve this is with a stripping bur. It is quite difficult to create a clearance of less than 0.2 mm with a stripping bur, as this is too technique-sensitive.
Steps must be taken to ensure that the enamel surface remains smooth after enamel reduction; therefore, polishing is important. Finishing the contacts where IPR has been performed is of paramount importance to avoid irregularities that can lead to periodontal problems, caries and discoloration. For finishing anterior teeth after IPR, we use a 15 μm extra-fine polisher strip in the Oscident handpiece. In order to reduce any sharp edges that may have been created by the files, we use a resin polishing bur on the edge of the contact (Figs. 12 & 13) with extreme caution, as this may further open the contact. As a final step, we apply a topical fluoride to the IPR-treated surfaces. After the agent is applied, the patient wears the aligner and is instructed not to rinse for 30 minutes (Figs. 14 & 15). Use of a fluoride-containing mouthwash at home is highly recommended. We have observed clinically that meticulous polishing of and topical fluoride application to IPR-treated surfaces are associated with reduced incidence of interproximal discoloration. It is unlikely the caries will develop if the interproximal reduction procedures are performed in the manner described.
The use of IPR is an effective means of treating cases of mild to moderate crowding with aligners. Align’s treatment planning software allows control of the amount and staging of IPR. The flexibility of observing the different IPR scenarios in real time via the live update new feature. ClinCheck software provides the user with an additional feature via the Bolton’s analysis table that allows more detailed and precise enamel reduction, taking into consideration the size of the teeth.
Enamel reduction can increase available space, but the quantity of enamel that can be removed without adverse consequences should be carefully evaluated. There is clinical evidence that the use of IPR can help the clinician better meet his or her original standards in treatment. However, the clinician must devote time and attention to detail for the IPR-treated case, as this is an irreversible procedure and must be done as meticulously as possible, always following the ethical practice of preserving tooth structure and finishing to perfection. The best IPR-treated cases are the ones that another clinician would not easily detect.
This article was published in aligners—international magazine of aligner orthodontics, issue 1/2022.
Tue. 9 August 2022
7:00 pm EST (New York)
Thu. 11 August 2022
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Thu. 11 August 2022
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