A patient comes in with a good bite. The upper teeth look relatively straight, but the lower front teeth are noticeably crowded. It seems like a straightforward orthodontic case.
Then the dentist plans the tooth movements, and something unexpected happens: small gaps appear between the upper teeth.
Why would straightening crowded lower teeth create spaces in an arch that originally had none?
This is one situation where interproximal reduction (IPR) may help. But deciding whether to perform IPR involves more than simply making room for crowded teeth.
Why Can Straightening Lower Teeth Create Upper Gaps?
Consider a patient with Class I posterior occlusion. The upper and lower back teeth already fit together well, but several lower incisors overlap and lean slightly inward.
To straighten them, the orthodontist may plan to move the lower incisors forward into a broader arch form.
However, moving the lower teeth forward can create interference with the upper teeth. The upper arch may also need repositioning to maintain a comfortable bite.
Here is where an unexpected problem can appear.
In one orthodontic teaching case, the upper lateral incisors measured less than approximately 6 mm wide. These teeth were relatively narrow compared with the patient's other anterior teeth.
As the upper arch was repositioned, small spaces appeared around the lateral incisors.
The original smile had no visible gaps, yet the proposed final alignment showed upper spacing.
This is an example of how a Bolton tooth-size discrepancy can become more apparent during orthodontic treatment.
For readers unfamiliar with tooth-size ratios, our guide to Bolton discrepancy and IPR explains the underlying analysis.
More Lower IPR or More Upper Spacing?
Imagine explaining this situation to the patient.
They came in wanting straighter teeth. Now they are being told that their treatment could create gaps between the upper teeth.
The patient does not want those gaps. But when the orthodontist suggests IPR on the lower teeth, they are also reluctant to have enamel removed.
In the demonstrated case, the clinician described this as having to pick your poison.
The expression captures a genuine treatment dilemma.
Option 1: Perform lower IPR. Creating space between selected lower teeth may reduce the amount of forward movement needed for alignment. This can also reduce the upper arch adjustments and spacing required.
Option 2: Accept some upper spacing. The orthodontist may plan space around the undersized lateral incisors and restore their width later with composite bonding.
Option 3: Combine both approaches. Limited lower IPR and upper restorative treatment may offer a suitable balance.
Interestingly, the orthodontist had already performed some lower IPR in the case, yet small upper gaps remained.
This highlights an important point: IPR does not necessarily eliminate the entire tooth-size discrepancy or remove every unwanted space.
Where Should Dentists Perform IPR on Crowded Teeth?
Not every interproximal contact requires the same amount of enamel reduction.
During treatment planning, the orthodontist examines tooth morphology, existing contacts, enamel thickness and the amount of space needed.
In the teaching case, several lower incisors had triangular shapes, while some contacts already showed small spaces.
Rather than reducing every contact equally, the clinician selected areas where controlled enamel reduction could contribute to alignment.
A planned total reduction of 0.5 mm at one contact, for example, might involve approximately 0.25 mm from each adjacent surface, provided sufficient enamel is available.
The key is to measure the actual reduction rather than assuming the instrument's head size determines the final space.
A 2025 systematic review involving 23 selected studies identified IPR as an established non-extraction space-management technique. However, the authors also emphasized that the available research was generally of low quality, highlighting the need for more rigorous clinical studies.
For anterior contact preparation, the MR.BUR One Slice IPR Kit provides different diamond bur head sizes to support selective enamel reduction.
The X02 can be used for initial contact access, while X03W, X04R and X05B provide different options for planned anterior IPR. X03SF is intended for super-fine finishing.
Instrument selection should follow the treatment plan, with appropriate cooling, measurement and finishing.
Can IPR Help Reduce Black Triangles?
IPR is not only considered when teeth are crowded.
In another orthodontic planning case, a patient had approximately 0.36 mm of mandibular Bolton excess, along with visible black triangles between the lower incisors.
These open gingival embrasures occur when the interdental papilla does not completely fill the space between adjacent teeth.
Triangular incisors can contribute to the problem because their contact points tend to sit closer to the incisal edges, leaving a wider space near the gums.
The orthodontist considered selective IPR to modify the proximal contours, followed by orthodontic space closure.
This approach may create a broader contact area closer to the gingival tissue.
Research published in Progress in Orthodontics in 2025 examined factors contributing to open gingival embrasures during orthodontic treatment, supporting the importance of evaluating tooth anatomy and the surrounding periodontal structures.
However, IPR cannot guarantee that black triangles will disappear. Gingival recession, papilla height and underlying bone support can limit the result.
For appropriate patients, tooth recontouring may contribute to improved contact geometry. For others, restorative or periodontal management may also be needed.
When Is Bonding Better Than More IPR?
The same 0.36 mm discrepancy case raised another interesting question.
Should the dentist reduce the lower teeth, or create space for bonding around an undersized upper lateral incisor?
The reviewing orthodontist preferred considering upper restorative space because the lateral incisor appeared relatively narrow.
He also examined where to position that space.
Opening it behind the canine could disturb an already satisfactory canine relationship. Creating space adjacent to the lateral incisor could instead allow its width to be improved with composite.
He even questioned whether planned IPR on the upper central incisors was necessary, because he considered their existing shape aesthetically acceptable.
This illustrates why the amount of discrepancy alone should not determine where IPR is performed.
Tooth proportions, occlusion, patient preferences and the intended restorative outcome all matter.
Choosing MR.BUR Instruments for Controlled IPR
For clinicians performing interproximal enamel reduction, the MR.BUR One Slice IPR Kit offers a selection of precision diamond dental burs for anterior and posterior IPR procedures, with head sizes ranging from 0.2 mm to 0.5 mm.
Each IPR dental bur is designed to support controlled contact access, planned enamel reduction, and precise interproximal adjustments during orthodontic treatment.
The clinical objective is not to remove as much enamel as possible, but to achieve the prescribed reduction while preserving suitable proximal tooth anatomy.
Straight Teeth Are Only Part of the Treatment Goal
A patient may begin treatment with crowded lower incisors and a seemingly ideal posterior bite, yet still require careful coordination between both arches.
Sometimes IPR is appropriate. Sometimes upper bonding is preferable. In other cases, both may be needed.
The decision should account for tooth-size proportions, arch movements, occlusion, periodontal health and the patient's expectations.
Successful orthodontic treatment is not simply about making teeth straight. It is about making them fit together.
And when IPR is part of that plan, controlled enamel reduction and appropriate finishing remain essential to achieving the intended result.
