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 Table of Contents  
Year : 2023  |  Volume : 12  |  Issue : 1  |  Page : 40

Evaluating orthodontic bracket slot dimensions and morphology: A narrative review

Department of Orthodontics, College of Dentistry, University of Baghdad, Iraq

Date of Submission29-Mar-2023
Date of Decision07-May-2023
Date of Acceptance24-May-2023
Date of Web Publication04-Sep-2023

Correspondence Address:
Mohammed Nahidh
Department of Orthodontics, College of Dentistry, University of Baghdad
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jos.jos_39_23

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The current article aims to review the previous studies that measure the orthodontic bracket slot dimensions and geometry. Searches in different databases, including PubMed Central, Science Direct, Wiley Online Library, the Cochrane Library, Textbooks, Google Scholar, and Research Gate, in addition to a manual search, were performed about the methods of assessing orthodontic bracket slot dimension up to March 2023. The irrelevant and duplicate studies were eliminated, leaving 35 studies for this narrative review. The findings indicate that the slots are oversized with diverging walls in most studies. Manufacturers must respect the standards during manufacturing brackets and adhere to the actual dimensions and tolerance values.

Keywords: Dimensions, geometry, orthodontic bracket slot, play, torque

How to cite this article:
Nahidh M, Yassir YA. Evaluating orthodontic bracket slot dimensions and morphology: A narrative review. J Orthodont Sci 2023;12:40

How to cite this URL:
Nahidh M, Yassir YA. Evaluating orthodontic bracket slot dimensions and morphology: A narrative review. J Orthodont Sci [serial online] 2023 [cited 2023 Oct 3];12:40. Available from: https://www.jorthodsci.org/text.asp?2023/12/1/40/385070

  Introduction Top

Today, orthodontists have several alternatives for correcting tooth irregularities. While conventional orthodontic brackets are the treatment of choice, hundreds of manufacturers provide different varieties concerning size, wings, slots, prescriptions, and ligation characteristics.[1]

In 1928, Edward H. Angle[2] invented the edgewise bracket system with a 0.022-inch slot. His appliance was suitable for both single and twisted gold wires. In 1953, Steiner[3] adopted 0.018-inch slot brackets to reduce the adverse effects of the harsh stainless-steel archwire that had replaced the gold archwire at that time.

Due to the improved orthodontic equipment, an orthodontist may choose between two orthodontic bracket slot sizes to treat a patient's malocclusion. Depending on the chosen option, they may be combined and matched in either 0.018-inch or 0.022-inch sizes. These two measures have a four-thousandths of an inch difference.[4]

For attaining the build-in tip and torque in the brackets, it is necessary to expose the exact measurements of the bracket slots. Although bracket slot measurements have long been assumed to be accurate, multiple investigations have demonstrated inconsistencies between the published size of orthodontic brackets and their actual size.

Kusy and Whitley[5] noted that orthodontists require precise dimensions to calculate the critical contact angle for binding. This angle is regarded as critical for the effective treatment of patients, as binding and sliding resistance can arise if the contact angle between the archwire and bracket rises; however, it can be somewhat compensated for by using slightly larger slots and smaller archwires.

Achieving an esthetically pleasing outcome requires incisors to have the correct torque.[6] Numerous variables, like the size of the bracket slot and archwire, impact the manifestation of torque. An archwire suitable for the bracket slot enables complete torque expression; nevertheless, a certain amount of “play” is necessary to enter a full-size rectangular archwire. Therefore, the vertical dimension or height of the slot must be more than the height of the archwire. If there is a significant difference in size between the bracket slot and the archwire, the computed torque will be erroneous.[5]

Different researchers have examined the impact of numerous factors on torsion play. These include bracket and archwire material,[7],[8] abnormalities in tooth morphology and bracket positioning problems,[9] as well as archwire beveling.[10],[11],[12]

The bracket slot dimensions are specified in thousandths of an inch. However, tolerance levels are not included in the manufacturer's product catalogs. Creekmore[13] postulated that the manufacturer tolerance was about 0.0005-inch (0.013 mm). Later on, Creekmore and Kunik stated that the manufacturing tolerances were 0.001-inch (0.03 mm) based on 3M/Unitek data without any information about the methods of obtaining these data.[14]

Sebanc et al.[10] used optical comparators to control the tolerance limits when producing metal-lined ceramic brackets, while Meling et al.[15] used gauge blocks to check the degree of fit between the bracket slot and the block in increments of 0.01 mm, but they faced a problem of varying measurements at a different area from the entrance to the middle of the slot.

Meling et al.[16] developed a novel method to estimate the bracket slot height when the wire actually made contact with the bracket with high accuracy depending on the relation between archwire height, width, edge bevel, and torsional play with the bracket slot height. Kusy and Whitley[5] established a precise specification of slot geometry and S.I. unit standardization; they are among the researchers who have defined the spectrum of slot and archwire manufacturing features.[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33],[34],[35],[36],[37],[38],[39],[40],[41],[42],[43],[44],[45],[46],[47],[48],[49],[50]

DIN 13971-2 summarizes the standards for orthodontic brackets adopted in 1993 and published in 2000 to control orthodontic brackets' nominal dimensions and tolerance limits.[51] DIN 13971-2 shows that the allowable tolerance limit was 0.04 mm. Therefore, the acceptable range for a 0.022-inch (0.559 mm) slot is 0.519 mm–0.599 mm.

In accordance with the ISO 27020 specification, the location for measuring the height of the bracket slot was determined to be roughly 100 μm from the outside and inner border of the slot to eliminate bias caused by the roundness of the slot angles. Moreover, 0.01 mm was chosen as the tolerance limit; therefore, the allowable range for a 0.022-inch (0.559 mm) slot is 0.549 mm–0.569 mm.[52]

Bennett and McLaughlin[53] contended that oversized slots violate the edgewise pre-adjustment assumption, designed to decrease wire bending. They emphasized that brackets should be manufactured with slots having parallel sides and accurate dimensions.

]Most bracket slot corners are rounded, and slot walls are not parallel, resulting in a trapezoidal slot form that makes it challenging to evaluate bracket slot measurements. Several apparatuses for measuring the size of bracket slot openings have been proposed in the literature. These comprise leaf gauges, pin gauges, digital gauges, a profile projector microscope, a stereomicroscope, an optical microscope, a scanning electron microscope, and a micro-CT scanner.[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33],[34],[35],[36],[37],[38],[39],[40],[41],[42],[43],[44],[45],[46],[47],[48],[49],[50]

This work aims to review prior studies examining the slot dimensions and morphology of different types of brackets including the metal and esthetic brackets, the conventional and self-ligating brackets, and the labial and lingual brackets with different slot heights.

  Methods Top

Various databases, including PubMed Central, Science Direct, Wiley Online Library, the Cochrane Library, Textbooks, Google Scholar, and Research Gate, were searched electronically as well as a manual search up until March 2023 for methods of assessing orthodontic bracket slot dimensions and morphology using the key words “orthodontic brackets”, “slot size”, and “slot dimensions” by one of the investigators (M.N.). About 1,845 articles were collected. The irrelevant and duplicate studies were eliminated, leaving 35 studies for this narrative review.

  Results and discussion Top

[Table 1] presents detailed descriptions of the orthodontic bracket brands, manufacturers, sample size, measuring apparatuses, and findings. Previous studies investigated several types of brackets and employed various instruments for assessing slot size. The nominal slot height indicated by manufacturers is 0.022-inch or 0.559 mm. According to DIN 13971-2[51] and ISO 27020,[52] the industrial slot height tolerance is 0.04 mm and 0.01 mm, respectively.
Table 1: Summary of the previous studies conducted from 1999 to 2023

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According to Cash et al.,[17] there were considerable disparities between the actual and nominal values of slot height in virtually all trials, with the bracket slot being large in most cases and ranging from 2.26% to 24%. Alternatively, few studies reported an inadequately sized bracket slot. Although undersized slots within the tolerance limitations are permissible according to the standard, they might hinder the proper seating of a full-size wire with a nominal cross-section equal to the nominal slot size.[29] In a clinical context, the full-size arch can also fit into smaller slots, given that modern archwires are often undersized compared to their nominal value.[43]

This difference may be attributable to several factors, including the type of bracket manufacturing methods (i.e. casting, milling, metal injection molding (“MIM”)), the type of bracket, the measuring apparatus, the location of measurement in the slot, the side of measurement (mesial, distal, or both), and even the batch of the same brand.

Methods of manufacturing orthodontic brackets

Due to their lengthier production cycles and lesser cost-effectiveness, investment casting, and machining or milling are less common manufacturing processes for bracket bodies than MIM.[54] Some bracket systems combine MIM to manufacture the bracket body with machining or milling for the bracket's slot. Unfortunately, most bracket manufacturing techniques are private, and manufacturers provide little information about them.

Jackson[36] attempted to create and test orthodontic brackets using 3D printing technology. He compared the dimensional correctness of two commonly made brackets (Damon® and Ti- Orthos®) to that of a unique one-piece 3D metal-printed orthodontic bracket, and discovered that the mean slot height of the 3D-printed slot was closest to the nominal value (0.022-inch) but had the greatest standard deviations in contrast to the other tested brackets, which were much lower.

Tepedino et al.[43] studied orthodontic brackets manufactured using a milling, MIM, and MIM bracket with a milled slot. They found that bracket slot heights are always oversized regardless of manufacturing methods, yet several manufacturers are more adherent to the nominal values. On the other hand, Park et al.[45] examined the slot size and walls parallelism of metal brackets produced by MIM and milling on a computer numerical controlled (CNC) machine. The CNC software should manage the operation, and it is designed to create vast quantities of brackets with higher quality and greater dimensional precision than casting or injection molding. The authors discovered that the whole sample of brackets had large slots and that just one of the seven analyzed systems had parallel walls, the rest having divergent walls. They could not determine that CNC milling was a more precise production technology than MIM. Other researchers did not account for production processes.

Martínez et al.[46] evaluated several orthodontic brackets made by MIM and discovered that four analyzed systems displayed a slot height mean value beyond the ISO 27020 tolerance limits.[52] Although MIM is the most cost-effective production method for brackets,[55] it requires an 18% to 20% larger mold to accommodate shrinkage after sintering. The shrinkage might vary based on many variables (alloy, powder type, de-binding procedure, sintering heat rate, and sintering hold time) that impact the final dimensions. A failure to calculate the real shrinkage after sintering or a lack of control during the final polishing step may contribute to the high occurrence of large slots. In addition, quality control employing gauges finds and rejects small brackets more quickly than big brackets.

Although the majority of manufacturers may not mention their technical tolerances for bracket slot variances, manufacturing errors may result from faults or problems in manufacturing methods or material type.[8],[12],[20] Orthodontic brackets manufactured by casting and milling are impacted by shrinkage, which causes flaws such as grooves and striations with porous slot wall surfaces. To overcome such manufacturing faults and guarantee that they do not interfere with the presence of archwires, producers deliberately expand the slot size and bevel the archwires' edges.[5] In addition, it has been asserted that European orthodontic bracket manufacturers use metric tooling. As a result of this disparity with American imperial-based tooling, the 0.022-inch slots in European-made brackets are oversized by 4.22% even before any manufacturing variation is identified.[56] Also, variations between batches by the manufacturer may contribute to the discrepancy.[46]

As the slot edges are beveled, it is often difficult to detect the edge of the slot, which might impact the computation of slot dimensions. Also, because the slot is slanted concerning the bracket base, care must be given to measure its edge, not its wall. These mistakes were eliminated by changing the illumination until it was possible to see the edge of the slot.

Side difference

In the majority of examined brackets, Tangri et al.,[23] Jackson,[36] Khan et al.,[37] and Lefebvre et al.[40] discovered a considerable variation in slot size between the mesial and distal aspects. Although the dimensions differed between mesial and distal sides, the difference was not statistically significant, and Khan et al.[37] stated that the mesial slot height was identical to the distal slot height in all tested bracket series. On the other hand, according to Jackson[36] and Alqahtani,[49] all bracket systems exhibited asymmetry.

Matasa[57] reported that irregularly cut slots inside the same twin bracket are ubiquitous, resulting in a 2.0-mm level variation between each pair of tie wings. In addition, excessive brazing material clogs the slot, making inserting the archwire difficult or impossible. Again, this is due to the lack of attention of the assembler, who either wrongly positioned the pieces or used excessive or badly placed brazing material.

A change in the size of the mesial and distal slots will vary the archwire tip generated by the bracket. This change in tip would depend on the width of the brackets and would be less evident in broader brackets than in small brackets or smaller-sized brackets such as the mandibular incisors and maxillary lateral incisors brackets.

Inter-batch variance

Numerous variables throughout the production process might lead to dimensional imprecision, and inter-batch variations may occur if unchecked. Only Martínez et al.[46] research examined the accuracy of different batches of brackets. This investigation revealed considerable batch-to-batch variation in 6 out of 12 systems, attributable to inadequate process validation or the absence of an efficient quality control mechanism.

The end objectives of process validation operations are the homogeneity within a batch and consistency between batches. A verified process is adequately shielded from sources of unpredictability that might impair manufacturing output.[58]

Apparatuses used for measurements and measurement techniques

Using micro-CT improves the precision of the orthodontic bracket slot measurement compared to other measuring techniques, such as leaf gauge, stereomicroscope, Maxtascan, scanning electron microscopy (SEM), microhardness tester, and digital gauges, which were utilized in earlier researches. No study had been conducted to validate the measurements for images taken via different apparatuses; each one had advantages and disadvantages.

During measurements, only a few researchers[30],[40],[46],[50] proposed leaving 100 μm (0.1 mm) between the inner and outside boundaries. Other researches did not provide specific measurement sites.

Slot Morphology (Geometry)

The design of the slot is crucial for maintaining good bracket-to-arch contact and for the full manifestation of the inherent prescription. Slot geometry can be assessed by measuring the height of the slot at the level of the base and face; moreover, some investigators used the angle between the walls of the slot[20] or the convergence angle between the base and the walls of the slot as an alternative method.[32] In a recent study, Shin et al.[59] measured the bracket slot base angle between two lines, one parallel to the slot base and the other coincident with the slot reference line on the bracket.

The top and lower walls of the slot must be as parallel as feasible, perpendicular to the bottom, smooth, and devoid of abnormalities or contaminants.[30] Most investigations highlighted the absence of parallelism in the slot wall surfaces. Lee et al.[32] and Lefebvre et al.[40] discovered divergent walls in most of their samples, 84%–85% and 100%, respectively. Cash et al.[17] discovered parallel walls in just 3 of the 11 systems examined, with most systems exhibiting converging walls. Park et al.[45] discovered diverging walls in seven of the eight bracket systems evaluated, with just one system having parallel walls. MIM and milled brackets were evaluated, and none showed capable of ensuring accurate geometry.[50]

  Conclusions Top

The findings of this narrative review demonstrated that virtually all brackets from different manufacturers are oversized and have divergent walls and that the great majority of the investigated bracket systems do not meet the DIN 13971-2 and ISO 27020 quality criteria. As orthodontic brackets are precision medical equipment, the specified standards, and quality controls must be implemented throughout production.

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Conflicts of interest

There are no conflicts of interest.

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