Dimensional parameters and the tolerance limits of brackets and archwires

Standard preadjusted edgewise appliances

At the time of Edward Angle, brackets used for standard edgewise appliance (SEA) were identical in terms of size, shape, form, and slot features (Angle, 1932). One of the main drawbacks of SEA is the need for wire bends to deliver the three main order bends (Table 1).



Until the 1970s, the use of SEA required extensive wire bending skills to deliver acceptable treatment outcomes (Proffit et al., 2006). Luckily, we witnessed the introduction of Andrews' six keys of occlusion (Andrews, 1972) as well as the preadjusted straight-wire appliance (SWA) that is considered one of the 20th century’s ground-breaking advancements in orthodontics (Andrew, 1979). Despite these revolutionary changes in fixed orthodontic therapy, the complete reliance on prescription is still suboptimal, in particular the third order bend expression.


Third order bend expression and torsional play

Expression of third order bend depends on several factors including but not limited to:

A. Manufacturer-related factors such as:

  • Method of bracket production: milling is more accurate and expensive than Metal Injection Moulding (MIM) (more research is required in this field)

  • Slot size precision and tolerance (Joch, 2010) (more research is required in this field)

  • Bevel angle (more research is required in this field)

  • Surface finish (Gioka, 2004)

B. Materials-related factors such as:

  • Type of materials used to manufacture brackets, wires and slot lining.

  • Modulus of elasticity of the wire and brackets

  • Height and depth of the slot (0.018 and 0.022) although literature showed that there is no evidence for higher treatment efficacy of one slot size over the other (Vieira et al., 2018, Yassir et al., 2019, Yassir et al., 2019).

  • Variable thickness of adhesive

C. Patient-related factors such as:

  • The angle between root and crown aka Collum angle (McIntyre and Millett, 2003)

  • Original malocclusion (Creekmore and Kunik, 1993)

C. Orthodontist-related factors such as:

  • Method of ligation

  • Choice of bracket prescription (MBT, Roth, etc)

  • Archwire selection

  • Treatment needs and intended biomechanics (Almuzian and Mcintyre, 2015, Khan, 2015)

  • Bracket position: variation in vertical bracket positioning can result in variable torque expression depending upon the teeth' morphology (Sardarian et al., 2014). Brackets placed more gingivally results in exaggerated buccal/ labial root torque expression and vice versa (Khan, 2015) as shown in figure 1.



Update

Progress in Orthodontic Journal in their September 2020 issue published an in vitro paper to answer some of the raised uncertainties (highlighted in red). The title of the paper is “Dimensional variability of orthodontic slots and archwires: an analysis of torque expression and clinical implications”. The study was undertaken by a team from Italy led by Dr Tepedino. The study analysed 0.022”x0.028” slot MBT brackets of UR1, UR3 and UR4 which are manufactured by three different companies using either metal injection moulding (MIM) or computer numerical control (CNC) milling. The study also analysed rectangular stainless steel archwires (size 0.019 × 0.025″ archwires, and 0.021 × 0.025″) produced by the same companies. The slot of the bracket were analysed using profile projector while archwire were observed with an optical Greenough Stereo Microscope.


In summary

  • Most of the bracket slot’s heights are constantly oversized,

  • Most of the archwires are slightly undersized but the discrepancies reduce as the wire size increase,

  • Most of the archwires have high edge bevels,

  • The discrepancies in the slot height, depth and bevel varies among different brackets (UR1, UR3 and UR4) produced by the same comapnies,

  • The torsional play of 0.019 × 0.025″ stainless steel archwire ranges from 10.7 to 16.9° which is twice more than that in 0.021 × 0.025″ wires, and

  • Method of bracket production (MIM or milling) has no effect on the degree of torsional play.

Therefore, wire bending, the use of auxiliaries, or special ligations are inevitable to overcome manufactural shortage in terms of the inherent torsional play.


What do you think?

Link to the study:

https://progressinorthodontics.springeropen.com/articles/10.1186/s40510-020-00333-5


PS: Credit to Haris Khan and Dalia El-Bokle

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