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Orthodontics

Authors: Dr. Dhaval Ranjitbhai Lekhadia

Abstract

MBT system has been the main stay in the practice of orthodontics since its introduction by McLaughlin, Bennett and Trevisi in 1993. Although this system has been used successfully to treat almost all types of cases presenting in an orthodontic clinic, it still presents with some drawbacks. A tendency of deep bite exists when using the flexible nickel titanium wires in the initial stages of treatment for levelling and aligning the arches. PASS technique which was introduced by Dr. Xu in 2015 is an excellent appliance system which overcomes the drawbacks seen with an MBT system. The purpose of this paper is to highlight the usefulness of PASS technique over MBT system and review the various advantages and disadvantages of both the systems.

Keywords: PASS; MBT; Anchorage; Tip; Friction

INTRODUCTION

The preadjusted edgewise Straight Wire Appliance was introduced in the 1970s.1 Since then, there have been many suggested modifications to the bracket prescriptions in terms of torque and tip values, often differing by only a few degrees. Over the years, clinicians have considered how bracket design can help achieve these objectives.2 Andrews made extensive measurements on untreated excellent occlusions.3 He determined the average tip and torque angles as well as in-out dimensions of the labial surface of each tooth relative to a flat labial arch wire plane. The MBT bracket system developed by McLaughlin, Bennett, and Trevisi is a third generation preadjusted edgewise appliance which introduced a range of improvements and specification changes to overcome the clinical shortcomings of earlier preadjusted edgewise bracket systems.4 Recently, PASS technique5 was introduced in august 2015 along with a case report of a 33 year old female patient substantiating the use of PASS technique. Although it is a relatively new technique, it seems promising in terms of anchorage control in maximum anchorage cases and control of overbite in cases with deep bite.

REVIEW

Molar anchorage loss has been shown to occur during the early stages of alignment with preadjusted appliances.6,7 The most commonly used archwires for initial alignment are made of nickel titanium, which has the elasticity to provide light, continuous forces but cannot accommodate the tipback bends needed to preserve molar anchorage. Among the methods developed to reinforce anchorage, headgear was once widely used but has not been entirely effective, even in compliant patients.8 Temporary anchorage devices (TADs) are more reliable but may be rejected by some patients and can only be placed in sites with optimal bone condition.

In recent years, self-ligating brackets have been advocated as a means of reducing wire-bracket friction, as well as allowing longer appointment intervals and fewer visits.9,10 Low friction is most advantageous during initial

alignment, however, whereas friction is needed for control in finishing and torque expression. If the friction level of a bracket could be adjusted for different treatment stages, orthodontic tooth movement would be more efficient.5

PASS

The Physiological Anchorage Spee-wire System (PASS) was designed to optimize natural anchorage preservation while controlling friction and utilizing the elasticity of nickel titanium wires for initial alignment.11,12 The PASS appliance has two main components: a crossed buccal tube (XBT) consisting of a −7° main tube and a −25° tipback tube, crossing at the mesial end of the molar; and a multilevel low-friction (MLF) bracket.

In the PASS technique, the initial archwire is inserted into the tipback tube, generating a protective moment for the anchor molars from the beginning of treatment. Engaging the wire into the anterior brackets from the gingival direction will help maintain the anterior overbite, which is important in most extraction cases.5 Molar anchorage is reinforced even when a light nickel titanium wire is used, without any wire bending. The double-tube design also facilitates the early use of a piggyback wire to control the upper-incisor vertical position during alignment and to promote bite opening in deep-bite cases. The effects of the XBT tube, with -25 degrees tip, on the vertical position of molars were not analysed in the case report by chen S et al.5

MULTILEVEL LOW FRICTION BRACKETS (MLF)

The MLF bracket5 has a constricted cervical area that can hold the ligature and keep it from compressing the archwire. A lighter archwire therefore experiences low friction during the alignment stage. As the archwire progresses to a larger size, the ligature contacts the wire and generates a controlled force for better expression of the bracket prescription. In addition, the vertical slots are sized at .020” for the incisors, allowing better torque control, and .022” for the rest of the teeth, providing optimal sliding mechanics.5

ANCHORAGE LOSS IN MBT SYSTEM

Anchorage is often required to retract protruding anterior teeth in the upper jaw or to relieve maxillary crowding. Unfortunately, owing to certain anatomical characteristics, anchorage loss occurs much more easily in the upper jaw than in the lower jaw. Generally, orthodontic extractions are advocated to facilitate stable relief of crowding by generating space limiting unwanted advancement of the anterior segments and arch dimensional change. The mesial angulation in-built in canine brackets predisposes to forward movement of the incisors during alleviation of crowding in the initial alignment phase.13 While many clinicians routinely use lacebacks to control incisor position during orthodontic alignment, they have not met with universal approval. Disadvantages of laceback use may include loss of anchorage posteriorly manifesting as mesial migration and tipping of first permanent molars, potential for plaque stagnation, and limited additional chairside time and complexity.14 Mclaughlin, Bennett and Trevisi advocated the use of headgears, Transpalatal bars, Lingual arches, etc to reinforce the anchorage but each of them have some drawbacks. Heather L15 et al evaluated the ability of TPA to enhance orthodontic anchorage during extraction treatment. The results of this retrospective cephalometric investigation indicated that the TPA has no significant effect on the vertical position of the maxillary first molars during extraction treatment. Mesial tipping of the UMs is a common observation during orthodontic treatment.16-22

PASS TECHNIQUE VS MBT SYSTEM

In a low-friction environment, gentle forces from lip pressure and tissue regeneration of extraction sites may be utilized to assist tooth movement.23 Because of the unique design of the XBT, the moment generated on a labially positioned canine—as commonly seen in a patient with anterior crowding— will result in guided distal drift of the canine without the use of laceback ties. Posterior anchorage is thus preserved while anterior crowding is relieved.24 The premolars are usually not bracketed for two to three months

to allow a favourable molar-canine moment to be created. In the vertical dimension, the reaction force from molar tipback is diminished by the canine and balanced by lip pressure, similar to the effect of light-force self-ligating systems. Therefore, no J-hook headgear (as in the Tweed technique) or Class II elastics (as in the Begg technique) are needed to prevent the incisor flaring that would otherwise occur under the heavy tipback forces generated by stainless steel archwires. The PASS technique avoids the need for mechanical anchorage reinforcement by taking advantage of the patient’s physiological anchorage, while the pre-treatment molar positions are maintained during initial alignment with small nickel titanium archwires inserted in the XBT tipback tubes.6 In a prospective clinical trial, the upper first molar tipped forward an average of 7.2° in the group treated with headgear7; therefore, keeping the upper first molar upright with the -7° XBT tube would save about 2mm of anchorage—nearly one-third of the premolar-extraction space in a maximum-anchorage case.

MLF BRACKETS VERSUS SELF LIGATING BRACKETS

The main difference between the MLF bracket and conventional self-ligating brackets is that instead of using a clip, the MLF provides more clearance for the archwire inside the slot. According to Kusy and Whitley, adding more clearance increases the critical contact angle for binding and thus reduces sliding resistance.25 Although the situation may not be ideal for every tooth as orthodontic correction proceeds, overall friction will still be reduced. They observed free sliding of the MLF bracket along a .014” archwire with an.008” ligature in vitro, but in vivo studies are still needed to substantiate these findings.5

CONCLUSION

Pass technique has definite advantages over MBT system in certain aspects. The use of flexible nickel titanium archwiresin MBT system results in a number of undesirable tooth movements in the initial stages. Among the various drawbacks, anchorage loss by mesial tipping of the upper first molars is of utmost importance when treating a maximum anchorage case. Cases with increased overbite are also difficult to treat with MBT system as the flexible nickel titanium archwires in the initial stage will cause over eruption of incisors leading to further increase in overbite. This results in “round tripping” of the case. PASS technique is a useful alternative to MBT system to avoid this form of round tripping and also to have maximum control of anchorage early in the treatment. Although there are less number of published case reports on the use of PASS technique, the appliance system looks promising, especially in treating maximum anchorage cases and deep bite malocclusions. Vertical control of molar in PASS technique has not been evaluated yet and further investigation is required to assess the same. Vertical control becomes important in patients with increased lower anterior facial height, increased mandibular plane angle and open bite due to over eruption of the molars. Both the systems presented in this paper are effective in treating most types of malocclusions encountered in daily practice but selection of appropriate case still holds good to use each of these systems to their maximum advantage.

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