Correction of Molar Rotations

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Maxillary molars consist of three roots, and due to the early loss of the deciduous second molars, the palatal root acts as a hinge for mesial rotation of molars. According to Lima et al (2015), the greater the severity of Class II malocclusion, the greater the mesiopalatal rotation of the molars, demonstrating contribution of rotation in the formation of malocclusion.

The maxillary first molars are often rotated with the mesiobuccal cusp displaced in a palatal direction. The consequences of the malposition are that the tooth occupies excessive space in the dental arch and that the buccal cusps occlude with a tendency to a Class II molar relationship. The palatal cusp, however, often occludes correctly in the fossa of the opposing molar.

Prevalence

• Patients with Class II malocclusion have high prevalence of mesiopalatal rotation of the maxillary first molars, ranging from 83 to 95%. 

• It apparently hinders proper dental intercuspation and often worsens the altered anteroposterior relationship.

Causes

• Maxillary molar rotation is a common finding in orthodontic patients with arch length discrepancies.

• Maxillary molar rotation on axis can result because of early extraction or proximal caries of primary second molar. 

• Difference in the tooth size and jaw size can also lead to such rotation. 

• Lack of buccal offset and end-to-end permanent molar relationship are the manifestations of such rotation and mesial drifting of upper first molar.

• Factors, such as the shape of the dental arch, the anatomy of the first molar crown, and variation in the size and position of the canines and premolar crowns may explain these differences.

How to assess molar rotation

There are different ways for assessing molar rotation proposed by various authors:

1. Friel (1959), used the median raphe as a reference and measured the angle between the raphe and a line through the mesiobuccal and mesiopalatal cusps of the molar. (Fig. 3) Values between 57°and 65° were considered normal. 

1. Henry (AJO 1956), measured the angle between the median raphe and a line through the buccal cusps of the molar. (Fig. 4) A molar with an angle of 11.24° was considered well positioned. The angle was given a negative sign if it opened anteriorly.

1. Orton (1966), used the angle between a line tangent to the buccal surfaces of the premolars and a line tangent to the buccal surface of the molar. (Fig. 5)

           Modification-The angle between a line through the contact points of the premolars and a line through the buccal cusps of the first molar. This angle is a modification of the one described by Orton (1966). The angle was given a negative sign if it opened posteriorly.

1. Ricketts (1969), evaluated the occlusal surfaces of the maxillary casts and proposed that in normal occlusion, the line (line of Ricketts) touching the tips of disto-buccal and mesio-palatal cusps of the maxillary first molar on one side should pass through the distal third of the canine (cusp tip of the canines) on the opposite side. (Fig. 6)

The distance was given a positive sign if measured distal (around 4mm) of the midpoint of the canine and a minus sign if measured mesial of the midpoint.

Cetlin and Ten Hoeves (JCO 1983), described that maxillary first molars are considered to be well positioned, when the buccal surfaces of molars are parallel to each other when viewed from the anterior.

Hansen and others (1997), studied the rotations of maxillary first molars by measuring the angle formed by the intersection of two lines, namely one formed by the union of two points corresponding to the tips of buccal cusps and the other to the midpalatal suture. The means observed for this angle were 14.08º and 12.76º for the right and left maxillary molars, respectively, in patients with acceptable occlusions.

Methods for derotation of maxillary molar

A mesio-rotated molar occlusal surface, occupying a larger space on the dental arch owing to its trapezoidal shape, reduces the available space in the dental arch, promoting the development of anterior crowding, besides hindering proper dental intercuspation and worsening the molar anteroposterior relationship.

The occlusal surface being trapezoidal in shape, with the long diagonal distance from distolingual to mesiobuccal, occupies more mesiodistal space in the dental arch when this tooth rotates mesially with the lingual root as the axis. By correction of these rotations, one to two mm of arch length per side and partial Class II correction can be achieved. These corrections also are needed to provide good intercuspation.

An upper first molar measures approximately 10 mm from mesial to distal. From its mesiobuccal corner to its distopalatal corner, it measures approximately 13 mm. For this reason, mesial derotation of upper molars may gain as much as 3 mm of space per side. 

Furthermore, several false Class II relationships (i.e., those in which the palatal cusp of the upper molar sits in the central fossa of the lower molars) may be resolved simply by rotating the upper molars distally. It should take 3 to 4 months to rotate the upper molars completely.

Careful analysis of dental casts and assessment of molar rotation is very important prior to the orthodontic treatment. Therefore, occlusal arch length and width assessment is crucial.

Upper first molar rotation has a considerable clinical relevance because it can influence three major aspects of the orthodontic treatment such as 

• the molar relation in the sagittal plane,  
• the available space in arch perimeter and 
• the long-term stability of orthodontic treatment outcomes related with better interdental contact points.

Ten Hoeve (JCO 1985), suggested that derotation of rotated upper first molars has gained in importance in non-extraction treatment. The theory is that derotation of rotated molars will result in some space gain in borderline cases, and can be a factor for a non-extraction treatment plan. 

It is estimated that due to mesiolingual rotation, 2 mm of mesiodistal arch space occupied by upper first molar increases and for 3 degrees of derotation, there is a net gain of 0.25 mm arch width.

Removable Appliances-Transpalatal Arch (TPA)

Class I molar relationships may convert into an end-on relation due to mesiopalatal rotation of maxillary first molars. One of the most efficient appliances for the derotation of molars is the transpalatal arch (TPA).

This appliance is favourable when the need for derotation is the same on both sides of the dental arch. Equal and opposite moments of rotation can then be used without the creation of forces in the mesiodistal direction.

Review of Literature 

• Lemons and Holmes (AJO 1961) reported that a gain of 1 to 2 mm of arch length per side is utilized by maxillary first molar rotation in Class II malocclusion cases.

• The transpalatal arch (TPA) for molar derotation was introduced to the orthodontic literature by Goshgarian in 1974.

• Cetlin and Ten Hoeve(JCO1982) described unilateral TPA activation as it causes successful derotation of molars on the affected side and least-possible or no changes on the unaffected side. It would generate distal force on one side and rotation on the other side

After the correction of rotation of the molar on one side, they recommended subsequent activation to derotate the molar on the other side a few months later. They stated that the TPA is an effective device to stabilize, rotate, and distalize the molars.

• According to Braun et al (AJO 1997), 2.1 mm of arch length can be gained when derotating upper first molars by approximately 20 degrees, with the application of a TPA and a distal force equivalent at the level of the maxillary first molar centre of resistance.

• Ingervall et al (EJO 1996), studied the moments and forces delivered by transpalatal arches, activated for symmetrical first molar rotation. They found that during the course of molar rotation contractive forces between the contralateral molars developed. These forces were not very large but needed to be compensated for, in order to prevent a tendency for crossbite of the molars to develop during their rotation.

Palatal Bar

Cetlin and TenHoeve (JCO 1983) have modified the Goshgarian’s anchorage appliance to make it a removable tooth-moving appliance called the palatal bar.

• It is made of a 0.036 inch stainless steel (spring temper) wire, doubled back at the ends to be inserted in a 0.036- × 0.072-inch horizontal lingual sheath.
• It incorporates a small “U-shaped” Coffin loop, which is positioned generally toward the mesial for two reasons:

(1) to make the palatal bar more comfortable and 

(2) to improve vertical upper molar control, because of forces exerted by the tongue during speech and swallowing is anterior to the centre of resistance of molars (this is an effect on upper molars only when the palatal bar is distant from the palate and low in the oral cavity).

Activation

Clinical Use of the Palatal Bar

• Used on both first and second permanent molars. 
• Second molars should be banded and moved with the palatal bar as soon as possible to facilitate distal movement of the first molars.
• Forces applied must be light and in one direction. For example, rotation and torque should not be attempted at the same time. 
• Over-activation leads to soreness, mobility, destruction of the lamina dura, and periodontal breakdown and does not produce results more quickly. 
• Reactivation is required approximately every 6 weeks. 
• Terminals must always be checked to ensure that they are passive to their tubes before additional force is added.
• Palatal bars must be tied. This can be done with an elastic or chain from the hook of the sheath to the curved end of the bar. Ligatures prevent it from being dislodged or, worse, swallowed.

Keles TPA

The Keles TPA, introduced by Ahmet Keles (WJO 2003) for effective & rapid derotation of molar, also can simultaneously derotate the molars bilaterally. It was fabricated in a 0.032×0.032” TMA wire. Hence it had a long range of action without the need for frequent reactivation.

The original Keles TPA was fabricated with 0.032×0.032” Burstone lingual arch system wire and used a precision lingual hinge cap attachment. 

Later it was modified by fabricating Keles TPA with 0.032”  Beta-Titanium Alloy (TMA) wire. This wire is adapted on the patient’s study model, about 2mm away from the palatal tissue. A helix with an internal diameter of 2 mm is incorporated bilaterally near the marginal gingiva.

The wire is extended mesially and then doubled back to form tab. TPA is checked on study model to ensure that it is passive. 

Modified Keles TPA is placed on a piece of white paper and two lines are drawn along the terminal ends (rotating component) of the TPA with a pen (Passive stage). 

Additional lines are drawn with a red pen, with a 20° angle passing through the distal end of the helix of the wire and the TPA is activated (Active Stage). 

It is checked on both sides and then inserted in the palatal sheath welded on to the molar band from the distal aspect.

The main advantage of Keles TPA over conventional TPA is the wire, that it allows constant and long-lasting, light force (low load deflection), without the need for frequent reactivation due to the incorporation of helices and the use of beta-titanium alloy.

Zachrisson (Zachrisson-type transpalatal bar [ZTPB])

The main differences between the design described by Zachrisson (Zachrisson-type transpalatal bar [ZTPB]) and the traditional Goshgarian-type transpalatal bar (GTPB) are in the amount and shape of the wire in the palatal loop.

The ZTPB is made indirectly, using a 0.036 inch (0.9 mm) Blue Elgiloy wire, because of its excellent formability it facilitates the making of the bar with low load deflection rate and allows greater flexibility to the bar, making the forces more constant and predictable.

The middle loop is larger and longer than the single round loop of the GTPB. The additional smaller loops are symmetrically positioned on either side of the middle loop. The middle loop is directed mesially, and the additional loops are directed distally. 

Although adaptations are required for individual palatal vault designs, the size of the central loop is generally about 9 mm, and the distance from the two farthest points is approximately 12 mm. The ends of the ZTPB are longer, double wire ends than those of the standard GTPB, to secure improved engagement to the lingual sheaths and make safe ligations possible. 

Activation

Other removable appliances (pre-formed):

Nitanium Molar Rotator

• Thermally activated nickel titanium provides the activating force and pre-programmed for total control. 
• Advised for patients requiring upper first molar rotation and stabilization. 
• Excellent for mixed dentition and adolescent cases. 
• Easily inserted into lingual sheaths on first molar bands. 
• For easy identification, each rotator has permanently stamped size numbers on the assembly and are available in 10 various sizes.

Quad helix appliances

• It is constructed from 0.035 inch (0.9mm) stainless steel wire and available in 3 different sizes. 
• The amount of activation is checked by inserting one side and observing the relationship of the retention to the lingual sheath on the opposite side.
• Activation should not exceed rotation of 20°. Appliance is monitored every 6 weeks.

Fixed appliance

Derotation is best achieved with fixed appliances incorporating springs or elastic forces using a couple.

Bracket position and its effect on molar rotation (Buccal Tube)

Bracket position is critical in the effort to achieve proper upper molar rotation. Whether a band or direct bond bracket is used, the position of the bracket is evaluated by viewing the bracket from the occlusal. 

If the most anterior portion of the bracket bisects the mesiobuccal cusp, the bracket is placed correctly. When the upper molar band fits well, the bracket is automatically placed in the correct position.

Moreover, selection of buccal tube prescription with a maximum offset of +10° or +14° mesial out rotation is of prime importance.

Archwire manipulation in distal molar rotation

When a patient present with severely mesially rotated molars, good bracket position may not be enough to gain proper rotation. Toe-in bends are routinely used to correct severely mesially rotated molars. 

A 2×4 set-up with 30° bends mesial to the molars is an effective molar rotator. This also promotes upper arch expansion, as a toe-in close to the molar not only distally rotates the molar but also expands it by moving the crown buccally. 

For these mechanics to be effective, the bend must be an off-centre bend. This means that the lateral segments must be bypassed (either left unbracketed or bypassed with a utility arch type bend). An additional benefit of lateral segment bypass is arch development. 

Mild rotation can be effectively treated using NiTi Arch wires.

• Their super elasticity allows easy engagement of wire into the bracket slot. 

• This brings derotation, alignment, and levelling of the teeth to a certain extent.

TADs

Temporary anchorage devices such as the mini-implants provide either direct or indirect anchorage for molar rotational correction. Depending on the position of the mini-screw and the attachment on the tooth, various combinations of movements can be accomplished. 

Conclusion

Thus, correct diagnosis and assessment of the severity of molar rotation in various malocclusions are important in the prognosis and are a determining factor in treatment selection.

References

• Dahlquist et al. The effect of a transpalatal arch for the correction of first molar rotation, European Journal of Orthodontics 18 (1996) 257-267.
• Giuntini et al, Mesial rotation of upper first molars in Class II division 1 malocclusion in the mixed dentition: A controlled blind study. Prog. Orthod. 2011, 12, 107–113
• Graber, Vanarsdall- Orthodontic Current Principles and Techniques (6th edition)
• Gündüz et al, An Improved Transpalatal Bar Design. Part II. Clinical Upper Molar Derotation—Case Report. The Angle Orthodontist: June 2003, Vol. 73, No. 3, pp. 244-248.
• Junqueira et al, Analysis of the rotational position of the maxillary first permanent molar in normal occlusion and Class II, division 1 malocclusion, Dental Press J Orthod 2011 Jan-Feb;16(1):90-8
• Kallury et al, Modified Keles TPA –An Effective Method for Molar Derotation, Orthodontic Journal of Nepal, Vol. 10 No. 1, January-June 2020.
• Lima et al, Correlation between the Rotation of the First Molars and the Severity of Class II Division 1 Malocclusion, The Scientific World Journal Volume 2015
• Robert A. Goshgarian: Orthodontic palatal arch wires, US Patent 3,792,529,1974;1974.