7: Molar Distalisation

7

Molar Distalisation

Whilst buccal mini-implants are relatively easy to insert, their interproximal position, in close proximity to the buccal roots of adjacent molar teeth, limits the amount of nearby tooth movement which may occur. Consequently buccal anchorage approaches provide only limited scope for molar distalisation and typically require a two phase distalisation process: initially for molar movement and secondly for retraction of the premolar and anterior teeth. Furthermore, the mini-implants need to be re-located between these two phases, risking anchorage loss during this switch-over process. Fortunately, the availability of the hard palate for bone anchorage substantially increases the options for upper molar distalisation. Therefore, the choice of mini-implant site depends on the planned amount of maxillary molar movement and on whether the mid-palate area is easily accessible (which may not be the case in high arched palate cases). For example, up to half a unit of distalisation may readily be achieved using direct traction from mini-implants sited in the palatal alveolus. A greater range of distalisation and probably better bodily control over the molars’ movements are both achieved through indirect palatal anchorage of a customised distaliser appliance. Importantly, this differs from the use of ’non-compliance’ distalisers since the incorporation of mini-implants avoids anchorage loss during both the distalisation and subsequent retraction phases, in the form of mesial movement/proclination of the anterior teeth and then molar anchorage loss.1,2

The options for distalisation of mandibular molars are limited to using anchorage sites in either the posterior alveolus (e.g. just mesial to the first molar) or the retromolar area. However, the latter site may be complicated by the presence of the third molar, difficult physical access and a lack of attached gingiva (a shallow buccal sulcus).

Clinical objectives

  • Prevention of mesial movement of the premolars and anterior teeth during molar distalisation.
  • Prevention of molar mesial movement (anchorage loss) during subsequent retraction of the anterior teeth.

Treatment options

  • Buccal alveolar mini-implant anchorage (direct or indirect) in either arch.
  • Palatal alveolar mini-implant anchorage.
  • Mid-palate mini-implant distaliser.
  • Premolar extractions (where none have previously been undertaken), avoiding the need for molar distalisation.
  • Headgear, possibly with an upper removable appliance (e.g. a ‘Nudger’), for upper molar distalisation.
  • Correction of the skeletal base (negating distalisation requirements) by orthognathic surgical or orthopaedic appliance treatments.

Key treatment planning considerations

  • The molar, canine and skeletal base relationships.
  • Eruptive status of the second molars: there is less resistance to molar distalisation before eruption of the second molars, but also a greater tendency to molar tipping (rather than bodily movement).3
  • Presence/absence of third molars: consider their removal (before orthodontics) if they are likely to interfere with distalisation.
  • Absence of posterior teeth (due to previous extractions or hypodontia).
  • Depth and shape of the palatal vault, i.e. a high arched palate makes access difficult for mid-palate insertion of both mini-implants and a distaliser.
  • Distalisation distance required and whether this differs for each side of the arch.
  • Avoid maxillary buccal alveolar insertions since there is very limited interproximal space for distal movement of adjacent roots past the mini-implant. Hence, this approach requires re-siting of the mini-implants after molar distalisation and before the second phase retraction of the premolar teeth.
  • Sufficient interproximal space is available between the maxillary palatal roots for both insertion and up to half a unit distalisation of teeth (before the mini-implant is approximated by roots mesial to it).
  • Mid-palate anchored distalisers require more initial clinical, and especially laboratory, time due to their fabrication and fitting requirements. However, once in situ they are simple to reactivate and provide the best range for distalisation. They may also provide optimum bodily molar movements, followed by simple retraction of the anterior teeth.4,5,6,7,8

Biomechanical principles

  • Forces applied at the coronal level will tend to distally tip molars during distalisation, then risk anchorage loss as the molars upright during retraction of the anterior teeth.1
  • Distalising forces directed at the furcation level will produce molar bodily movement.1
  • First molars tend to distally tip when the second molars are unerupted.3
  • There is a tendency to molar crossbites as the molars distalise. This is exacerbated when a rigid transpalatal arch (TPA) is used, or when insufficient compensatory transverse coordination is incorporated in the fixed appliance archwires or a distaliser appliance’s wire frame.

Mid-treatment problems and solutions

  • If there are signs of molar tipping then over-correct their distalisation, in anticipation of molar relapse during retraction of the anterior teeth.
  • It may be necessary to accept buccal crossbites, especially if a rigid TPA is used, until after the anchorage requirements have been met. However, if alveolar mini-implants are being used for direct traction then arch coordination can be attempted, e.g. by expansion of the maxillary archwire.
  • Patients should alert the orthodontist as soon as they become aware of the breakage of any components to prevent anchorage loss or unfavourable tooth movements.

Mandibular arch distalisation

This involves mini-implant insertion in either posterior interproximal sites or the retromolar area.


Clinical steps for mandibular distalisation

Pre-insertion

1. Level and align the arch to be ready for the working archwire, e.g. a 0.019 × 0.025 rectangular stainless steel wire.
2. Determine the optimum insertion site, e.g. distal to the terminal molar (Fig. 7.1) or mesial to the first molar (Fig. 7.2), depending on the ease of access to the retromolar site and the available interproximal space, respectively. The retromolar site may not be suitable if the third molar is present or if it has been removed within the previous few months.
3. Consider fabrication of a stent, especially if the insertion site is difficult to access or deliberately close to a molar root, by taking an impression one to two weeks pre-insertion. The fixed appliance brackets may be blocked out with soft wax except those adjacent to the insertion site, and ideally the clinical level of the mucogingival junction should be measured or photographed.

Mini-implant selection

4. Select a narrow, short neck mini-implant (e.g. Infinitas 1.5 mm diameter, 6 or 9 mm length, version). The length selection depends on the anticipated cortical depth and physical access (depth of insertion), e.g. a short length is appropriate in the buccal alveolus and a long length in the retromolar area.

Insertion

5. Identify the insertion point in either the retromolar (Fig. 7.1) or alveolar interproximal areas. The latter insertion should be made just mesial to the first molar roots to allow for subsequent distal movement of the adjacent premolar towards the mini-implant (Fig. 7.2). Ideally the insertion should be through attached gingiva, but if this is not feasible then plan to first use a soft tissue punch.
6. Superficial anaesthesia of the insertion site.
7. Determine the angle of insertion (in the vertical plane): an interproximal mini-implant is inserted perpendicular to the buccal surface, whereas a retromolar one is placed at a small oblique angle from the axial plane such that the head is slightly buccal to its body position.
8. Manual insertion is feasible where there is sufficient physical access, i.e. adequate stretching of the cheek and buccal sulcus. Otherwise contra-angle handpiece insertion is recommended, especially in the retromolar area, but with resultant loss of full tactile control.
9. Limited pre-drilling (e.g. using a cortical bone punch) is recommended prior to mini-implant insertion, except in adolescent patients with less dense mandibular cortex.
10. Complete the insertion to the point that the mini-implant neck is partially submerged but its head is fully accessible for traction purposes. This is best performed gradually and with intermittent release of the screwdriver during the final turns, so that over-insertion is avoided. If the cortex gives rise to high insertion torque then remember to offset this by partial derotation of the mini-implant (provided that more than 1 mm of insertion range remains).
11. Percuss the adjacent teeth to gauge root proximity and take an intra-oral radiograph if a problem is suspected.

Post-insertion

12. Immediately load the mini-implant with light traction for the first 5–6 weeks, e.g. a lightly stretched powerchain. Subsequently apply direct traction to either an anchor tooth or the archwire using a NiTi coil spring or elastomeric attachment (Fig. 7.2a), or alternatively indirect anchorage may be performed by connecting the anchor tooth and mini-implant with a ligature wire or traction auxiliary (Fig. 7.1b).

Figure 7.1 (a) Pre-treatment view showing the Class II malocclusion with absent lower right second molar, moderate crowding and lingual displacement of the lower right second premolar. (b–d) A mini-implant in the lower second molar site providing indirect anchorage using a ligature wire to the first premolar then canine brackets, whilst a compressed coil spring distalises the first molar. (e, f) Alignment of the second premolar after molar distalisation had created ample space.

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Figure 7.2 (a, b) Pre-surgical decompensation in this Class II case required incisor retraction, but the lower first premolars were already absent. These views show a mini-implant sited close to the right first molar root to provide sufficient range of arch distalisation before premolar–implant proximity occurs. Elastomeric traction applied to an archwire hook provides en masse retraction and a modest molar intrusive side-effect. (c, d) Traction completed prior to a mandibular osteotomy.

c7-fig-0002

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Case examples
1. Terminal mandibular molar distalisation (Fig. 7.1)

  • An adult with a Class II division 2 malocclusion with absent lower right second molar and lingual displacement of the lower right second premolar tooth (Fig. 7.1a).
  • The lower arch, except the right second premolar, was aligned ready for insertion of a 0.019 × 0.025 steel archwire (Fig. 7.1b).
  • A 1.5 mm diameter, 9 mm body length, long neck Infinitas mini-implant was inserted on the buccal aspect of the alveolar ridge and distal to the lower right first molar. It was used to indirectly anchor the right premolar then canine teeth (Figs 7.1b–d).
  • Anchorage was discontinued once molar distalisation had created space for alignment of the second premolar (Figs 7.1e, f).
2. En masse lower arch distalisation (Fig. 7.2)

  • An adult with a Class II division 2 malocclusion with absent first premolars, recent removal of the lower right third molar, and requiring incisor decompensation for a mandibular advancement osteotomy.
  • Treatment commenced with arch alignment to accommodate a 0.019 × 0.025 stainless steel archwire, but with the lower first molars mesially tipped to increase the interproximal space on their mesial aspects (Fig. 7.2a).
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Dec 31, 2014 | Posted by in Orthodontics | Comments Off on 7: Molar Distalisation
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