Introduction
The evolution of temporary skeletal anchorage devices (TSADs), spanning miniscrews, miniplates and diverse appliance designs is a testament to their precision in addressing a wide range of orthodontic challenges where conventional anchorage control is insufficient or cases are borderline for orthognathic surgery.
Extra-alveolar miniscrews are strategically placed in bone locations with superior bone quality and quantity. Notably, they reduce the risk of root interference and demonstrate efficacy in three-dimensional tooth control. Specific extra-alveolar miniscrew appliances are ( Fig. 73.1 ):
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Infra-zygomatic crest (IZC),
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Buccal shelf (BS),
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Anterior subapical and
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Midpalatal screws.
Extra-alveolar miniscrews.
(A) Infra-zygomatic crest miniscrews. (B) Buccal shelf miniscrews. (C) Anterior subapical miniscrews. (D) Midpalatal miniscrews.
IZC miniscrews facilitate total arch distalisation with reduced root interference, providing a predictable retraction of the maxillary dentition. BS miniscrews in the mandibular buccal bone excel in maximal retraction and class III correction.
Anterior subapical miniscrews, with a high success rate and diverse design possibilities for palatal appliances, address anterior dumping effectively. The procedure involves subapical insertion techniques to achieve effective intrusion. The combined intrusion and retraction force system using IZC and anterior subapical miniscrews contribute to gummy smile correction and active vertical control.
Midpalatal miniscrews, with a high success rate and diverse design possibilities for palatal appliances, play a key role when buccal miniscrews are not secure to provide optimal anchorage in achieving desired tooth movements. The author strategically integrates buccal and palatal miniscrews based on clinical needs, optimising treatment outcomes.
The evolution of TSADs in orthodontic treatment since the first report in 1983 by Creekmore, titled ‘Possibility of Skeletal Anchorage’, has been marked by a diverse array of devices designed to address various challenging scenarios encountered in orthodontic treatments, including miniscrews, miniplates, TSADs-reinforced distal jets, TSADs-reinforced pendulum appliances, mesial or distal-sliders and miniscrew-assisted rapid palatal expanders (MARPE). These innovations reflect a growing recognition of the versatility and efficacy of TSADs in tackling a spectrum of orthodontic challenges. Temporary anchorage devices (TADs) and temporary skeletal anchorage devices (TSADs) are often used interchangeably. However, we typically refer to mini screw implants as TADs and reserve the term TSAD for more advanced anchorage devices, such as bone plates, infrazygomatic crest screws (IZC), and similar devices.
Definition and rationale
Extra-alveolar miniscrews are classified based on their positioning, strategically targeting areas with superior bone quality and quantity to enhance stability and success rates compared to interdental regions.
The extra-alveolar miniscrews can be safer during miniscrew insertion than the interdental miniscrews because of the reduced proximity to roots. The risk of interference between the miniscrews and moving roots is greatly reduced by placing the miniscrews in the extra-alveolar areas instead of interdental areas. This deliberate placement in extra-alveolar regions gives orthodontists greater freedom to exert three-dimensional control over tooth positions, further optimising treatment precision and outcomes.
Among the various positions for extra-alveolar miniscrews, the most widely adopted locations include the IZC, BS, mid-palatal areas encompassing paramedian palatal zones, anterior subapical regions, retromolar spaces, ramal areas and tuberosity.
Infra-zygomatic crest (IZC) miniscrew
The IZC is a bony ridge on the maxilla, specifically over the maxillary molar areas, one of the facial pillar bones in the skull. The use of IZC miniscrews proves advantageous in reducing failures associated with root proximity, and they exhibit the capability to withstand appropriate orthodontic mechanics. This enables the achievement of optimal tooth movements without encountering interference between the miniscrews and the roots in motion ( Fig. 73.1 A).
IZC location
Typically, the miniscrews are placed between the maxillary first and second molars, with the specific location determined by the optimal amount of attached gingiva. Alternatively, a secondary placement option may be considered between the maxillary second premolar and the first molar. Following local anaesthesia, the insertion point is carefully chosen within the attached gingiva zone to prevent soft tissue irritation.
IZC insertion method
The IZC miniscrews, the author uses, are made of stainless steel, 2 mm in diameter and 10 mm in length. The procedure for inserting IZC miniscrews is straightforward. The author consistently employs a screwdriver for insertion without pre-drilling. The initial orientation of miniscrew insertion is perpendicular to the bony surface to minimise the risk of slippage. Once initial bony engagement is achieved, the miniscrew is adjusted to an oblique direction, creating a 60-degree angle between the miniscrew and the occlusal plane. The miniscrew is then inserted fully, ensuring appropriate exposure of the miniscrew head for elastic attachment.
IZC advantages over interdental miniscrew implant (MSI)
The IZC miniscrew seems equally effective in maximal anchorage control compared to interdental miniscrews. However, for total arch distalisation, the IZC miniscrew may be the preferred option due to the reduced risk of interference with moving roots, achieved by situating the miniscrews outside the dental roots. Additionally, the placement of IZC miniscrews is no more challenging than that of interdental miniscrews; the procedure remains essentially the same, provided the concept of extra-alveolar miniscrew is well understood.
Total arch distalisation with IZC MSI
With this concept, total arch distalisation can be achieved after complete space closure of the upper arch if there is an indication to continue the retraction of the maxillary dentition ( Fig. 73.2 ). Cone beam computed tomography (CBCT) might be considered to check any possible anatomical constraints for targeted tooth movements, such as the terminal alveolar trough, palatal plate or incisive foramen.
Treatment of class I bimaxillary protrusion with IZC miniscrews.
Pre-treatment profile and occlusion. In a severe protrusion case, interdental miniscrews for maximal retraction may not be sufficient for profile improvement. Miniscrew repositioning might be necessary for further retraction after complete space closure. Total arch distalisation after complete space closure is possible with IZC miniscrews.
The pre-treatment panoramic radiograph reveals the extraction of mandibular molars of poor prognosis, in addition to the removal of four first bicuspids and maxillary third molars, which may benefit the profile improvement in such a severe protrusion case shown in the pre-treatment lateral cephalogram.
The IZC and BS miniscrews were placed for maximal retraction.
The additional extraction of mandibular molars (37 and 46) due to poor prognosis may lead to more retraction in the lower arch and, consequently, an increase over jet.
Pre and and post treatment position of incisors.
With the help of the IZC miniscrews, total arch distalisation after complete space closure in the upper dentition maintained an optimal overjet at the end of treatment.
Profile and occlusion following IZC-supported orthodontic treatment.
Post-treatment records reveal a significant enhancement in facial profile and the establishment of a well-aligned occlusion.
Cephalometric superimposition after orthodontic treatment supported by the infrazygomatic crest (IZC) screws.
Cephalometric superimpositions reveal a substantial maxillary incisor retraction of 12 mm, exceeding the extraction spaces of premolars. Additionally, there is a 2.6 mm distalisation of maxillary molars, indicating total arch distalisation after complete closure of spaces in the upper dentition. The mandibular incisors underwent significant retraction of 11.5 mm. The movements of the mandibular molars were not discussed for the involvement of substitution.
Careful clinical monitoring, including assessments of profile improvement and cephalometric superimpositions, plays a crucial role in identifying the optimal conclusion of total arch distalisation. However, it is helpful for clinicians to understand that 3.5 mm upper molar distalisation should be a reasonable expectation for total arch distalisation with IZC miniscrews. Sugawara et al. reported an average amount of 3.78 mm at the crown level and 3.20 mm at the root level for the maxillary molar distalisation with miniplates. It is also possible to achieve the same amount of tooth movements with the interdental miniscrews, only that repositioning of the miniscrews might be necessary.
IZC success
The reported success rate of IZC miniscrews varies widely, ranging from 78.2%, according to Uribe et al., to 95%, as reported by Chang. , Notably, the distinct difference in success rates may be attributed to variations in the definition of success. The author examined the success of IZC miniscrews based on clinical results, considering any removal before achieving the intended treatment goals as a failure. The observed success rate was as low as 73.9%, potentially influenced by repeated attempts in cases of initial failure. Notably, instances of failure in IZC miniscrews might experience recurring challenges in subsequent trials, possibly due to unfavourable conditions in the same patients. Addressing the issue of repeated failure in IZC miniscrews, a plausible solution may involve a shift to alternative priorities, such as utilising palatal miniscrews and associated palatal appliances.
Buccal shelf miniscrew implants
BS miniscrews are the counterparts of the IZC miniscrews in the mandibular arch, which are located outside the buccal alveolar ridge between the mandibular first and second molars. ( Fig. 73.1 B) The buccal shelf refers to the bony prominence or ledge on the buccal aspect of the alveolar bone in the posterior region of the mandible. This anatomical structure provides a stable and well-supported area for the insertion of orthodontic miniscrews. The bone structure of the buccal shelf is typically very dense, which enhances stability and minimises the risk of screw displacement.
The author employs the same miniscrew specifications for both IZC and BS miniscrews, conducting direct hand drilling under local anaesthesia with a screwdriver without the need for pre-drilling. The insertion point is maintained as close to the mucogingival junction as possible to avoid soft tissue irritation and consequent soft tissue coverage. In cases where the buccal shelf has a wide and flat platform, the insertion pathway is kept perpendicular to the occlusal surface and parallel to the roots of the mandibular molars. Conversely, when dealing with a steep and narrow buccal shelf, the author may opt to create an indentation using a high-speed diamond round bur through the attached gingiva. This technique aids in securing the miniscrew to prevent slippage during BS miniscrew insertion.
Unlike IZC miniscrews, the pathway of BS miniscrews cannot be altered due to the robust bone density of the mandibular buccal shelf. The author prefers stainless steel miniscrews based on their durability, minimising the risk of breakage during insertion. The BS miniscrew is then fully inserted, exposing the miniscrew heads at approximately the bracket level. The author always initiates force application immediately after the miniscrew insertion.
The success rate of BS miniscrews was reported as high as 93% by Chang et al., while the author’s statistics were around 80.7%.
Clinical applications of BS miniscrew implant:
The BS miniscrews are primarily utilised for the maximal retraction or total arch distalisation of the lower dentition without extraction spaces for class III correction ( Fig. 73.3 ) .
A case of adult class III malocclusion treated with buccal shelf screw. Pre-treatment.
A severe class III malocclusion with a concave facial profile and mandibular prognathism. The patient declined the orthographic surgery.
Pre-treatment cephalogram and OPG.
Four wisdom teeth noted in the pre-treatment panoramic radiograph. These were suggested to be removed. The skeletal discrepancy of class III malocclusion cannot be improved without orthographic surgery.
Occlusion after a camouflage treatment with BS miniscrew retraction of the mandibular arch.
With the understanding of compromised treatment results, a camouflage treatment was undergone with BS miniscrew retraction of the mandibular arch and class III elastics, as evident in the sixth-month progress records.
Post-treatment profile and occlusion.
Post-treatment records show an acceptable overjet and overbite and an improved occlusion. The facial profile remained concave with slight improvement.
Normalised relationships of the anterior teeth following orthodontic treatment with buccal shelf (BS) screws.
A comparison of the overjet before and after treatment.
