Skeletal Anchorage in Orthodontics. Temporary Anchorage Devices (TADs): Mini-Screws

Orthodontic treatment relies on the application of forces generated by orthodontic appliances to move teeth into their desired positions. These forces are directed toward specific anatomical regions that act as support areas. However, the forces used to move teeth also create equal and opposite reactions on these support structures, which can lead to unwanted movement. Anchorage in orthodontics refers to the resistance provided by anatomical structures to counteract these reactive forces, ensuring controlled and predictable tooth movement.

The introduction of skeletal anchorage has transformed modern orthodontics by offering a dependable and efficient method for tooth movement without the risk of undesirable reciprocal forces. Temporary anchorage devices (TADs), especially mini-screws, have gained widespread popularity due to their minimally invasive nature, straightforward placement, and adaptability in various clinical scenarios. 

The use of implants as a means of ensuring orthodontic anchorage offers significant advantages in orthodontic treatment by eliminating the need for traditional intraoral and extraoral appliances.

The effectiveness of implant anchorage led to the development of specialized orthodontic mini-screws. An orthodontic mini-screw is an intraosseous implant with a diameter of less than 3.0 mm, designed to serve as an anchorage point in orthodontic treatment due to its primary mechanical stability.

Miniscrews are typically made from titanium alloys (ASTM grade 5, Ti-6Al-4V), known for their strength and biocompatibility. The addition of other metals enhances mechanical properties such as strength. The force capacity of TADs depends on their size and material composition. Titanium alloy mini-screws, for instance, can withstand 200–400 grams of force, though additional implants or splinting techniques can increase their load-bearing potential.

Each screw consists of three primary parts:

• Head: Designed for orthodontic applications, allowing for attachment of ligatures or elastics.
• Core: The central part that provides stability.
• Thread: Wrapped around the core, facilitating bone engagement.

The screws vary in diameter (1.2 mm to 2.0 mm) and length (5 mm to 9 mm), with longer screws used in areas with thicker gingival tissue.

Miniscrews are categorized into:

• Self-drilling (drill-free): These have a cutting flute at the tip, allowing insertion without pre-drilling, reducing bone trauma.
• Pre-drilling (drilled): These require a pilot hole before placement, typically used in dense bone areas.

The success of orthodontic treatment with mini-screws depends on several factors, one of the most crucial being their stability, whether used for direct or indirect anchorage. Loss (rejection) of an orthodontic mini-screw does not lead to irreversible changes as with dental implants but requires the orthodontist to modify the treatment plan or place a new mini-screw, typically in a different area. TADs provide stable anchorage by integrating with bone, allowing orthodontic forces to be applied without relying on adjacent teeth for support. This eliminates the risk of anchorage loss, which is a common limitation in traditional orthodontic treatments.

Mini-implants have found widespread use in treating various dentofacial anomalies, including bilateral protrusion of the incisors, vertical incisor disocclusion, as well as for aligning axial positions, incisor intrusion, molar distalization, and the traction of impacted teeth. Unlike conventional mechanics, which often produce extrusive forces, TADs facilitate intrusion by being strategically placed in apical positions relative to brackets. This capability makes them highly effective for vertical control and open-bite correction.

During orthodontic treatment, maintaining bone volume and quality is essential – especially in cases of missing teeth, premolar agenesis, or extensive tooth movement. Without proper bone support, undesirable resorption or inadequate anchorage can compromise treatment outcomes. Would you like to learn how to effectively use microimplants to preserve bone and enhance skeletal anchorage? Join the course “Microimplants as Bone Maintainers and Skeletal Anchorage: Insertion Protocols” and get the protocols which will help you achieve predictable outcomes, enhanced anchorage control, and superior biomechanics in orthodontic treatment!

Indications for Mini-Screw Use 

Mini-screws are indicated in various orthodontic treatments, including:

• Closing spaces caused by missing teeth
  • Braces: TADs allow front teeth to move backward without shifting molars forward, maintaining strong anchorage.
  • Aligners: Clear aligners alone may struggle to close large extraction spaces efficiently. TADs can reinforce the movement and prevent unwanted side effects.
• Intruding over-erupted molars
  • Braces: TADs can intrude overerupted upper front teeth (for deep bites) or posterior teeth (for open bites).
  • Aligners: TADs are sometimes combined with aligner cutouts and elastics to push teeth upward more predictably.
• Distalizing molars for Class II correction
  • Braces: TADs can be placed in the posterior maxilla to move upper molars backward, helping correct Class II malocclusion (where upper teeth are too far forward).
  • Aligners: TADs can serve as anchorage points for elastics or power chains to achieve the same distalization effect.
• Correcting open bites
  • Braces: TADs help intrude molars, allowing the front teeth to naturally close.
  • Aligners: TADs can be used with elastics to enhance anterior contact, a movement aligners alone may struggle with.

Managing midline discrepancies and asymmetries

• Braces: TADs can be placed on one side of the mouth to shift the midline without unwanted movements on the opposite side.
• Aligners: TADs can provide an anchorage point for interarch elastics to correct asymmetries more effectively.

Providing anchorage for posterior tooth protraction in non-extraction cases

Aligners are a powerful tool, but their predictability has limitations – especially in complex cases. By combining microimplants (TADs) with aligners, you can achieve superior control, improved biomechanics, and more predictable outcomes for challenging cases like open bites, crossbites, Class 2 and 3 malocclusions, impacted canines, and gummy smiles. We invite you to join our course “Aligners and microimplants in one protocol” to master advanced biomechanics of aligner therapy, microimplant placement and anchorage strategies, and receive step-by-step protocols for treating vertical anomalies and complex malocclusions!

The most common sites for mini-screw integration include the palatal area, the palatal slope of the alveolar ridge in the upper jaw, the retromolar region in the lower jaw, and the buccal cortical plate of both jaws. Critical anatomical factors to consider when choosing an implantation site include the anatomy of soft tissues, inter-radicular distance, the morphology of the maxillary sinus, the location of nerve trunks, and the buccal-lingual depth of the bone tissue.

Studies of computed tomography scans of the upper and lower jaws of patients have been conducted to define “safe zones” for mini-screw placement. The most favorable zones for mini-screw placement in the upper jaw were identified on the palatal side between the second premolar and first molar, and between the first and second molars; on the vestibular side between the canine and first premolar, and between the first and second premolars. In general, the study concluded that the more mesial and superior the implantation zone, the safer it is. For the lower jaw, the safe zones for mini-screw placement were identified between the first and second premolars, and between the first and second molars.

Clinical Protocol for Mini-Screw Placement 

Required Instruments

1. Pilot Drill: Used for pre-drilling in dense bone areas.
2. Hand Driver: A manual tool for controlled insertion.
3. Contra-angle Driver: Used with a low-speed handpiece for precise placement.
4. Tissue Punch: Creates a small opening in the gingiva when necessary.
5. Tweezers or Forceps: For handling and positioning the miniscrew.
6. Torque Wrench: Measures insertion torque to ensure proper stability.

Successful mini-screw placement requires proper planning and adherence to specific protocols:

1. Site Selection: Ideal sites include interradicular spaces, the infrazygomatic crest, and the palatal region.
2. Preoperative Assessment: CBCT imaging is recommended to evaluate bone density and anatomical limitations. TADs should be placed in areas with sufficient cortical bone thickness to ensure primary stability.
3. Placement Technique: Mini-screws can be inserted with a self-drilling or pre-drilled approach, depending on bone density. Placement is generally flapless, but a small incision may be made in non-keratinized gingiva to prevent soft tissue entrapment.
4. Loading Protocol: Immediate or delayed loading is determined by primary stability and clinical needs.

An effective treatment plan requires prioritizing tooth movements, designing an appropriate force system, and ensuring three-dimensional control of tooth positioning. Two primary types of mechanics are employed:

• Force-Driven Mechanics: Utilizes a single-force system for predictable outcomes, ideal for molar intrusion and significant tooth movements.
• Shape-Driven Mechanics: Relies on archwire configurations, useful for fine-tuning tooth positions but less effective for complex movements.

        5. Postoperative Care: Regular monitoring for inflammation, mobility, and integration with surrounding tissues.

Complications and Risk Factors 

While mini-screws have a high success rate, potential complications include:

• Soft tissue inflammation and infection
• Mini-screw mobility or failure due to inadequate stability
• Root contact, leading to resorption or loss of anchorage
• Soft tissue overgrowth, requiring re-adjustment or removal

While certain risks exist, they can be minimized with proper planning, careful insertion, and strict adherence to oral hygiene and follow-up care. 

Clinical Application of Temporary Anchorage Devices for Molar Intrusion, Distalization and Protraction

Molar Intrusion with TADs

Molar intrusion is often indicated for correcting deep bites, open bites, or occlusal plane discrepancies. Overcorrection and functional improvements (e.g., controlling tongue thrust) are essential for long-term stability. Molar intrusion is one of the most challenging tooth movements. TADs enable force-driven mechanics, which are highly efficient for this purpose. For maxillary molar intrusion, palatal root control is critical, as the center of resistance lies on the palatal side. In the mandible, lingual forces are less necessary due to the natural lingual inclination of molars.

Molar Distalization with TADs
Molar distalization is commonly used to address crowding or Class II malocclusions. Distalization is ideal for cases requiring up to 3 mm of space per side. TADs allow for precise control of distalization forces. Palatal TADs in the maxilla are particularly effective for applying direct distalization forces while controlling the molar’s mesiodistal axis. Buccal TADs are simpler to place and can achieve significant distal movement.

Molar Protraction with TADs

Molar protraction is often used to close edentulous spaces. However, biological factors, such as alveolar bone quality and gingival health, play a significant role in success. In the mandible, protraction is more challenging due to slower bone turnover and higher risks of attachment loss.

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Mini-screws represent a significant advancement in orthodontics, offering a reliable solution for skeletal anchorage. Their advantages, including minimal patient compliance requirements, ease of placement, and versatility, make them an indispensable tool in modern orthodontic practice. However, proper case selection, precise placement, and vigilant post-procedural care are crucial for ensuring success and minimizing complications. Future research should focus on improving mini-screw design, materials, and integration techniques to further enhance their clinical applications.