Mini-screws in orthodontics

    Mini-screws in orthodontics

Introduction :

              In orthodontics, anchoring is the key to all mechanics, without which it is impossible to create dental movements. When traditional dental anchoring is insufficient, we can now use mini bone screws.

             After several years of use, mini-screws have already established themselves as an undeniable development, with a considerable impact on our therapeutic decisions.

             Their use has significantly affected orthodontic biomechanics and now allows us to define new treatment methods.

1. Limitations of conventional orthodontics:

• Patient cooperation determines the success or failure of treatment 
• unfavorable skeletal patterns, mainly very hyperdivergent patients, at the limit of surgical indication
• the lack of therapeutic alternatives in certain critical situations: 

Ex: extensive edentulism, weakened periodontal support, multiple agenesis,

            Asymmetries, certain Class III malocclusions, or failures    

            therapeutics.

2.  Definition :

  Miniscrews are temporary bone anchoring devices that are a tool in carrying out orthodontic treatments. 

        Miniscrews constitute a so-called absolute anchor.

3. Anchorage concept in Orthodontics:

   According to Newton’s third principle: “For every action there corresponds a reaction of equal intensity and opposite direction.”

  Regardless of the orthodontic mechanical therapy used, applying a force to a tooth, a group of teeth or an arch generates a force of the same intensity, the same line of action, but in the opposite direction on the support structure.

        The orthodontist must therefore control his force system to avoid unwanted movements in all three spatial directions.

4.  General principles:

•  The purpose of miniscrews is to provide support for tooth movement when conventional anchorage is non-existent or faulty.

•  The miniscrew must not undergo any deformation or fracture during screwing or during loading.
• The material used must be biocompatible and avoid corrosion (titanium alloy). 
• The installation of the miniscrews is temporary; they will be removed after use, which lasts on average 3 to 8 months.

5.  Description:

          The number of mini-screws available on the market is increasing and varies depending on their materials, dimensions and shapes.

• Diameter and length of mini-screws:

     Diameters vary between 1.2 and 2 mm, and lengths range from 7 mm to 12 mm. 

    – A small diameter will be recommended at the inter-radicular level in order to respect a safety margin with the roots. 

    – A large diameter will allow the application of higher orthodontic loads but must be used away from the roots.

• Shape of mini-screws:

The mini screws have three parts:

1- a cylindrical-conical intraosseous part: perhaps self-drilling or self-tapping.

2- the head of the mini-screw: simple, in the shape of a Bracket, allows connection with different orthodontic devices (elastic, ligature).

3- the transmucosal neck: between the head and the intraosseous part, it helps prevent mucosal irritation. 

6. Clinical protocol for the placement of miniscrews:

– Location of the miniscrews depends on: -Positioning

                          – Choice of diameter and length

                          – Gum quality

                          – Insertion into an inter-radicular space 

-Surgical insertion protocol

-Loading the mini-screw

1- Location of the miniscrews:

1.1. Positioning:

• The positioning of the screw follows the clinical indication and the biomechanical strategy developed to correct the malocclusion. 
• The first step is an X-ray examination.
•  The screws are located in the alveolar area away from the roots and nerve structures, i.e. in the edentulous, retromolar, interradicular, vestibular or palatal sectors.

1.2. Choice of diameter and length:

• The choice of the diameter and length of the miniscrew is based on the radiographic analysis and the insertion axis offered by the recipient site.

• The insertion is monocortical. 
• To optimize anchor stability, the longest and widest miniscrew that the recipient site can accept should be chosen.

1.3. Gum quality:

• This is an essential factor in choosing the insertion site.
•  It is recommended, for better healing, that the neck of the miniscrew emerges in the attached gingiva which is keratinized and adherent to the periosteum. 
• Insertion into the mobile free gum is possible but there is a risk of inflammatory and painful healing and of the screw head being buried under the gum.

1.4. Insertion into an inter-radicular space: 

To avoid possible root damage, it is important to have at least 1.5 mm of bone between the miniscrew and the desmodontal ligament of the adjacent teeth.

2- Surgical insertion protocol:

• The insertion procedure is sterile, the recipient site is disinfected with 0.2% chlorhexidine.
• Probing the gum to quantify its thickness and thus choose the type of miniscrew neck.
• The insertion site is locally anesthetized
• The face is made through the gum into the cortical bone with a manual screwdriver (self-tapping miniscrew) or a low-speed contra-angle under irrigation (self-drilling miniscrew).
• Clinical control of primary stability

The screw’s mobility in tension and compression must be tested using a probe and a percussion test: a dull, metallic sound attests to primary stability.

• Radiographic control of primary stability

The insertion axis and the correct positioning of the miniscrew must be checked radiologically.

3- Loading the screws:

• The installation of the force application system, between the screw and the tooth to be mobilized, can be done the same day. This involves immediate loading or after gingival healing. 
• The forces applied must not exceed 150g

NB: – The screws are removed using a sterile surgical procedure and do not require local anesthesia in 90% of cases.

 – Bone and gum healing after removal is rapid (6 to 8 days).

7.  Indications and contraindications:

• Intraosseous anchors are indicated when the possibilities of conventional anchors are exceeded, essentially in cases of deficient periodontium in adults.

Dental indications: 

It is orthodontic biomechanics that guides the positioning of the miniscrew.

     All sagittal, vertical or transverse intra- or inter-arch dental movements are possible, for example:

• Retraction of canines
• Contraction of the incisor block
• Version or straightening of molars
• Ingression of the incisor block or molars
• Traction of impacted teeth
• Correction of the occlusal plane
• Delaire mask traction
• Mini-implant assisted maxillary disjunction 

Contraindications:

  – Destruction, loss or poor bone quality likely to affect the stability of the anchor.

 – Location near nerve pathways, nasal and sinus cavities

 – Insufficient inter-radicular bone width.

 – Acute or chronic local infections or systemic conditions (heart disease, unbalanced diabetes, constant hemodialysis,

 – Coagulation disorders, reduced immune defenses, etc.).

8.  Benefits :

• Simpler equipment, sector system 
• Absence of iatrogenic effect on neighboring teeth 
• Better control in all three dimensions of space, especially in the vertical direction which is difficult to control in traditional techniques
•  Better control of the orientation of the occlusal plane by eliminating the parasitic effects of intermaxillary traction by elastics
•  Faster results than with traditional orthodontic treatment

9.  Mini-screw failures:

• The majority of failures are related to a loss of primary stability before orthodontic loading

• The more the force intensity is distributed over a large number of screws, the less failure there is. 
• Screws placed perpendicular to the tooth axis have a higher than average failure rate.
• Poor hygiene and degree of inflammation

 Conclusion : 

Mini-implants, or mini-screws, have now demonstrated their effectiveness, providing us with effective anchoring in almost all clinical situations .

 Their use, however, requires the mastery of new, essentially biomechanical parameters, the application of codified protocols adapted to the type of movement sought.

 Mini-screws in orthodontics

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 Mini-screws in orthodontics