February, 2012

Factors Affecting Success of Mini-implants – A Review

Dr Nagarathna KN

PG Student ,Department of Orthodontics

Dr Syamala Reddy Dental College, Bangalore

Dr Prashanth Kamath,

Professor and HOD,Department of Orthodontics

Dr Syamala Reddy Dental College, Bangalore

Dr .Renu Prasad,

Professor ,Department of Orthodontics

Dr Syamala Reddy Dental College, Bangalore

Dr .Arun Kumar BR ,

Senior Lecturer ,Department of Orthodontics

Dr Syamala Reddy Dental College,Bangalore

Dr. Mamatha Thomas,

Senior Lecturer , Department of Orthodontics ,

Dr Syamala Reddy Dental College,Bangalore

Corresponding author: Dr Nagarathna KN

PG Student,phone no : 91-9986175541,   E-mail:  nagarathna_kn@yahoo.com

Abstract; Mini-implants have become a routine anchorage method in orthodontic practice given their high predictability and scientifically proven benefits. The small size of miniscrew implants allows them to be placed into bone between the teeth, thus expanding their clinical applications. With more patients treated with screw implants as anchorage, their stability is gathering attention. Despite their tremendous success in facilitating treatment outcomes, the implant failure rates are widely variable and could be as high as 10-30%.This article describes the various factors affecting success of mini-implants.

Key words: Miniscrew success, Stability, Safe zones.

Introduction:

Use of mini-implant anchorage in orthodontics has gained increasing popularity in both clinical applications and research since its introduction. Despite their tremendous success in facilitating treatment outcomes, the implant failure rates are widely variable and could be as high as 10-30%.¹

Success in mini-implant orthodontics is defined as a mini-screw with minimal mobility and inflammation and the ability to obtain full functional correction either through direct or indirect anchorage.²

Successful mini-implants require adequate primary and secondary stability.Table1,shows comparisons of primary and secondary stability.³

Table no. I :comparision of Primary stability and Secondary stability

Various factors affecting success;

I.            Implant dependent

II.            Operator dependent

III.            Patient dependent

I .Implant dependent factors

Shape

1. Conical
2. Cylindrical

Dimension

1. Length
2. Diameter
3. Head
4. Trans gingival collar

Shape: Results from insertion torque measurements suggest that a conical screw design will provide greater primary stability than cylindrical screw types. The cylindrical screw design’s superiority was evident in the pullout tests. All the miniscrews’ primary stability rose after drill-free insertion.⁴

Dimension of mini-implant; Mini-implants with increased diameter, tapering, and double threading can maximize interlocking between bone and implant, thereby increasing primary stability.

1. Length: A longer screw with a small diameter is more subjected to bending or breakage. Screws that are longer than 10 mm could result in greater risk of iatrogenic perforation (i.e. On the lingual side of the mandible or into the maxillary sinus . Table  II describes the  Ideal implant length and site of placement.⁵

Table II: Ideal implant length and site of placement

2. Diameter: An increase in screw diameter can efficiently reinforce the initial stability of miniscrews, but the proximity of the root at the implanted site should be considered. Screws with diameters of 1.2, 1.5, and 2.3 mm have acceptable levels of success.⁴

3. Miniscrew head: The head must be of sufficient dimension to accept and hold any coupling elements selected for a particular application. Different head designs also require different dimensions. A small diameter and lower profile of the miniscrew head are important for oral hygiene and patient comfort.⁴

4. The Trans-gingival Collar: The trans-gingival collar (trans-mucosal collar/neck) is the sensitive part of implant. Any perforation in the soft tissue provides a potential entry point for microorganisms and could give rise to infection. Figure 1 shows the thickness of soft tissues and length of the trans-gingival collar.⁴

Figure 1: A-standard type, B- wide collared type, C-Long collard type

  I.            Operator related factors

These include selection of implant site, bone density, soft tissue considerations, Placement technique, direction of placement, implant placement torque, loading  protocol, avoiding soft tissue over growth,  using mini-plates, sterilization and finally clinicians  experience.

1. Selection of implant site: In order to avoid root contact, implants should be placed in safe zones. The minimal space requirement between roots is 0.5 mm mesial and distal to the implant or 1 mm more than the implant diameter.  In the maxilla, the more anterior and the more apical, the safer the location becomes.

Table III shows the order of the safer sites, available in the inter-radicular spaces of the posterior maxilla.⁶ The order of the safer sites available in the inter-radicular spaces of the posterior mandible are as follows:⁶

Table III: The order of the safer sites, available in the inter-radicular spaces of the posterior maxilla:

1. Between the 1^st  and 2^nd  molar.
2. Between 1^st  and 2^nd   premolar.
3. Between the 1^st molar and 2^nd  premolar at 11 mm from alveolar crest.
4. Between the 1^st premolar and canine at 11 mm from the alveolar crest.
5.  Between the 1^st premolar and canine at 11 mm from the alveolar crest.

2. Bone density:Stationary anchorage failure is often a result of low bone density due to inadequate cortical thickness. Bone density is classified into 4 types D1, D2, D3 and D4. D1, D2, D3 are optimal for self-drilling miniscrews.⁷

Implant placement in D4 not recommended due to the reported high failure rate. Table  IV shows the various Bone types and corresponding bone density, Figure 2 and 3 shows the distribution of  bone density in the maxilla and mandible.

Figure 2: Distribution of bone density in maxilla and mandible

Figure 3 : Distribution of bone density in palate

Table IV:Bone type and corresponding bone density

3. Assessment of miniscrew site: Periapical radiographs and OPG provide two-dimensional image of three-dimensional anatomic structures. The use of CBCT with 3-D images would provide more accurate and reliable results .(Figure 4 and 5)

Figure 4: In IOPA ,the possiblility of root contact is not clear.

Figure 5: In CBCT view: The image is clear and free of distortion

4. Soft tissue considerations: Thin and keratinized mucosa is the preferred area for implant placement. Table V shows comparision of mucosal site and prognosis.

Table V: comparision of mucosal site and prognosis

5. Placement technique: A small amount of local anesthetic is sufficient, profound anesthesia of the teeth is not required. There are two methods of implant placement, the Surgical technique and the Self-drilling method.⁸

1.Surgical technique: Ideally a pilot drill should be 0.2 to 0.5 mm less than the implant diameter, and the depth should be less to obtain proper initial mechanical stability. Table VI shows Implant diameter and pilot drill size.⁹

Table VI: Implant diameter and pilot drill size

A slow drill speed (800rpm) should be used.  Excessive pressure of the drill and worn drills are contraindicated. Heat production should be minimized.  47°C is the critical temperature that can cause bone damage. Copious irrigation with saline solution is recommended.^10,11 It is safer to use a manual screw driver, if the clinician feels any resistance from roots, the implant can be withdrawn and redirected.

2. Self drilling method: The self-drilling Implant has high placement torque and high bone-implant contact values. This procedure is contraindicated in the posterior and inferior aspects of the mandible since they have been reported to have a high breakage rate.

3. Direction of placement: Angulation of the bone surface needs to be moderate, a 45 degree angulation relative to the occlusal plane is considered acceptable .¹²

4. Insertion angle of miniscrews shows that an oblique insertion is advantageous to avoid possible root damage, but it is sometimes difficult because of possible slippage on bone surface (Figure 6). Excessive angulation may weaken the cortical bone structure and part of the threaded portion may be exposed on buccal side. (Figure 7).¹²

Figure 6: Insertion angle of miniscrews

Figure 7: Exposed threaded portion

5. Implant placement torque: Motoyoshi et al recommended an implant placement torque range of 5 to 10Ncm. Very high insertion torques leads to higher failure rates due to excessicve bone compression.¹³

6. Loading  protocol: involves immediate loading or a waiting period of 2 weeks to apply orthodontic forces.¹⁴ Most mini-implants can withstand 100 to 200 g of  horizontal  immediate loading successfully. But the position of micro-implants should allow adequate distance from vital organs in expectation of some implant displacement.

7. Minimizing soft tissue over growth: This can be done by placing of a healing abutment cap, a wax pellet, or an elastic separator. Using Chlorhexidine mouthwash slows down epithelialisation.

In regions of loose alveolar mucosa partial insertion with a longer miniscrew (10 mm) is done leaving 2 or 3 threads of the shaft exposed to oral cavity. This helps to minimize the possibility of soft-tissue coverage.

8. Using mini-plates: The connection of two mini-implants with mini- plate provides a stable anchorage system and improves the versatility of the device.¹

9. Screw implants on the right side of the jaw had a higher failure rate, and the mandible

had a higher failure rate than the maxilla. ¹⁵

10.Vertical skeletal pattern is an important factor for the success of orthodontic mini-implants placed in posterior buccal areas.¹⁶

11. Sterilization and asepsis are mandatory throughout the procedure.

12. Clinician experience and skill do contribute to the success of mini implants.

III -Patient dependent factor-

Oral hygiene is the only patient dependent factor.  Poor oral hygiene leads to: Peri implantitis,  Epithelial infiltration, Bleeding on probing, Suppuration, Loss of  bony support, Mobility and finally Implant failure. ⁷

Along with regular tooth-brushing, Chlorhexidine (0.12%, 10 ml) mouthwash is recommended. Patient should be explained about the importance of oral hygiene and motivated  at every visit.

Conclusion;

Orthodontic mini-implants are a powerful aid for the orthodontic practitioner in resolving challenging malocclusions but, Implant failure might delay treatment time. A good knowledge of factors affecting miniscrew success will help us to increase their success rate, thereby achieving desired treatment results and save chair-side time.

References;

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