Orthodontics IMPLANTS AS ABSOLUTE ANCHORAGE DEVICES IN ORTHODONTICS
IMPLANTS AS ABSOLUTE ANCHORAGE DEVICES IN ORTHODONTICS
Anchorage consideration forms an integral part of orthodontic treatment planning and specific measures and precautions have been indicated to ensure adequate anchorage control so that reactionary forces produce minimal reciprocal tooth movement i.e. anchorage loss. Traditionally a variety of extra oral and intra-oral devices have been used to conserve anchorage but the cumbersome designs and demand for patient cooperation are limitations that have been difficult to overcome. Of late, the incorporation of dental implants and temporary anchorage devices (TAD) into orthodontic treatment make possible infinite or absolute anchorage which has been defined in terms of implants as showing no movement (zero anchorage loss) as a consequence of reactionary forces. A Temporary anchorage device (TAD) is a device that is temporarily fixed to bone for the purpose of enhancing orthodontic anchorage either by supporting the teeth of the reactive unit or by obviating the need for the reactive unit altogether, and which is subsequently removed after use. They can be located transosteally, subperiosteally or endosteally; and they can be fixed to the bone either mechanically (cortically stabilized) or biochemically (osseointegrated). As presently defined, all TAD’s are invasive devices and are best reserved for problems that cannot be effectively managed with conventional mechanics. Most common TAD’s include the Miniscrews, Microscrews, Miniature Implants (Mini-Implants), Palatal Implants, Modified Bone Plates and Retromolar Implants. In addition, a TAD may be a temporary prosthetic component that is removed after treatment. Therefore temporary anchorage devices can range from non integrated miniscrews to implant supported prosthesis with temporary orthodontic attachments. Most current miniscrews are stainless steel, titanium or titanium alloys and are manufactured with a smooth machined surface that is not designed to osseointegrate. By definition, TAD’s are temporary devices; no long term esthetic goal is planned. Thus most TAD’s are removed after orthodontic treatment. However some osseointegrated TAD’s may be covered with soft tissue or retained for sustained prosthetic function.
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Diagrammatic representation of a TAD (Spider Screw System). |
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CLASSIFICATION OF IMPLANT ANCHORAGE DEVICES
Transosseous anchor systems:
1. Temporary palatal endosseous implants
2. Bone plates
3. Bone screws (TAD)
4. Subperiosteal palatal onplants Bone Screw (TAD)
a) Based on the head type
i. Head with a hole in the neck: (e.g.: Dual-Top Anchor System, Lin/Liou Orthodontic Mini Anchorage Screw (LOMAS), Leone System.
ii. Head with button like design: (e.g. Absoanchor, The Aarhus system)
iii. Head with bracket like design: (e.g. Temporary MiniOrthodontic Anchorage System TOMAS)
iv. Head with a hook: (e.g. New LOMAS)
b) Based on the different types and lengths of transmucosal parts/neck portions
c) Based on the body shape and surface
i. Tapered micro implant / Cylindrical micro implant.
ii. Self tapping or Smooth surfaced miniscrew - which requires pre-drilling followed by screw insertion.
iii. Self-drilling miniscrew (The Aarhus system, Dual-Top Anchor System,LOMAS)
PLACEMENT PROTOCOL
The first aspect of any micro-implant placement is the identification of the exact location that the implant should be placed. A peri-apical radiograph taken using the paralleling technique or an orthopantamograph is sufficient for determination of placement site. Adequate site of implant placement is goverened by:
1. Indication, system used, and required mechanics.
2. Placement in attached gingiva, clear of the frenulum.
3. Sufficient interradicular distance to avoid encroachment into periradicular space.
4. Avoiding anatomical structures such as neurovascular bundles, maxillary sinus, foraminas and the nasal cavity.
5. Adequate cortical bone thickness. Placing the implant in areas of favorable bone thickness ensures better primary stability and long-term success. The following locations are only few of the many available implant sites in the oral cavity.
Maxillary Implant Sites:
(a) Paramedian of mid-sagittal region of the hard plate.
(b) Zygomatic buttress of the maxilla.
(c) Area below the anterior nasal spine.
(d) Maxillary tuberosity.
(e) Edentulous alveolar ridges.
(f) Interradicular spaces, both buccal and lingual. Mandibular Implant Sites:
(a) Retromolar area.
(b) Symphysis or parasymphysis area.
(c) Buccal cortical (shelf) area.
(d) Edentulous alveolar ridges
(e) Interradicular spaces, both buccal and lingual.
IMPLANT POSITION:
Maxillary Micro Implant:
Maxillary arch is made up of thick porous cortical bone with coarse trabeculae only in the anterior region, but most of the arch (posterior and distal part) is made up of thin porous cortical bone with fine trabeculae or fine trabecular bone. Because of this bony type in the maxilla, the micro-implants sites need a 300-400angulation to the long axis of the teeth, either buccally or lingually. As a general guide the implant can be placed at 7mm from the interdental papillae (Fig 1.) between the molar and premolar and at an angulation of 450.
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Fig1: adequately positioned microimplant between maxillary premolar and molar |
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Mandibular Micro-Implants:
Most of the mandibular arch is made up of thicker cortical bone. Micro-Implants in these thicker cortical regions require only 100-200 of angulation to the long axes of the teeth, ie almost parallel to the tooth roots so that most of the inserted portion of the implant will lie in the cortical bone.
(1) Surface anesthesia for only mucosal drilling is indicated so that any encroachment into the periodontial ligament space of the adjacent teeth will cause pain and indicate the need to change direction of drilling or implant placement.
(2) Infection control measures are similar to that for any surgical procedure.
(3) Depending on the implant site, one of the following 2 surgical procedures can be performed. Attached Gingiva: - flap elevation or sutures are not needed in case the implants are placed though the attached gingiva. A circular hole approximating the size of head of the implant can be prepared though the attached gingiva with a punch. Alveolar Mucosa:- A 3mm vertical or horizontal incision along the mucogingival junction with a No 15 surgical blade, then elevate a mucoperiosteal flap to expose the underlying bone.
(4) When ever pilot drilling is used, it should be 0.2-0.3mm thinner than the screw and should be inserted to a depth of no more than 2-3mm. Drill the pilot hole just inside the cortical bone to allow seft-drilling of the bone screw and better mechanical retention. Keep the drill speed low under thorough irrigation with normal saline to avoid overheating and bone necrosis.
(5) If resistance or pain occurs during the micro-implant insertion, it might be probably due to the contact of the adjacent tooth roots. In these instances, the micro implant is withdrawn and reinserted at a different angulation. If the inserting micro implant is loose, then remove that implant and replace it with the next larger size implant at the same site. Following insertion, the head of the micro implant remains outside the mucosa, with the base of the head resting on, but not compressing the mucosa.
(6) Antibiotics have been recommended by several authors, but need not be routinely prescribed.
BIOMECHANICAL CONSIDERATIONS IN MINISCREW ANCHORAGE
Depending on the clinical situation, implants for anchorage are loaded either directly or indirectly. Direct loading implies that the forces needed for the desired tooth movements are directly introduced onto the implant. Indirect loading implies that the teeth intended to act as reactive unit are indirectly stabilized by the mini screws.
The loading protocol for the implants and the screws are not uniform and various authors have been proposed different views on this. Loading protocol for prosthodontic implants involve a waiting period of a minimum of 2 months before loading, whereas for screws immediate loading or a waiting period of 1 week is recommended.
The amount of the orthodontic forces used for various tooth movements ranges from 50gms to 300 gmm, whereas for orthopedic forces it might be littlie more than this.
CLINICAL APPLICATIONS
Micro implants have been recently used in most of the clinical situations because of their small size, ease of placement and versatile designs of the heads (with slots). Their use has been reported of management of non extraction cases with molar distalization, entire arch distalization, closure of extraction space, symmetric incisor intrusion, correction of canted occlusal plane,alignment of the dental midlines, individual canine retraction, extrusion of impacted canines ,molar intrusion, molar mesialization,inter maxillary anchorage (Figures 3 to 5).
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Fig. 2: TAD placed at junction of free and attached gingiva. |
Fig. 3: Closed coil spring used for retraction |
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Fig. 4: indirect canine retraction with open coil spring |
Fig. 5: canine retraction with opencoil spring with |
SUCCESS AND FAILURES
In most of the repoted cases the overall success rate was 91.6%, with a mean period of force application of 15 months. Miniscrew or microscrew implants, however, used as orthodontic anchorage should be loaded early to reduce treatment time and should be removed after treatment.
The factors associated with the failure of screw implants are maily related to mobility of the implant, infections and the site of placement 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. This might be explained by better hygiene on the left side of the dental arch by right-handed patients, who are most of the population. The mandible is expected to have a higher success rate because it has a thicker and denser cortical bone than the maxilla but the results are however, opposite. The assumed reasons might be overheating of the bone during drilling and irritation during chewing. Because the mandible has denser bone, there is a greater chance of generating heat greater than 47°C, which is the critical temperature that can cause bone damage. In addition, screw implants placed in the posterior part of the mandible can easily be irritated by food during chewing. Excluding mobility, inflammation around the screw implants was added as a relative risk factor. For screw implants used as orthodontic anchorage mobility might not represent failure unlike that for dental implants. By using comparatively low force (less than 200 g), the screw implants that showed minimal mobility could be set as anchorage. If heavy force is applied to screw implants, their mobility might be increased, and they can fail by not becoming sufficiently osseointegrated to the bone. In the animal studies of Ohmae et al and Deguchi et al, stable screw implants showed osseointegration from 25% to 40%. Deguchi et al postulated that less osseointegration does not necessarily indicate a negative finding. When an excessive load is applied, partly osseointegrated screw implants can become severely mobile and eventually fail. Dental implants are usually loaded in all directions in addition to vertical occlusal forces, but orthodontic screw implants are usually loaded with unidirectional lateral forces. Therefore, minimal mobility can be allowed in orthodontic screw implants. Mobility can also be increased if the implant is placed too close or abuting the root of the adjacent teeth as constant movement of the teeth under occlusal forces will loosen the implant. Encroachement onto the root either as a result of implant placement of movement of the root onto the implant may lead to root resorption though small amounts of resorption may heal and not lead to any clinically significant effects.
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Fig. 6: Implant encroaching onto root surface of the 2nd molar. |
Fig.7: Infection and gingival overgrowth seen in relation to a TAD |
Inflammation or peri-implantitis (Fig. 7) can damage the bone surrounding the neck of screw implants. With progressive damage of the cortical bone, screw implants can be endangered. To ensure success, it is important to prevent inflammation around the screw implants Local inflammation can be exaggerated not only by oral hygiene but also by weak nonkeratinized soft tissue around the neck of the screw implant Nonkeratinized mucosa is a risk factor for miniscrews.The highest success rate (100%) of screw implants placed is in the maxillary palatal area where there is thick keratinized mucosa. In addition, the screw implants in closed group, in which the head of screw was covered by soft tissue, had greater success than the open group hence the overlying soft tissue on the head of screw implants might be a barrier against inflammation.
ONSET OF FORCE APPLICATION:
Success with respect to the onset of force application might indicate that immediate loading of screw implants is possible after placement without a discernible deterioration of stability, though some operators prefer to wait for a period of 1week before loading the implants. This period of consolidation may be more desirable with the use of self tapping implants that the self drilling type.
PERIOD OF FORCE APPLICATION:
The mean period of force application to the miniscrew or microscrew implants was 15 months, which is sufficient to provide proper anchorage in most orthodontic patients. The most critical time period demanding anchorage control for successful orthodontic treatment is for anterior tooth retraction in extraction patients. This usually takes 10 to 12 months of microscrew implant anchorage sliding mechanics.In nonextraction treatment; the distal movement of the posterior segment can be obtained within 10 months. This is because the posterior segment can be distalized together and not 1 tooth at a time. Therefore, microscrew implants seem to cover the critical time period requiring absolute anchorage.
ANGULATION DURING IMPLANT PLACEMENT:
The contact surface of the screw implants to the cortical bone is increased by placing them at angles. Therefore, clinicians can place long screw implants with angulations to bone surface without decreasing stability, and the capability of using long screws might influence success positively. Though this has been shown otherwise by Petry et al who reiterate that placement of the TAD at 900 to the cortical plate provides the most retention. They also showed that increased penetration depth increases retention and increased head distance from the cortical plate decreases retention.
LENGTH AND DIAMETER OF IMPLANT:
The length of dental implants was reported to have a positive effect on stability. From a clinical point of view, smaller diameter screws are easier and less traumatic to place and use. Screw implants with a diameter over 1.2 mm can be recommended as orthodontic anchor screw implants.
REMOVAL OF IMPLANT:
If there is too much osseointegration, clinicians might have difficulty in removing the screws or they can fracture during removal. Fractured segments can be retrieved by loosening it by drilling around it or they can be left in place and covered with soft tissue.
CONCLUSION
The success of implants being used as anchors has widened the horizons of the orthodontist, which should be explored to the best possible advantage for treating cases. This could help in providing the aesthetically conscious adult patient orthodontic care, which was once compromised or denied altogether due to the lack of posterior teeth, which serve as anchors during orthodontic treatment.
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