Anchorage control is a challenging problem in orthodontics. The palatal implant Orthosystem (from Straumann) is a temporary anchorage device, which can provide the necessary anchorage control in the upper jaw. This article describes general features and specific indications of this implant.

By Dr Karlien Asscherickx

Anchorage in orthodontics
In cases where the jaws are in the correct position and only the teeth need to be moved, the main objective of the orthodontist is to move certain teeth, while often other teeth should remain in position. Newton’s third law states that every action creates a reaction, which is equal in size and opposite in direction. In orthodontics, the reaction forces are those forces that are generated when one tooth, a group of teeth or a skeletal unit are moved in a certain direction. Only when the reaction forces are dealt with in a sufficiently robust way will the desired movements occur. Anchorage can be defined as the sites which provide resistance to the reaction generated by the forces applied in order to move certain teeth. These sites can be extra-oral or intra-oral.

Extra-oral appliances like headgears can be used to enhance the anchorage potential of posterior upper teeth. Transpalatal arches can be used for the same purpose. Lingual arches can enhance the anchorage potential of lower posterior teeth. Intermaxillary elastics can be used to move teeth in one jaw, while using the teeth in the opposite jaw as anchorage unit. The major disadvantage of all these auxiliary devices is that most of them depend on patient compliance. If the appliance is not worn as instructed, the treatment outcome can be compromised. Another disadvantage is that none of these appliances can guarantee stationary stability. Anchorage loss, with undesired tooth movements, can occur. The only possibility of obtaining stationary anchorage is by using ankylosed teeth or implants as the anchor.

Implants for orthodontic anchorage purposes
Implants or implant-like devices can be used in orthodontics to provide the required anchorage, since they show no movement as a consequence of reaction forces. Classical endosseous dental implants, smaller palatal implants, mini-plates and mini-screws have all been described as possible tools to provide anchorage control.

The palatal implant Orthosystem (from Straumann, Basel, Switzerland) is such a temporary anchorage device, designed specifically to be used in the palate. It was described in 1996 by Wehrbein and coworkers. The implant can replace compliance-dependent extra-oral anchoring aids and makes a bonding of well aligned mandibular dentition as well as use with class II elastics unnecessary.

Palatal implant, design and procedure
The fixture is designed for one-stage application. The endosseous part of the implant is cylindrical and made of pure titanium [Figure 1a]. It has a diameter of either 4.1 or 4.8 mm and a length of 4.2 mm. The implant features a sand-blasted, large-grit, acid-etched (SLA) surface to enhance osseointegration. Above the polished transmucosal neck there follows an abutment, on which the desired suprastructure is soldered or laser-welded.
The implant is placed under local anesthesia. After a punch of the palatal mucosa with a trephine bur, the mucosa is removed with a curette. A pilot drill and a standardised profile drill are used to prepare the implant bed. The implant is hand-turned as far as possible, and if necessary a ratchet is used to tighten the implant in to its final position. After insertion, a screw is placed in the implant.

In adult patients, the median palatal suture zone is the area of choice for placement of palatal implants [Figure 1b]. In adolescents, however, the paramedian region is preferred to avoid possible growth impairment of the maxilla in transverse direction by placing an implant in the median palatal suture. We observed this problem when implants were inserted in the median palatal suture in adolescent beagle dogs.

After a healing period of 12 weeks, the implant is normally osseointegrated and can be used for anchorage purposes. An impression is made and an analogue is placed in the impression cap. The impression is poured out into a study cast and the suprastructure is made in the laboratory, according to the guidelines of the orthodontist, as a function of the purpose of the anchorage.

Since no findings on “sleeping orthodontic implants” have been published to date, the implant should be retrieved after use. As the implant is osseointegrated, this procedure necessitates the use of a special drill (cylindrical and open) with which the implant and the surrounding bone are removed. Healing is uneventful in most cases. After one week the palatal mucosa is closed and three months after explantation, the original rugae are restored. On histological slides (animal study) we observed that the extraction site heals with the formation of new bone, with preservation of a suture in adolescents.

Clinical indications
The palatal implant can be used to provide either direct or indirect anchorage. In indirect anchorage, the implant is connected with a bar to the teeth, which should remain positionally stable. An example of indirect anchorage is an implant-anchored transpalatal arch (TPA). The arch can be connected to the upper second premolars, in order to retract the frontal segment in case of extraction of first premolars [Figure 2].

In direct anchorage, the reaction forces are acting directly on the implant. Different designs of suprastructures have been developed at our department according to the desired tooth movements. Two designs are explained in detail.

The modified Implant-Anchored Distal Jet (IAD) [Figure 3] (designed by B. Vande Vannet) is used to distalise upper teeth and can replace a headgear. It should be kept in mind that with the IAD, no skeletal growth modification potential is present and therefore it should be carefully considered versus headgear therapy in growing individuals. Upper molars are banded and connected to each other by a transpalatal arch on which tubes are welded. A suprastructure is made on the implant, consisting of a small bar with two openings pointing distally, which can host boll pins. The boll pins pass through the tubes on the transpalatal arch. If molars should be distalised, coil springs are put on the boll pins to put force on the palatal arch in a distal direction. As the molars distalise, the removable transpalatal arch can be adapted, to provide expansion of the molars. Tipping of the molars is prevented, because the force is applied at the level of the point of resistance of the molars. Rotation of the molars can be prevented by bending anti-rotation in the transpalatal arch. Once enough distalisation is achieved, the molars can be held in place by the appliance by putting a closed coil spring over the boll pins.

The main advantage of this appliance is that only in the final phase should all teeth be bonded. In addition the invisibility of the appliance is important, especially for adult patients. We observed that as the molars moved distally the premolars distalised spontaneously, probably by stretching of the transseptal fibres.

A similar design is the Implant-Anchored Mesial Jet (IAM) [Figure 4] (designed by J. Aerts). Upper molars are banded and connected to each other by a transpalatal arch on which hooks are welded, pointing distally. The suprastructure is a bar with hooks pointing mesially. Coil springs can be placed from the hooks on the transpalatal arch to the hooks on the suprastructure to mesialise the molars. As the molars mesialise, the removable transpalatal arch can be adapted to reduce the intermolar distance. There is the same advantage as with the IAD: only in the final phase is bonding of all teeth necessary. The IAM is especially indicated in cases where extractions have been performed previously and no retraction of the front teeth is desired during the space closure. In cases of agenesis of upper lateral incisors, this appliance can provide the necessary anchorage to mesialise canines, premolars and molars.

Discussion
In order to serve as absolute anchorage, palatal implants must maintain positional stability under orthodontic loading. Osseointegration is therefore a prerequisite. Primary stability is necessary to prevent connective tissue encapsulation and the possible premature loss of the implant. The implants should therefore be inserted by an experienced surgeon.

In contrast to dental implants, which are loaded intermittently in an axial direction, palatal implants are loaded continuously in an horizontal or oblique direction. In animal studies, the effect of continuous loading (as is the case in orthodontics) on osseointegration has been investigated. It appeared that loading with light forces (as used in orthodontics) significantly influenced the turnover of alveolar bone in the vicinity of the implants and no implants lost osseointegration.

The fact that palatal implants osseointegrate makes them superior to mini-screws in the maxilla for certain indications. Anchorage moments can be applied, while only simple forces may be applied on mini-screws. This requires a perfect positioning of the mini-screws in relation to the desired tooth movement.

Only a few publications report on the success rate of palatal implants. Since these implants are temporary devices, the term of success is different from that of dental implants. A palatal implant can be termed successful if it could be used as planned throughout the orthodontic treatment. Success rates reported for palatal implants are all above 90%. This is substantially higher than the success rates reported for mini-screws. Loss of the implant mainly occurs during the healing phase (before orthodontic loading) and is in most cases attributed to bad oral hygiene or excessive tongue pressure on the implant, causing mobility and preventing adequate osseointegration. It is of utmost importance to discuss this with the patient before starting the procedure.

A disadvantage of the implant is the delay of loading. To allow for adequate osseointegration, a healing period of at least 10 weeks is allowed before the implant is loaded. Another disadvantage is the need for a laboratory phase to have the suprastructure fabricated.

Since the suprastructure is attached to the implant with a screw, according to the desired tooth movements, the design can be changed during treatment using the same palatal implant. Furthermore, one implant provides anchorage control in the upper arch for both right and left sides at the same time.

The insertion site in the palate (median or paramedian) is a safe site, in terms of injury to other structures. No roots are present in this region. The implant should be inserted at a safe distance distally to the incisive canal. If it is doubted that there is enough bone height in the median palatal suture, a short implant should be used in order to avoid perforation of the nasal cavity.

Although the insertion and removal procedures are more invasive compared to the procedures for mini-screws, they are well accepted by patients in terms of pain and discomfort. We have positive experience with palatal implants in patients with ages ranging from 10 years to 53 years. If the procedure is well explained, usually no problems occur.

Conclusion
Orthodontic palatal implants, such as the Orthosystem, with a rough surface and rotation resistant suprastructure provide a new dimension  in orthodontic anchorage. They reduce the need for patient compliance and offer increased clinical flexibility and effectiveness. They provide reliable absolute orthodontic anchorage, making treatment outcome more predictable than ever. Many more applications and designs of suprastructures will appear over time.

References
1. Wehrbein H, Merz BR, Diedrich P, Glatzmaier J. The use of palatal implants for orthodontic anchorage. Design and clinical application of the Orthosystem. Clinical Oral implants Research 1996;7:410-416
2. Wehrbein H, Feifel H, Diedrich P. Palatal implant anchorage reinforcement of posterior teeth: a prospective study. American Journal of Orthodontics and Dentofacial Orthopedics 1999; 116:678-686
3. Asscherickx K, Wehrbein H, Sabzevar MM. Palatal implants in adolescents. A histological evaluation in beagle dogs. Clinical Oral Implants Research;19:657-664
4. Asscherickx K, Vande Vannet B, Bottenberg P, Werhbein H, Sabzevar MM. Clinical observations and success rates of palatal implants. American Journal of Orthodontics and Dentofacial Orthopedics 2010;137:114-122

The author
Dr Karlien Asscherickx
Vrije Universiteit Brussel
School of Dentistry
Brussels
Belgium