19: Mini-Screws and Palatal Implants for Orthodontic Anchorage

CHAPTER 19

Mini-Screws and Palatal Implants for Orthodontic Anchorage

Marc Schätzle

Anchorage is one of the limiting factors in orthodontics, and its control is essential for successful orthodontic treatment. The term orthodontic anchorage was first introduced by Angle< ?xml:namespace prefix = "mbp" />1 and later defined by Ottofy.2 Orthodontic anchorage denoted the nature and degree of resistance to displacement of teeth offered by an anatomic unit when used for the purpose of tooth movement. The principle of orthodontic anchorage has been implicitly explained already in Newton’s Third Law (1687) according to which an applied force can be divided into an action component and an equal and opposite reaction moment. In orthodontic treatment, reciprocal effects must be evaluated and controlled. The goal is to maximize desired tooth movement and minimize undesirable effects.

Basically, each tooth has its own anchorage potential as well as a tendency to move when force is applied toward the tooth. When teeth are used as anchorage, the inappropriate movements of the anchoring units may result in a prolonged treatment time and unpredictable or less-than-ideal outcomes.

Orthodontic anchorage is oriented to the quality of the biological anchorage of the teeth. This is influenced by a number of factors, such as the size of the root surfaces available for periodontal attachment, the height of the periodontal attachment, the density and structure of the alveolar bone, the turnover rate of the periodontal tissues, the muscular activity, the occlusal forces, the craniofacial morphology and the nature of the tooth movement planned for the intended correction.3 To maximize tooth-related anchorage, techniques such as differential torque,4 placement of roots into the cortex of the bone,5 and distal inclination of the molars6,7 may be used. If the periodontal anchorage is inadequate with respect to the intended treatment goal, additional intraoral and/or extraoral anchorage may be needed to avoid negative effects. Although the teeth are the most frequent anatomic units used for anchorage in orthodontic therapy, other structures such as the palate, the lingual mandibular alveolar bone, the occipital bone, and the neck are also alternatives.

Additional anchorage such as extraoral and intraoral forces are visible and compliance dependent and are associated with the risk of undesirable effects such as tipping of the occlusal plane, protrusion of mandibular incisors, and extrusion of teeth.

Implants, mini-screws, and ankylosed teeth, as they are in direct contact with bone, do not possess a normal periodontal ligament. Consequently, they do not move when orthodontic forces are applied8 and hence can be used for “absolute anchorage” that is independent of the patient’s compliance.

The aim of this chapter is to present skeletal anchorage to be integrated into orthodontic treatment as “absolute anchorage,” thereby avoiding the disadvantages listed previously.

1 Historically, what kind of anchorage devices preceded mini-screws and palatal implants?

The first known attempt to achieve skeletal anchorage was made in 1945 by Gainsforth and Higley.9 They placed vitallium screws in the ramus of dog mandibles and then immediately applied elastics from the screw to the maxillary arch wire in order to tip or retract the canines (Fig. 19-1). Tooth movement was successfully accomplished in two cases, but an effective force could not be maintained longer than 1 month in any case. This may have been due to infection and the immediate loading of the screw.

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FIG 19-1 Skull of dog with orthodontic appliance using vitallium screw anchorage. Force is applied through traction of the elastic that connects two hooks. The arch wire is welded to the canine band and slides freely in the perforated buccal flange of the molar overlay.

(From Gainsforth BL, Higley LB: Am J Orthod Oral Surg 1945;31:406-416.)

Later, skeletal anchorage systems evolved from two lines. One line originated from dental implants, which have a solid scientific base of clinical, biochemical, and histological studies. The other one developed from screws used in traumatology and orthognathic surgery.

Linkow10 reported using blade implants as anchorage to retract teeth with rubber bands; however, long-term results were never presented. Later Ödman et al.11 and Shapiro and Kokich12 suggested using endosseous implants. This resulted in the development of specially designed implants to the retromolar area13 and to the palatal site of the maxilla introduced by Triaca et al.14

The other category developed from surgical screws. The first clinical report of the use of a temporary anchorage device (TAD) appeared in 1983, when Creekmore and Eklund15 used a vitallium bone screw to treat a patient with a deep overbite. The screw was inserted into the anterior nasal spine; 10 days after its placement, the screw was used to intrude the upper incisors by an elastic thread from the screw to the incisors. Kanomi16 first described a mini-implant specially designed for orthodontic use.

2 How can the skeletal anchorage be classified?

Currently available skeletal anchorage devices can be classified either as biocompatible or biological. Ankylosed teeth are biological anchorage units.

Biocompatible skeletal TADs can be further subclassified based first on the nature of mechanical retention in the bone (modification of surgical fixation methods), such as fixation wires,17 fixation screws,15 fixation screws in combination with mini-plates18 and mini-screws.16 The second basis is the biological osseointegration such as endosseous prosthetic implants11,12 (Fig. 19-2). Since orthodontic patients do not usually display edentulous alveolar bony ridges for the insertion of an implant, special implants for orthodontic anchorage purposes were developed for the retromolar13 and palatal areas.14

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FIG 19-2 Biocompatible temporary anchorage devices.

(From Cope JB. Semin Orthod 2005;11:3-9.)

From a clinical point of view, it is relevant whether implants are to be used only as TADs or subsequently to be used as abutment for supporting prosthetic appliances. These aspects determine insertion sites, implant types, and dimensions, as well as type of orthodontic anchorage. Moreover, the fact that these devices may have to be placed in a growing patient is of particular importance.

Another device, the Onplant®,19 placed subperiostally, is a smooth titanium disc with a hydroxyapatite-coated surface that is supposed to connect to the bone. Because of the submerged installation, the monitoring of the healing process of these Onplants may be troublesome and their osseointegration may be questioned.20

3 What is the definition of temporary anchorage devices?

A TAD is a device that is placed into bone in order to enhance orthodontic anchorage. They either support the anchorage teeth or act by themselves as the anchorage element/unit. They are temporary and are subsequently removed after use. They can be located on the bone surface (transosteal), under the periosteum (subperiosteal), or inside the bone (endosteal) and can be fixed to bone either mechanically (cortically stabilized) or biologically (osseointegration). It should also be pointed out that dental implants placed for the purpose of supporting prosthesis, regardless of the fact that they may be used for orthodontic anchorage, are not conceptually considered TADs since they are not removed after orthodontic treatment. Importantly, the incorporation of dental implants and TADs into orthodontic treatment makes absolute anchorage possible, which has been defined in terms of implants as showing no movement (no anchorage loss) as a consequence of reaction forces.21

4 Where can implants used as temporary anchorage be placed?

Since orthodontic patients do not normally display edentulous alveolar bony ridges for the insertion of an implant, implants for orthodontic anchorage must be placed in areas other than the usual topographical locations foreseen for the replacement of missing teeth. Besides the installation of orthodontic anchorage implants into the retromolar area of the mandible,13,22 the midsagittal palatal region14,19,23 had initially been proposed.

Incomplete closure of the median palatal suture during childhood and early adolescence, however, prevents placement of orthodontic implants into the midsagittal region of fully grown adolescents and adults because of possible developmental disturbances of the palatal suture.23,24 The paramedian insertion site is therefore a potential alternative in young patients. Furthermore, the exact site chosen for palatal implants should be carefully evaluated to avoid perforations of the inferior nasal turbinate.23

5 Where can mini-screws or similar devices be placed as temporary anchorage?

The introduction of small temporary orthodontic anchorage devices such as mini-screws (<2 mm) in various lengths16,25 and titanium pins,26 as well as L-shaped mini-plates with the long arm exposed into the oral cavity18 and the zygomatic anchors27 both fixed by bone screws, offered new additional insertion sites: the interradicular septum,16,26 the supra-apical and infra-zygomatical area,16,18,25,27 and the mandibular symphysis.25

Through an analysis of panoramic radiographs and computed tomographic (CT) images, adequate bone for mini-screw placement exists primarily in the maxillary (mesial to first molars) and mandibular (mesial and distal to first molars) posterior regions. Typically, adequate interradicular bone distance was found more than halfway down the root length, which is likely to be covered by movable mucosa.28,29 Inability to place mini-screws in attached gingiva may necessitate design modification or oblique insertion direction to decrease soft-tissue irritation.29,30 The absence of keratinized mucosa around mini-screws significantly increases the risk of infection and failure.

6 What kind of imaging measures are recommended prior to palatal implant insertion?

Presurgical dental CT and/or lateral cephalograms have been recommended to evaluate vertical bone volume of the hard palate, which determines whether palatal implants can be used. Examinations of the palate have shown that the vertical bone volume commonly decreases posteriorly.

Dental CT of the alveolar process is well established for the evaluation of the alveolar bone volume before implant placement.31 It can also be used to assess the hard palate and is currently the most accurate tool for measuring the vertical bone volume at this site. Bernhart et al.32 found the greatest mean of about 6 to 9 mm posterior to the foramen incisivum in the midsagittal plane. To avoid the midpalatal suture, the suitable area for implant placement is located 6 to 9 mm posterior to the foramen incisivum and 3 to 6 mm paramedian. If the suitable bone volume for an insertion of implants is defined as 4 mm or more, Bernhart et al.32 found that 95% of the patients in their study had enough bone vertically for accommodating palatal implants with a length of 4 mm, which correlates with the clinical experience of Schiel et al.33 However, a preoperative diagnostic evaluation is recommended in order to avoid perforation of the inferior nasal turbinate.

Wehrbein et al.34 insisted on obtaining precise information for the intended implant site before placing palatal implants to avoid perforations of the nasal cavity. For this purpose, in lateral cephalograms the vertical bone volume along the palatal suture was evaluated presurgically. Since these are already used for orthodontic diagnosis and treatment planning, patients are spared from additional radiation exposure.

The vertical bone level in the anterior and middle thirds of the hard palate is at least 2 mm higher vertically than seen on lateral cephalograms. A safety margin of at least 2 mm is recommended when planning treatment on the basis of lateral cephalograms to avoid potential complications.34 But it must be considered that even though some implants are projecting beyond the nasal floor in lateral cephalograms, they can cause false-positive results and could not be related to actual penetrations into the nasal cavity (Fig. 19-3).35

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FIG 19-3 Most palatal implants are installed satisfactorily when the location of entry into the cortical bone is at the anteroposterior level of the maxillary first and second premolars—perpendicular to the palatal surface.62

(From Männchen R, Schätzle M: Clin Oral Implants Res [submitted].)

7 Are imaging measures needed prior to mini-screw insertion?

A panoramic radiograph, normally available from pretreatment diagnostic records, is usually sufficient for establishing the insertion areas outside the alveolar process. Where a mini-screw is to be inserted into the alveolar process, a periapical radiograph taken with an acrylic or putty-based template serves as a guideline for establishing the exact height and orientation of the mini-screw.17

In addition, devices such as an adjustable surgical guide,

Jan 1, 2015 | Posted by in Orthodontics | Comments Off on 19: Mini-Screws and Palatal Implants for Orthodontic Anchorage
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