Activator designs
During the course of treatment with an activator, articular and periodontal tissue remodeling takes place slowly over the course of 2 or 3 years. The optimal activator effect happens during exfoliation; treatment should be finished after eruption of the maxillary second molars.
With slight reshaping, suitable additional elements can be attached to the activator. Guide loops, spikes, and screw components for active appliances are used. These active components should be attached so that, like an activator, they only take effect when the mandible is pushed into the construction bite position by muscle activity. If malpositions of individual teeth or groups of teeth are to be corrected or if massive dental arch expansion needs to be carried out, there is the option of using active appliances before inserting the activator.
For prognathism treatment in which the mandible needs to be corrected over a greater distance than joint tolerance will allow, the activator can be separated horizontally in the area of the locked occlusion and fitted with an adjustment screw.
For this occlusal correction, the activator halves perforated in the region of the locked occlusion are pushed against each other in the sagittal direction so that the maxillary dental arch is brought forward and the mandibular arch backward. The posterior teeth in this region remain held in a splintlike fashion. In the anterior region, the mandibular plate part is milled lingually so that the lingual bow under tension presses the mandibular incisors lingually. The mandibular incisors must not be "nested" in a crowded state. A maxillary labial bow is additionally fitted and keeps the upper lip from lying closely against the maxillary incisors.
Protrusive (bite-jumping) and retrusive doubleplates are two-part appliances that can be pushed against each other on a guide plane (Fig 10-76).
The retrusive or protrusive movement is produced with active appliances by different mechanisms: elastics, screws, activatable springs, and inclined guide planes. In the activator, the force for correcting an occlusal discrepancy must be applied by the masticatory muscles, which always want to pull the mandible into a habitual position. Two special screws are suitable for bite jumping or retrusive double-plates: (1) the prognathism screw and (2) the simple expansion screw. The screws are not used as on an active plate but are adjusted depending on the progress of the correction.
The simple expansion screw is placed with a sagittal direction of thrust at the dorsal edge of the maxillary plate. The screw section directed backward is held with an additional small plastic block from which two wire brackets run along the palate to the mandibular plate. If the expansion screw is now opened, the mandibular plate pushes in a distal direction (Fig 10-77). The advantage is that the tongue is not severely crowded (when a special prognathism screw is used, the tongue can be extremely crowded anteriorly). This is a screw whose movable part is bent over so that it extends downward into the mandibular plate.
Open bites can also be treated with an activator. The aim of the treatment must be to lengthen the anterior teeth and their alveolar processes while preventing further growth of the posterior teeth. The latter goal is achieved by interocclusal splints for the posterior teeth, while the anterior teeth are free-ground into the alveolar region. If lingual tipping of the anterior teeth is to be achieved, a rest can remain cervically while the labial bow lies tight to the tooth incisally. The direction of movement must be unimpeded, and there must be no anterior crowding. The activator with lateral interoc-clusal splints has another effect: When the mouth is closed, the mandible can be tipped around the interocclusal splint as the blocked incisors are approached and the condyles are lifted out of the fossae. Functional adaptation due to lengthening of the condyles and remodeling processes in the angles of the mandible are possible.
Several modifications of the activator are described in the literature as maxillomandibular orthodontic appliances.The following section provides brief descriptions of several different activators.