He took the mechanical braces off from his daughter, and he had her wear the "Biofunctional Retainer" throughout the summer in the mandibular arch.
There he met Karl Haupl with whom Viggo devised the name "Activator" to describe his appliance.
Haupl believed that "Shaking of Bone Hypothesis" by Wilhelm Roux was the functional concept that described how the appliance would work.
In addition, an adolescent or adult patient with retrognathic mandible, well aligned maxillary and mandibular dentition were also other indications of this appliance.
The initially theory consisted of Haupl-Andersen's ideas who believed that Isometric Muscular Contraction caused by Myotatic reflex activity was the primary way functional adaptation of the appliance took place.
Petrovic, in the 1970s, performed studies which found that Lateral pterygoid muscle played an important role on Mandibular condyle cartilage growth.
Proponents of this view believed that Viscoelastic Properties of Muscle and stretching of soft tissues was the primary way of functional adaptation.
Activator appliance initially started out as one block of acrylic which fit in both maxillary and mandibular arch.
The lower arch would see the horseshoe shaped lingual plate acrylic extending from distal of the last erupted molar.
He stated that the clasps allowed the activator to attach to the maxillary dentition, and thus make it more stable.
Robert Shage from LSU modified activator by having lower incisors bite on a plane formed by acrylic to impede the growth in occlusal direction.
The occlusal acrylic on the posterior teeth was grounded away to assist in eruption of the molars, premolars.
Along with their activator properties, ideal for correction of class II malocclusion, being based on tooth size, these appliances aptly coined EGAs (Eruptive Guidance Appliances) also function as a positioner along with correcting overbite and mild to moderate crowding.
The palatal area in this modification remains free of acrylic, making the appliance more convenient for patients and them being able to wear it for longer periods of time.
In this modification, the maxillary and mandibular active plates are joined at the 1st permanent molar region using a U-shaped bow.
The activator is bonded to the maxillary arch and the forward guidance of the mandible can happen due to the lingual flanges of the appliance.
This type of activator is particularly suitable for treatment in the early mixed dentition[7][8][9][10][11][12][13] but can also be used in other stages of dental development.
In addition to guiding the mandible to a Class I relationship, it can also be used to align teeth and to correct crowding.
In this type of approach, we can see that the inner bows are completely embedded in the labial side of the maxillary splint.