"The possibility exists that different functional appliance designs act in dissimilar ways and are not directly comparable. Variation in appliance action - such as the amount of mandibular advancement, types of construction bites, and prescribed time of wear also brings about different results. Furthermore, the duration of treatment varies from individual to individual. Despite all of these problems there is still convincing evidence supporting the concept that functional appliances do create an orthopaedic effect in specific individuals. Growth and development, which affect considerable change in our work area, cannot be overlooked. Indeed, guiding growth and development is a major part of the correction of any basal malocclusion. Change of function in the young growing skeleton leads to changes in form and structure, whereas in old matured bones it induces predominantly structural transformation."