When looking closely at the illustration, previously shown in
Figure 2, one can notice that the maxillary molars migrate mesially during the observation period. This is in part due to forward growth of the maxilla in part to mesial migration within the maxilla. This observation is important as it demonstrates one of the main goals of functional appliance treatment, namely to prevent the normal mesial migration of the upper posterior teeth in addition to restraining forward growth of the maxilla. Additionally, it is one of the goals of treatment to prevent further proclination of the upper incisors during the treatment period. Most of these goals can be achieved by using fixed appliances and headgear, however in patients that are in the early mixed dentition this approach would be limited to a two by four fixed appliance combined with the headgear. However, this approach does not address the lip dysfunction often seen in these patients, and the fixed appliances increase the risk of caries in these young patients. Alternatively, a functional appliance can be used to achieve similar results but with several additional benefits to be discussed further in the following.
An approach that has been popular for many years for early interception of a Class II, Div. 1 malocclusion is to treat the malocclusion with a combination of a bite plate and headgear. The bite plates function is to correct the deep overbite and the cervical headgear to restrain forward growth of the maxilla, hopefully thereby correcting the Class II malocclusion. Although this may work well in cases with good facial growth and a favorable growth pattern, it can also result in downward growth of the mandible in some instances. The example in
Figure 3 shows a patient with a Class II, Div. 1 malocclusion, treated with headgear and bite plate, where the mandible didn’t growth forward as expected but instead descended vertically, possibly caused by too much tooth eruption relative to the amount of vertical condylar growth during the treatment period. The lack of posterior tooth contact, resulting from the separation of the posterior teeth by the bite plate, permits too much eruption of both the upper and lower posterior teeth. The additional extrusive component of the cervical headgear only adds to the problem with this unintended outcome. As a result of the appliance combination the AFH (anterior face height) increase in this patient is greater than the increase in PFH (posterior face height). The latter mostly comes from the amount of condylar growth.
Figure 3. A patient With a Class II, Div. 1 Malocclusion
The illustration in
Figure 4 show the components that make up the AFP and the PFH. The green arrow indicates the condylar growth component that together with lowering of the temporo-mandibular fossa during growth make up the PFH increase. The red arrow represents the sutural lowering of the maxilla that together with the eruption of the maxillary and mandibular molars (blue arrows) make up the AFH change. So in patients where limited growth intensity is expected this combination of appliances, headgear and bite plate, should in general not be used and other approaches with better control of the vertical component of AFH may be more useful.
Figure 4. The Components That Make Up the AFP and the PFH
It is an unfortunate fact that the amount of condylar growth in a juvenile patient in general is unpredictable and that the annual growth of the condyle can vary from as little as 1mm to as much as 4 - 5 mm per year. The following graphs show these changes in subjects with untreated Class II malocclusion (
The amounts of annual condylar growth in girls and boys during the juvenile growth period are seen in
Figure 5, (Kim et al. 2002) ( 6). The subjects all had a Class II malocclusion and did not receive any treatment during the observation period.
Figure 5. The Amounts of Annual Condylar Growth in A, Girls and B, boys during the juvenile growth period
The average growth at the condyles is about 3 - 4 mm per year, with some individual variations. Additionally the growth intensity fluctuates from year to year in each individual. These facts should be taken into consideration when planning treatment and discussing the estimated length of treatment with the patient’s parents. The limited amount of growth during the juvenile period presents a further challenge namely that it necessitates good vertical control to reduce the eruption of the posterior teeth that otherwise could increase the AFH and prevent forward growth of the mandible.
Functional appliance treatment has been used, as previously mentioned, especially in Europe for many years and quite successfully. It has also to some degree been used in the US with good results, but has lately fallen behind compared to fixed appliance treatment, especially in cases where early-interceptive treatment was indicated. Before we discuss the general function of these appliances it may be a good idea to review the advantages and disadvantages of functional appliances (
7). As seen in Figure 6 there are some positive and some negative issues relating to these appliances. A couple of important issues are listed that make these appliances preferable in patients with poor oral hygiene and concerns for caries.
Figure 6. A, Some Positive and B, Some Negative Issues Relating to Functional Appliances
One of the main advantages of functional appliances over fixed appliances is that these appliances are primarily used after school and at night which may be an important consideration in some patients. One clinical advantage, not to be ignored, is that adjustments require very little chair time. An important benefit of functional appliances is that because of the postural position of the mandible they remove any possibility of lip dysfunction while they are in the mouth, which often can be a problem in patients with a large overjet, especially during fixed appliance treatment.
On the negative side it must be mentioned that these appliances do not work well in patients with speech problems and those that are primarily mouth breathers. Also if there is severe crowding of the front teeth it may be more practical to treat with limited fixed appliances, at least initially, before the functional appliance is inserted to correct the skeletal part of the malocclusion.