Correction of Skeletal Discrepancies
Correction of a Class II Buccal Segment Relationship
Based on the findings by Roux that functional occlusal stimuli affect the growth of the maxilla and mandible, Andreesen and Häupl introduced the now-classical concept of functional orthodontics in 1935. Most of the functional appliances were originally removable, and patient compliance was an issue. Adequate wearing time is of paramount importance for achieving the treatment aims, and this issue led to the development of the noncompliance myofunctional systems that are now routinely used in many orthodontic practices. It was the rediscovery of the “Herbst appliance” (originally dating from 1904) by Pancherz in the 1970s that led the way to noncompliance class II correctors during the past 20 years. Other examples of noncompliance functional appliances are the Mandibular Anterior Repositioning Appliance (MARA), the Functional Mandibular Advancer (FMA), and other successors to the Herbst appliance such as the Sabbagh Universal Spring (SUS) and the Forsus spring. All of these appliances have been used successfully, and they have been extensively described in the literature. Apart from the very effective therapeutic principles, all noncompliance functional appliances can also demonstrate unwanted side effects such as significant proclination of the lower incisors. This is a well-known unwanted side effect that is particularly prevalent with use of Herbst appliances.
Functional Mandibular Advancer
The Functional Mandibular Advancer (FMA) was developed by Kinzinger in 2002 ( Fig. 7.43 ).16 , 17 , 19 The concept is similar to the twin-block concept in that it has guide planes that train the mandible forward on final closure, achieving the desired myofunctional effect (see Case Study 7.16). In the FMA, however, the guide planes are positioned buccally and are usually part of a cast framework.
Modification of FMA. During function, the molars that retain the appliance undergo intrusive forces and tipping moments. The mandibular molars, for example, tend to tip mesially, which can lead to proclination of the anterior segment. These effects can be prevented by using skeletal anchorage to maintain the position of the lower molars. A modified cast FMA is currently being developed to allow skeletal anchorage.
Orthodontic mini-implants are inserted distal to the mandibular second premolars to reinforce anchorage of the lower molars. The mini-implants are placed before the impressions for the fabrication of the modified FMA are taken, which allows the implants to be placed in the ideal position. The device is then applied in the post-insertion situation.
After insertion of the mandibular mini-implants, upper and lower alginate impressions and a construction wax bite are taken. The impression of the lower jaw needs to be sufficiently precise to reproduce the position of the mini-implants exactly. The laboratory will then produce a cast design including molar bands, guide planes, and connectors to the mini-implant head ( Fig. 7.43e ). After cementing of the elements, the residual space between the mini-implant head and the connector is filled with a flowable composite.
Case Study 7.16 (Fig. 7.44)
Patient: K.I., female, age 20.
Diagnostic records: models, panoramic radiograph, lateral cephalometric radiograph, intraoral/extraoral photographs.
Main findings: increased overjet, asymmetric occlusion with mandibular midline deviation to the right.
Treatment aims: establish bilateral class I occlusion and coincident midlines using the Functional Mandibular Advancer (FMA).
Appliances: lingual self-ligating brackets, individually fabricated FMA.
Archwire sequence: 0.012 SE and 0.016 SE preformed lingual archwires, 0.016 SS and 0.018 SS custom-bent archwires.
Alternative treatment strategy: different Herbst appliance modifications, orthognathic surgery.
Active treatment time: 14 months.
Retention: vector retention with directional component (for example, retainer with a built-in FMA), bonded retainer.
Careful diagnosis of the masticatory apparatus, including palpation of the muscles of mastication and the temporomandibular joint, should form part of the initial diagnostic work-up in all patients, particularly when a temporomandibular disorder is suspected. The clinical diagnosis can be supported by diagnostic imaging (magnetic resonance imaging). This patient presented with postero-superiorly directed loading of the joint, leading to anterior disk displacement with reduction, which was revealed by MRI. The treatment aim was stable repositioning of the disk. Anterior repositioning of the mandible should lead to recapturing of the disk.