Biological Basis


Biological Basis

There is evidence that specific types of malocclusion are associated with particular relapse patterns. Independently of the original malocclusion and completed therapy, however, the following four general factors that determine the stability of the orthodontic outcome can be identified.

Active Tooth Movement

Orthodontic movement of teeth has an impact on the surrounding gingival, periodontal, and bony structures of the teeth. Applying force to teeth leads to periodontal changes that result from a number of complex processes and adaptive responses at molecular level.14 Subsequent to active orthodontic treatment, consolidation of the result in terms of tissue changes at the cellular level occurs after 3-4 months. Adaptation of the elastic fibers and collagen network of the gingiva takes longer, however. It takes between 6 months and 1 year for these to reorganize, depending on the patient’s age.18 , 20


The supracrestal fibers are able to induce postortho-dontic tooth movement for up to 2 years.18 , 20

Functional Parameters of the Orovestibular System

Orthodontic treatment has an immediate impact on the stomatognathic system, particularly when the treatment is undertaken during growth; a fine-tuned and highly individual interplay between the functionality of the chewing apparatus, the tongue, and the facial musculature develops during this period in particular. All kinds of orofacial functions and dysfunctions, such as parafunc-tions or habits, can have an impact on the stability of a patient’s postorthodontic occlusion. In particular, functional changes in habitual tongue positioning can be a challenge for the stability of the final orthodontic result; the tongue is a relatively powerful, subconsciously controlled muscle ( Fig. 9.1 ). The section on “Retention Following Treatment for Anterior Open Bites” below provides suggestions for retention strategies that are largely compliance-free.

Patient’s Age

Some authors claim that a patient’s residual growth may influence the orthodontic treatment results. Particularly in treatment for patients with underlying skeletal problems, there is often a need for greater retention until the end of the active growth period.3 As growth patterns are genetically predetermined, there is a risk of long-term relapse. In fact, growthlike changes in the maxillofacial skeleton appear to persist well into adulthood.

Although growth changes do not appear to be a predominant factor for relapse in adult patients, it is important to remember that this group has a reduced ability to respond to orthodontic stimuli. Cellular activity is reduced in comparison with adolescent patients, and the individual cellular response to orthodontic treatment tends to decrease with increasing age. This appears to be the case for both hard and soft tissues. Orthodontists need to be aware that when treatment for adult patients is being planned, the stability of the desired result will be affected by the patient’s reduced ability to adapt to the changes achieved. In addition, all orthodontic treatment changes are also affected by a reduced capability of tissue repair (with all the associated consequences) as the patient gets older.

Fig. 9.1a, b Parafunctional habits contributing to relapse. a The patient had a class II occlusion and an open bite due to tongue interposition at the beginning of treatment. During the course of treatment, the situation improved and the tongue thrust disappeared, possibly due to simultaneous speech therapy. At the conclusion of treatment, the patient was given information about the risk of relapse. The result remained stable 1 year into retention. b The return of the parafunctional tongue habit led to a relapse of the open bite, with marked proclination of the incisors.
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Jul 7, 2020 | Posted by in Orthodontics | Comments Off on Biological Basis
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