Orthodontic Relapse: What is Relapse, Types, Causes of Relapse
What is Relapse?
Relapse is the return, following correction, of the features of the original malocclusion, by definition of the British Standards Institute. However it may be more commonly described as any change from the final tooth position at the end of treatment. According to this definition, relapse could be a return to the original malocclusion, but could also include any other tooth movements caused by factors unrelated to the orthodontic treatment.
Types of Relapse:
There are four main reasons as to why relapse can occur following adequate orthodontic treatment. These are:
* Periodontal or gingival factors
* Occlusal factors
* Soft tissue factors
Causes (Aetiology) of Relapse:
Periodontal and Gingival Factors:
Periodontal ligaments (PDL) that attach onto the tooth insert into the surrounding alveolar bone, into the gingivae, and also into adjacent teeth Figure 1. They lie in many different orientations in order to resist changes in the tooth’s position relative the periodontium due to forces from all directions acting on the tooth.
The aim of orthodontic treatment is to move teeth, and so to do this, the orthodontic appliance must overcome the periodontal support to the tooth. The alveolar bone remodels fairly quickly, and can change to accommodate the change of position relative to the jaw. However, the PDL do not remodel so quickly and will still be applying a force to resist the changes in the tooth position relative the periodontium.
After treatment, the periodontal fibres that attach onto the tooth and insert into the alveolar bone need to remodel to the new position of the tooth within the periodontium. Without the retention of the tooth’s position, the periodontal ligaments exert an unopposed force on the tooth, causing it to revert it back to its original position. This effect can be exaggerated in the case of severely rotated teeth, which are more prone to relapse. In Figure 2 the orthodontic treatment has applied a force to overcome the resistance from the PDL. Once the orthodontic force is removed, the opposing force from the PDL will be unbalanced, and will cause the tooth to begin to relapse back to its original position within the periodontium.
The PDL principal fibres take 3-4 months to remodel. Re-organization of the collagen fibres in the gingivae takes around 6 months but the dentogingival and transeptal fibres take more than 8 months to remodel.
Bone is the basic support to the tooth, and the PDL fibres co-ordinate the more intricate support to the teeth. As the tooth moves within the jaw, the bone remodels to accommodate the new tooth position. If the tooth position is not retained, the bone will not remodel to the desired extent, and relapse occurs via the effect given from PDL. Bone remodelling takes about 1 month to complete.
To help prevent relapse of a deep overbite, it has been shown that establishing the incisal edges of the lower incisors in a position 0-2mm anterior to the midpoint of the root axis of the centroid (most prominent upper central incisor) helps improve stability. Prevention of over-eruption is achieved in these cases by creating a good occlusal stop with an inter-incisal angle of approximately 135o.
Relapse can be reduced in some cases without the need for post-treatment retainers or adjunctive treatment. For example, in the case of correcting a simple anterior crossbite, the lower anterior teeth can aid the retention of the corrected upper tooth where there is a sufficient overbite.
Soft tissues can influence the position of teeth. The tongue applies forces which would have the teeth move buccally/labially, and the cheeks and lips oppose those forces. The teeth naturally lie in a theoretical area of balance between those two sets of forces, called the neutral zone. Soft tissues can influence the position of teeth and it is therefore highly beneficial that at the end of treatment the teeth are positioned within the neutral zone. The tongue applies forces which would move the teeth buccally/labially, and the cheeks and lips oppose those forces. Providing the periodontal ligament is in a healthy state the teeth should remain in a position of equilibrium between the soft tissues.
The lower lip is especially muscular with the forces of the orbicularis oris, depressor anguli oris and depressor labii inferioris. It can become particularly problematic with movement of the lower labial segment. Any excessive retro-/proclination of the lower incisors after orthodontic treatment will most likely relapse.
Changes in intercanine width are considered to be more unstable than changes in intermolar width. This should be considered in the treatment planning, and the archform and dimensions of the lower arch kept the same with the upper arch planned around it.
Growth is a process that continues throughout life. Mandibular height very slowly increases after puberty throughout life. In the context of relapse, excessive forward growth rotation of the mandible can result in an increase in overbite, and anterior crowding. Excessive backwards growth rotation of the mandible (matrix rotation with fulcrum around the condyle), can reduce the overbite with possible development of a skeletal anterior open bite, and the patient may also become more class II.
Between the ages of 13 and 20 years, the upper arch length and intercanine width show significant reduction, and can produce anterior crowding. In the lower arch, the intercanine width and arch length peak at the age of approximately 8 years old, and also show significant reductions from then onwards.