Orthodontic forces and tooth movement.
A scientific essay in Medical Sciences
DOCTORAL THESIS defended in public on 1st of October 1996
The correction of malposed teeth is one of the major goals in the treatment of orthodontic patients. The elimination of crowding and spacing, the correction of rotations and abnormal tooth positions, and the alignment of teeth to a proper arch form are restricted to the dento-alveolar part of the maxillofacial complex. An effective orthodontic therapy aims at producing a maximum amount of tooth movement with as little damage as possible to the root, periodontal ligament, and alveolar bone. The present study deals with the relationship between the magnitude of constant and continuously acting orthodontic forces and the rate of bodily tooth movement. The histologic changes in the periodontal ligament during different phases of tooth movement are studied with light microscopy. Finally, the relapse of tooth movement after treatment is studied when no retention measures are taken.
Chapter 1 deals with clinical, biological and biomechanical considerations, which are important in the study of experimental tooth movement.
Chapter 2 describes the time dependent behaviour of orthodontic elastics tested in different media in vitro. Six different types of elastics were tested under four experimental conditions: in artificial saliva in the dark, in distilled water in the dark, in air in natural daylight, in air in the dark. Force measurements show that, after a great initial loss of 13% of the initial tension, elastics kept in artificial saliva and distilled water can produce almost constant forces for at least three weeks. This means that, if in clinical orthodontics constant forces are preferred, there is no rationale for daily renewal of elastics, if they are used with a static loading pattern. Because a large variation was found in initial force levels between elastics of the same type, they should always be measured if force values are critical. When elastics are kept in natural daylight, the force decay is significantly larger than when they are kept in the dark.
Chapter 3 describes the spontaneous migrations of teeth in the mandible of beagle dogs after extraction of the mandibular third premolars. In humans, teeth adjacent to an extraction site, move towards each other and the extraction space is normally reduced. In our experimental group however, a significant increase of most of the interdental distances was found. This unexpected finding may be explained by the diverging eruption pattern of the mandibular teeth, especially the canine, which has a mesial inclination. Growth of the mandible may provide the additional space required for the mandibular teeth to spread out. Furthermore the tongue may play a role, especially if it fills up the space at the extraction site.
Chapter 4 describes the process of bodily orthodontic tooth movement in young adult male beagle dogs during a period of 112 days. The mandibular second premolars were moved distally with elastics exerting forces of 50, 100, or 200 cN. Tooth movement was measured twice a week with a digital calliper. In the time-displacement curves, four distinct phases could be discerned. No significant differences between the three force groups were found in the duration of each phase, nor in the mean rate of tooth movement during each phase. Large individual differences were found in the rate of orthodontic tooth movement. However, mean rate of tooth movement at the left and the right side, although different forces were used, were highly correlated. Maximum rate of tooth movement was about 2.5 mm per month in all force groups. It was concluded that not the magnitude of the orthodontic forces that were used, but individual characteristics are decisive in determining the rate of orthodontic tooth movement.
Chapter 5 describes the relapse directly after active orthodontic tooth movement during 112 days. There was no period of retention and the orthodontic appliances were left in place. Time-displacement curves showed a rapid initial relapse, followed by a gradual decrease in the rate of relapse to a final stable position. The mean amount of relapse was about 40% of the tooth movement produced before. Mean duration of the relapse period was 78 days. No significant differences between the force groups were found in mean amount of relapse and mean duration of relapse. Significant positive correlations between the amount of active tooth movement on the one hand, and the amount of relapse and the duration of the relapse period on the other hand were found. The mean amount of relapse of the right and the left side was significantly correlated.
Chapter 6 describes the histologic changes in the periodontal ligament and alveolar bone during bodily orthodontic tooth movement and subsequent relapse in beagle dogs. At the pressure side of the periodontal ligament, normal tissue structure was lost soon after the start of the experiment. This was followed by undermining resorption in the hyalinized areas. Later, only direct osteoclastic activity was found, which was limited to continuously changing local bony protrusions. Root resorptions were present in all force groups, initially as small local spots, which increased with time to extensive resorptions, especially in the middle part of the root. At the tension side osteoid was deposited around newly formed collagenous fibres within 7 days. This was followed by the deposition of trabecular bone in finger-like bony protrusions, oriented in the direction of the stretched fibres. A cementum layer with increasing thickness was deposited along the root surface. Histologic changes in the periodontal ligament were independent of force magnitude. Root resorption seemed to increase with the amount and duration of tooth movement. During relapse after some time reversed cellular activity was observed at the pressure and tension sides and even root resorption was seen at the former tension side. When teeth had come to a standstill, there was still cellular activity to reorganize the tissue structure in the periodontal ligament.
In chapter 7 the results of the previous chapters are discussed. Compared to tipping orthodontic tooth movement, bodily tooth movement offers advantages with respect to stress distribution, biologic reactions and tissue damage in the periodontal ligament. Moreover experimental bodily tooth movement offers experimental results that are reproducible and can be a basis for extrapolation to future research. The optimal force theory and the hypothesis that a linear relationship exists between the stress in the periodontal ligament and rate of orthodontic tooth movement are rejected by the present findings. It seems that the discussion on optimizing biomechanical therapy should not concentrate on changing force magnitude and stress levels in the periodontal ligament, but on increasing metabolic activity and cellular processes that might be responsible for the large inter-individual differences in rate of orthodontic tooth movement. The time-displacement curves of the relapse without retention represent the long term recovery of the periodontal ligament and alveolar bone and suggest visco-elastic properties. The mean relapse of 40% of the active tooth movement stresses the "strength" of the information that is stored somehow in the biologic system. Clinical implications of the present findings include the recognition of large individual differences in rate of bodily tooth movement, which is independent of force magnitude as used in this experiment. On the basis of the present findings suggestions for further research are given.