M. von Böhl
Changes in periodontal ligament and dental pulp after experimental orthodontic tooth movement
A scientific essay in Medical Sciences
DOCTORAL THESIS defended in public on 14th of November 2011
The appearance of hyalinization in the PDL space during orthodontic tooth movement, even when using low forces, is still an encumbrance in daily clinical practice. Also the relationship between the appearance of hyalinization and local morphological changes of the bone and periodontal ligament is still not elucidated. Finally the impact of orthodontic force application on pulp tissue and its possible relationship with age is still unclear. Therefore the purpose of the study was to investigate hyalinization and dental pulp changes during experimental orthodontic tooth movement.
Chapter 1 elucidates the background of the study and gives an overview of current orthodontic literature about hyalinization and dental pulp changes during experimental tooth movement. Five aims of the study presented in this thesis were specified:
- To perform a systematic review of the literature on hyalinization in relation to experimental tooth movement in animals and humans.
- To evaluate rate of tooth movement and tissue reactions in the PDL after application of low and high orthodontic forces.
- To investigate the relation between local morphological changes in the periodontal structures and individual differences in rate of tooth movement.
- To investigate the relationship between pulp reaction to orthodontic force application and force level in humans.
- To evaluate the effect of age on pulp tissue after experimental tooth movement using a standardized appliance, in which magnitude and direction of force was kept constant.
In chapter 2 the literature concerning hyalinization in relation to experimental tooth movement in animals and humans is presented. A structured search of electronic databases as well as hand searching retrieved 70 publications concerning the subject. After application of inclusion and exclusion criteria, 39 studies remained, of which three were in humans. Articles on animal experiments were in the majority with most studies performed in rats. Among other data force magnitude, type of tooth movement, duration of the experimental period, and moment of first and last appearance of hyalinization were extracted from the included studies. The heterogeneity of the published studies and the limited data on appearance of hyalinization made it impossible to perform a meta-analysis. Therefore, the literature was systematically reviewed. It appears that there are no major differences in tissue reaction between species during experimental tooth movement. Although hyalinization is considered to be an undesirable side-effect of orthodontic tooth movement, little attention has been paid to the phenomenon itself and its possible relationship with stress/strain levels in the periodontal ligament (PDL) and alveolar bone or the rate after the initial phase of tooth movement. There is a need for well-designed experimental studies to elucidate the role of hyalinization in orthodontic tooth movement.
In chapter 3 the results of a study into histological changes in the periodontal structures of beagle dogs after using high and low continuous forces during experimental tooth movement are described. An orthodontic appliance was placed on the second premolar and the first molar in the mandible and a continuous and constant reciprocal force of 25 cN on one side and 300 cN on the other side was applied. Tooth movement was recorded weekly. Dogs were sacrificed after 1, 4, 20, 40, and 80 days for histological evaluation. Hematoxylin and eosin (HE) staining was used for tissue survey, staining for alkaline phosphatase as a marker was used for active osteoblasts, and tartrate-resistant acid phosphatase staining was used for osteoclasts. After 24 hours, the remodeling process had already started at the pressure and tension side, and in some samples hyalinization was found. In contrast to earlier studies, hyalinization was found throughout the entire experimental period, both in molars and in premolars. In the periodontal ligament of some teeth, small patches of hyalinization were found at the pressure side, mostly located buccally or lingually of the mesiodistal plane, whereas others showed large areas of necrotic tissue. It is concluded that hyalinization limits tooth movement, but there is no relationship with the force level.
Chapter 4 elucidates morphological differences between the periodontal structures of beagle dogs showing different rates of tooth movement under identical experimental conditions. An orthodontic appliance was placed on the mandibular second premolar and the first molar to exert a continuous and constant reciprocal force of 25 cN. Tooth movement was recorded weekly. The dogs were killed after 1, 4, 20, 40, and 80 days for histological evaluation. Haematoxylin and eosin staining was used for tissue survey, alkaline phosphatase staining was used as a marker for active osteoblasts, and tartrate resistant acid phosphatase staining was used for osteoclasts. After 24 hours, osteoclastic and osteoblastic activity had already increased at the pressure and tension sides, respectively, and, in some samples, hyalinization was found. In case of fast-moving teeth, areas of direct bone resorption at the pressure side and deposition of trabecular bone at the tension side were found throughout the experimental period. In the periodontal ligaments of teeth showing little movement, small patches of hyalinization were found at the pressure side, mostly located buccally or lingually of the mesiodistal plane. These phenomena were found in both molars and premolars and at all time points. It is concluded that small focal hyalinizations might be a factor that could explain individual differences in the rate of tooth movement.
In chapter 5 a systematic literature review is presented that was conducted to investigate the relationship between orthodontic force level and pulp reaction in humans. An electronic database search was performed in PubMed/old MEDLINE, Web of Science, EMBASE, CINAHL, Scopus, and the Cochrane Library (DARE, CENTRAL) until 15 April 2011. Publications before 1954 and the most recent ones in main dental and orthodontic journals were hand searched. Additional studies were identified by hand search of reference lists of relevant articles from both the electronic and hand search. Search terms included biomechanics, force, orthodontic*, tooth movement, dental pulp, pulpal reaction. Two independent observers assessed eligibility for inclusion, extracted the data, applied quality indicators and graded level of evidence. 23 studies matched the inclusion criteria. The outcomes concerned histological and cell-biological parameters after orthodontic tooth movement in 18 studies and pulpal blood flow in 5 studies. The methodological quality of most studies was graded moderate to low. Due to a lack of high quality studies there is no conclusive scientific evidence for a relation between force level and dental pulp tissue reaction in humans. There is contradictory scientific support for a force dependent reduction of blood flow.
In chapter 6 the effect of age on rat pulp tissue and the possible additional effect of experimental tooth movement was investigated, using a standardized appliance exerting a force of constant magnitude and direction. In 2 groups of 30 rats, aged 6 and 40 weeks respectively, a standardized orthodontic force system was placed. At one side of the maxilla 3 molars together were mesialised with a force of 10 cN. The other side served as a control. Parasagittal histological sections were prepared after tooth movement for 1, 2, 4, 8, and 12 weeks. The pulp tissue was characterized for the different groups, with special emphasis on cell density, inflammatory cells, vascularity, and odontoblasts. Dimensions of dentin and pulp tissue were determined, and their relation with duration of orthodontic force application and age were evaluated. The results showed that application of a light force in young and adult rats did not lead to long-lasting or irreversible changes in pulpal tissues, except for a tendency towards a higher vascularity. The dimensional variables related to occlusal wear showed significant changes over time. It was concluded that the dimensional variables showed the most apparent age-related changes. These changes were caused by occlusal wear and secondary dentin formation. Orthodontic tooth movement per se had no long-lasting effect on the pulp tissue neither in the young nor in the old group.
Chapter 7 is the general discussion. The background of the topic selection of this thesis was elucidated. The results from the different studies were related and extrapolated. Suggestions for further investigation were made and the direction of future orthodontic research was briefly discussed.