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The purpose of this study was to perform a meta-analysis on the literature concerning hyalinization in relation to experimental tooth movement in animals and humans. 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.

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It is generally accepted that the effect of orthodontic tooth movement on the dental pulp in adolescents is reversible and that it has no long-lasting effect on pulpal physiology. However, it is not clear yet if the same conclusion is also valid for adult subjects. Thus, in two groups of rats, aged 6 and 40 weeks respectively, 3 molars at one side of the maxilla were moved together in a mesial direction with a standardized orthodontic appliance delivering a force of 10 cN. The contralateral 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 the pulpal horns was determined and related with the duration of orthodontic force application and age ware evaluated. We found that neither in young nor in adult rats, force application led to long-lasting or irreversible changes in pulpal tissues. Dimensional variables showed significant age-related changes. In conclusion, orthodontic tooth movement per se has no long-lasting or irreversible effect on pulpal tissues, neither in the young nor in the adult animals.

ZusammenfassungZiel der vorliegenden Studie war die Untersuchung histologischer Veränderungen im Bereich der parodontalen Strukturen bei Hunden der Rasse Beagle, bei denen Zähne experimentell mit starken und mit schwachen kieferorthopädischen Kräften bewegt wurden. Mit Hilfe einer am zweiten Prämolar und am ersten Molar befestigten kieferorthopädischen Apparatur wurden im Unterkiefer kontinuierliche reziproke Kräfte von 25 cN in einer und 300 cN in der anderen Kieferhälfte ausgeübt. Die Bewegung der Zähne wurde wöchentlich gemessen. Nach jeweils einem, vier, 20, 40 und 80 Tagen wurden Versuchstiere für die histologische Untersuchung getötet. Zur Untersuchung des Gewebes wurden die Schnitte mit Hämatoxylin-Eosin eingefärbt, aktive Osteoblasten wurden mit alkalischer Phosphatase und Osteoklasten mit tartratstabiler saurer Phosphatase markiert. Die Umbauvorgänge im Knochen auf der Druck-und der Zugseite der Zahnwurzeln hatten nach 24 Stunden bereits eingesetzt und in einigen Schnitten fanden sich schon hyalinisierte Areale. Im Unterschied zu bisherigen Studien ließen sich sowohl bei Prä-molaren, als auch bei Molaren Hyalinisierungen über den gesamten Versuchszeitraum hinweg nachweisen. Bei einigen Zäh-nen fanden sich hauptsächlich lingual und bukkal neben der mesiodistalen Mittellinie im parodontalen Faserapparat nur kleine hyalinisierte Bereiche, bei anderen Zähnen wiederum ausgedehntes nekrotisches Gewebe. Die Studie kommt zu dem Schluss, dass Hyalinisierungen zwar die Zahnbewegung einschränken, allerdings existiert hier kein Zusammenhang mit der Stärke der ausgeübten Kräfte. AbstractThe aim of this study was to evaluate histological changes in the periodontal structures of beagle dogs after using high and low continuous forces during experimental tooth movement. An orthodontic appliance was placed on the second premolar and the first molar by exerting a continuous and constant reciprocal force of 25 cN on one side and 300 cN on the other side of the mandible. Tooth movement was recorded weekly. Dogs were sacrificed after one, four, 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.