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ABSTRACT
Teeth move through alveolar bone, whether through the normal process of tooth eruption or by strains generated by orthodontic appliances. Both eruption and orthodontics accomplish this feat through similar fundamental biological processes, osteoclastogenesis and osteogenesis, but there are differences that make their mechanisms unique. A better appreciation of the molecular and cellular events that regulate osteoclastogenesis and osteogenesis in eruption and orthodontics is not only central to our understanding of how these processes occur, but also is needed for ultimate development of the means to control them. Possible future studies in these areas are also discussed, with particular emphasis on translation of fundamental knowledge to improve dental treatments.
KEY WORDS: dental follicle, periodontal ligament, osteoclastogenesis, osteogenesis, RANKL, OPG, CSF-1, bone remodeling, bone formation, bone resorption.
INTRODUCTION
Given the breadth of the two topics, tooth eruption and orthodontic tooth movement, this review will focus more upon what is currently known about their molecular mechanisms, commonalities, and differences, instead of a lengthy historical review. For the latter, readers are referred to other reviews (Cahill et al., 1988; Marks and Schroeder, 1996; Wise et al., 2002; Krishnan and Davidovitch, 2006; Masella and Meister, 2006; Meikle, 2006).
Theories of Tooth Eruption
For the past 70 years, various theories have been presented on the mechanisms of tooth eruption. That numerous theories abound may be due to the enormous success of orthodontics in moving teeth with force application. However, tooth eruption is a fundamental developmental and physiologic process, and force plays a secondary role. Regardless, some of these theories are discussed in the section of this review entitled "Motive Force of Tooth Eruption". Previous reviews in the past 20 years also have considered the various theories of eruption (e.g., see Cahill et al., 1988; Marks and Schroeder, 1996; Wise et al., 2002).
In this review, emphasis will be on the dental follicle and its role in initiating eruption by regulating alveolar bone resorption and alveolar bone formation. This focus emanates from the pioneering work of Sandy Marks, Jr., and Don Cahill, who demonstrated that the dental follicle was required for eruption. Their surgical studies utilizing dog premolars showed that removal of the follicle from the unerupted tooth prevented the tooth from erupting (Cahill and Marks, 1980), whereas leaving...