Periodontal collagen turnover in orthodontic tooth movement
A scientific essay in Medical Sciences
DOCTORAL THESIS defended in public on 12th of November 2015
Chapter 1 gives an overview of clinical, animal experimental and in vitro cell culture research in the field of orthodontics with emphasis on the biological processes in tooth movement. The main cues for regulating tooth movement are generated in the periodontal ligament (PDL). A key process enabling tooth movement is the turnover of collagen fibers in the PDL. The main enzymes that degrade collagen fibers during tooth movement are the matrix metalloproteinases (MMPs) and the cysteine proteinases. The production of these enzymes can be stimulated by growth factors and cytokines but also by hormones. The modulation of MMP production and activity might affect orthodontic tooth movement. Increased degradation of collagen might enhance tooth movement, while reduced degradation might prevent relapse. Up to now, the exact turnover rate of the collagen fibers in the PDL and gingiva under physiological conditions is still unclear as are the effects of orthodontic tooth movement on turnover.
Chapter 2 is a literature review on the mechanical and biological signaling pathways during orthodontic tooth movement. A new theoretical model is developed that clarifies the complex cascade of events taking place after the application of an orthodontic force. This theoretical model describes the induction of orthodontic tooth movement in four stages: 1. Matrix strain in the PDL and alveolar bone resulting in fluid flow; 2. Cell strain inducing the production of signaling molecules; 3. Activation of fibroblasts and osteoblasts in the PDL and osteocytes in the alveolar bone, and osteoclast recruitment; 4. Coordinated remodeling of the PDL and the alveolar bone that enables tooth movement. This theoretical model might help researchers and clinicians to understand the complex biological consequences of the application of an orthodontic force.
In chapter 3, the effects of the protein hormone relaxin on MMP production by human PDL cells is studied in vitro. A key physiological role of relaxin is the stimulation of collagen degradation in the pelvic ligaments to facilitate delivery. Similar effects might stimulate the remodeling of the PDL and thus enhance orthodontic tooth movement. In this study, human PDL cells were incubated with relaxin, and total MMP activity, MMP-2 and MMP-9 expression, and α-smooth musle actin expression (α- SMA), were determined. A dose-dependent increase of MMP-2 and α-SMA expression was found, which indicates an activated state of the PDL cells.
Chapter 4 describes the effects of a chemically modified tetracycline (CMT-3) on orthodontic tooth movement in rats. CMTs are able to inhibit MMPs, but lack the antimicrobial activity of unmodified tetracyclines. The inhibition of MMPs in the PDL might decrease relapse by inhibiting tooth movement after treatment completion. Standardized orthodontic tooth movement was performed in rats and molar displacement was measured. CMT-3 inhibited orthodontic tooth movement in a dose-dependent manner. The number of osteoclasts at the resorption side was also lower and these cells seemed to contain less MMP-9. CMT-3 therefore seems to inhibit orthodontic tooth movement in rats by reduced osteoclast recruitment and activity at the resorption side. In addition, CMT-3 might inhibit MMP activity in the PDL and thereby further inhibit tooth movement.
Chapter 5 investigates the turnover of periodontal collagen fibers in rats under physiological conditions by means of 3H-proline labeling and autoradiography. In order to label most of the collagen fibers, weanling rats were injected with 3H-proline at regular intervals. Autoradiography of histological sections was performed at sequential time-points after the last labeling and the half-life of collagen was determined at multiple locations along the root surface. The half-life was longest in the supra-alveolar region but even there it was shorter than two weeks, which indicates a high turnover of collagen fibers. These findings indicate that throughout the PDL, collagen fibers are completely remodeled during long- term tooth movement and thus cannot be responsible for relapse. The local variations in the turnover of collagen fibers in the PDL might be caused by local differences in strain levels induced by physiological forces such as chewing.
Chapter 6 describes the turnover of collagen fibers during orthodontic tooth movement in rats. It was hypothesized that the collagen turnover would be higher during orthodontic tooth movement due to increased remodeling of the PDL and that the turnover would be higher at the resorption side than at the apposition side of the moving tooth. Again, weanling rats were repeatedly injected with 3H-proline to label PDL collagen. An orthodontic appliance was then placed around one maxillary molar block of each rat to induce tooth movement. As in the previous study, autoradiography was performed on histological sections of the moved molars and the control side to determine collagen half- life. Unexpectedly, the turnover of collagen was not increased during orthodontic tooth movement. Also, collagen turnover was not higher at the resorption side than at the apposition side of the root. These results suggest that the physiological turnover of collagen in the PDL is enough to accommodate orthodontic tooth movement.
Chapter 7 is the general discussion. The studies described in this thesis were performed against the background of the two main clinical problems in orthodontics; long treatment duration and relapse after treatment. Tooth movement might be enhanced by drugs such as relaxin, although recent in vivo studies suggest that relaxin is not clinically effective. Inhibition of relapse by MMP inhibitors such as CMTs might be feasible but further research in animal models is required and the feasibility of clinical application of these drugs should be tested. The in vivo studies on collagen turnover in the PDL indicate that the rate of turnover is not affected by tooth movement. This unexpected finding suggests that the solution to both key issues in orthodontics does not lie in the modulation of collagen turnover but possibly in the modulation of other biological processes such as osteoclast differentiation, recruitment and activity.