All PhD Theses

Y. Ren

Age effect on orthodontic tooth movement


A scientific essay in Medical Sciences

DOCTORAL THESIS defended in public on 7th of October 2003


This study was performed to investigate the effect of age on the efficiency of orthodontic tooth movement based on critical literature reviews, studies on a standardized orthodontic animal model and a non-invasive clinical investigation.

Chapter 1 elucidates the background of the study and gives an evaluation of the level of evidence of current orthodontic literature.

In chapter 2 a systematic review was performed on the optimum force for orthodontic tooth movement. Over 400 articles both on human research and animal experiments were found in Medline and by hand searching of main orthodontic and dental journals. Articles on animal experiments were in the majority. A wide range of animal species was used such as rat, cat, rabbit, beagle dog, monkey, mouse, and guinea pig. Besides variation in species, there was also a wide range in force magnitudes, teeth under study, directions of tooth movement, duration of experimental period, and force reactivation. Furthermore hardly any experiments were reported that provide information on the relation between the velocity of tooth movement and the magnitude of the applied force. Data from human research on the efficiency of orthodontic tooth movement appeared to be very limited. The large variation in data from current literature made it impossible to perform a meta-analysis. Therefore literature is systematically reviewed. It appeared that no evidence about the optimal force level in orthodontics could be extracted from literature.

Chapter 3 describes a mathematic model that was developed to describe the relation between the rate of orthodontic tooth movement and the magnitude of the applied force. Initially, data were extracted from experimental studies in beagle dogs, in which controlled standardized forces were used to move mandibular second premolars distally. Trend-fitting by iterative non-linear regression analysis provided an equation describing the relation between force magnitude and rate of tooth movement in beagle dogs. Similar techniques were subsequently used for the analysis of the limited available literature data on human canine retraction. The results showed that the maximum rates of tooth movement in humans and in dogs are very similar. A threshold for force magnitude that would switch on tooth movement could not be defined. The model showed that a wide range of forces can be identified that leads to maximum rate of tooth movement.

In chapter 4 a systematic review was performed on the use of rats as a model for experimental tooth movement. The literature from 1981-2002 indicates that in 57% of the animal studies on orthodontic tooth movement, rats are used. But in many of these studies the experimental set-up was poorly documented. Only 3 out of 159 studies fulfilled the inclusion criteria for a good animal model: force magnitude less than 20 cN; moving molar(s) mesially; experimental duration longer than 2 weeks; and no extra experimental condition such as drug intervention. As more than one quarter of the studies on tooth movement in rats used elastics to produce an orthodontic force, and as the forces they produced and their force decay are unknown, we tested their mechanical characteristics. Elastics stored under dry conditions or in water showed significant force decay from around 45 N to almost 0 within the first 0.2 mm of decompression. In regard to the above-mentioned shortcomings of using rats as a model for tooth movement, a newly designed experimental appliance for tooth movement in rats was proposed and evaluated.

Chapter 5 presented the study of age effect on orthodontic tooth movement in rats. In orthodontic practice orthodontic procedures in adults seem to be more time-consuming than in juveniles. This might be related to delay in the initial tissue response or to a slower turnover of the bone and periodontal ligament in adults. To study this problem orthodontic tooth movement was carried out in two groups of 30 rats, aged 6 weeks and 9 – 12 months respectively. At one side of the maxilla three molars together were mesialised with a standardized orthodontic appliance delivering a force of 10 cN. The other side served as a control. The results showed a faster initial tooth movement in juvenile than in adult animals. However, once tooth movement had reached the linear phase, the rate of tooth movement was the same in both groups. The results indicate that besides a delay in the onset of tooth movement in adult animals, tooth movement could be equally efficient in adults once tooth movement had started.

As a continuity of chapter 5, chapter 6 presented a study on osteoclast differentiation during tooth movement in young and adult rats. Osteoclasts were identified by ED1 staining and the numbers at the mesial and distal sides of the roots of the second and third molars were counted. The results showed that in control distal sides, the number of osteoclasts seemed decreased slowly by age. In experimental mesial sides, young rats showed increased number of osteoclasts and reached a peak at week 2. In adults this increase started slower. In young rats a positive correlation between the rate of tooth movement and the number of osteoclasts was found. The results indicate that orthodontic forces induced faster osteoclast differentiation in young rats. This may explain the initial delay in tooth movement in adult animals as well as in adult patients as often reported.

Chapter 7 compared cytokine levels (PGE2, IL-6 and GM-CSF) in crevicular fluid between juveniles and adults during initial orthodontic tooth movement. 43 juvenile and 41 adult patients took part in the study. One of the lateral incisors of each patient was tipped labially, the other served as control. GCF samples were taken before force activation (t0) and 24 hrs later (t24). Mediator levels were determined by radioimmunoassay (RIA). PGE2 concentrations were significantly elevated at t24 in juveniles and adults; while concentrations of IL-6 and GM-CSF were significantly had elevated only there is something wrong with this sentence in juveniles. Total amounts of all three mediators in GCF significantly increased at t24 in both groups. This implicated that in early tooth movement mediator levels in juveniles are more responsive than in adults, which agrees with the finding that the initial tooth movement in juveniles is faster than in adults and starts without delay.

Chapter 8 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 evidence-based research were proposed, specifically, perspectives in GCF studies and future orthodontic research were discussed.