Video microscope observations of human premolar eruption

Mechanism and control of tooth eruption: overview and clinical implications

OBJECTIVES

To review pre- and post-emergent eruption, with particular emphasis on distinguishing isolated molar ankylosis from primary failure of eruption (PFE) and genetic considerations in eruption problems.

MATERIAL AND METHODS

Radiographic review of eruption failure patients; animal and human experiments; high precision observations of movements of erupting teeth.

RESULTS

In pre-emergent tooth eruption, the controlling element is the rate of resorption of overlying structures. A path is cleared, and then the erupting tooth moves along it. This has clinical importance in recognizing the cause of eruption problems, particularly PFE, in which all teeth distal to the most mesial involved tooth do not erupt or respond to orthodontics. In our study of by far the largest sample of PFE cases yet reported, familial cases of PFE accounted for approximately (1/4) of all cases examined. Candidate genes now are being evaluated. In post-emergent eruption, control seems to be light forces of long duration that oppose eruption, rather than heavy forces of short duration such as those during mastication. Studies of human premolars in their passage from gingival emergence to the occlusal plane show that in this phase eruption occurs only during a few hours in the early evening. The critical hours for eruption parallel the time that growth hormone levels are highest in a growing child. In this stage intermittent force does not affect the rate of eruption, but changes in periodontal blood flow do affect it.

Long-wave cycles in the position of erupting human premolars

OBJECTIVE

Unusual long-wave cycles in the position of an erupting human premolar were first noted by Trentini et al. [Trentini CJ, Brown WH, Paterson RL, Proffit WR. The application of Moire magnification to high precision studies of human premolar eruption. Arch Oral Biol 1995;40:623-9]. This report characterises them in detail and evaluates their significance relative to human tooth eruption.

DESIGN

The magnitude and frequency of Trentini cycles were examined in high-resolution recordings of premolar eruption in patients without and with periapical infiltration of a vasoconstrictor or vasodilator, and the possibility of artifact due to the natural frequency of vibration of a tooth-PDL system or to building vibration was evaluated.

RESULTS

The cycles are characterised by a magnitude of 1.3 +/- 0.5 microm and a frequency of 1.8 +/- 0.5 cycles per minute, with greater variation between than among subjects. They cannot be explained as a natural frequency phenomenon, and vibration from microseism is highly unlikely. The cycles disappear when blood flow to the periapical area of the tooth is interrupted and reappear when blood flow resumes.

CONCLUSIONS

The cycles appear to reflect a physiologic rhythm, probably related to blood flow in the periodontal ligament. Because they are similar whether or not a tooth is erupting at a particular time, however, it appears that they are not a direct reflection of a blood flow-related eruption mechanism.

Response of erupting human second premolars to blood flow changes

The effect of infiltration of a vasodilator and a vasoconstrictor [2% lidocaine (lignocaine) without or with 1:100,000 adrenaline (epinephrine)] above the root apex of human second premolars in the prefunctional stage of post-emergent eruption was evaluated for 11 teeth in eight children. On two consecutive days, 30 min of high-resolution data on changes in tooth position were collected for each participant during each of four sessions, two in the afternoon and two in the evening, using an optical instrument based on Moiré magnification with 0.05 microm resolution. The immediate reaction to the 1.8 ml injection was extrusion of the tooth, which lasted 2-4 min. After that, in the majority of teeth receiving a vasodilator, the reaction was an increased rate of eruption. In teeth receiving a vasoconstrictor, a decrease in eruption or intrusion was noted after the initial extrusion. The low-frequency Trentini cycles characteristic of erupting teeth were immediately obliterated in all teeth receiving injections. In the next few minutes, the cycles reappeared in teeth receiving the vasodilator, but did not reappear or reappeared only partially in teeth receiving the vasoconstrictor. It appears that vascular changes can influence the rate of prefunctional post-emergent eruption.

Eruption pattern of autotransplanted premolars visualized by radiographic color-coding

Eruption patterns and root growth were visualized with the use of a new technique, radiographic color-coding, for comparison of the development of autotransplanted premolars with contralateral control teeth. Rates of eruption and root growth were studied. The eruption pattern and rate was assessed relative to the first molar. Maximum rates were found to occur between 30 and 60 days after transplantation. There were no significant differences between transplants and their contralaterals. Two distinct categories of eruption patterns were demonstrated. One group showed a tendency toward an initial rate of transplant eruption that was somewhat faster than that of the contralaterals. The other group showed initially retarded eruption. Possible explanations were discussed. Because no significant differences between the transplants and the contralaterals were observed, it was concluded that autotransplantation is a sound treatment option for substitution of missing teeth, at least from a tooth development point of view.

Response of erupting human premolars to force application

To test the hypothesis that human tooth eruption requires a critical time period during which no force is applied to the erupting tooth, the eruption of a maxillary second premolar in the prefunctional stage was recorded during the afternoon or evening hours in eight children, using an optical device based on the principle of Moiré magnification, while intermittent loads of 300-400 mN were applied to the teeth. When a tooth was erupting actively, light force applications either had no discernible effect or decreased the eruption rate for 2 min or less. When a tooth was intruding spontaneously rather than erupting, a varied response was observed, but the rate of intrusion never increased after force application. Often intrusion showed or halted, and especially during the evening, eruption was likely to occur after a force application during an initial period of intrusion. The conclusion is that, although light force applications significantly displace an erupting premolar, they have little or no effect on net eruption, and that a critical time period without force application is not necessary for eruption to occur.

The rhythms of human premolar eruption: a study using continuous observation

New optical technology now allows for precise observation of erupting human premolars. Continuous overnight tracking of an erupting tooth with a video microscope system shows that eruption occurs almost exclusively during the early evening. During the day, slight intrusion is likely to occur. When Moiré magnification is used to provide extremely high resolution (0.05 micrometer), slight movements of the erupting tooth in concert with the arterial pulse can be observed, and a previously unknown cyclic movement with a period of 20 to 50 seconds is seen consistently. Rhythms in skeletal growth and tooth eruption suggest that treatment to influence jaw growth and move teeth may be most effective at specific times of the day.

Continuous overnight observation of human premolar eruption

Such observation was made possible by transmitting the image of a mobile ceramic ruling on the erupting maxillary second premolar to a video-microscope via a coaxial fibreoptic cable. The cable was inserted into a reference bar secured to the adjacent first molar and first premolar. The image of the ruling was superimposed with the image from a surveillance camera focused on the patient and continuously recorded on video-tape along with the participant's blood pressure, pulse rate, electromyographic activity and occlusal contact sounds. Overnight data from 12 individuals clearly revealed a circadian rhythm in eruption during the prefunctional spurt. On average, the maxillary second premolar erupted 41 microns during an 11-h overnight observation, with almost all the eruption occurring in the late evening from 8 p.m. to 1 a.m. After 1 a.m., eruption typically ceased, with a tendency for intrusion to occur until 7 a.m. Sleep increased the rate of eruption during the late evening, but did not influence the eruption rate during the early morning. Haemodynamic changes, including blood pressure and pulse rate, did not have a significant impact on the rhythm of eruption. The observed eruption rhythm is most probably caused by changing hormone levels and their effect on the periodontal ligament. The late-evening eruption of human premolars coincides with the late-evening secretion of growth hormone and thyroid hormone typically found in humans.

Tooth eruption: theories and facts

The mechanisms of tooth eruption (i.e., the answer to the question of how and why teeth erupt) has been a matter of long historical debate. This review focuses on human and other mammalian teeth with a time- and spacewise limited period of eruption and analyzes recent observations and experimental data on dogs, rats, primates, and humans in a framework of basic biological parameters to formulate a guiding theory of tooth eruption. Acknowledging basic parameters (i.e., that teeth move in three-dimensional space, erupt with varying speed, and arrive at a functional position that in inheritable) eliminates a number of previously held theories and favors those that accommodate basic parameters, such as alveolar bone remodeling in association with root elongation, with possible correction factors in the form of cementum apposition and periodontal ligament formation. We have critically analyzed, summarized, and integrated recent findings associated with preeruptive movements of developing teeth, the intraosseous stage of premolar eruption in dogs, molar eruption in rodents, and premolar and molar eruption in primates. The variable speeds of eruption are particularly important. We conclude with basic principles of tooth eruption--that is, the type of signals generated by the dental follicle proper, the conditions under which teeth are moved and the clinical understanding to be derived from this knowledge.

High-resolution observations of human premolar eruption

Using an optical instrument based on the principle of Moire magnification to obtain resolution of less than 0.1 mu m, eruption of maxillary second premolars was observed during the prefunctional phase of eruption in 10 children. The participants were observed on four occasions for approx. 30 min each, once in the afternoon and once in the evening on two consecutive days. For all participants, a pulsatile movement of the erupting tooth was noted in concert with the arterial pulse. Significant variations in short-term eruption rates and patterns were observed, with a mean rate of 0.28 mu m/min over continuous 20-min periods but a range from -0.91 to 2.29 mu m/min. During most sessions net eruption occurred, but in several of the 30-min periods there was little movement or intrusion. An unexplained cyclic phenomenon was observed consistently, which had a period of 20-50 s and a range of magnitudes from 0.12 to 2.22 mu m. It seems clear that a discontinuous pattern of eruption occurs in short-term as well as longer-term observations.

The application of Moire magnification to high-resolution studies of human premolar eruption

The optical phenomenon of Moire magnification can be used to provide real-time measurements of the position of an erupting human premolar, with a resolution of 0.1 micron or better. A new instrument for this purpose, which allows repeated measurements in the same individual, is described. Cyclic changes in the position of the erupting tooth in concert with the heartbeat, with a magnitude of 0.05-0.35 micron, can be discerned. An unusual cyclic rhythm, with a period of 20-50 s, was observed.

Axial movements of rat mandibular incisors measured under artificial respiration with halothane anaesthesia

The axial movements of rat incisors were recorded continuously for over 20 h. The rats were anaesthetized with halothane delivered by intratracheal intubation using an artificial respirator. A haemostatic clamp was used to immobilize the jaw and the displacement detector exhibited high resolution, good linearity and low drift. The previously impeded and unimpeded eruption rates (the average extrusive movements) were 406 and 548 microns/24 h, respectively. The previously unimpeded rate was lower than the normal unimpeded rates obtained by other investigators. It appears that the eruptive movement of the previously unimpeded incisor was inhibited by halothane anaesthesia. Circadian rhythm was not observed in the eruption rates of previously impeded or unimpeded incisors. Following the i.p. injection of a lethal dose of pentobarbital sodium, the incisors retracted. The average intrusive movement was significantly greater in the previously impeded group (29 microns) than in the previously unimpeded group (12 microns) at 30 min after the lethal injection. It is assumed that the elimination of the pushing force at death may cause the stretched periodontal fibres to recoil, thereby pulling the tooth in an intrusive direction. It is also likely that the mechanical strength of the ligament is reduced in the previously unimpeded group by repeated shortening of the incisors; therefore the force needed to retract previously unimpeded incisors may be less than that needed to retract previously impeded incisors.

A rationale for a simplified occlusal design in restorative dentistry: historical review and clinical guidelines

An occlusal contact pattern in which the number of occlusal contacts has been substantially reduced as compared with traditional schemes is described. Concepts that may have had a justification in balanced occlusions have been needlessly transferred to anterior disclusion mechanics. No natural dentition presents occlusal contacts as described in many texts and yet stability is established. The temporomandibular joint does present structural changes that should be accounted for when an occlusal anatomy is designed. The force vectors that are active on teeth are not directed along the longitudinal axes of the roots only, and thus occlusal contact locations will not determine the direction of functional forces. The stability of the teeth on the arch depends primarily on the forces of eruption from the periodontium and the balance between the resting pressures of the muscles of the cheeks and the tongue. The mechanics of the stomatognathic system are not as accurate as their counterpart on an articulator. The variability of the guiding surfaces inherent to the temporomandibular joints should be incorporated into an occlusal design. Occlusal contacts that do not fulfill a justifiable purpose may be eliminated, and the number of contacts may be reduced to one per tooth.

The daily rhythm of tooth eruption

A video microscope system, which can resolve tooth movements to 1 to 2 microns, was used to monitor the eruption of 17 human maxillary second premolars in the prefunctional phase. Eruptive movement was detected by the change in position of an optical ruling on the erupting tooth relative to a reference ruling on the occluding adjacent teeth. Eruption was observed as the teeth erupted toward the occlusal plane over approximately a 41-hour period, from late Friday afternoon until Sunday noon. Half hourly observations were made during a 6 to 8-hour continuous laboratory session, which was during the day (9 AM to 5 PM) for nine of the subjects, and during the evening (5 to 11 PM) for eight. A similar pattern of eruption was seen for all subjects. The tooth intruded from before to after dinner on the first day, erupted significantly overnight, ceased eruption and approximately maintained its position during the next day, and erupted again during the second night. On the average, a slight intrusion was observed during the day, and statistically significant intrusion was associated with breakfast and dinner. Eruption occurred during the evening observation period, and the rate of evening eruption was significantly greater in children who were supine and relaxed than those who were upright and active. The 24-hour eruption rate was slower for teeth that were within 1.5 mm of the occlusal plane. The circadian eruption rhythm may be related to fluctuations of hormonal levels that affect metabolic activities within the periodontal ligament. It is also possible that a transient reduction in pressures by the cheeks, lips, and tongue during periods of rest allows eruption to occur then.

The basic and applied biology of tooth eruption

The dentition and the alveolar process of each jaw develop simultaneously so that, by the time the crown is completed and eruption begins, the crown is enclosed in a crypt within alveolar bone. Thus, the eruption of a tooth to its functional position involves discretely localized, bilaterally symmetrical bone resorption to produce an eruption pathway and bone formation to fill in the space previously occupied by the crown and growing roots. Studies of crypt surfaces during eruption confirm this polarization of alveolar bone metabolism around a tooth with respect to both bone cells and mineralized surface topography. Experimental studies of tooth eruption have shown that the dental follicle, the dense connective tissue investment of the tooth, is necessary for eruption and that neither bone resorption nor bone formation occur without the adjacent part of the dental follicle. Early in eruption the coronal part of the follicle accumulates mononuclear cells which have cytochemical and ultrastructural features of osteoclasts and the apical part of the follicle, a site of intense cell proliferation, binds epidermal growth factor (EGF). The dental follicle contains a variety of proteins and the concentration of several change during eruption. Prominent among them are a reduction in matrix metalloproteinases and an increase in protoglycans as eruption proceeds. The contribution of these changes to those in cell proliferation, migration and differentiation during tooth eruption present experimental opportunities for developmental biologists. The rate-limiting factor of the earliest (intraosseous) stage of tooth eruption is bone resorption and eruption can be accelerated or retarded by the local delivery of factors which increase or decrease the activity of osteoclasts.(ABSTRACT TRUNCATED AT 250 WORDS)