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.

Occlusal forces in normal- and long-face children.

In children aged from six to 11 yr, forces of dental occlusion during swallowing, simulated chewing, and hard biting are similar for normal- and long-face individuals. Forces in the normal- and long-face children are similar to those in long-face adults, but are about half those in normal adults. It appears that individuals with the long-face pattern fail to gain strength normally in the mandibular elevator muscles.

The effect of intermittent forces on eruption of the rabbit incisor.

Rabbits were prepared so that a variable capacitance displacement transducer (VCDT) could be used to monitor the eruptive movements of a previously-amputated lower incisor, and a strain gauge transducer mounted on a relay-controlled plate was used to place precisely controlled intermittent forces opposing the erupting tooth. Forces of one or three g were applied under time conditions of one second on, nine seconds off (1:9); one second on, three seconds off (1:3); and one second on, one second off (1:1). The data indicate that with force present for 10% of the time (1:9), there was little or no effect on eruption rate. In contrast, 50% time (1:1) produced essentially the same intrusive effect as continuous force, and 25% time (1:3) gave an intermediate response. The data are consistent with the hypothesis that force duration is more important than force magnitude for the vertical equilibrium, as is the case for facio-lingual equilibrium.

Occlusal forces in normal- and long-face adults.

Using both quartz- and foil-based piezo-electric force transducers, occlusal forces during swallow, simulated chewing, and maximum effort were evaluated in 19 long-face and 21 normal individuals. Forces were measured at 2.5 mm and 6.0 mm molar separation. Long-face individuals have significantly less occlusal force during maximum effort, simulated chewing, and swallowing than do individuals with normal vertical facial dimensions. No differences in forces between 2.5- and 6.0-mm jaw separation were observed for either group.

Lip and tongue pressures related to dental arch and oral cavity size in Australian aborigines.

Although the oral cavity and dental arches of the Australian aborigine are large, studies of lingual and labial pressures indicate that the tongue is neither unusually large nor strong. The Australian aborigine's pharyngeal cavity is smaller in height and depth than that of the American; just the opposite is true for the oral cavity. To the extent that environmental factors are important at all, the resting pressure of the lips, not tongue pressure during swallowing, is probably the significant determinant of dental arch dimensions.

Video microscope observations of human premolar eruption.

It is now possible to observe with an accuracy of 1-2 microns the pattern of eruptive movements of a human premolar as it moves from gingival emergence to the occlusal plane. This was accomplished by use of a video microscope to track the position of an optical ruling on the erupting tooth relative to a reference ruling mounted on adjacent teeth that were in occlusion. Net eruptive movements averaging 25-75 microns occurred from one day to the next until the tooth was almost in occlusion; then the eruptive rate slowed. Eruption occurred almost entirely at night. During the day, there was a marked slowing or cessation of eruption and often intrusion, which appeared to be related to masticatory activity.

Variables affecting measurements of vertical occlusal force.

Previous studies of occlusal force have provided conflicting results. The purpose of these studies was to determine whether the extent of vertical opening, contralateral occlusal support, or head posture influenced vertical occlusal forces during swallowing, simulated chewing, and maximum biting effort. Three samples of subjects with normal vertical facial proportions--one each of children, adolescents, and young adults--were evaluated to determine the effects of changes in small (2.5 vs. 6.0 mm) vertical separation of the first molars. A sample of young adults was used to evaluate changes in large (10-40 mm) vertical openings, and a sample of adolescents was used to investigate the effect of contralateral support and head posture. All between-group comparisons were evaluated using non-parametric statistics. For the small vertical openings, there was significantly more vertical occlusal force at 6.0 than 2.5 mm in children during swallowing and chewing but not during maximum biting effort. In adults, there was significantly more force during swallowing at 6.0 than at 2.5 mm separation, but no differences in chewing or maximum biting. Increasingly large vertical openings resulted in a progressive increase in maximum bite force to a maximum at about 20 mm, followed by a decrease and then a second increase to near-maximum force at about 40 mm for young adults. There were no significant differences in vertical force with or without contralateral support or between flexed, normal, and extended head postures at either of the small openings.

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.

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.

The effects of continuous axially-directed intrusive loads on the erupting rabbit mandibular incisor.

A cantilever beam strain-gauge transducer was used to apply continuous light intrusive loads (0.2-0.4 and 2.5 g) to unimpeded mandibular incisors. Tooth position was continuously monitored using a variable capacitance displacement transducer. After 4 days of unimpeded eruption, the animals were divided into three experimental groups: no axial loading (6 rabbits); 0.2-0.4 g of axial loading (3 rabbits); 2.5 g of axial loading (3 rabbits). During a 4-h period, light forces (0.2-0.4 g) slowed eruption to a stop, whereas a heavier force (2.5 g) intruded the teeth. The data support Burn-Murdoch's (1981) (Archs oral Biol. 26, 939-943) contention that the eruptive force of unimpeded continuously-erupting incisors is significantly less than previously thought.

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 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.

Accuracy of movement tracking with a miniaturized second-generation Moiré device.

The availability of a Nanoform 600, a machine used in the contouring of optical components, with movement capabilities of 12.5 A, made it possible to determine the sensitivity, accuracy and movement tracking ability of a high-resolution measuring device using the principle of Moiré magnification. The Nanoform 600 was programmed to make a series of movements measured by the Moiré device. Comparison of the Moiré measurement with the programmed movements verified that the Moiré device is capable of reliably tracking movements as small as 0.05 microm. Increasing the rate of movement had little effect on linearity. Tracking movements over a 350 microm distance at rates of up to 340 microm per second resulted in a 0.23% error. Intentionally defocusing the Moiré signal did not appreciably affect sensitivity.

Referring adult patients for orthodontic treatment.

BACKGROUND: An increasing number of adults are undergoing orthodontic treatment, but they still make up only a small proportion of all adults who could benefit from such therapy. The purpose of this article is to illustrate the appropriateness and value of orthodontic treatment in adults. METHODS: The authors review and summarize previous studies regarding adult orthodontic treatment. Data from these studies are compared to determine the prevalence of orthodontic treatment in adults, the reasons why many adults do not seek treatment and the outcomes of orthodontic treatment in adults. In addition, the authors describe a case to demonstrate how a combination of orthodontic therapy and orthognathic surgery can facilitate and complement prosthetic rehabilitation. RESULTS: Numerous studies indicate that two-thirds to three-fourths of adults possess some form of malocclusion, yet adults make up only 15 percent of all orthodontic patients. Reasons given by adults for not seeking treatment include a lack of awareness that orthodontic treatment can be performed in adults, apprehension about possible pain or discomfort and concern about social acceptance. However, most patients who underwent orthodontic treatment reported only mild discomfort of one to two days' duration, and only 20 percent of patients felt that the appliances had an adverse social effect. Overall, patients reported high levels of satisfaction with treatment. Orthognathic surgery in addition to orthodontics may be required for the most severe problems, and is compatible with subsequent fixed and removable prosthetic treatment. CONCLUSIONS: Adjunctive and comprehensive orthodontic treatment is feasible for adults of all ages. Correction of malocclusion makes it possible to improve the quality of periodontal and restorative treatment outcomes, in addition to providing psychosocial benefits. CLINICAL IMPLICATIONS: General dentists are often the first dental professionals to suggest orthodontic treatment and to refer patients to orthodontic specialists. Moreover, with the growing emphasis on cosmetic dentistry, more adults are likely to seek information regarding orthodontic surgery. This article provides general dentists with the information needed to communicate with patients about treatment.

Orthodontic care for medically compromised patients: possibilities and limitations.

Medically compromised children and adults are increasingly likely to seek orthodontic care as improved medical management creates more long-term survivors. For the majority, treatment of orthodontic problems is feasible, but special precautions usually are required. These include medical consultation to establish the patient's prognosis, maintaining a current knowledge of drug therapy, and modifications in office procedures. Patients with a history of multiple transfusions should be presumed to be hepatitis carriers until proved not to be, and special precautions to protect office staff members and other patients should be taken. Decreased resistance to infection is a common complicating factor in medically compromised patients. Dentists must therefore avoid mucosal irritation and carefully monitor periodontal health. The practitioner should be alert to side effects of drug treatment such as xerostomia and depressed immune response, and be aware of the particular features of the underlying disease. Bleeding disorders, which can be managed by replacement of missing clotting factors, do not contraindicate orthognathic surgery. The major contraindication is poor anesthetic risk, which almost always is true for patients having sickle cell anemia because of poor blood oxygen saturation. Because orthodontic treatment can provide positive benefits, it should not be withheld solely because of the presence of a serious medical problem. With appropriate management, successful orthodontic treatment can be done for most patients.

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.

Forty-year review of extraction frequencies at a university orthodontic clinic.

In a review of consecutive charts at 5-year intervals from the orthodontic clinic at the University of North Carolina, the number of patients with extraction of all four first premolars increased from 10% in 1953 to 50% in 1963, remained at 35% to 45% until the early 1980s, then declined sharply to the 1950s level by 1993. Extraction for camouflage of Class II malocclusion (maxillary first premolars alone or maxillary first-mandibular second premolars) reached 16% in 1968, then declined, but not as dramatically, and presently is as frequent as the extraction of four first premolars. The rate of extraction of other teeth, done for a variety of individual reasons, has remained almost constant at about 15% for the past 40 years. Thus the total extraction percentage was 30% in 1953, peaked at 76% in 1968, and declined again to 28% in 1993, with almost all the change in the percentage of four first premolar extractions. The increase in first premolar extractions occurred primarily in a search for greater long-term stability; the recent decline seems due to a number of factors. Greater concern about the impact of extraction on facial esthetics, data to suggest that extraction does not guarantee stability, concern about temporomandibular dysfunction, and changes in technique all seem to have played a role. With appropriate orthodontic mechanics, many patients with Class I crowding can be treated satisfactorily with or without premolar extraction.

Equilibrium theory revisited: factors influencing position of the teeth.

The major primary factors in the dental equilibrium appear to be resting pressures of tongue and lips, and forces created within the periodontal membrane, analogous to the forces of eruption. Forces from occlusion probably also play a role in the vertical position of teeth by affecting eruption. Respiratory needs influence head, jaw and tongue posture and thereby alter the equilibrium. "Deviate swallowing" is more likely to be an adaptation than a cause of tooth changes. Patients with failure of eruption have been recognized and alterations in the eruption mechanism may be more important clinically than has been recognized previously.

Soft tissue limitations in orthodontics: treatment planning guidelines.

Orthodontists have traditionally viewed structural discrepancies as the major limitation of treatment. In reality, it is the soft tissues that more closely determine therapeutic modifiability. The boundaries of dental compensation for an underlying jaw discrepancy are established by pressures exerted on the teeth by the lips, cheeks, and tongue; limitations of the periodontal attachment; neuromuscular influences on mandibular position; and the contours of the soft tissue facial mask. The ability of the soft tissues to adapt to changes in tooth-jaw relationships are far narrower than the anatomic limits in correcting occlusal relationships. The tolerances for soft tissue adaptation from equilibrium, periodontal, and facial balance standpoints are in the range of 2 to 3 mm for expansion of the mandibular arch and even less for changes in condylar position. Thus, analysis of the soft tissues is the critical step in orthodontic decision making, and this can only be accomplished through physical examination of the patient. Although quantitative measurements cannot be rigorously applied, guidelines for soft tissue assessment, with particular emphasis on facial esthetics, are proposed. From this perspective, a contemporary philosophy of orthodontic practice is offered, with general indications and contraindications for nonextraction, extraction, and surgical treatment.

Communication in orthodontic treatment planning: bioethical and informed consent issues.

Orthodontic treatment planning is an interactive process in which the patient or parent and the orthodontist serve as co-decision makers. As in most partnerships, there is a natural tension between the orthodontist and the patient because of differences in their frames of reference. The orthodontist generally is influenced more by the objective findings (the problem list), whereas patients are guided more by subjective issues related to their perceived needs, desires, and values. The art of careful probing and listening to the patient as part of the treatment planning process is an essential skill. One of the most difficult situations in contemporary orthodontics is presented by the patient with a jaw discrepancy for which the alternative treatments are orthodontic camouflage through dental compensation or surgical-orthodontic correction. Computer imaging to simulate the probable treatment outcomes can facilitate communication about these alternatives by eliminating misconceptions. Full disclosure and the consideration of all viable treatment alternatives have great benefits from a risk management standpoint, in addition to their bioethical merits.