Evidence-Based Considerations for Determining Appointment Intervals

Alger routinely scheduled visits at six-week intervals, as opposed to the traditional four-week interval (although many patients were not seen for three to six months during summer holidays). Several practice-management issues arose as a result of these scheduling procedures, including fee-collection problems, since most patients were used to paying at their monthly visits.² Alger concluded that extraction patients were probably best seen at four-week intervals, and that patients undergoing rapid maxillary expansion needed to be scheduled every one to two weeks for assessment of the expansion. He also surmised that patients who had poor oral hygiene or were undergoing compliance-dependent treatment (such as headgear or elastics) might need to be seen more frequently to foster psychological reinforcement and encouragement.

In an informal survey of 59 randomly selected practitioners in 2005, Sheridan found that the most common interval between appointments was five to six weeks, followed closely by seven to eight weeks.³ A four-week interval was uncommon, and only a few respondents reported booking patients at intervals longer than nine weeks or shorter than three weeks. The most frequently reported reasons for lengthening the period between appointments were the availability of highly resilient wires, constant-force springs, self-ligating brackets, and non-compliance mechanics such as the Herbst^* appliance, as well as time constraints in families with two working parents and busy after-school schedules.

Sheridan's respondents noted that the treatment stage generally dictated the appointment interval: patients undergoing initial leveling and alignment could easily go six to eight weeks between visits, but the demands of finishing required a shorter interval of four to five weeks.³ Cases involving maxillary expansion, impacted teeth, or periodontal complications also needed more frequent appointments. Risk/benefit analysis indicated that longer appointment intervals reduced overall chairtime and overhead, freed up patients' schedules and resulted in less time out of school, and allowed practitioners to serve more patients. Disadvantages included a higher proportion of cases that were out of control or overcorrected, longer overall treatment periods, fee-collection problems, and inadequate monitoring of patients with poor oral hygiene.

Periodontal Considerations

The most commonly accepted hypothesis of how teeth move is the pressure-tension theory. According to Proffit, when light, prolonged pressure is placed on teeth, the periodontal ligament (PDL) is partially compressed on the pressure side and stretched on the tension side, resulting in tooth movement within the socket.⁸ This histophysiological phenomenon occurs within hours of force application. Frontal resorption results when the force levels are kept low and continuous, thus facilitating gradual tooth movement. Higher force levels create undermining resorption due to a delay in breakdown of the adjacent bone, resulting in tooth movement marked by alternating periods of movement and stasis. Although Proffit questioned whether the forces generated in clinical practice can produce the most efficient tooth movement, he strongly advised the use of low and continuous pressure to ensure optimal tooth movement and patient comfort.

Cellular activity is slower in elderly individuals than in children. Moreover, the PDL does not regenerate when periodontal tissue is inflamed. In patients with active periodontal disease, orthodontic tooth movement exacerbates the disease process, resulting in rapid and severe periodontal destruction.⁹ In almost every case, however, orthodontics is associated with small amounts of alveolar crestal bone loss, slight increases in gingival recession, and transient increases in pocket depths.¹⁰

The appointment interval has a definite effect on periodontal status. Orthodontic appliances are plaque traps that are either in contact with or in close proximity to the supporting periodontal structures for extended lengths of time. Because patient hygiene is monitored less often when the appointment interval is longer, there is a greater risk of delay in diagnosing periodontal disease.

Root Resorption

Root resorption is an iatrogenic consequence of orthodontic treatment. A systematic review of 24 articles concluded that while the causes of root resorption are multifactorial, there are direct correlations among overall treatment duration, the use of fixed appliances, dental root morphology, individual susceptibility, and the type of tooth movement.¹¹ Fortunately, this effect is usually of minor clinical significance. One study found mild root resorption of 1.9%, 3.2%, 4.9%, and 8.6% in orthodontic patients at the beginning of treatment, the six- and 12-month marks, and the end of treatment, respectively.¹² When moderate resorption was discovered, the authors diminished the forces applied to the teeth, employed resting periods, and reduced the overall treatment time. They recommended that patients who exhibit root resorption within the first year of treatment should be observed monthly.

Since root remodeling is always a concomitant of orthodontic tooth movement, the permanent loss of tooth structure essentially results from a lack of cemental repair. Overly frequent activation of orthodontic appliances interferes with the normal process of breakdown and repair,^8,13 which suggests that a longer appointment interval can prevent or at least minimize the resorptive process. When an appliance delivers light, continuous forces that produce frontal (as opposed to undermining) resorption, there is little need to activate it more often.

Decalcification

Enamel decalcification in the form of whitespot lesions appears when plaque is retained on the tooth surface for a prolonged period of time. Half of all orthodontic patients exhibit at least one area of decalcification, with most lesions appearing in the maxillary anterior segment or mandibular posterior area.¹⁴ Ranging in intensity from slight discoloration to cavitated lesions, decalcification has the potential to reach depths of 75-100 microns, with 25% mineral loss in as little as four weeks. The incidence of decalcification does not appear to differ between patients treated for short (15-month) vs. long (36-month) periods of time.¹⁴ Since the major risk factor is the patient's oral hygiene, a longer appointment interval may compromise the practitioner's ability to diagnose and manage this problem in a timely manner.

Treatment options for decalcification include observation (with the expectation of eventual remineralization) and restorative procedures.^15,16 Decalcification can be minimized by prescribing a .2% sodium fluoride rinse, particularly one with a low pH,¹⁷ or by using a fluoride-releasing glass ionomer adhesive.¹⁸

Biomaterials

The development of round and rectangular nickel titanium wires and coil springs revolutionized clinical orthodontics. A nickel titanium archwire has two clinically relevant properties: shape memory¹⁹ and superelasticity.²⁰ The nonlinear unloading characteristics of the wire, as it changes phases from martensitic to austenitic, results in the transmission of less force to the teeth over a greater working range, thus inducing a more physiologic movement (frontal resorption) while improving patient comfort. Increasing the interbracket distance or bypassing specific teeth during initial leveling and alignment also reduces force levels. Because the wire stiffness increases toward the end of activation, archwires do not need to be changed as often; according to Viazis, two-month appointment intervals are usually sufficient.²¹ 

From a scheduling perspective, the longer a low force can be delivered, the more efficient tooth movement becomes. Therefore, the orthodontist should use appropriately sized multistranded stainless steel archwires (in cases of mild interdental distortion), superelastic nickel titanium archwires (in cases of mild-to-moderate interdental distortion), or copper nickel titanium archwires (in cases involving severe interdental distortion or requiring early torque control) and should leave each wire in place longer before progressing to the next size. Overtreatment with unnecessary or frequent archwire changes will inhibit the full expression of the wire's working range and hence its biomechanical advantages.²²

Elastomeric chain and nickel titanium coil springs are the two other force-delivery systems most commonly used to move teeth. In a study comparing these two modules, a chain was placed in one quadrant and an open-coil spring applied to the contralateral side, with each stretched to produce the same amount of force.²³ Patients were scheduled at four-to-six-week appointment intervals (although some did not return for four to five months). Elastomeric chain retained an average of 53% of its initial force over one to 15 weeks, while nickel titanium coil springs maintained an average of 52% over a one-to-22-week period. In both samples, the degree of degradation correlated with the strength of the initial force. Space closure occurred at the rate of .21mm per week for elastomeric chain and .26mm per week for the coil spring--virtually the same--and both systems had sufficient residual force to accommodate appointment intervals longer than six weeks.

Orthodontic brackets have been intentionally omitted from this review. Opinions are currently mixed on whether one type of bracket is more efficient than another.^24,25

Recommendations

The evidence indicates that when scheduling orthodontic appointments, one size does not fit all. Appropriate intervals can be as short as one to two weeks or as long as eight to 10 weeks, based on the following considerations:

1. Patient age. Because of a slower cellular response and a higher risk for periodontal disease, adult patients should be seen at shorter intervals (four to six weeks) than children.

2. Type of archwire and force-delivery system. Nickel titanium archwires should be used for initial leveling and alignment and left undisturbed as long as possible. During space closure, appointment intervals of six to eight weeks are generally acceptable, regardless of the force-delivery system, as long as sliding mechanics are employed. The interval can be extended to as long as 10 weeks under clinically appropriate circumstances.

3. Periodontal status. Patients with a history of periodontal disease or poor oral hygiene should bescheduled at intervals of four weeks  or less. Whether to extend the treatment interval to the eight-week range for specific procedures, such as space closure with sliding mechanics, should be premised on the patient's periodontal health. 

4. Extraction vs. nonextraction. Longer appointment intervals (six to eight weeks) may be routinely used in nonextraction cases. Patients undergoing extraction treatment should be scheduled in four-to-six-week blocks unless straightforward sliding mechanics are used for space closure, in which case a six-to-eight-week interval is acceptable.