“Orthodontic therapy can produce dire consequences that will later require the services of the periodontist or even the prosthodontist. The limitations of orthodontic treatment are stringent. The [useful] role of tooth sacrifice [and/or orthognathic surgery] must be recognized. Traditionally, orthodontists had their patients wear retaining appliances indefinitely after the removal of tooth moving appliances and attributed any change back toward the original malocclusion to ‘the wisdom teeth’ or ‘the lack of patient cooperation wearing the removable retainer as instructed.’ ”

The Periodontal Ligament and Bone Response to Sustained Orthodontic Force

The specific anatomic response to orthodontic forces is dependent on the force magnitude. “Heavy” forces often result in pain, necrosis of cellular elements within the PDL (i.e., hyalinization), and remodeling with the resorption of the alveolar bone that is being compressed (i.e., the hyalinization zone). It may also result in the resorption of the root apex, remodeling with the loss of crestal bone, and fenestrations or dehiscence through the cortical plates. When light and constant forces are applied to the tooth, there is compression without necrosis of the PDL elements and remodeling of the tooth socket without significant irreversible necrosis. As long as the tooth is being moved into adequate alveolar bone and the forces are light, the root of the tooth is less likely to resorb, and the alveolar crest height and cortical plates are for the most part preserved.³⁰ The objective of the orthodontic movement of a tooth is to produce the desired new tooth position as quickly as can be done, with as little damage as possible to the tooth, the PDL, the alveolar bone, and the adjacent teeth.

There are two major theories to explain the observed process of orthodontic human tooth movement: the bioelectric theory and the pressure-tension theory. The bioelectric theory holds that the changes in bone metabolism that are required to effectively move teeth and their sockets are controlled by the electric signals produced when alveolar bone bends and the crystalline structure is distorted. The pressure-tension theory presumes that cellular changes occur when pressure or tension is applied within the PDL or tooth socket, which then produces chemical changes. The pressure and tension that occur within the PDL result in specific blood flow changes and the release of chemicals that affect cellular function, thereby causing the observed changes in bone architecture. These two theories of bone metabolism are also discussed and debated within the orthopedic literature, and they are not mutually exclusive. Many scientists and clinicians believe that both biologic mechanisms play a role in the remodeling and repair of bone. The pressure-tension theory represents the classic theory of tooth movement. It proposes that chemical rather than electric signals are the stimuli for the cellular differentiation required to alter the tooth socket and ultimately to move the teeth. In either case, when an alteration in blood flow within the PDL occurs and is sustained through pressure that is applied directly to a tooth, the tooth shifts position within the PDL space, thereby compressing the ligament on one side and stretching it on the other. Blood flow is decreased where the PDL is compressed, although it may be increased or maintained in areas in which the PDL is under light tension. Interestingly, if the PDL is overstretched (i.e., if “heavy” tooth forces are applied), then the blood flow may also be decreased on the tension side. It is easy to understand how these alterations in blood flow can create changes in the chemical environment. Oxygen levels fall in the areas of compression, although they may even increase on the tension side of the tooth socket. The changes in oxygen content cause a series of biologic responses that bring new cells into the area either through migration or differentiation.

The Amount of Orthodontic Force Applied to the Teeth

The heavier the sustained pressure, the greater the reduction in blood flow where compression is occurring in the PDL.25,166,174 When only light prolonged force is applied to a tooth, the partially compressed PDL experiences diminished blood flow, and the interstitial fluids are expressed (pushed out) from the PDL space as the tooth moves within the socket.¹²⁵ Animal experiments confirm that, within a few hours, there will be increased levels of cyclic adenosine monophosphate (cAMP).¹⁷⁵ cAMP is known for its importance in the alteration of cell functions. Within 4 hours of continued sustained low pressure, mesenchymal cell differentiation occurs. From a clinical perspective, it is known that, if orthodontic appliances are used for less than 4 to 6 hours per day, not enough sustained force will be applied, and little or no tooth movement will occur.^143,148

For the tooth to actually move further than just within the socket itself, osteoclasts must arrive (i.e., migrate or differentiate from mesenchymal cells) and then remove bone from the area adjacent to the compressed part of the PDL. Osteoblasts are required to form new bone on the tension side. Prostaglandin E (a family of prostaglandins that are often used pharmaceutically in medicine) has been shown to increase its levels during this phase of orthodontic tooth movement. It is known that prostaglandin E stimulates both osteoclastic and osteoblastic activity, thereby leading to the belief that it is an important mediator of tooth movement.

It is known that the osteoclasts attack the compressed area of lamina dura to remove bone.¹⁸⁴ The radiographic appearance of a widened PDL (on the tension side) in the presence of light prolonged orthodontic forces indicates that tooth movement is soon to begin.⁶⁶ On the compression side, osteoclasts are removing the lamina dura. On the tension side, remodeling activity involves osteoblasts laying down new bone. The course of events is different if the prolonged force applied to the tooth is too great. If the force transmitted to the periodontal ligament totally occludes the blood vessels, thereby cutting off all blood supply to the area, generalized necrosis occurs within the compressed area. Although some avascular areas in the PDL will occur even with light forces, if the predominant forces are “heavy,” then the response is necrosis. Irreversible damage can then occur, and this includes remodeling with the loss of crestal bone, root resorption, and cortical plate dehiscence. The histologic findings within the PDL—even with light forces during the compression process—show the disappearance of cells within the vessels. The histologic appearance is descriptively said to be “hyalinized” because of its visual resemblance to hyaline connective tissue. After this hyalinization process begins, after several days of delay, the remodeling of bone that borders the sporadic necrotic areas of the PDL will occur as osteoclasts (derived from adjacent undamaged areas) arrive and do their work. The process of bone removal is called undermining resorption. This term is visually descriptive when it is viewed under the microscope, because the attack by the osteoclasts is from the underside of the lamina dura. After the hyalinization (i.e., the compression of blood flow with isolated necrosis) and the undermining resorption (i.e., the osteoclastic activity on the underside of the lamina dura) have “softened” the lamina dura, tooth movement is soon to occur. The chronology of tooth movement with frontal resorption is different than that of undermining resorption. With frontal resorption, a steady attack on the outer surface of the lamina dura results in smooth, continuous tooth movement. With undermining resorption, there will be a delay in the removal of the bone adjacent to the tooth. When heavy force (i.e., pressure) is again reinitiated, there will be a similar delay in tooth movement until a second round of undermining resorption can occur.¹⁷¹

The orthodontic movement of a tooth is clinically more efficient when there are only minimal areas of PDL necrosis. As a result, only limited levels of pain are experienced by the individual undergoing treatment. Unfortunately, neither aspect is completely avoidable. To date, a completely smooth progression of tooth movement with consistent light force over time has proven to be unattainable within the human biologic system. One important compounding factor is that the individual who is undergoing orthodontic tooth movement must also continue with the everyday functions of mastication, speech, swallowing, and breathing within a 24-hour cycle. These functions also place forces on the teeth, which may either conflict with or expedite the applied orthodontic forces. Another factor is the relative ineffectiveness of the appliances (i.e., the materials) used to apply sustained appropriate levels of force. To review, in clinical orthodontic practice, on one end of the spectrum, too much sustained force is detrimental to the tissues; alternatively, too little force will not produce the tooth movement that is desired.²⁵

Control of Orthodontic Tooth Movement

The basic principle of the ideal orthodontic movement of a tooth includes the application of just enough force to stimulate required cellular activity without the complete occlusion of the blood vessels within the PDL. For example, it is estimated that as little as 30 g of force may be required to tip a single rooted tooth. For controlled resorption and remodeling to occur, force is first delivered to the tooth and then to the PDL, which results in compression, tension, or both at the appropriate locations of the lamina dura. Depending on the desired three-dimensional repositioning of the tooth, different amounts of pressure (i.e., the force per unit area) are applied to different locations along the lamina dura.

The different basic forms of tooth movement include tipping, rotation, and translation. Tipping results from a single force applied at a distance to the center of resistance. Although the center of resistance of a tooth is generally located about halfway down the root, the center of rotation for tipping movements is apical to that. Here, the crown will move in one direction, and the apex will move in the opposite direction. When the tooth tips in this fashion, the PDL is compressed most strongly near the root apex on the same side as the force being applied as well as at the crest of the alveolar ridge on the opposite side. Progressively less pressure is created as the center of rotation is approached, where there is minimum pressure applied. During translation movement of the tooth, the PDL space is loaded uniformly from the alveolar crest to the apex on the side of tooth compression. A pure translational force will allow for the bodily movement of the tooth rather than tipping. The forces that are required for the bodily movement (i.e., translation) of incisors up to multi-rooted teeth is in the range of 70 to 120 g. This represents the maximum orthodontic tooth movement force to be used in clinical practice. The optimal force for the orthodontic rotation of a tooth around its long axis (i.e., 35 to 60 g) is less than that required for the bodily movement of a tooth. The optimal force required for the orthodontic extrusion of a tooth is in the range of 35 to 60 g. The intrusion of teeth requires the carefully controlled application of consistent very light forces that are applied to the tooth in a specific way. It is difficult to apply controlled intrusion forces to a tooth (i.e., 10 to 20 g) without also applying tipping forces. Small implants (i.e., temporary anchorage devices [TADs]) can be used to help this aspect of tooth movement and to provide greater control, as described later in this chapter.

Control of Force Duration and Decay in the Orthodontic Movement of Teeth

A basic principle in the process of orthodontic tooth movement is the application of sustained force. The force must be present for a significant percentage of the time during the whole process of tooth movement. Animal experiments suggest that, only after force is maintained for approximately 4 to 6 hours, will levels of cAMP be measurably increased in the PDL. Appropriate levels of cAMP are thought to be necessary for the stimulation of cellular differentiation. Human clinical experience suggests that a force duration of at least 4 to 8 hours (per 24-hour cycle) is required to effect tooth movement. The teeth can be moved more rapidly if force is maintained for longer durations. In fact, the application of continuous light force (24 hours per day) would produce the most efficient tooth movement. Through technical advances in materials and appliances, the ability to apply continuous force and to control the duration of that force to each tooth has greatly improved. For example, the nickel-titanium alloys and the self-ligating brackets that are in use today have advanced the process of the sustained duration of light force to achieve tooth movement over time. Office visits can often be spaced every 6 to 10 weeks. Applied orthodontic forces are classified as either continuous or interrupted (intermittent). In general, the orthodontist would like to apply continuous force (or at least some appreciable fraction of the original applied force) from one patient visit to the next. Orthodontists also use intermittent forces for specific purposes. The so-called activated appliances (e.g., removable plates, headgear, elastics) may be used intermittently.

Medication Effects on the Rate of Orthodontic Tooth Movement

The bisphosphonates act as inhibitors of osteoclast-mediated bone resorption, which by definition have an effect on bone remodeling.5,102 There is evidence to suggest that tooth movement in patients who are receiving parenteral bisphosphonate therapy may be retarded, with a subsequent risk for osteonecrosis of the jaws.¹¹³

Bartzela and colleagues conducted a systematic review of the literature regarding the effects of medications and dietary supplements on the rate of experimental orthodontic tooth movement.²³ The results of their literature search indicated the following:

The Role of Anchorage in the Orthodontic Movement of Teeth

The term anchorage as it applies to orthodontics refers to the nature and degree of resistance to displacement provided by an anatomic unit (e.g., molars and premolars, temporary skeletal anchorage devices, hard palate) when it is used for the purpose of effecting tooth movement.¹³⁹

The principle of anchorage in the orthodontic movement of teeth is an application of Sir Isaac Newton’s realization that action and reaction are equal and opposite. Restated, this means that, for every force that is directly applied, there is an equal and opposite (reciprocal) force. Depending on how the orthodontic force is applied to a tooth and given the fact that each tooth has a different resistance value to tooth movement (as a result of the nonuniform nature of the teeth and the periodontium), the orthodontist may choose to use certain teeth or groupings of teeth (e.g., ligating molars together as a unit) for “anchorage” when moving other teeth into a more desirable position. Sources other than teeth may also be used for anchorage, including TADs, headgear, a facemask, the hard palate, or the lingual or buccal alveolar supporting bone of the mandible.

Dental anchorage is most typically used to overcome resistance to the orthodontic movement of teeth. To assess the anchorage requirements of the desired tooth movement, the clinician must first evaluate the part of each tooth that is anchored into the alveolar bone. The overall anchorage requirements primarily include the number of roots and the shape, size, and length of each root of the teeth that require movement. This is primarily the sum total of the surface area of the root portions of the teeth that are enveloped by alveolar bone and that are to be moved. A tooth (root) with a large surface area (enveloped by alveolar bone) is more resistant to displacement than one with a smaller surface area. A multi-rooted tooth is more resistant to displacement than a single-rooted tooth. A long-rooted tooth is more difficult to move than a short-rooted tooth. A triangular-shaped root offers greater resistance to movement than a conical-shaped root. Other factors are also important, including the following: (1) the relationship with the contiguous teeth; (2) the ongoing forces of occlusion; (3) the age of the patient and the soft-tissue forces on the teeth (i.e., cheeks, tongue, and lips); and (4) the individual’s unique tissue response. Thus, molars are generally good teeth to use for anchorage, because they have deeper and longer roots, they have a greater root surface area, and they are less susceptible to tipping forces. As stated previously, anchorage may also be increased by ligating multiple teeth together, thereby summing their individual resistance. Anchorage forces may also be provided by the hard palate, the buccal cortical plate of the posterior mandible, the head (headgear), the face (a face mask), or implants into bone (TADs).

Reciprocal Tooth Movement

When planning orthodontic therapy, the clinician must consider not just the tooth movement that is desired but any reciprocal effects that may occur throughout the dental arches as well. Reciprocal effects must be controlled to achieve the desired repositioning of all of the teeth and to establish preferred occlusion. In a completely reciprocal situation, Newton’s theory is easy to understand. When the forces applied to similar teeth and to similar arch segments are equal, there will be an equal force distribution in each PDL with this same degree of movement. A classic clinical case analysis that is frequently used to understand the “reciprocal/nonreciprocal” concept is to consider the type of tooth movement that occurs when an elastomeric chain is placed across a first premolar extraction site in the maxilla. This pits the central incisor, the lateral incisor, and the canine in the anterior segment in opposition to the second premolar and the first molar in the posterior segment. The same total force would be felt by the three anterior teeth and the two posterior teeth as the action of the chain is equal in each segment. However, equal reciprocal movement would require the same total PDL surface area over which the force is distributed. Because the measured PDL area for the two posterior teeth is larger than the three anterior teeth, the movement will not be reciprocal. In this typical, real-life clinical setting, the anterior teeth move back into the bicuspid extraction sites more than the posterior teeth migrate forward. If it is desired that the anterior teeth move entirely into the extraction space, reinforced anchorage will be required. This may be accomplished by incorporating the second molar into the posterior unit. This changes the root surface area ratios, which results in additional retraction of the anterior teeth. Other methods of reinforcing posterior anchorage include the use of extra oral force, such as headgear, TADs, and palatal support through the use of Nance appliances or a transpalatal bar.¹⁴⁸

Cortical Walls as Limiting Factors to Orthodontic Tooth Movement

Cortical bone is more resistant to resorption; therefore, tooth movement is slowed whenever the root to be moved contacts it. This can be problematic, for example, when there is a dense cortical layer of bone formed within the alveolar process of a previously (long-ago) extracted tooth. It can be more difficult to close an “old” extraction site, because tooth movement is slowed to a minimum when the root encounters cortical bone along the resorbing side of the alveolar ridge.118,126 Tooth movement is also greatly slowed and root resorption more likely when a tooth (or tooth root) is forced against a cortical plate (e.g., a labial plate, a lingual plate), such as may occur during attempts to correct incisor inclination.

In theory, the optimal force level for the orthodontic movement of a tooth is the lightest force (and the resulting pressure) needed to produce the near-maximum preferred response.¹⁷⁶ Forces that are more than “optimal”—although effective for the production of tooth movement—are both unnecessary and possibly traumatic to the tooth, the periodontium, the alveolar ridge height, and the cortical plates.

Negative Sequelae of Orthodontic Forces

Mechanical Techniques Used in Orthodontic Force Control

Orthodontic tooth movement is best produced by light continuous force. Orthodontic appliances in clinical practice strive to use light continuous forces that are neither too great nor too variable over time.¹⁵⁶ The use of sound mechanical principles; an understanding of the available options with regard to materials, appliance design, and anchorage control; and the prevention of a rapid decrease in the orthodontic forces are more or less achievable.

The potential response of each tooth to the mechanical forces that can be orthodontically applied (e.g., elastic modules, wires, springs) must be considered by the clinician when he or she is designing appliances (e.g., fixed, removable) and then orchestrating anticipated dental changes. The orthodontist delivers mechanical therapy with the placement of brackets and bands on the teeth; the use of wires that connect teeth together; the use of springs, coils, and elastics; and with dental and non-dental anchorage control. However, this is always done with an understanding of the overall clinical objectives and pitfalls of treatment.

At times, it may be tempting to pursue an improved occlusion by moving teeth outside of the alveolar housing rather than coordinating treatment with either needed orthognathic procedures or with selective extractions to create necessary space40,74,98,124,137,219,235 (see Figs. 5-3 and 5-4). These two biologic treatment pitfalls contribute to many of the everyday frustrations that are experienced in clinical practice.^{31,41,47,60–62,67,132,133,149,168,195,212,214} The orthodontist should remain vigilant when assessing the degree of dental compensation introduced while attempting to correct the occlusion.^{2,24,42,65,82,169,170,194,196,206}

Fixed Appliances Used in Orthodontic Treatment

Edward Angle is considered the father of modern orthodontics (see Chapter 2). His treatment was based on the development of appliances from which all contemporary orthodontics stem.^{15,16,138,228} Current fixed appliances in orthodontics are all variations of Angle’s edgewise appliance system.^36,112,233 By the late 1800s, orthodontic fixed appliances depended on a rigid framework that was positioned on the outside circumference of the teeth.²¹⁴ The teeth were then tied to the rigid framework to move them out to the desired arch form (i.e., expansion) as dictated by the appliance. Angle’s “E” arch was an innovation on earlier designs. Bands were placed on the molar teeth, and a heavy U-shaped arch wire was attached to the banded molars posteriorly. The arch wire was positioned around the labial side of the individual teeth, which were then ligated to it. The wire was gradually unscrewed at the molar location, which resulted in expansion as the arch perimeter increased. The teeth were forced to expand with the labial wire. Unfortunately, the “E” arch was only capable of tipping the teeth into the new position. In addition, it could not precisely differentially position individual teeth over the dental arch. Angle then began to band each individual tooth (not just the molars), and he refined the connection of each banded tooth to the labial arch wire. This allowed him to move each tooth toward the wire at different rates to round out the arch. This technique required ingenuity and a sophisticated mechanical understanding of the process. Accomplishing these maneuvers had a steep learning curve, and it was not easily grasped by clinicians in practice. Angle’s next appliance innovation modified the tube (band) that was placed around individual teeth to include a vertically positioned rectangular slot. The ribbon arch was made of gold wire, and it remained on the labial side of the arch.⁶³ The gold wire was inserted into the slot (bracket) on each banded tooth and held in place with a pin. This improved the efficiency of the alignment of malpositioned teeth. Unfortunately, these appliances still did not achieve consistent bodily movement of the teeth. Tipping of the teeth continued to be problematic. To overcome the deficiencies of the ribbon arch, Angle reoriented the slot (i.e., the shape and location of the bracket on the labial side of each banded tooth) from vertical to horizontal and then inserted a rectangular labial arch wire. He also coined the term edgewise. The edgewise appliance allowed for the improved control of crown and root positioning in all three planes of space. By the early 1930s, Angle’s edgewise appliance became the mainstay of fixed orthodontic appliance therapy.²²² The rectangular arch wire was tied into a rectangular slot on the labial side of each tooth with additional wire ligatures; pins were no longer used. This further improved the control of the root position (i.e., it limited tipping) during the movement of the teeth. Eyelets were added to the corners of the bands on each tooth, which could be tied into the labial arch wire as needed for rotation control. Angle was the first clinician to place attachments on each individual tooth to achieve the precise repositioning of each tooth toward a labial arch wire.

Interestingly, Angle’s edgewise appliance system was also able to provide the level of control of root position that was necessary during the process of tooth movement to close an extraction site. If Angle had known that his appliance would be used in this way, he likely would have prevented its development. Charles Tweed, who was initially an Angle disciple, adapted the edgewise appliance for this purpose.²¹⁴ He did so after he observed significant relapse in many of his own non-extraction (i.e., arch expansion) cases. After he made a decision in favor of extraction, he was able to move the teeth bodily into the bicuspid extraction site with molar anchorage control. The teeth were then able to be retracted into the extraction sites with closing loops, one at a time (canines first, incisors second). In Australia, Begg adopted the ribbon arch appliance (rather than the rectangular wire) to close extraction sites.²²³ He felt that it provided better control of the root position. He first tipped the anterior teeth with light continuous forces and then used torquing and uprighting auxiliaries to complete tooth alignment. When this was coupled with the detorquing of the incisors, root resorption sometimes occurred.

Orthodontic appliances were further modified to allow for rotational control without the need for additional ligatures.11–14,54,107,108,111,190,191 This was accomplished by using either twin brackets or extension wings with single brackets that could contact the underside of the arch wire. Further modifications included the alteration of the bracket slot dimension, self-ligating brackets, and the addition of lingual appliances. With improvements in dental materials, bonding attachments (i.e., brackets) that were applied directly to the tooth surface (rather than the banding of teeth) became popular. Bonded attachments have no interproximal component and therefore require no separation of teeth, and they are less painful for the patient. They are more efficient for the clinician to put on and to remove, and they are more aesthetic because they are less visible to the casual observer at conversational distance. Clear and tooth-colored appliances have also been developed. An additional advantage to direct bonded brackets and tubes is improved access for oral hygiene and periodontal maintenance.

Contemporary edgewise appliances still remain an integral component of orthodontic treatment.⁹⁹ Brackets or tubes are customized for each individual tooth, thereby allowing for the minimum number of bends in the arch wire that are necessary to produce an ideal arrangement of the teeth. This “straight wire” approach requires some bending of the wire to allow for a passive fit of the arch wire when the teeth are ideally aligned.

Auxiliary attachments are components of the edgewise appliance system and include the following: (1) headgear tubes; (2) auxiliary edgewise tubes; (3) auxiliary labial hooks; and (4) lingual arch attachments. Headgear tubes can accept heavy auxiliary labial arch wires. Auxiliary edgewise tubes allow for the application of segmental arch mechanics. They are generally used for the intrusion of teeth, and they are placed gingival to the plane of the main arch wire. Auxiliary labial hooks can be used for intra-arch elastics, but they require meticulous oral hygiene. Lingual arch attachments can also be used for adjunct anchorage control.

Removable Appliances Used in Orthodontic Treatment

The use of removable appliances to guide tooth movement has a long history and persuasive clinical advocates. There have been periods of intense use followed by waning interest. During the late 19th and early 20th centuries, the use of fixed appliances was limited by available materials, the need for labor-intensive “chair time,” and an incomplete mechanical understanding of their potential by most clinicians. Removable orthodontic appliances have several immediate advantages, including that they can be fabricated in a laboratory (thereby reducing “chair time” for both the patient and the dentist) and that they can be removed at socially sensitive times or to make oral hygiene easier. Removable appliances that are used for the movement of teeth have disadvantages, including their dependence on consistent patient compliance. They are difficult to construct, thus making precise complex tooth movements problematic. Even with excellent patient compliance, carrying out complex tooth movement remains an issue.

During the early 1900s, removable upper and lower arch appliances used to produce the desired tooth movement were often made of heavy gold wires as a framework from which lighter gold finger springs were placed. The first molars were banded, and wire ligatures were used to tie in heavy labial and lingual arch wires. Interestingly, the use of removable appliances for the movement of teeth was favored in Europe during the first half of the 20th century. This was primarily for economic reasons, and there was a lack of a strong clinical advocate for fixed appliances on that side of the Atlantic. During the same time frame, Edward Angle’s profound influence on clinicians in the United States encouraged the preferred use of the edgewise fixed appliance system.

Functional appliances represent a second category of removable design (although they may also be fixed). They are constructed primarily with the hope of changing the position of the mandible (i.e., to hold the lower jaw open and to posture it forward) as well as to limit or enhance the eruption of maxillary molars. The history of functional appliances can be traced back to the mid 1800s, when Norman Kingsley introduced the “bite-jump” appliance (1879).¹¹⁴ The “monobloc,” which was developed by Robin in the United States during the early 1900s, was followed by the “activator,” which was developed in Norway by Andresen during the 1920s; both of these devices became widely accepted. Harvold reintroduced removable functional appliances to North American orthodontists during the 1960s, around the same time that the use of fixed appliances finally began to catch on throughout Europe. Harvold’s animal model studies confirmed that mandibular posture did in fact affect maxillomandibular growth^92–97 (see Chapter 4). This led to a belief that functional appliances could be used to “alter back” abnormal jaw growth tendencies. During the 1970s, the Frankel regulator by Rolf Frankel of East Germany popularized the concept of a tissue-borne functional appliance. By the 1980s, William Clark of Scotland introduced the Twin Block, which is a two-piece, tooth-borne appliance. The Twin Block was easier to wear than the Frankel regulator, and it was designed to accomplish the same objectives. Pancherz then popularized the Herbst fixed functional appliance during the 1980s, because it circumvented the problem of patient compliance. This appliance was originally developed by Emil Herbst at the beginning of the 20th century. Proffit has stated that, despite continued strong advocates of functional appliances, “the results of human clinical trials and retrospective analysis have not confirmed sufficient success with the use of functional appliances to make them mainstay treatment.”²¹⁰ Although there is limited evidence-based research to support functional appliances, their continued use in everyday orthodontic practice remains a reality.

Siara-Olds and colleagues recently completed a prospective, randomized, controlled clinical study to assess the early and long-term treatment outcomes of tooth-borne functional appliances.²⁰¹ They tested the use of four different functional appliances in a treatment group, and they also followed a group of untreated control subjects with Class II malocclusions. The treatment group (n = 80) was subdivided evenly into four subgroups in accordance with the type of functional appliance being used: the Bionator (n = 20); the Herbst fixed functional appliance (n = 20); the Twin Block (n = 20); and the mandibular anterior repositioning appliance (n = 20). The functional appliance treatment was carried out on individuals with Class II malocclusions who were between the ages of 8 and 12 years. Each randomized Class II malocclusion subject received comprehensive orthodontic treatment with bands and brackets to complete their treatment after the use of the specific functional appliance. The investigators compared lateral cephalometric radiographs that were taken at standard intervals (i.e., before treatment, after functional appliance therapy, and after the completion of comprehensive orthodontics). The four subgroups were also compared with an untreated control group who had similar Class II malocclusions who were not treated with functional appliances, bands, or brackets. The results of the study confirmed that the use of the functional appliances did not alter long-term mandibular growth patterns as compared with the results seen among the control subjects with Class II malocclusions. Interestingly, the use of a functional appliance in combination with bands and brackets did demonstrate effects, including the following: (1) the restriction of maxillary horizontal growth; (2) the creation of a steeper occlusal plane (with the Herbst and mandibular anterior repositioning appliances); and (3) the steepening of the mandibular plane angle (i.e., clockwise rotation) with the labial version of the mandibular incisors with the Twin Block appliance.²⁰¹ In review, the study by Siara-Olds, et al. confirms that functional appliances for the purpose of correcting micrognathia have negligible impact on the growth potential of the mandible long term.

A third category of removable appliances are used for the retention of the achieved orthodontic results. A spectrum of retention appliance designs are used in everyday orthodontic practice. With regard to the issue of retention, Melsen stated the following: “We [orthodontists] deliver the product, but the final result, as when you buy a car, depends on life-long retention and life-long maintenance. Other than diamonds, everything has to be maintained; true stability only occurs post mortem.”¹⁴²

We can also add Invisalign, which is a product that involves a series of removable clear aligners, as a fourth category to the list of removable orthodontic appliances. It is a method that is used for the purpose of orthodontic tooth movement. Its current level of popularity is at least partially the result of an effective worldwide marketing campaign. Clinical judgment is required to determine when this option is an appropriate form of treatment for the correction of specific—but limited—patterns of malocclusion. Nevertheless, as of 2013, it is estimated that approximately 2 million individuals have completed or are currently receiving Invisalign treatment.

Accelerated Osteogenic Orthodontics

Finding a technique that enhances the speed of orthodontic tooth movement without causing damage has been an objective of clinicians for more than 100 years. Attempts to modify the balance between resorption and deposition and to bypass the waiting time for the alveolar cortex to resorb and thus move the teeth faster without causing irreversible damage to the periodontium has been the focus of much research over the years.28,29,49,78,79,81,90,101,200,217,218,234 Corticotomy, which is the intentional injury of the cortical bone, was described in the mid 1800s as a surgical approach to correct malocclusion. Osteotomies into the cortical alveolar bone were made, and then the teeth were splinted into new positions. This concept received renewed attention when Kole published a series of clinical articles describing a different approach to the treatment of orthodontic patients.^116,117 In 1975, Duker used a dog model to demonstrate rapid segmental alveolar movement after corticotomy.⁶⁸ In 1986, Anholm and colleagues also described corticotomy-facilitated orthodontics in clinical practice.¹⁷ William and Thomas Wilcko further modified the protocol for corticotomy-facilitated orthodontics; they trademarked their technique in 1998.^230,231 That same year, the Wilcko brothers received an endorsement for their technique from the American Academy of Periodontics. Their technique involves elevating a full-thickness mucosal flap off of the alveolar plate (either labial or lingual/palatal) directly over the teeth to be moved. They then place corticotomies (perforations) through the alveolar bone adjacent to the teeth to be moved with the use of a burr on a handpiece. The Wilckos also recommend placing bone graft material over the surgical perforations in the cortical plate to improve alveolar volume. They then replace the mucosal flap into its anatomic location with sutures. They further claim that the technique enhances difficult tooth movement (i.e., molar intrusion) and that it can provide relative anchorage (i.e., a corticotomy limited to the anterior region will provide relative anchorage from the posterior teeth). The theory behind this technique is that injury to the bone (i.e., the cortical perforations) creates a “regional acceleratory phenomenon.”²³⁰As a result, the body rapidly sends cells to the site to heal the injured bone. This surge in cellular healing activity is thought to promote faster tooth movement. The Wilckos called their technique accelerated osteogenic orthodontics.²³⁰

The animal research by Lino and colleagues¹³⁰ and the experimental study completed by Sebaoun and colleagues¹⁹⁷ describe a plausible biologic explanation for the greater speed of tooth movement that is seen with these techniques. Lino’s group used adult beagle dogs (n = 12). The mandibular right and left first premolars were to be moved mesially with the use of a calibrated spring that attached to the mandibular canines. On one side of each mandible, a flap was elevated, and holes were placed in the alveolar bone (corticotomies) to create a regional acceleratory phenomenon. No corticotomy was performed on the opposite (control) side. The PDL on the mesial (compression) side of each premolar was evaluated histologically at weeks 1, 2, 4, and 8. In addition, the amount of mesial bicuspid tooth movement on each side (i.e., the experimental side versus the control side) was compared at the 8-week interval. Histologic comparisons of the two sides showed very distinct differences. At 1 week, bicuspid tooth compression produced a typical hyalinization of the PDL on both sides. “Hyaline formation” within the PDL, which is referred to as a “sterile necrosis,” occurs in response to compressive forces. This is the expected response when significant compressive forces are produced in the PDL. It is known that this sterile necrosis interferes with bone resorption; thus, at the 1-week interval, no tooth movement was experienced on either side. At 2 weeks, hyaline was still present on the control side, so no bone resorption of the tooth socket wall had yet occurred; however, root resorption was evident. At 2 weeks, on the experimental (corticotomy) side, the hyaline had already been removed from the PDL. Presumably, macrophages (the presence of which had been stimulated by the regional acceleratory phenomenon) were in place very early to perform this removal and thus, in the process, no root resorption occurred. At 4 and 8 weeks after the initiation of treatment on the control (non-corticotomy) side, the hyaline had finally been removed and the alveolar bone resorption had begun, thereby allowing mesial bicuspid tooth movement to proceed. During this waiting period, extensive root resorption had occurred on the control side. Interestingly, at the 4- and 8-week post-treatment intervals on the experimental (corticotomy) side, there was no hyaline present in the periodontium, the socket wall had appropriately resorbed, and the bicuspid tooth was able to move mesially without root resorption.

In the rat model, Sebaoun and colleagues studied the alveolar response to corticotomy as a function of time and proximity to the surgical injury.¹⁹⁷ Maxillary buccal and palatal cortical plates were injured (corticotomy) in healthy adult rats (n = 36) adjacent to the upper left first molars. Twenty-four of the animals were euthanized at 3, 7, and 11 weeks after injury.

Histomorphometric analysis was performed to study alveolar spongiosa and periodontal ligament dynamics. Both catabolic activity and anabolic actions were also measured. A separate group of animals (n = 12) were fed with calcein fluorescent stain. After the same surgical and orthodontic maneuvers, they were processed for non-decalcified fluorescent stain histology. The results of the study demonstrated that, at 3 weeks, the surgical (corticotomy) group had significantly less calcified spongiosa bone surface, greater periodontal ligament surface, higher osteoclast number, and greater lamina dura apposition width. The catabolic activity (i.e., the osteoclast count) and the anabolic activity (i.e., the apposition rate) increased by threefold, the calcified spongiosa bone surface decreased by twofold, and the PDL surface increased by twofold in the corticotomy group. Interestingly, the surgical injury to the alveolus (corticotomy) that induced a significant increase in tissue turnover by week 3 dissipated to a steady state by postoperative week 11. In addition, the impact of the injury (rapid tooth movement) was localized to the area immediately adjacent to the decortication injury. The authors concluded that selective alveolar decortication induced increased turnover of alveolar spongiosa and that the activity was localized. The escalation of demineralization–mineralization dynamics is the likely biologic mechanism that explains the rapid tooth movement that occurs after selective alveolar decortication.

In review, it appears that the completion of a corticotomy of the alveolar process immediately brings macrophages into the field of bony injury.¹¹⁶ The macrophages are then able to remove the hyaline (necrotic material) from the periodontium more quickly than occurs in a non-corticotomy setting. The early presence of macrophages can explain the rapid degree of bone resorption and minimal root resorption during orthodontic tooth movement that was documented by Lino and colleagues. When Lino’s group compared the amount of tooth movement in their experimental and control groups, the teeth on the corticotomy side moved twice as fast as the teeth on the control side. The enhanced tooth movement after corticotomy should continue as long as the surgical site healing remains in progress (only approximately 2 to 3 months). The potential for morbidity to the teeth and periodontium that is associated with the surgical procedure, must be considered in light of any advantage of reduced time to reach treatment objectives.

Temporary Skeletal Anchorage in Orthodontics

TADs that are made of titanium and temporarily fixed into alveolar bone have been shown to provide effective supplemental or “maximum” anchorage for the movement of teeth while avoiding unwanted negative reciprocal responses in the dentition.* Retromolar titanium (dental) implants simply for the purpose of temporary skeletal anchorage to facilitate orthodontic movement of the teeth were first placed by Roberts and colleagues in 1989.^185–187 Wehrbein and Merz placed titanium implants directly into the hard palate and then tied the molar teeth to them.^224,225 The purpose of the temporary hard-palate implants was to prevent the movement of the molar teeth that were to be used as direct anchorage for the distal movement of anterior teeth into bicuspid extraction sites. Standard titanium bone fixation plates (i.e., mini-plates) were next used as TADs. The mini-plates were fixed to the zygomatic buttress with titanium screws. The end of the titanium plate then perforated through the oral mucosa to provide a point of attachment for orthodontic wires, springs, coils, and elastics.

Mini-plates and mini-screws made of titanium alloy are becoming a frequent temporary source of skeletal anchorage (i.e., TADs) for orthodontic tooth movement. Advantages of these TADs include being relatively simple to place, providing proximity for orthodontic attachments, allowing for immediate loading, requiring no laboratory work, not being socially embarrassing for the patient to use, and being relatively low in cost. Alternatively, TAD use may be complicated by early “loosening” or injury to the teeth and the periodontium. Depending on their location of placement, TADs may also provide vector challenges for accomplishing the task at hand. Although the clinical role of TADs in everyday orthodontic practice has not yet been fully clarified, there are potential applications for a wide spectrum of purposes, including the following:

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