The Contour Appliance Revisited: Evolution, Advancements, and Applications in Orthodontics

Orthodontic treatment has witnessed numerous developments and innovations over the years, aimed at enhancing patient comfort, reducing treatment duration, and improving treatment outcomes. Among these, the contour appliance has emerged as a versatile and effective tool for various dental applications, primarily in orthodontics. The contour appliance, a pressure-formed thermoplastic dental device, is fabricated using a plaster cast of a patient's dental arch. Primarily utilized for orthodontic applications, the contour appliance's inner surface mirrors the precise impression of the complete dental arch, earning it the moniker "the tray." The tray forms the foundation for various tooth-aligning systems, including the Essix and Invisalign systems. Orthodontic movement of teeth is facilitated by generating a series of contour appliances that correspond to consecutively modified models, reflecting incremental changes in the malposed teeth's position. This is achieved by excising malposed teeth from the plaster model, repositioning them, and securing them with sticky wax. The contour appliance is highly regarded for its invisibility and ease of use. A properly designed tray, fabricated with an appropriate thermoplastic material, can generate adequate and controlled forces to facilitate most orthodontic procedures.

HISTORICAL PERSPECTIVE ON THE CONTOUR APPLIANCE

Pressure forming of calendered thermoplastics was initially developed by the packaging industry in the late 1950s [1,2]. In 1957, the author (HIN) began experimenting with this technology and, subsequently, developed the contour appliance for various dental applications in 1958 [3]. The chosen plastic and appliance design were contingent upon the specific dental application. Available thermoplastics at the time included acetates, butyrates, styrene, high impact styrene, and polyethylene, offered in clear or colored rolls. The food packaging industry's utilization of these thermoplastics suggested their non-toxic nature.

An accurate impression of the dental arch, encompassing the surrounding supporting structures, was essential for plaster cast creation, on which the appliance was designed and fabricated. The contour appliance has been employed for diverse applications, such as orthodontic tooth moving appliances and retainers represents a collection of commercial thermoplastic appliances that are based on the contour appliance). Additionally, other applications include periodontal splints, temporary bridges, surgical packs, and cosmetic covers for malformed or discolored

teeth (Fig. 1(a, b)) [4,5,6]. Each application necessitated specific appliance modifications. For instance, discolored teeth required space for a veneer, which was achieved by implementing wax corrections on the model before pressure forming, followed by painting tooth-colored, self-curing acrylic on the tray's inner surface. Thermoplastics have also been employed for bite blocks and mandibular repositioning appliances. Corrections for these purposes were performed on study models prior to final appliance construction. A modified technique for mandibular repositioning has been employed in pulmonary medicine for sleep apnea treatment [7].

The availability of appropriate equipment was crucial for fabricating these dental appliances. Tronomatic Engineering Company of New York, a packaging equipment manufacturer, developed a sample-making machine replica featuring a 1/2 H.P. motor and a vacuum reservoir tank. This machine utilized the drape-forming method, capable of heating the thermoplastic to its thermoforming temperature and instantaneously forming an appliance on a plaster cast without causing damage or dislodging teeth held in place by wax (Fig. 2). This was possible due to the time lag required for the wax to soften. Vacuum pressures of up to 28 inches of mercury

(equivalent to nearly -15 PSI) were achieved using this equipment. Vacuum forming, an atmospheric-air pressure forming technique,

was preferred over compressed air, owing to its simplicity, cost-effectiveness, and adequacy for the task at hand. Vacuum cleaner motors, generating -2 PSI to -4 PSI, were deemed insufficient, while compressed air up to 100 PSI was considered excessive.

SPECIALIZED APPLICATIONS IN ORTHODONTICS

The concept of excising malposed teeth from a dental cast, repositioning them on the cast, and then applying force to correct their positions likely predates 1900. However, suitable materials for implementing this change were unavailable until 1926 when Remensnyder introduced minor tooth movement using a gum massaging appliance, rejuvenating the concept [8]. The fusion of this idea with thermoplastics usage eventually materialized [3].

The Kesling Positioner, a short-term (90 days) finishing appliance made of black vulcanized rubber, is often cited as a predecessor of thermoplastic appliances [9, 10]. Designed to close interdental spaces after debanding patients treated with the Begg appliance and make minor interarch occlusal corrections, the Positioner covered both dental arches and was activated upon

the patient biting into the rubber (Fig. 3). The appliance was to be worn four hours a day and at night. Patients were unable to speak, eat, or breathe comfortably while wearing the positioner, necessitating the drilling of holes for air passage into the oropharyngeal space. An acrylic and wire retainer was subsequently fabricated following this treatment phase.

The contour orthodontic appliance was conceived with the understanding that a new tray would be formed at each visit. Teeth requiring straightening were numbered, and the laboratory technician excised them from a duplicate plaster cast. The teeth were then repositioned on the plaster cast using wax. The orthodontist later repositioned the teeth in corrective increments (Fig. 4). Prior to treatment initiation, a template was created from acrylic, impression plaster, or hard wax. This template functioned similarly to the acrylic plate on a Hawley retainer, guiding the retraction of maxillary teeth.

The teeth were then held in their new positions with base plate wax or sticky wax, and a new thermoplastic appliance was fabricated by the chair-side assistant or laboratory technician during the patient's visit. The same work model was used from start to finish, ensuring

consistency throughout the treatment process (Fig. 5(a,b)). The tray was carefully trimmed near the mucobuccal fold to avoid impinging on the musculature or engaging deep undercuts that would make it difficult to remove the tray from the plaster model. The tray was cleaned with a mild alkaline solution to remove any residual monomer or wax, and the orthodontist was ready to insert the appliance and check the occlusion. This method proved to be very cost-effective, as it eliminated the need for multiple appliances and

reduced the overall treatment time.

Tooth movement occurred as a result of the pressure applied by the corrected appliance to the malposed teeth. The thermoplastic's resilience allowed it to snap over the teeth and mucosa, exerting pressure until the teeth attained their predetermined positions. This type of appliance was particularly effective for dental arch contraction but required more appliances for expansion.

A patient with a relatively simple problem is presented to illustrate these principles. The patient had a lower lateral incisor crowded out of the dental arch and in crossbite with the maxillary lateral incisor. The lower incisor was slenderized with a rotary disc on the mesial and distal surfaces to facilitate its movement into the dental arch. Treatment time was two months. The appliance was extended a convenient distance past the gingiva over the mucosa in a straight line for maximum control and power (Fig. 6(a-d)). An upper lingual

arch wire was inserted to control the maxillary incisors during treatment.

When the required correction was too great to perform in one step, adjustments were made in increments. Partial and progressive adjustments were made by moving the teeth gradually and holding them in place with wax. A new appliance was made for each step during the patient's scheduled visit. When the teeth were properly aligned, the final appliance was used as a retainer, ensuring that the newly positioned teeth would remain in their corrected positions.

The periphery of the tray was trimmed according to the specific orthodontic use. However, for orthodontics, it is important to emphasize that the tray should not be festooned to follow the margins of the gingiva around the teeth.

Thermoplastic appliances often fail because they are festooned, which leads to a significant loss of power and stability. Any intended

couple is robbed of the force from the gingival vector when the plastic is trimmed at its gingival aspect.

Moreover, a festooned tray proves to be unhygienic, requiring more appliances to achieve the desired tooth movement. In contrast, the contour appliance creates a powerful couple with its center of gravity in the crown of the tooth, providing better control and stability. A metal appliance, such as an edgewise appliance, has its center of rotation outside of the tooth in the center of the channel that holds the wire. This external center of rotation is a major disadvantage when it comes to uprighting a tooth.

Consequently, the festooned system is not typically done in-house and is usually sent to a commercial laboratory, making it more

expensive and time-consuming. Emphasizing these drawbacks is essential in understanding the limitations of festooned aligners. A properly fabricated appliance for both dental arches, which takes into account these considerations, can be seen in Figure 7.

The contour appliance can be readily adapted for inter-maxillary elastics or extra-oral force systems. For instance, hooks or buttons may be welded to the thermoplastic or placed directly on the teeth for intermaxillary force. Additionally, molar bands with tubes for an extraoral appliance may be cemented to the first molars before thermoforming (Fig. 8). However, the contour appliance was primarily intended for intra-maxillary corrections when used without adjuncts. While it is especially well-suited for handling Class I crowded extraction cases, it is essential to consider other more efficient systems for Class II or Class III patients.

THE PHYSICS

In the realm of orthodontics, a pivotal capability of an orthodontic appliance is its capacity to alter the axial inclination of a tooth in labiolingual or buccolingual directions, a property referred

to as torque. A crucial aspect of achieving torque is the appliance's ability to generate a couple and maintain stability (Fig. 9 (a, b, c)). A contour appliance possesses these attributes, despite reports suggesting the inefficacy and unreliability of thermoplastic appliances in correcting axial inclination [11,17]. It is crucial to note that such studies

focused on aligners, which are designed to follow the gingival margins of teeth. When shortened, the gingival vector of a couple loses significant power, but torque remains achievable if the tray is not festooned [4,8, 9,11,12,13]. Although thermoplastics may require a longer duration to achieve results, their simplicity renders them indispensable in orthodontics.

In the context of Newtonian Mechanics, a couple is characterized as two equal, non co-linear forces acting oppositely on a free body, generating a moment equal to the sum of the individual moments (Fig. 9a). The moment of force (M=FxD) represents the rotational

impact on a body when a force is applied, with F representing the force and D the perpendicular distance from the force vector to the rotation center. When two unequal, non co-linear forces act oppositely on a free body, translation occurs in addition to rotation (Fig. 9b), with the magnitudes dependent on force disparities and locations (Fig. 9c). A judicious combination of net force and net moment facilitates simultaneous controlled translation and rotation.

The way in which a couple is generated by a thermoplastic appliance, such as the Nahoum dental contour appliance, is significantly different from the process by which a couple is generated by a fixed metal appliance. The thermoplastic material applies force directly to the crown of the tooth on both sides—labial and lingual or mesial and distal, depending on the required tooth movement (Fig. 10). The resulting moment is then transmitted through the crown of the tooth to the root, with the center of rotation located within the

crown. The thermoplastic appliance is created by forming it over a model that includes the necessary corrections. Due to the elasticity

of the plastic material, the appliance can comfortably fit over the entire dental arch. Force is selectively applied to the teeth targeted for movement, with contact on one side of the tooth and space on the opposite side, allowing the tooth to move into its predetermined position.

In contrast to thermoplastic appliances, generating couples with fixed metal appliances, such as edgewise brackets, involves a different process. When a tooth is fitted with an edgewise bracket that is cemented to the crown and firmly ligated to an edgewise wire containing torsion or tip back, a couple is created within the bracket channel, with the moment resulting in a center of rotation within the channel itself (Fig. 11(a, b)). This assembly shifts the natural center of gravity or center of rotation of the tooth, largely determining the location of the center of rotation for the given assembly. The force is then distributed along the tooth's root. The force within the bracket can be as much as 4,000 Gm, and the calculated force 10mm away on the root is only about 100 Gm. This entire process can be quite complex and confusing, especially when considering the three-dimensional nature of orthodontic tooth movement [14].

It is imperative to acknowledge that Newtonian Physics pertain to free bodies, whereas teeth in vivo, embedded in bony sockets and suspended by a fibrous periodontal ligament, are not free bodies. Teeth in vivo possess an interface combining dynamics principles on one side and a blend of hydraulics and deformable media principles on the other. Partial differential equations expressing the mathematics of deformable media may be unsolvable [14], and the center of rotation and resistance of a tooth remain unknown. Consequently, moments generated when applying a known force to a tooth cannot be determined, with finite element analysis offering limited utility in certain biological functions [15].

The significance of time, frequently overlooked, is essential: a force vector necessitates a time measurement to exist. When time equals zero (0), the vector exists only in conceptual models. The removability of trays or aligners allows for patient non-compliance, impacting the prescribed duration of appliance use.

It is also important to recognize that there are no accurate sensors capable of detecting the force applied by an enveloping thermoplastic appliance on a tooth. Since it is impossible to calculate the forces applied to the teeth with thermoplastic appliances, and because we cannot precisely predict how cells will respond to external pressures, the most prudent method of treatment involves periodic observations by the orthodontist. The observed changes will then dictate the next adjustment in the orthodontic treatment plan.

THE BIOLOGY

The biological process behind orthodontic tooth movement is intricate and involves sophisticated cellular communication and feedback mechanisms that are not yet fully understood. Teeth and their supporting structures are naturally designed to withstand masticatory forces. However, it is established that certain forces, depending on their characteristics, can induce tooth movement. Factors such as force location, magnitude, activation duration, and the patient's biological state influence these orthodontic forces. While force application on teeth is well comprehended, the tissue's biological response remains enigmatic, involving the transformation of a physical force into a complex, unpredictable molecular event.

Upon application of an orthodontic force, hypoxia occurs within the periodontal ligament to a certain extent. The force's magnitude determines whether this hypoxia leads to a mild inflammatory response or severe necrosis of the periodontal tissues. The functions of inflammatory cells diffusing into the periodontal space are gradually being unravelled. Research is ongoing to understand cellular communication and regulation of bone remodelling during orthodontic tooth movement. Additionally, studies are investigating the influence of hormones, vitamins, the piezoelectric effect, and medications on tooth movement. A significant factor, the patient's genetic code, or Genetic Orchestrated Directions (GOD), remains largely unexplored. This factor includes the patient's biological age and growth potential.

Tissue remodelling in response to orthodontic tooth movement is crucial and can be facilitated by thermoplastic appliances extending beyond the teeth. The force delivered by thermoplastic appliances is generally gentler and more distributed over a larger area compared to metal appliances. This promotes tissue oxygenation, which in turn favors tissue repair and bone formation. A well-designed thermoplastic appliance aids in maintaining proper oral hygiene, helping to prevent inflamed, overgrown, fibrous gingiva, which can impede tooth movement and appropriate remodelling [15,16].

The remodelling process primarily involves new bone formation, altering the alveolar process. The periodontal ligament, being highly vascular, transmits force to the alveolar bone, serving as an interspace separating biological processes, with gingiva and its supporting structures on one side of the membrane and alveolar bone and its supporting structures on the other. Alveolar bone forms in response to tooth eruption and is contiguous with the maxilla or mandible. It is highly sensitive and readily responds to orthodontic forces and other functional demands.

Root resorption, when present, is likely related to cementum, an acellular structure lining tooth roots, unassociated with issues within the periodontal ligament. Orthodontics has been implicated in inflammatory root resorption. Regrettably, early detection of iatrogenic root resorption with 2-dimensional radiographs is challenging, and while 3-dimensional cone-beam computed tomography offers improved accuracy, it involves significantly higher radiation exposure. Nevertheless, root resorption is a valid concern warranting close monitoring. In time, due to different force distribution, thermoplastic appliances may prove less prone to root resorption than their metal counterparts [16,17,18,19].

AUTOMATION

In 1958, with the advent of thermoplastic contour appliances for orthodontics, computer-aided design (CAD), three-dimensional (3D) printing, and Artificial Intelligence (AI) were either in their nascent stages or entirely unknown. By 1997, automation in the form of computer graphics emerged in the orthodontic field, alongside the development of highly efficient and compact thermoforming machines and advancements in thermoplastic materials. Today, most dental offices are equipped to produce thermoplastic trays in-house. This technological progression raises several pressing questions: Can a computer algorithm substitute the discernment of a trained orthodontist when treating a patient? Is it possible for a computer to predict a specific tooth's response based on central tendency measures (a feat even an orthodontist cannot achieve)? Can issues arising between visits, such as pain, loose teeth, occlusal interference, periodontal recession, root resorption, habits, ankylosis, etc., be entrusted to a computer or a commercial enterprise? Should clinicians create a new, adjusted appliance during each visit or rely on prefabricated aligners? The most prudent treatment approach appears to involve the orthodontist conducting meticulous periodic observations, recognizing that the observed changes dictate subsequent adjustments. This decision must be made by the clinician for each patient individually.

SUMMARY

The introduction of thermoplastics in dentistry in 1958 marked the beginning of an entirely novel modality, with its primary appeal in cosmetic value and simplicity. The concept has since evolved rapidly, incorporating new methods and materials, more efficient forming machines, and cautious attempts at automation. The contour appliance boasts numerous advantages, including patient compliance, comfort, convenience, and ease of oral hygiene maintenance. It also remains under the operator's complete control and is cost-efficient. For orthodontic patients, a new appliance is crafted on an adjusted model during each visit, contingent upon the patient's progress. The thermoplastic extends beyond the teeth without interfering with the functioning musculature, akin to a full denture, maximizing stability and controlled orthodontic force.

The concept of the periodontal ligament as an interspace is introduced. A mechanical force applied to a tooth results in compression or stretching of the tissues between the tooth and the ligament. The periodontal ligament processes this force, converting the mechanical stimulus into a biological response, allowing tooth movement within the alveolar process. The orthodontic mechanical force alone does not propel the tooth through the bone.

CONCLUSION

In conclusion, the contour appliance represents a versatile, cost-effective, and efficient solution for various orthodontic procedures. Leveraging the pressure-forming technique and utilizing a range of thermoplastic materials, these appliances can be customized to meet specific requirements. By understanding the intricacies of appliance design and execution, dental practitioners can deliver improved outcomes for their patients. The history and development of the contour appliance and its continued adaptation in dental practice demonstrate the value and potential of this innovative approach to orthodontic treatment.

REFERENCES          

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6.  Hirshfeld L, Geiger A. Minor tooth movement in general practice, ed 2, St. Louis: Mosby, 1966.

7. Marklund M, Steinlund H, Franklin K A, Mandibular advancement devices in 630 men and women with obstructive sleep apnea and snoring. Chest 2884; 125:1270-8.

8. Remensnyder, O.  A gum-massaging appliance in the treatment of pyorrhea. Dent Cosmos 1926; 28:381-384.

9. Kesling H. The philosophy of the tooth positioning appliance.  Am J Orthod 1945; 31:297-304.

10. Nahoum HI. Forces and moments generated by removable thermoplastic aligners. Am J Orthod Dentofacial Orthop. 2014 Nov;146(5):545-6.

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12. Proffit WR, Fields HW.  The biologic basis of orthodontic therapy, In; Rudolph P. editor. Contemporary orthodontics.  3rd ed. St Louis: Mosby 2000 p. 296-325.

13. Simon M, Keilig L, Schwartz J, Jung BA, Bourauel C,. Forces and moments generated by removable thermoplastic aligners: Incisor torque, premolar derogation, and molar distalization. Am J Orthod Dentofacial Orthop. 2014; 145: 728-35.

14.  Bhatia AB.  Mechanics of deformable media. East Sussex, United Kingdom: Taylor and Francis; 1986.

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FIGURE LEGENDS

Figure 1 (a,b). (a) A patient with dysplastic teeth, (b) Labial view of thermoplastic with tooth colored acrylic ‘paint’. The difference in intensity of the color is obtained by the thickness or layers of ‘paint’.

Figure 2. The vacuum forming process.

Figure 3. The Kesling Positioner (labial view).

Figure 4. Teeth are repositioned on the cast with pink baseplate wax.

Figure 5 (a,b). (a) A clear plastic appliance made on a cast where the maxillary incisors were repositioned and held in place with wax. (b) Note that a thin film of molten wax was spread around the teeth.

Figure 6 (a-d). A simple correction with a thermoplastic appliance. (a). The mandibular right lateral incisor is in cross-bite and was slenderized with a carborundum disc on the mesial and distal surfaces, (b) occlusal view of the cast, (c) The incisor was repositioned and held in place with wax. A clear plastic appliance was made on the altered model, (d) View of the appliance in place after 2 months. The appliance was not scalloped.

Figure 7. A Finished case. Note that the appliances are not festooned.

Figure 8. A clear plastic appliance that was made on a model with bands on the maxillary first molars. A facebow, for extra oral force, is inserted into the buccal tubes of the bands through an opening in the plastic.

Figure 9. (a, b, c,). Couples and adjustments for translation. (a) Pure rotation, (b) unequal forces resulting in translation with no rotation, (c) A couple with a greater third force resulting in translation, no rotation.

Figure 10. A couple in a thermoplastic envelope. Note the space for the tooth to move into. The center of rotation is in the crown.

Figure 11. (a, b). (a) cross section of an edgewise bracket bonded to an incisor and an activated wire, (b) when the wire is inserted in the bracket, the force within the bracket is enormous compared with the force at the root, 10 mm. away or at the apex. The center of rotation is in the center of the wire.