As an intermaxillary relationship, MIP is simple to perform and provides predictable results, allowing for less extensive treatments by preserving the present occlusal references. Assembly in a semi-adjustable articulator (SAA) and dental treatment in MIP will be indicated when the stomatognathic system is in a physiologic state and a reduced number of teeth need to be restored within a conformative approach. In these cases, it is critical to take care not to inadvertently remove stabilizing contacts from this position during tooth preparation or occlusal adjustment, as well as not to introduce interfering contacts with the patient’s envelope of function due to the risk of triggering functional disturbances with uncoordinated neuromuscular response or dysfunction^27–29.

When treatment planning complex cases in which many teeth require prosthetic preparations involving the occlusal surfaces (full crowns, onlays, overlays, and veneers), the MIP is not the most frequently indicated position precisely because of the difficulty in maintaining occlusal references. Also, when a change in VDO is planned, the MIP cannot be used due to the loss of occlusal references that results from the intentional opening of the mandible in relation to the maxilla to establish the new planned working position.

Centric relation

In this author’s opinion, the best definition of CR is published in the eighth edition of The Glossary of Prosthodontic Terms³⁰. In this work, CR is described as the maxillomandibular relationship in which the condyles articulate against the thinnest and most avascular portion of the respective discs, with the sets in an anterosuperior position against the posterior slopes of the articular eminences, regardless of tooth contacts [Figure 9-02]. According to Okeson¹⁵, CR can be defined orthopedically as a stable musculoskeletal position of a “closed package” between the condyle, disc, and articular fossae, mediated by the jaw elevator muscles during masticatory function and parafunctional activities. The CR will be reproducible and verifiable when the tissue of the TMJs is structurally intact and healthy, in addition to the absence of inferior and superior lateral pterygoid muscle hyperactivity.

[Figure 9-02] Centric relation (CR) is the maxillomandibular relationship in which the condyles articulate against the thinnest and most avascular portion of the respective discs, with the sets in an anterosuperior position against the references in restorative treatments, at different times, by the posterior slopes of the articular eminences. Its registration is independent of the presence of teeth or the position of the occlusal contacts.

Although it has scientific relevance and clinical value, there is no consensus on the definition and position of CR, with controversies in academia and clinical practice^{2,6,7,31–33}. Many concepts were based on beliefs to establish exact and immutable constants and did not consider the variability of biologic systems³⁴.

Amid so many discussions, the essential aspect of CR is its operative utility or therapeutic convenience due to its consistent clinical reproducibility. This makes it possible to use it as a reference position for different dentists³⁴. This position also provides physiologic stability of the condyle-disc complex, minimal activity of the lateral pterygoid muscles, and excellent control of mandibular movements, as it is an orthopedic “limit” position¹⁵.

Adapted centric position

The ACP^8,9 is defined as the maxillomandibular relationship in which the condyles articulate against the posterior slopes of the articular eminences in TMJs that have suffered varying degrees of structural deformation, but which have adapted in order to withstand the applied forces without causing discomfort^8,9[Figure 9-03A–C]. As with CR, this position occurs independently of tooth contacts. A wide variety of intraarticular disorders are due to adaptive responses with structural remodeling, which may be asymptomatic or minimally evident for an extended period. However, they need to have their stability carefully controlled^9,35,36.

[Figure 9-03A–C] The adapted centric position (ACP) may be present in typical clinical situations such as anterior disc displacements with reduction [A,B] and without reduction [C]. This situation may not constitute an impediment to the continuation of the treatment, provided that the dentist is confident about the clinical diagnosis and the stability of the condyle-disc set.

According to Dawson²⁵, the condyles will be in ACP if five criteria can be observed: 1) Condyles comfortably seated in the highest portion of the articular eminences; 2) Medial poles of the condyles supported against the bone surfaces (the disc may be partially interposed); 3) Inferior lateral pterygoids without signs of hyperactivity; 4) Condyle-fossae relationships with controllable stability; 5) TMJs without signs of strain or discomfort when subjected to load testing.

Assessing the pathophysiologic status of the TMJs is critical to maintaining the function, comfort, and longevity of any treatment, as structurally compromised TMJs result in an unstable occlusion. This status should be performed by a combination of history, observation of jaw movement pattern, palpation of the TMJs, palpation of the masticatory muscles, mandibular manipulation, and vertical load testing (more details in Chapter 2)^15,25,37. If necessary, complementary imaging tests, such as computed tomography (CT) or magnetic resonance imaging (MRI), should be requested.

Some temporomandibular disorders (TMDs) have a significant prevalence in the population. In MRI scans, anterior disc displacements were evidenced in 77% to 89% of patients with TMD symptoms, and 30% to 39% of those without symptoms^38,39. Lamot et al⁴⁰ identified anterior disc displacements with or without reduction in 75% of symptomatic patients, and osteoarthritis in about 52% of symptomatic and 10% of asymptomatic patients⁴¹.

The interpretation of these indices proves that a significant percentage of patients who need restorative treatment have already undergone various processes of functional and structural adaptation of the TMJs. Thus, as long as the stomatognathic system is in a physiologic state, there is no report of joint or muscle pain when submitted to the vertical load test, and there are no restrictions on mandibular movements, the patient can be considered in a position to start the restorative treatment. However, according to Dawson⁹, it is essential to recognize that TMJs with deformed tissue may not be as stable as those with healthy condyle-disc sets and should be placed under a permanent monitoring system.

Therapeutic position

The TP can be defined as the unstable and nonreproducible maxillomandibular relationship observed in patients with discomfort or painful symptoms in the TMJs [Figure 9-04A–D]. These manifestations may result from hyperactivity of the lateral pterygoid muscle, disc disorders, trauma, or pathologic changes that interfere with joint function^8,9. In these cases, the TP will be used as a starting point for the treatment, from which pain relief, function restoration, and condyle stabilization in ACP or CR should be sought. Any definitive therapy should be postponed until the proper diagnosis is established, etiologic factors analyzed, and pain or dysfunction properly controlled^8,9,25,42.

[Figure 9-04A–D] As it is an unstable position, the therapeutic position (TP) is the starting point to be used in patients with discomfort or painful symptoms in the TMJs resulting from hyperactivity of the lateral pterygoid muscle [A], disc disorders [B], trauma [C], or pathologic changes [D]. In order to obtain predictability, the rehabilitation should only continue when there is a remission of the symptoms presented and the condition is stable due to the preliminary therapies proposed by the interdisciplinary team.

Many cases are transient and reversible, while others are due to TMJs with active instability processes. In these situations, occlusal devices such as a jig¹⁶, fronto-plateau⁴³, Kois therapeutic deprogrammer⁴⁴, or interocclusal splints²⁶ may be indicated for the differential diagnosis of joint disorders and muscle dysfunctions, or even as a treatment mechanism.

More complex situations can occur when evaluating unstable TMDs. Unlike transient disorders, which can return to normality or adapt to a condition that allows sustainable stability, unstable TMJs are at risk of progressive and uncontrolled deformation, as in cases of avascular necrosis, degenerative joint processes, and rheumatoid arthritis, among others. An experienced interdisciplinary team must address such cases through therapies aimed at reducing pain and inflammation and controlling forces in the TMJs.

Transient and reversible therapies with stabilizing interocclusal splints will help to assess the adaptive capacity of the TMJs and their ability to remain in this adapted position within their physiologic limits⁴⁵. In more severe cases, a repositioning interocclusal splint in an anterior maxillomandibular relationship may be indicated to allow decompression of the retrodiscal region by changing the position of the condyles, enabling tissue adaptation or repair of the affected areas^46,47. This splint should be used for several weeks, 24 hours (h) a day, even while eating, and can be progressively adjusted, according to the remission of symptoms, in search of ACP or, in some cases, CR. Guidelines for a soft diet should be provided to the patient to minimize forces and reduce the compression of vascular and innervated tissue in the retrodiscal region, in addition to the prescription of anti-inflammatory medications.

CR–MIP discrepancy

Another historic debate concerns the three-dimensional (3D) discrepancies between the positions of CR and MIP. Different authors have reported varying percentages of coincidence between these positions, being 12%⁴³, 10%⁴⁸, 8%⁴⁹, 10% to 14%⁵⁰, 15%⁵¹, 22%⁵², and 25%⁵³, depending on the sample studied. The discrepancies reported were from 0.5 to 2 millimeters (mm) in the anteroposterior direction, 1 to 2 mm vertically, and 0.5 mm transversally^50,52–60 [Figure 9-05].

[Figure 9-05] The Posselt diagram⁵⁴, in its sagittal view, represents the positions and movement limitations of the mandible. 1. CR. 2. MIP. 3. Edge-to-edge. 4. Maximal protrusion. 5. Maximal opening. 6. Maximal rotation.

Some studies have demonstrated discrepancies of 0.5 to 2 mm in the anteroposterior direction, and 1 to 2 mm in the vertical direction in the CR position.

The actual clinical significance of the CR–MIP discrepancy as a diagnostic factor needs to be reassessed. In a systematic review of the literature, Jiménez-Silva et al⁶¹ concluded that the available scientific evidence does not confirm a causal relationship between the presence and magnitude of discrepancies between CR–MIP and TMDs. Most individuals have suboptimal occlusal relationships and do not show signs and symptoms of functional or structural disorders. The adaptive response of the patient’s stomatognathic system – TMJs, neuromuscular system, and teeth – needs to be taken into account due to the crucial role played by its constant biologic activity³⁴.

Despite the quest for a single and exact reference, authors report a possible “tolerance zone” of tenths of a millimeter^8,28,62–67 in the CR position. According to Hobo et al⁶⁸, considering CR as a centric point may even seem like scientific naivety, as TMJs have a viscoelastic nature due to the resilience of the articular discs and mandibular ligaments, in addition to the presence of synovial fluid^3,65,69–71.

Furthermore, the condyles are not symmetric structures but present irregular sizes and shapes, modified by a continuous remodeling process resulting from functional or parafunctional activities. They rotate and translate over discs and articular eminences with potentially irregular surfaces, making it practically impossible to determine an actual axis of rotation by positioning the condyles in the articular fossae². Schluger et al⁷² admit that the term “centric” is the most controversial in dentistry, not only from a semantic but also from a conceptual point of view.

Due to neuromuscular responses, the discrepancy between CR and MIP may not have any harmful effect, as long as a structural adaptation has occurred such as occlusal wear and tooth or functional movements. Often, the long phase of growth and development of the individual’s stomatognathic system will have provided time for a gradual and effective adaptation, with no signs or symptoms of pain or dysfunction observed. Eliminating such discrepancies through occlusal adjustment or restorative treatments has proved ineffective and provided questionable stability⁶⁵. The same cannot be said about adding iatrogenic occlusal contacts by restorative treatments. Electromyographic studies have reported that premature contacts introduced in CR were able to generate incoordination of the masticatory muscles^{8,15,17,18,20–24,26}.

Intermaxillary registration

The intermaxillary registration is defined as the registration of any mandible position with the maxilla¹. This author considers this to be one of the most neglected procedures in dentistry. Dentists who carry out the registration in a hasty or inattentive way will need to spend a significant amount of time adjusting the occlusal contacts, mutilating the anatomy of the restorations, or even potentially repeating procedures, which causes frustration for all those involved. Therefore, the dentist should have appropriate knowledge of the critical factors related to this procedure and how to optimize its results.

The accuracy of an intermaxillary registration is influenced by the biologic characteristics of the stomatognathic system^73–76, the technique used^69,77–79, the properties of the material^80–82, and the adaptation between the material and the model^83–87.

Intermaxillary registration in MIP

As explained above, MIP is indicated for patients whose stomatognathic system is in a physiologic state, in which a sufficient number of occlusal contacts will be maintained in the proposed treatment, and where the maintenance of the occlusal relationships will be ensured.

Thus, the stability of the MIP should be investigated clinically and in the dental laboratory. In the mouth, the MIP is tested using a thin articulating tape (Shimstock; Hanel Coltène/Whaledent, Switzerland). The patient is asked to occlude with the tape interposed on the most posterior teeth and on an anterior tooth so as to provide a stabilization tripod. If there is a precise and stable intercuspation, without tilting, no registration material that could interfere with the mandibular closure trajectory or the seating of the models during articulator assembly should be used⁷⁸. Using the registration material over all teeth in the MIP is unnecessary and costly when the contacts of the remaining teeth provide occlusal stability^78,88.

With the absence of several teeth, it is necessary to assess whether the remaining occlusal contacts can provide a stabilization tripod, providing vertical support and horizontal stability that prevents the rotation or slipping of the models⁸⁹. Suppose one or two contacts of this tripod are absent or not conveniently distanced due to the condition or position of the teeth. In that case, the registration material should be used only on the area(s) that require support.

In digital workflows, the intraoral scanner allows the registration of the MIP with precision and speed by using varied tissue references and appropriate algorithms^90,91. It is necessary to ask the patient to bring their teeth into contact with a constant but not excessive force, and to perform the registration scan close to the teeth to be treated in that quadrant, or bilaterally in cases of full arches⁹¹. Repeating this quick procedure once or twice more is recommended to check the accuracy of the registration.

The digital workflow allows for precise work at the occlusal level, as it minimizes the introduction of clinical and laboratory errors inherent to any prostheses such as mandibular deflection when occluding on a material of variable plasticity, distortion of the registration material, adaptation of this material to the model, or the expansion of the plaster when mounting on the articulator. It also reduces the working time, as it is not necessary to wait for the plaster to set in order to assess the accuracy of the registration [Figure 9-06].

[Figure 9-06] The digital workflow for intermaxillary registration can minimize clinical and laboratory errors that generally accumulate in the various steps of analog workflows.

Intermaxillary registration in CR

Several studies have reported concern about the imprecision of intermaxillary registrations in CR and its possible consequences during the assembly of models on the articulator and the preparation of prostheses^92–98. McKee⁹⁹ observed low reproducibility in the registration techniques used by dentists, emphatically recommending teaching and training methods for obtaining reliable registrations in CR.

Intermaxillary registration methods in CR

Initial clinical recommendations

The dentist should previously instruct the patient clearly and briefly about the importance of the procedures to be performed and train them in the technique to be used in order to obtain accurate, reproducible, and consistent contacts. Such guidelines should be provided because patients who have received adequate instructions tend to be more relaxed, confident, and collaborative.

The registration in CR can be performed using mandibular manipulation techniques, performed by the dentist or with the aid of auxiliary intermaxillary registration devices (AIRDs), which provide condylar positioning through the action of the jaw elevator muscles.

Mandibular manipulation with chin support

The technique of mandibular manipulation with chin support¹⁰⁰ is the most common technique for the registration in CR¹⁰¹. However, it can also lead to more significant inaccuracies, mainly due to an inappropriate magnitude or direction of forces on the chin. As with other techniques that require mandibular manipulation, it requires knowledge, experience, and “sensitivity”¹⁰¹ on the part of the professional.

The dentist should be in front of the patient, who is seated or lying down with the head in a neutral position and aligned with the body. The dentist must then support – without force – the thumb and forefinger of their dominant hand on the patient’s chin and make short mandibular opening and closing movements that are consistent and free of muscle tension. Attention is recommended not to force the condyles later, as mandibular reflex movements may prevent correct seating^102,103 [Figure 9-07A,B].

[Figure 9-07A,B] Mandibular manipulation with support on the chin¹⁰⁰ is adequate for registration in CR, as long as the patient’s muscular conditions allow it. The dentist coordinates the opening and closing movements with their index finger and thumb, without the use of force.

Mandibular manipulation with chin and posterior support

The technique of mandibular manipulation with chin and posterior support¹⁹ is a variation of the previously suggested method. The dentist’s index and middle fingers apply a force on the ramus of the mandible in a superior direction to seat the condyles more adequately during intermaxillary registration [Figure 9-08A,B]. In this technique, the dentist’s position is similar to the manipulation technique with chin support. The relevant difference is that an evenly distributed force is applied to the three support points that constitute the stabilization tripod, gently directed superiorly, presumably facilitating the anterosuperior positioning of the condyles in the articular fossae.

[Figure 9-08A,B] The technique of mandibular manipulation with chin and posterior support¹⁹ was proposed to seat the condyles more adequately during the intermaxillary registration. Like the technique explained above, it requires operator training in order to perform it correctly.

Dawson’s bimanual mandibular manipulation

Bimanual mandibular manipulation, a technique recommended by Dawson²⁵, provides accurate and consistent intermaxillary registrations in CR^72,99,104. This, however, requires a learning curve combined with the sensitivity and experience of the professional⁷⁵ [Figure 9-09A,B].

[Figure 9-09A,B] The bimanual mandibular manipulation technique recommended by Dawson²⁵ can produce accurate intermaxillary registrations in CR. However, according to this author’s experience, it demands excellent knowledge, experience, and gentleness on the part of the operator for mandibular manipulation to achieve its purposes.

The dentist should be positioned behind the patient, with the patient lying down and the head aligned with the body. Care is recommended when positioning the fingers on the anterior border of the chin and the inferior border of the mandibular ramus due to possible patient discomfort or pain, which would interfere with the intermaxillary registration process. The jaw should be opened and closed slowly in several short, tension-free arcs of rotation. The force applied to the condyles should be incremental and directed superiorly, not posteriorly. As both of the operator’s hands are used for this manipulation, an assistant should set the registration material over the teeth. As it constitutes a manually stabilized position, this technique, like the previous ones explained, is not indicated for obtaining digital registrations with intraoral scanners [Figure 9-10A–D].

[Figure 9-10A–D] All auxiliary CR intermaxillary registration devices are practical tools for the differential diagnosis of problems in the masticatory muscles or TMJs, in addition to providing accurate registrations in analog and digital workflows. In a simple line of reasoning, the use of these “deprogramming” devices aims to remove the “obstacles,” that is, the premature occlusal contacts and interferences, that may be present and that are preventing the condyles from seating properly in their respective fossae, thus allowing the neuromuscular system the possibility of positioning the mandible independently of the present occlusion.

The manipulation difficulties with this method are related to the correct positioning of the operator’s hands and the appropriate mandibular movement, in addition to the magnitude, moment, and direction of forces applied to seat the condyles in the articular fossae. A harmonious combination of sensitivity¹⁰¹, gentleness, and timing¹⁰⁵ is required when manipulating the mandible, so that the inferior lateral pterygoid muscle is not reflexively activated by applying force at the incorrect time or in the wrong direction. When not performed as recommended by this author, the hyperactivity of the mandibular lateral pterygoid muscles makes the patient tend to resist manipulation or even protrude the jaw through myotatic or protective reflexes¹⁵.

Auxiliary intermaxillary registration devices in CR

Neuromuscular “deprogrammers” have been suggested in the literature to overcome some of the clinical difficulties of mandibular manipulation procedures^{3,16,26,44,106,107}. The primary objectives of these AIRDs for CR would be to avoid occlusal contacts and temporarily suppress dental proprioceptive stimuli that guide the trajectory of mandibular closure. This allows access to the respective condyles of the articular fossae by the action of the mandibular elevator muscles and the relaxation of the mandibular lateral pterygoid muscles^{7,16,22,43,96,106-115}.

However, the need to “deprogram” the neuromuscular system of all patients before performing the intermaxillary registration in CR remains controversial. Although some authors have stated that neuromuscular deprogramming is key to reproducing the intermaxillary registration^{16,106,107,116-119}, others have considered some of the evidence to be wrong⁷. According to Rinchuse and Kandasamy⁷, there is a lack of biologic basis and scientific method in determining which patients require neuromuscular conditioning, its real benefits^98,120, and the time required for its effective action¹²¹. Many questions persist regarding the type of device indicated (previous partial overlay or full overlay deprogrammers), the relevance of the fractions of millimeter differences observed between the various studies, and the stability of the position obtained; in the same way as the differences seen result from effective deprogramming or from an inherent variability of the technique used⁷.

Lucia¹⁶ mentioned the difficulties of mandibular manipulation and suggested using an acrylic resin device on the maxillary incisors to remove the proprioception of the tooth contacts of the other anterior and posterior teeth that could influence the mandibular closure trajectory. That author assumed that if the center of condylar rotation could be “frozen” in a convenient VDO with such a device, CR registration would be performed accurately and consistently. According to Lucia, the time of approximately 20 minutes (min) required for making and adjusting this jig would provide the disruption of the neuromuscular reflex pattern. It would act as training for the patient to acquire a consistent mandibular opening and closing pattern, with the condyles sitting on the fossae.

Calagna et al¹²¹ stated that factors such as emotional tension, TMJ pain, neuromuscular conditioning, mandibular manipulation mode, and registration material would influence the accuracy of intermaxillary registration. Their study concluded that using an interocclusal splint for 24 h led to more consistent results than a jig, Myomonitor (Myotronics Noromed, USA), bilateral occlusal stimulation, and free jaw closure. Williamson et al¹²², based on the work of Jarabak¹²³, concluded that a short period of 5 min using a leaf gauge would be sufficient to remove the aberrant electromyographic patterns presented by the patient.

Roth and Rolfs¹²⁴ stated that “true” CR could only be obtained using an interocclusal repositioning splint, called a craniomandibular orthopedic device, for a prolonged period. Although these authors emphasize that its main indication would be for symptomatic patients and for those with mandibles that are difficult to handle, asymptomatic patients would also benefit from achieving a more stable CR. They recommended that such splints be used 24 h a day, adjusted regularly, and monitored for 3 months.

Donegan et al¹²⁵ and Carr et al¹²⁶, using different assessment technologies – electromyography and 3D electrogastrography, respectively – questioned the use of a leaf gauge for 15 min to “deprogram” the elevator muscles. In that study, using the leaf gauge did not alter the electromyographic activity of the masseter and anterior temporal muscles. It also did not alter the accuracy of the mandibular closing position. According to these authors, the terminology “programmer,” “deprogrammer,” or “reprogrammer” would be ambiguous in a procedure considered “enigmatic” and in an indication considered clinically difficult.

Kinderknecht et al⁹⁸ demonstrated that a time of use of around 30 min of the anterior deprogrammers produced results statistically similar to a period of 12 h. They concluded that the routine use of deprogrammers for occlusal registration in healthy patients did not seem justified. These authors also stated that it is difficult to estimate whether the variations found during the research were due to the biologic individuality of the TMJs, such as their anatomy, viscoelasticity of the articular disc, ligaments, and synovial fluid, or whether they were related to the pattern of muscle activity obtained.

Contin et al¹²⁷ confirmed the findings of Lucia¹⁶, noting that the mandibular position did not change with the time of use of a jig in patients without signs and symptoms of TMDs, but recommended stabilizing interocclusal splints for symptomatic patients. According to these authors, the clinical time for making the jig would be enough for effective deprogramming, and they suggested using a cotton roll between the teeth each time it was necessary to remove the jig from the mouth for adjustments in order to reduce the influence of proprioceptive reflexes during the procedure.

Becker³⁷ concluded that significant muscle relaxation and a reproducible arc of closure in CR could be achieved in 10 min with a jig¹⁶ or leaf gauge¹⁰⁷. That author stated that both the clinician and the patient could feel the benefits of its use, and recommended the routine use of deprogrammers before performing intermaxillary registrations, even to aid mandibular manipulation. Also, according to this author, neuromuscular deprogrammers help to diagnose TMD, causing painful sensitivity in intraarticular disorders, but relaxing the masticatory muscles and improving mandibular dynamics in muscle dysfunctions.

Karl and Foley¹¹⁴ suggested using anterior deprogrammers in patients with difficulties in mandibular manipulation, as they are practical and effective in helping to seat the condyles in the fossae by contracting the mandibular elevator muscles^{16,74,114,117,118}.

Kois¹¹⁵ explained that, as it is impossible to predict each individual’s response to the neuromuscular system, the anterior deprogrammer must be extended for the time necessary until the patient verifies whether there is a consistency in the perception of the first contact point when removing the device. According to that author, this period can vary from a few minutes to several days, depending on the individual’s neuromuscular condition.

Wise²⁹ suggested that a distinction should be made between different degrees of difficulty in mandibular manipulation in the patients examined. For that author, patients with “easy” manipulation could have their CR position recorded using traditional unilateral^19,100 or bilateral²⁵ manipulation techniques, “average” patients through anterior deprogrammers, and “difficult” patients through correctly adjusted stabilizing interocclusal splints²⁹.

Santosa et al⁷¹ demonstrated no significant differences in the 3D displacement of the condyles with the use of a jig or leaf gauge with maximal clamping force compared with the MIP recorded under clamping. According to these authors, the jig and leaf gauge significantly decreased the anterior bundle of the temporal muscle activity with the clamping forces in MIP. The jig was also able to decrease the activity of the posterior bundle of the temporal muscle.

[Figure 9-11A–E] A classic clinical way of making a jig¹⁶. The incisors are isolated with solid Vaseline, and a portion of chemically activated acrylic resin is applied over them. When the exothermic polymerization reaction begins, it must undergo strip movements and be left to cool, and not contract on the incisors. The jig should then be adjusted to contain a single occlusal contact point centered on a flat platform parallel to the mandibular occlusal plane.

The following is a brief description of the main positive and negative aspects of each type of AIRD:

Jig

A jig¹⁶ is a partial overlay device positioned over the maxillary central incisors. It has a flat lingual platform parallel to the mandibular occlusal plane to provide multidirectional freedom to mandibular movements. It can be made directly in the mouth, or indirectly on plaster models, with chemically activated acrylic resin. It can also be digitally designed, milled, or 3D printed.

The clinical time for making and adjusting a jig is 20 to 30 min, for those with training. Prefabricated versions of the device are currently available in various shapes, but adjustments to individual patient characteristics are limited and can take considerable time. The increase in VDO required for making a jig is around 1 mm in the region of the second molars when the objective is to assemble the study models in semi-adjusting articulators (SAAs) for diagnosis or when their height is adequate to establish a preliminary VDO increase in the jig, depending on the treatment plan [Figures 9-12A–F and 9-13].

[Figure 9-12A–F] A jig can also be made indirectly, analogically or digitally, but it must preserve the same principles of shape and adjustment.

[Figure 9-13] A jig¹⁶ allows control of the amount of VDO increase for diagnosis and restorative planning.

After its adjustment, a jig should provide a single, centered contact that allows a stable stop of the mandible at a predetermined VDO. As reported by Lucia¹⁶, by “freezing” this maxillomandibular relationship, the registration was accurate and was facilitated with the selected materials. This author further suggests that the procedure be repeated three times, and that two of them should be identical [Figure 9-14A,B]. According to him, checking the occlusal contacts in the mouth and the models is essential after assembly on an articulator. The above-described characteristics of a jig make it suitable for interocclusal registrations with intraoral scanners [Figure 9-15A–E].

[Figure 9-14A,B] A jig is a highly recommended device within digital workflows as it “freezes” the mandibular position while the intraoral scanner records the maxillomandibular relationship.

[Figure 9-15A–E] A jig can be added to the mock-up for diagnostic purposes and to help determine the maxillomandibular relationship and vertical dimension of occlusion (VDO) when a reorganization of the patient’s occlusal scheme is indicated.

The jig design may vary, depending on the condition and position of the anterior teeth, and its effectiveness will depend on the patient’s collaboration when its use is indicated for a long period of time [Figure 9-16A–C]. Due to the risk of overeruption of teeth without occlusal contacts, it is recommended not to use a jig for more than 16 continuous hours daily. As with the other previously mentioned devices, it is not indicated for cases of TMJ dysfunction or painful symptoms. Its use can also be uncomfortable in patients with deep overbites.

[Figure 9-16A–C] A leaf gauge¹⁰⁶ can be prefabricated or made by the dentist. Its use is simple, involving a sufficient number of plastic strips between the maxillary and mandibular anterior teeth for the disclusion of the posterior teeth.

Wilson and Banerjee¹⁰⁴ stated that the use of a jig associated with non-forced mandibular manipulation on the chin has the potential to increase the consistency of intermaxillary registrations. The author of the present work adopts this practice.

Leaf gauge

A leaf gauge¹⁰⁶ is a device made of flexible plastic strips with a length of approximately 5 centimeters (cm), a width of 1 cm, and a thickness of 0.1 mm. It can be prefabricated (Huffman Dental, USA; Great Lakes Orthodontics, USA) or made by the dentist. As with other previously mentioned devices, it is indicated for cases of muscle dysfunction but not for intraarticular disorders [Figure 9-16A–C].

A leaf gauge is versatile and allows immediate application; however, its use may be restricted, depending on the patient’s overbite and degree of tooth mobility, or even when the anterior teeth are absent. For its use, the patient should be instructed not to occlude with excessive force due to the possibility, mentioned in the literature¹²², that some degree of condylar distalization may occur.

A leaf gauge is contraindicated for patients with a deep overbite or an inclined wear pattern on the incisobuccal surface due to the possibility of a restricted envelope of function. These individuals require the use of a device that provides a certain amount of anterior freedom, such as a jig, fronto-plateau, Kois therapeutic deprogrammer, or even an interocclusal splint. A leaf gauge does not allow measuring the consistency of the contact of the mandibular incisors on their sliding surfaces in the same way as a jig or other AIRDs are able to do this.

A leaf gauge is simple, using a sufficient number of plastic strips between the maxillary and mandibular anterior teeth for the disclusion of the posterior teeth. The number of strips may vary according to the purposes of their use. For mounting on an SAA for diagnostic purposes, an increase of about 1 mm in the interocclusal distance of the second molars is sufficient.

For restorative planning, a sufficient number of strips are used to establish a preliminary increase in the patient’s VDO, according to the treatment goals.

The “relaxation” process of the mandibular lateral pterygoid muscles can be accelerated to a certain extent by asking the patient to perform the following exercise for a few minutes: hold the leaf gauge between the anterior teeth, slide the mandible forward then backward, and contract the jaw elevator muscles as if to occlude in this position, with controlled force. This causes the action of the masseter, temporal, and medial pterygoid muscles to direct the condyles to the CR whenever the TMJs are in a healthy state or are physiologically adapted.

Anterior repositioning occlusal splints

Anterior repositioning occlusal splints are partial overlay devices that have contacts only on the anterior teeth^{15,26,43,131,132}. They can be tooth-supported only, such as a fronto-plateau, or tooth-mucosal–supported, such as a Hawley plate¹³¹, Sved plate¹³², or Kois therapeutic deprogrammer¹¹⁵, among others.

A fronto-plateau is a device that, like a jig, can be made directly in the mouth with acrylic resin, or indirectly, pressed onto the plaster model, milled, or printed. Unlike a jig, which covers only the central incisors, a fronto-plateau covers the entire anterior region, from canine to canine. It has the same indications and limitations as all anterior devices such as the possibility of the extrusion of the posterior teeth. By distributing the forces over the six anterior teeth instead of only two (as with a jig), a fronto-plateau provides more stability and comfort when prolonged use of the device is required in patients with masticatory muscle dysfunction [Figure 9-17A,B]. This device can also be made from a 1-mm–thick pressed acetate plate, with the addition of chemically activated acrylic resin in the anterior region to create a flat platform.

[Figure 9-17A,B] A fronto-plateau is a device that can be made directly in the mouth with acrylic resin, or indirectly, pressed onto the plaster model, milled, or printed. It provides more stability and comfort when prolonged use of the device is required in patients with masticatory muscle dysfunction, provided that the occlusal contacts are fitted adequately over it.

Kois therapeutic deprogrammer

A Kois therapeutic deprogrammer^44,115 has the same advantages as the AIRDs already described. In its original configuration, it has a design similar to a Hawley plate¹³¹, with the addition of a flat anterior platform, 3-mm wide, which, after adjustment, allows the disclusion of all teeth except for the mandibular central incisors, and jaw movement in a planned VDO. This device can be made with chemically or thermally activated acrylic resin [Figure 9-18A,B] or be produced through a digital workflow, milled, or printed¹³³ [Figure 9-19A–G]. As it has a more significant extension and support area than a jig, its design provides comfort and allows prolonged use.

[Figure 9-18A,B] A Kois therapeutic deprogrammer^44,115 has a design similar to a Hawley plate, with the addition of a flat anterior platform, 3-mm wide, which, after adjustment, allows the disclusion of all teeth except the mandibular central incisors, and the jaw movement in a planned VDO.

[Figure 9-19A–G] A Kois therapeutic deprogrammer may be designed according to a digital workflow¹³³, following the same fundamental principles that provide effective action [A–D]. From there, it will be milled [E] or printed and fitted in the mouth [F,G]. (Digital workflow by Dr. Florin Cofar, Romania.)

Like any anterior partial coverage device, it is indicated for muscle dysfunctions but not for intraarticular TMJ disorders. Since its manufacture is indirect, it incurs laboratory costs and needs an additional clinical appointment for its installation and adjustments. An additional advantage of this device is the possibility of its use as a mandibular positioning aid during occlusal adjustment procedures, as a jig or fronto-plateau tend to fracture at the final moment of the adjustments. A Kois therapeutic deprogrammer should not be used for more than 16 continuous hours per day due to the risk of overeruption of teeth without occlusal contacts.

Stabilizing interocclusal splint

The stabilizing interocclusal splint^15,25,26,124 is indicated in cases of TMJ disorders, as the posterior occlusal support provided by the device minimizes the overload on the TMJs that occurs with other AIRDs. Another relevant benefit is that the stabilizing interocclusal splint is also effective for muscle dysfunctions by suppressing the occlusal contacts present in the patient and providing a therapeutic occlusal regimen.

The manufacture of this device is usually indirect, demanding laboratory costs and additional clinical time for its installation, adjustments, and follow-up. The effectiveness of a full-coverage interocclusal splint will depend on the quality of its fit and the accuracy of the adjustments of centric occlusal contacts, laterality, and mandibular protrusion. The option of performing it directly on a 1.5- or 2.0-mm–thick acetate plate is a viable, low cost, immediate alternative. It requires training and skill on the part of the professional to add acrylic resin to the required areas and for its full completion [Figure 9-20A–C].

[Figure 9-20A–C] The interocclusal stabilizing splint is indicated in cases of TMJ disorders due to the minimization of the overload provided by its posterior occlusal support. Although it is also effective in masticatory muscle dysfunction, its adjustment is more complex and time consuming than the previously described devices.

The stabilizing interocclusal splint should be used until satisfactory remission of joint and muscular signs and symptoms is achieved, allowing the condyles to reach CR or ACP in a reproducible and comfortable way. Maxillary or mandibular splints are equally effective, and their indication largely depends on the preferences of the dentist or patient. Mandibular splints can be used when the patient needs to use them during the day, as they interfere less with esthetics and phonation.

The splint can be segmented so that only its anterior part is used as an indexing factor to accurately perform the intermaxillary registrations. Splint segmentation is not necessary because the device software uses reference points on the buccal surface of the teeth and buccal soft tissue to perform intermaxillary registrations with intraoral scanners.

The influence of the patient’s posture

The interaction between the patient’s body posture and occlusal contacts is an old and controversial concern. Brodie¹³⁴ stated that the mandibular rest position was determined by the balance between the masticatory and cervical muscles. McLean et al¹³⁵ observed no change in occlusal contacts in MIP during voluntary mandibular closure in terms of different patient postures, probably due to the action of neuromuscular and periodontal proprioceptors. On the other hand, Mohl¹³⁶, in accordance with the work of Posselt¹³⁷, suggested that the change in head posture can alter the closure trajectory from the mandibular rest position, but not the MIP.

Tripodakis et al⁸⁶ found MIP, on average, 0.8 mm anterior to CR. These authors stated that they did not observe location discrepancies or differences in the reproducibility of MIP and CR that were related to the patient’s posture. They concluded that the condylar position depends on the mandibular manipulation method and state of hyperactivity of the masticatory muscles, which is capable of eliminating the effect of body posture variation.

Visscher et al¹³⁸ concluded that the relationship between head posture and mandibular dynamics is linked to the contraction or stretching of the head and neck muscles and ligaments that act on the TMJs, in addition to the effect of gravity on the hard and soft tissue. According to these authors, the alterations found were of a small magnitude and had no clinical relevance.

According to van’t Spijker et al¹³⁹, it is unclear whether intermaxillary registration for extensive oral rehabilitation should be performed with the patient sitting or lying down. However, static and dynamic occlusion should be evaluated and adjusted in both positions.

Given the lack of consensus in the literature, intermaxillary registration can be performed with the dental chair in an upright or reclined position. The fundamental factor is that the patient’s head is aligned with the body axis, with adequate occipital support, avoiding neck hyperextension, which can generate mandibular retrusion by the contraction of the jaw depressor muscles.

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