Interocclusal splints

Interocclusal splints can be divided into types according to their indication:

Pretreatment or diagnostic interocclusal splints: These are used in the preliminary phase of treatment, in a temporary, conservative, and reversible way, to investigate the influence of existing occlusal factors on the health status of the temporomandibular joints (TMJs) and neuromuscular system. Their purpose in this case is to provide an occlusal situation that allows the condyles to assume a more orthopedically stable position within the articular fossae, with minimal lateral pterygoid muscle activity.

Diagnostic interocclusal splints can be used to simulate occlusal relationships intended for the planned restorative treatment. Thus, the neuromuscular responses and degree of condylar repositioning are evaluated through an optimized arrangement of occlusal forces and individualized adjustments. The distribution of forces throughout the splint has the potential to control the magnitude and direction of forces on the TMJs, allowing the repair, remodeling, and adaptation of joint structures²³.

It is recommended to use interocclusal splints during pretreatment until the painful symptoms have subsided, the function is comfortable, and the distribution and position of the contacts prove stable in the control appointments, which may require several months in severe cases. In such situations, pretreatment splints can be used for up to 24 hours (h) a day – except when the patient is eating or the splint is removed for cleaning – for a few weeks.

Diagnostic interocclusal splints are also used to determine the characteristics of the patient’s envelope of function and parafunction through the analysis of the wear patterns visualized on it that may interfere with the planning of the new occlusal scheme and selection of the restorative material.

Posttreatment or stabilization interocclusal splints: These are recommended based on the analysis of the anamnesis, clinical examination, and tests that prove significant parafunctional activity. Stabilizing interocclusal splints are not intended to prevent or cure bruxism but rather to redistribute the forces affecting the teeth and restorations^24,25. Another indication of these splints is to complement some occlusal requirements that cannot be fully satisfied in the treatment such as the absence of occlusal contacts in a patient with Angle Class II occlusion or in the case of a patient with an open bite.

Ideal occlusal requirements for an interocclusal splint

The interocclusal splint generally used by this author is the Michigan stabilizing splint^6,26. This is a rigid and smooth maxillary splint, generally fabricated from clear acrylic resin and made according to the following specifications [Figures 14-01A–D, 14-02A,B, and 14-03A,B]:

[Figure 14-01A–D] The Michigan stabilizing splint covers all the teeth and a small part of the palatal tissue.

[Figure 14-02A,B] In the Michigan stabilizing splint, uniform contacts of equal intensity of all the teeth against the smooth surface of the interocclusal splint in centric relation (CR), adapted centric position (ACP), or maximal intercuspal position (MIP) occur. The canines should carry the lateral and protrusive functional guidance, adding a certain “freedom in centric.”

[Figure 14-03A,B] Protrusive functional guidance can be performed symmetrically by the central and lateral incisors.

1. Uniform contacts of equal intensity of all the teeth against the smooth surface of the interocclusal splint in centric relation (CR), adapted centric position (ACP), and/or maximal intercuspal position (MIP). In addition to the absence of deflective contacts, the uniform distribution of the occlusal forces between several teeth minimizes the force on each of them, providing stability for the neuromuscular system and the TMJs.

2. Functional guidance with smooth and sufficiently inclined angles to provide disclusion of the posterior teeth during lateral and protrusive movements. According to Klineberg and Eckert²⁷, gentle guidance inclination generates less force on the neuromuscular system and requires less energy expenditure for mandibular movement. The canines should carry functional lateral and protrusive guidance, adding a certain “freedom in centric”⁶. The incisors can assist in protrusive movements, depending on the configuration of the dental arches.

3. Posterior teeth without any contacts in lateral and protrusive movements. These contacts should be avoided because they increase muscle activity^{1–3,28–30} and change the system of forces on the TMJs.

Types of splints – hard versus resilient

In general, hard interocclusal splints control and direct occlusal forces more effectively. Resilient splints, on the other hand, dampen occlusal forces but tend to increase muscle forces. Hard interocclusal splints can be made clinically with chemically activated acrylic resin, resulting in a device with poor physical characteristics due to the incorporation of bubbles and inadequate polymerization. Splints pressed in a dental laboratory with a heat-activated acrylic resin exhibit superior mechanical properties and longevity.

Dentists may prefer resilient interocclusal splints due to their ease of fitting and initial patient comfort. They offer protection against trauma and fatigue of restorative materials but do not allow for the introduction of precise occlusal contacts. Resilient splints increase temporal and masseter muscle activity³¹, and many patients with parafunctional activity report the habit of squeezing them. Okeson³¹ observed greater effectiveness in the remission of TMD symptoms with the use of hard splints, and 50% of individuals who used resilient splints showed increased muscle activity.

As they are made of porous material, resilient splints accumulate bacterial biofilm, acquiring an unpleasant appearance and odor in the short term. Therefore, they are not very durable. Some patients with intraarticular pain report comfort with the use of these splints due to the posterior support they provide. However, patients with muscle dysfunction complain of significant discomfort.

Currently, the development of interocclusal splints using CAD/CAM technology is increasing in clinics and prosthetic laboratories, resulting in some notable advantages. In a digital workflow, splints can be fabricated subtractively through milling or additively through the use of a 3D printer^32,33. The crimping of prefabricated monolithic blocks of polymethyl methacrylate (PMMA), with a high percentage of crosslinks and low porosity, is capable of producing interocclusal splints with clinically acceptable adaptation and retention³³, which is also in line with the clinical experience of this author. This workflow contains a smaller number of laboratory steps that are subject to the introduction of errors during the assembly of plaster models on an articulator and duplication of models for pressing, in addition to the cycle and contraction of the polymerization process of the work.

Berli et al³² evaluated interocclusal splints fabricated through pressing, milling, and printing, and concluded that the flexural strength and hardness of pressing and milling were similar. In contrast, the mechanical properties of the printed splints were impaired by water sorption and degradation of the chemical bonds of the polymeric network resulting from the aging process.

Design of interocclusal splints

Maxillary versus mandibular

The choice between maxillary or mandibular interocclusal splints may be a matter of dentist or patient preference. Maxillary splints are easy to adjust, provide tooth containment, and protect the maxillary anterior teeth, which are more subject to tension or shear stresses. Mandibular splints tend to interfere less during phonation, which makes them suitable for patients who need to wear them during the day; however, they can be structurally fragile or generate harmful forces in the maxillary anterior teeth, causing periodontal problems.

Although the occlusal requirements for adjustment are the same, it should be borne in mind that the location of the functional guidance adjusted in the maxillary interocclusal splint is similar to the adjustment of the guidance in the teeth. In contrast, in mandibular splints, the guidance is located buccally to the centric contact [Figure 14-04A–D]. For either one or the other, the guidance should have the lowest possible inclination capable of discluding the posterior teeth, and contain an area of 0.5 to 1 mm of centric freedom to minimize the functional forces when discluding the posterior teeth.

[Figure 14-04A–D] The mandibular stabilizing interocclusal splint is similar in design to the maxillary one [A,B]. It should also have contacts of equal intensity evenly distributed throughout the splint, but its functional guidance should be located buccally to the centric contact [C,D].

The maxillomandibular relationship should also be considered when deciding between a maxillary or mandibular interocclusal splint. Patients with noticeable overjet, for example, require an increased volume of buccal acrylic on a mandibular splint, which causes significant discomfort. As an additional factor to consider when making this decision, it is suggested that the interocclusal splint be used in the arch that provides excellent stability, and in the one that contains restorations to be protected from functional forces. The arch containing teeth with mobility or an irregular occlusal plane may also benefit from the stabilizing effects of such a device³⁴.

In maxillary interocclusal splints, all the mandibular anterior teeth and only the buccal cusps of the mandibular posterior teeth should contact the device. On mandibular splints, the maxillary canines and maxillary palatal cusps should contact the splint. In these splints, the contact of all the maxillary incisors will depend on the maxillomandibular relationship and overjet. When the overjet is significant, an uncomfortable buccal extension will be necessary.

Manufacturing of interocclusal splints

1. Impression or scanning of models: Correctly manipulated impressions with good-quality alginate provide satisfactory results for the fabrication of interocclusal splints. It is recommended to use hard and perforated trays, as they provide stability and material locking while removing the impression from the mouth. The impression should be disinfected and immediately poured with high-strength type IV plaster.

The maxillary and mandibular arches should be scanned carefully with an intraoral scanner if a digital workflow is adopted.

2. Intermaxillary registration: This is a critical step in the fabrication process of interocclusal splints [Figure 14-05A]. As explained in Chapter 9, intraarticular or muscular problems can impair the accuracy of this registration. In these cases, the intermaxillary registration should be performed in the “closest” position possible to CR, in which the patient does not feel pain or discomfort^38,39. In severe cases of TMDs, if the patient has many symptoms, the models can be mounted in MIP without needing any registration that could bother the patient at this time. With the evolution of the treatment and remission of the symptoms caused by the periodic adjustments of the interocclusal splint, the condyles will be able to settle more comfortably and stably in the articular fossae.

[Figure 14-05A–H] Intermaxillary registration with silicone [A]. Use of the facebow to mount the maxillary model on the semi-adjustable articulator (SAA) [B]. Incisal pin increased on the SAA according to the thickness planned for the device [C]. Mounting the models with care to maintain the accuracy of the occlusal contacts [D,E]. Interocclusal splint previously adjusted by the dental laboratory technician (DLT) on the SAA [F–H].

Another clinically relevant point is to perform the intermaxillary registration in a VDO that considers the thickness of the splint to be constructed. In general, it is recommended that the splint has a thickness of around 1.5 mm in the region of the second molar⁶ to allow the necessary adjustments without loss of resistance⁴⁰ and to provide comfort and ease of adaptation.

Within a digital workflow, an anterior registration device such as a jig⁴¹ or fronto-plateau^42,43 should be used to maintain the static mandibular position during intraoral scanning in the VDO planned according to the thickness of the splint.

3. Mounting the models on a semi-adjustable articulator using a facebow: The mounting of the plaster models with the use of a facebow provides greater precision in the transfer of the mandibular arc of rotation [Figure 14-05B]. However, in this author’s experience, the use of a facebow is not of paramount importance when the intermaxillary registration is performed in the VDO planned for the splint by eliminating the effect of the difference between the mandibular arc of rotation in the mouth and on the articulator.

4. Fabrication of interocclusal splints – laboratory steps: Interocclusal splints can be accurately fabricated through either an analog or a digital laboratory workflow. With the impression of the dental arches and the special plaster working models mounted on an articulator [Figure 14-05A–E], the laboratory steps can proceed with an analog workflow [Figure 14-05F–H]. Depending on the process selected by the dental laboratory technician (DLT), these models could be scanned by a bench scanner, and a digital workflow could produce the splint in the same way that it would be executed from the files received from intraoral scans of the dental arches and intermaxillary registrations performed by intraoral scanners [Figure 14-06A–O].

[Figure 14-06A–I] Determination of the device insertion axis [A]. Demarcation of the splint dimensions: 1/3 to 1/2 of the incisocervical extension of the buccal surface (or about 3 mm) should be covered. Regarding the lingual aspect, it is recommended to cover approximately 5 mm in the palatal region [B]. Carrying out the relief of the retentive areas [C]. A minimum thickness of 1.5 mm is recommended in the region of the second molars of the splint [D–F]. Final check of splint shape, bilateral occlusal contacts, and effective functional guidance [G–I].

[Figure 14-06J–O] Milling of the splint in polymethyl methacrylate (PMMA) [J]. Final appearance of the device after milling and laboratory finishing [K]. Intraoral installation of the splint [L–N]. View of the splint after making the occlusal adjustments [O].

Clinical adjustment of interocclusal splints

The adjustment of maxillary interocclusal splints is explained below, as they are the splints most frequently used by this author. The adjustment of mandibular interocclusal splints should comply with the exact same adjustment requirements and extra care should be taken to eliminate any vibration in the maxillary anterior teeth.

Clinical steps: Evaluate the relationship of the interocclusal splint with the model: the plaster model should be intact, which denotes the care of the DLT. Note the extent of the coating, thickness, retention, stability, and degree to which it has been polished. The general occlusal requirements, such as occlusal contact distribution and effective guidance, can be fulfilled on the SAA.

• Wash and sanitize the splint with disinfecting agents.
• Inform the patient that the interocclusal splint will be installed carefully and that it should be tight, but not so “tight” as to be uncomfortable.
• Insert the splint into the patient’s dental arch. It should sit passively, with good retention and without tipping. Many patients may complain that the splint feels “tight” initially. Avoid performing adjustments to relieve this at this moment, as this initial tension may be dispersed after a few minutes. If the patient continues to report discomfort, cautious adjustments should be made.

Splints that have a tilt should be adjusted internally. These adjustments are not always straightforward due to the difficulty of locating the exact point of interference. It is suggested to use a light body silicone impression manipulated with its catalyst to verify the points of intimate contact with the abutment teeth. Fine articulating tapes can also be used for this purpose, but in this author’s opinion they are less accurate. Usually, the areas responsible for the misfit are proximal and occlusal due to the possibility of distortion or incorporation of bubbles in these regions.

Performing the occlusal adjustment on the splint:

1. Occlusal contacts of equal intensity in opening and closing

The fundamental objective is that all the teeth touch simultaneously and comfortably. Depending on the clinical picture, it is not possible to determine the CR position at this time. Ask the patient to make rapid opening and closing movements, and mark the contacts with a thick dark horseshoe-shaped articulating tape of about 100 to 200 micrometers (μm).

Use a “mini-cut”–type bur to grind the acrylic resin, keeping the bur parallel to the surface of the splint so as not to create depressions in these areas [Figure 14-07A–C].

[Figure 14-07A–C] Initially, the centric contacts are marked on the splint using a dark horseshoe-shaped articulating tape [A]. The adjustment should be performed with a “mini-cut”–type bur positioned parallel to the surface, and grinding should be done in a way that does not create depressions in these areas [B]. The objective at this point is to make the contacts with as uniform a distribution and visual appearance as possible [C]. Ask the patient whether they are experiencing the sensation of simultaneous and balanced contacts on the device.

When it can be seen that all the teeth have similar contact intensity and are well distributed with the patient’s free movements, the adjustments can be started in CR, providing there is no joint pain or muscle hyperactivity. This can be done with mandibular manipulation techniques^6,44,45 or by using a leaf gauge⁴⁶.

2. Adjustment in lateral movements

Adjustments should be made for lateroprotrusive and lateral movements on the right and left sides. Ask the patient to make broad movements from the inside to the outside, and mark contacts with a thick articulating tape in a different color from that used for the centric adjustment, such as red, to facilitate the interpretation of the contacts. A clear, complete, and unbroken line should be obtained in the canine region, without any contact being identifiable in the posterior teeth on the working or non-working sides [Figure 14-07D–F].

[Figure 14-07D–F] Use an articulating tape of a contrasting color to mark the eccentric contacts on the right and left sides in order to proceed with the adjustments of the functional guidance in lateral movements.

Readjust the lateral excursions now under dentist-guided movements to avoid the tendency of patients to make lateroprotrusive movements instead of lateral ones when they do so freely. Finally, adjusting the movements from the outside to the inside is recommended, placing the articulating tape with the canines on top and asking the patient to occlude, simulating masticatory movements forcefully. These contacts will strongly mark the functional guidance; later contacts may appear due to the action of the elevator muscles.

Note: The inferior lateral pterygoid muscles are responsible for the mandibular movements from the inside to the outside, excursive, or centrifugal. The powerful jaw elevator muscles are responsible for functional movements from the outside to the inside, incursive, or centripetal⁴⁷. Due to the significant difference between the muscular actions involved, the tooth contacts may present different magnitudes, locations, and directions. Therefore, both movements should be performed, and adjustments made for both.

3. Adjustment in protrusive movements

The patient should be asked to make backward and forward movements, and the resulting contacts should be marked with an articulating tape of the same color used for the adjustment of excursions in lateral movements.

In the Michigan interocclusal splint^6,26, the canines perform the anterior guidance. These should be adjusted to create a symmetric movement of the mandible toward the anterior region, with equal intensity and without deviations until the edge-to-edge position. For the effectiveness of the splint, not all the anterior teeth need to perform anterior guidance, as this would result in significant adjustment difficulties and offer no benefits when the canines are in good structural and periodontal condition. As with lateral movements, no posterior teeth should touch during the protrusive movements. The adjustments to the splint should be performed while keeping the bur parallel to the guidance angle, taking care not to reduce its height [Figure 14-07G,H].

[Figure 14-07G,H] It is initially recommended to use canine guidance due to its effectiveness and ease of adjustment. Adjustments are made to the protrusive movements, and the guidance can be located both in the region of the canines and the central incisors, the symmetric distribution of these forces being essential.

In the event of the functional clinical examination of the patient showing parafunctional wear facets, specific adjustments related to the patient’s envelope of function should be performed such as the adjustment of extreme excursive movements (cross-over)^34,48,49.

4. Adjustment of postural freedom in centric

It is recommended to ask the patient to assume a sitting position. With a horseshoe-shaped articulating tape of 100 to 200 μm in place, ask them to open and close the mandible until the splint has been touched several times (“tap-tap”) [Figure 14-07I]. The contacts on the front of the splint should be adjusted if they are more intense⁶, providing additional comfort to the patient.

[Figure 14-07I] The adjustment of postural freedom in centric is performed after changes in the distribution or intensity of occlusal contacts are noticed, especially in the anterior region, when the patient is in an upright and sitting position or even with the head bent forward as if eating a meal.

Clinical suggestions

• Refine all adjustments with an articulating tape of 40 to 60 μm.
• Adjustments in CR as well as lateral and protrusive movements can be performed with the aid of electronic occlusal adjustment devices such as the T-Scan (Tekscan, USA) or OccluSense (Jean Bausch, Germany)^50,51, generating additional information for the procedure such as the exact moment and distribution of forces of the occlusal contacts. In this author’s experience, this gives the dentist greater confidence regarding the accuracy of these adjustments.
• Finish the adjusted surface with sandpaper strips or rubber cups. Meticulous polishing with polishing tips or abrasive pastes on a prosthetic lathe is highly recommended for splint effectiveness and patient comfort. Thus, the splint should be delivered to the patient impeccably fitted, smooth, and polished [Figure 14-07J–M].

  

  [Figure 14-07J–M] The finishing is carried out with sandpaper strips parallel to the surface of the splint. The polishing is performed on a low-rotation lathe with a pumice stone and Spanish white until a completely smooth surface is obtained so that friction during parafunctional movements is reduced and patient comfort is optimized.

• At the end of the session, ask the patient how the newly adjusted splint feels. They should feel that all the teeth are touching simultaneously, and the jaw is able to slide in all directions freely and unhindered.
• Provide postoperative guidance [Appendix 14-I].

[Appendix 14-I]

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