Occlusal splints: the role and fabrication of stabilization splints

An occlusal splint (synonymously referred to as an occlusal device) has been defined as, ‘any removable artificial occlusal surface affecting the relationship of the mandible to the maxillae used for diagnosis or therapy.’¹ A plethora of occlusal splints has been reported in the literature, with variations in design and indications. While the evidence-base to support the application of occlusal splints for the treatment of certain conditions may be lacking, they have been used to facilitate diagnostic evaluations prior to embarking on complex restorative treatment plans often involving changes to the occlusal scheme, for the preventive management of tooth wear, the protection of dental restorations/restorative materials, to help with the taking of occlusal records and for the treatment of temporomandibular disorders (TMD).¹

Occlusal splints may be categorized by their:

• Level of coverage (full versus partial);
• Consistency (hard or soft);
• The dental arch to which they are fitted;
• Whether they reposition the mandible into a predetermined position (repositioning splint) or stabilize the mandible into a desired reference position (stabilization splint).

This article provides an overview of the more commonly prescribed occlusal splints, with focus on the stabilization splint, and includes the indications, design and fabrication techniques of this device.

An overview of occlusal splints

Level of coverage

Partial-coverage appliances include anterior and posterior bite planes that respectively cover specific anterior or posterior teeth. Examples of anterior bite planes are the traditional anterior bite plane, comprising a horseshoe-shaped appliance covering the palatal surfaces of six to eight anterior maxillary teeth, and mini anterior appliances that engage between two and four maxillary incisor teeth, such as the sleep clench inhibitor (SCi) and the anterior midline point stop (AMPS). Contact between the anterior teeth and an SCi appliance has been described to result in the reflexive relaxation of the masticatory muscles,² which may help with the management of bruxism and TMD. The latter may take place by the stimulation of periodontal proprioceptors.³

An example of a posterior bite plane appliance is the mandibular orthopaedic repositioning appliance, MORA, typically comprising bilateral hard acrylic tables that are fitted over the mandibular posterior teeth, joined by a lingual bar.

Owing to limited tooth contact during mouth closure when a partial coverage splint is in place in the oral cavity, with more prolonged use, uncontrolled tooth movement may unfortunately happen (Figures 1 and 2). This may result from the intrusion of the dento-alveolar segments at the teeth contacting the splint and supra-eruption at the non-contacting areas – a concept that is used by Dahl appliances that are prescribed to facilitate the minimally invasive restoration of localized tooth wear. However, it may take several consecutive days without any tooth contact, for supra-eruption to commence.⁴ As a result of the relatively smaller sizes of some partial coverage splints, accidental inhalation and ingestion are additional risk factors, with the need for careful case selection.^5,6 Any risks should be appropriately discussed as part of attaining informed consent, to include the need for vigilant monitoring.⁶

Examples of full-coverage splints are discussed below.

Consistency

Soft splints are vacuum/thermoformed devices made from ethylene-vinyl acetate (EVA). While they offer the merits of lower financial cost, better tolerance and ease of fabrication compared to hard, full-coverage stabilization splints, they have several disadvantages. These include the tendency to wear down/perforate (especially among those displaying severe bruxism), discolouration, the perception of bulk, challenges with adjustment and food entrapment. Furthermore, due to the flatter nature of these devices that are formed from a blank sheet of material and without any clearly defined occlusal surface form, it is usual to find the absence of even occlusal contact during mouth closure. Initial contact is most likely to take place at the posterior aspect of the splint. Such appliances have, therefore, been classed as partial occlusal contact splints.³ The latter may result in increased masticatory muscle activity,⁷ with the exacerbation of any existing bruxism (possibly due to the presence of premature posterior occlusal contacts), as well as the risk of uncontrolled tooth movement and the subsequent formation of a posterior open bite. A previous investigation involving the cross over between hard and soft occlusal splints among 10 patients diagnosed with bruxism, where subjects were initially fitted with hard occlusal appliances followed by soft occlusal appliances, showed the use of the harder appliances to significantly reduce nocturnal muscle activity among eight participants. However, the use of the soft appliance resulted in a significant increase in muscle activity among five of the participants, and a significant reduction was only reported with one of the participants.⁸ Other studies have reported the absence of any substantial difference in the clinical effects and changes in patient symptoms with the use of either a hard or a soft occlusal appliance.^9,10 A systematic review published in 2021 also reported the lack of sufficient evidence to recommend occlusal splint therapy over no treatment or other forms of treatment (to include, other oral appliances, transcutaneous electrical nerve stimulation (TENS), pharmacological intervention, or behavioural therapy) for the management of bruxism. ¹¹

Soft occlusal splints are perhaps, therefore, best used for the short-term protection of teeth/restorations from the effects of bruxism. However, patients must be advised of the risks as discussed above and clearly informed of the importance of periodic monitoring.

Hybrid/bilaminar splints with a soft inner layer (to offer a more comfortable fit) and a harder outer layer to offer a higher level of resilience, have gained popularity. In the experience of the authors, separation of the two layers in areas of higher loading (especially where the thickness of the splint may be reduced) may limit the life expectancy of such appliances. The presence of a flexible, softer inner layer also limits the scope for the clinician to accurately verify the presence and the future maintenance of a precise occlusal prescription.

Repositioning/stabilization splints

Stabilization splints (also referred to as flat plane splints, flat plane stabilization appliances, muscle relaxation splints or gnathological splints) aim to provide a temporary and removable ideal occlusal scheme. This has been suggested to reduce abnormal muscle activity and provide neuromuscular a balance.¹² Stabilization splints are based on the principles of the mutually protected occlusal scheme (Table 1).¹³

Thus, with a stabilization splint in place in the oral cavity, during the retruded arc of closure, even occlusal contact with all the antagonistic teeth is expected, with lighter occlusal contact at the anterior dentition compared with the posterior teeth. Wearing of a full-coverage stabilization splint should enable the smooth and ready separation – disclusion – of the posterior teeth upon lateral and protrusive mandibular movement with occlusal contact only taking place only between the anterior teeth and the occlusal device. The latter is achieved by the preparation of an anterior ramp that permits canine-guided disclusion.

Unwanted movement of antagonistic teeth should be prevented by the presence of a complete/full-arch design, as displayed by a stabilization splint. The use of a more resilient material for the fabrication of stabilization splint, typically, heat-cured polymethyl methacrylate (PMMA), often of a transparent variety, should culminate in a durable appliance that may be suitably contoured to the desired prescription, offering the ease of chairside adjustment and minimal wear of the antagonistic surfaces. However, compared to soft occlusal splints, stabilization splints are more costly, require more chairside time and the necessary levels of operator and technical skill.

With the increasing popularity of computer-aided design and computer aided-manufacturing (CAD/CAM) in dentistry, it is also possible to use alternative, ‘more flexible’ tooth-coloured, polycarbonate-based materials to construct stabilization splints that offer acceptable levels of fracture resistance at comparatively lower levels of thickness. This may prove invaluable among patients where compliance and tolerance with bulkier appliances may be problematic.¹⁴

Anterior repositioning appliances (ARAs) are prescribed for the treatment of painful disc displacement.^15,16 They aim to recapture an anteriorly displaced disc or to position the condyles in a physiological position that will prevent the impingement of the retrodiscal tissues,¹⁵ helping to prevent disc displacement, which may manifest as a TMJ click and symptoms of pain. An ARA device will hold the mandible in a protrusive position, with maximum occlusal contact in this position. Such splints often require expert attention and may be associated with the risk of developing posterior open bites.

Maxillary/mandibular splints

The maxillary hard acrylic stabilization splint is commonly referred to as a Michigan splint (Figure 3), while the mandibular hard acrylic stabilization splint is often termed a Tanner appliance (Figure 4). While both offer an analogous outcome, the latter are perhaps more suitable for patients displaying a Class 3 incisor relationship because it may prove easier to develop the desired occlusal scheme, or where the tolerance of a maxillary appliance may be problematic and/or where there may be aesthetic and phonetic concerns. ²

Soft splints may be fitted at either arch; however, repositioning splints are usually fitted to the maxillary arch.

Full-coverage stabilization splints

Indications and possible modes of action

The reasons for prescribing a full-coverage stabilization splint are listed in Table 2.^17,18,19

The precise method(s) by which a stabilization splint can fulfil some of the roles listed in Table 2, are unknown. Proposed theories include causing occlusal disengagement, restoring the vertical dimension of the occlusion (VDO), causing muscle relaxation, establishing ideal jaw relationships and/or by allowing joint unloading or TMJ repositioning.²⁰

For stabilization splint devices, it has been suggested that the effect of posterior disclusion provided by the inclusion of the anterior ramp, with canine-guided occlusion results in decreased elevator muscle hyperactivity.²¹ Electromyographic studies have reported a reduction in anterior temporalis and masseter muscle activity when clenching on a canine ramp, opposed to performing the same role during group function or during centric occlusal contact.^21,22 This may enable the components of the TMJ and masticatory muscles to re-establish a balanced state where pathology may have developed due to disharmony. In general, canine teeth have been described to have the longest roots, more dense alveolar bone support and the largest number of periodontal proprioceptors.¹⁵ The higher level of proprioception has been postulated to have a calming effect on the elevator muscles on stimulation,¹⁵ while their anatomical form and distance from the TMJs (fulcrum in a Class 3 lever system) helps with the absorption of mechanical loads applied during vertical and lateral movements of the mandible.

Clinical protocol for the fabrication of a stabilization splint: the conventional approach

Please also refer to the video that accompanies this article that can be found at: https://youtu.be/P0zMaemvAHA

The account below relates to the construction of a Michigan splint. However, these concepts are also applicable for the fabrication of a mandibular stabilization splint.

Attaining accurate impressions and accurate occlusal records is imperative for the provision of an appropriate standard stabilization splint. The use of a custom tray with an appropriate impression material, an additional cured polyvinylsiloxane (PVS)-based product, is ideal for the construction of the working model. However, metal rim lock impression trays with an accurate alginate-based impression material may prove more time and cost effective, providing the impressions are cast in a timely manner using a suitable, dimensionally stable material. Alginate is usually sufficient for the opposing arch impression. The use of a PVS material additionally enables a duplicate cast to be poured to help verify the fit of the finished splint, as the working cast is frequently damaged during construction. Occlusal surfaces must be clean, and suitably dried prior to impression taking. Impressions must be closely inspected and any excessive material at the borders, carefully trimmed away.

Once the impressions have been cast, the casts should be permitted to dry for 24 hours after pouring prior to the undertaking of any anatomical articulation, to provide the desired level of abrasion resistance.

Appropriate standard facebow record and intra-occlusal records are required to permit the accurate mounting of the casts onto a suitable type of semi-adjustable articulator. The clinical techniques for the taking of a facebow record and inter-occlusal records to permit mounting on a semi-adjustable articulator have been described.²³ These records must be accompanied by a clear and comprehensive laboratory prescription.

Stabilization splints made using PMMA are usually fabricated to provide a minimal thickness (inter-occlusal clearance) of 1.5–2.0 mm. In the event of a patient displaying pathological tooth wear, the thickness of the appliance will be dictated by the space requirements being proposed for the rehabilitation. For patients with bruxism, given that it has been shown that an increased vertical dimension of 4.4–8.2 mm was more effective in producing muscle relaxation versus the use of a thinner, 1-mm appliance,²⁴ a minimum of 4-mm increase in the vertical dimension of the occlusion has been described to be necessary when the splint is in situ.²⁵ Splint thickness may be increased if the dimension is not deemed sufficient.²⁵

An outline of the splint should be scribed onto the maxillary cast. The outline should ideally extend 3–4 mm onto the palate, and 3 mm onto the buccal cusps of the posterior teeth and 2mm of overlap of the incisal edges of the anterior teeth. Gross undercut areas may require blocking out; however, the unnecessary blocking out of all undercuts on the proximal, buccal and palatal surfaces should be avoided because the engagement of the acrylic into these interstitial areas will provide the necessary mechanical retention. Where the retention form may be inadequate, Adam's cribs may be added to the baseplate at the first molar teeth area.

Sheets of softened pink baseplate wax should then be adapted to a suitably dampened maxillary cast, conforming to the outline previously scribed. The articulator is then closed, ensuring the incisal pin contacts the incisal table. The latter can be verified by a positive tugging action on a piece of thin 8-µm articulating foil positioned between the pin and table. Indentations will now form into the wax base, forming the centric stops. Excess wax should be carefully trimmed away. The occluding surface should be relatively flat, eliminating the potential for the cusps of the opposing dentition to be locked, permitting freedom in movement across the occluding surface. A minimum of one centric stop should be present per opposing tooth. This may be verified using articulating paper.

Wax is then carefully added at the canine areas, placed anterior to the established centric stops to avoid an unwanted alteration in the occlusal prescription established so far. The riser is built at an approximate angle of 45° to the occlusal surface. The canine riser, once formed, should provide guidance to the mandible upon protrusive and excursive movements, concomitantly ensuring separation of all other teeth. Canine risers are then joined together by the process of adding wax in the anterior segment, resulting in the formation of a shallow concave anterior ramp. The anterior ramp should provide immediate disclusion of the posterior teeth upon mandibular protrusion, with anterior guidance being shared equally between the anterior teeth. The wax pattern may now be tried-in, if desired.

On satisfaction, the wax pattern is processed and finished, typically using a transparent heat cured acrylic. It is also advisable to prepare a duplicate cast for processing, to enable the processed splint to be repositioned onto the mounted casts for verification and the making of any final adjustments.

The splint should be carefully inspected prior to fitting onto the patient's maxillary arch. If the splint is tight, areas should be carefully relieved, using an acrylic trimming bur. The use of an occlusal indicator medium such as Occlude Marking Spray (Pascal Co Inc, Bellevue, WA, USA) to identify any areas of interference can prove particularly helpful in some cases. When the appliance may show signs of minor instability or rocking, relining of the splint at the area of instability with an appropriate chairside, cold-cured acrylic reline material may be appropriate.

Once the splint has been satisfactorily seated and deemed to be adequately retentive, a type of suitable articulating paper such as GHM Occlusion foil 12 µm (Hanel, Coltene Whaledent, Germany) can be used to mark up the centric stops (Figure 5). For a right-handed operator, using a pair of Miller's forceps in their left hand to support the articulating paper, the patient should be requested to ‘close together.’ With the clinician's right hand positioned on the patient's chin, the mandible should be gently guided along its retruded arc of closure. With the opposing teeth contacting the splint, the patient should be asked to rub slightly back and forth onto the splint, making partial protrusive and excursive movements of the mandible (Figures 6–10).

The splint is then removed from the patient's mouth. Using a sharp surgical pencil, desired areas of occlusal contact should be marked-up; ideally, a minimum of one contact should exist between opposing functional cusps. Using an acrylic trimming bur or a wheel, unwanted occlusal contacts should be carefully removed avoiding the creation of any unwanted indentations into the occluding surface. The splint should also be regularly checked (using an Ivanson's calliper) to ensure that it does not reach any less than a minimal thickness of 0.5 mm in any area, which is necessary to make sure that it remains sufficiently robust to withstand occlusal loading.

Where an obvious lack of occlusal contact exists, there will be a need to add suitable increments of cold-cured acrylic on to the occluding surface of the splint, making sure that the patient is carefully manipulated into position, maintaining the established vertical dimension. Once the presence of centric stops has been identified between each occluding pair using articulating paper, these should be re-assessed using an 8-µm articulating foil. Contacts should be lighter between opposing anterior teeth.

To aid the visualization of the occlusal scheme provided by the stabilization splint, using a different colour of the type of articulating paper described above, the presence of a suitable canine rise should be ascertained. Where satisfactory, the canine rise should permit posterior disclusion on protrusion and lateral excursive movements. Where it may be deemed insufficient, cold-cured acrylic may be carefully added, ensuring the avoidance of a very steep rise which may otherwise be poorly tolerated by the patient. Finally, using a third colour of articulating paper, the presence of evenly shared anterior guidance on protrusion should be checked and the appliance amended by the addition or subtraction of acrylic. A higher level of accuracy with the verification of the desired occlusal form of a stabilization splint may of course be offered using computerized occlusal analysis (T-Scan III, Tekscan, Boston, USA). However, at present, it is likely that many dental practitioners in the primary care setting will have limited access to such types of equipment.

A review should be arranged 7 days after fitting of the device.²⁶ The comfort, compliance with wear and the occlusal contacts should be checked and appropriate records kept at each review appointment. The need for some initial occlusal adjustment may be likely, due to mandibular repositioning that will accompany muscle relaxation. Where the splint has been prescribed as a night guard or for the protection of teeth/restorations, further reviews may not be required until the next routine examination provided the occlusion has remained stable at review. When a stabilization splint is prescribed for the treatment of TMD or for pre-restorative stabilization, further reviews should be arranged at weekly intervals and the same checks performed. Adjustments to the occlusal form of the device should be made until the occlusal contacts are consistent between consecutive visits at a stable retruded position and the clinician and patient are satisfied with the standard of splint provided.²⁶

When a stabilization splint has been prescribed to ascertain the patient's tolerance to a new occlusal scheme, the splint should be worn continually other than when eating and performing oral hygiene for a period of 1–3 months. For the management of nocturnal bruxism, the splint should be worn every night, until further review.

The use of CAD/CAM for the fabrication of a stabilization splint

CAD/CAM may also be used for the fabrication of stabilization splint appliances. This approach has several advantages,¹⁴ including:

• Avoiding the risk of polymerization shrinkage that takes place during the curing of PMMA materials and the effect of this on the accuracy of the definitive appliance.
• Opportunity to use prefabricated materials manufactured to industrial standards, such as tooth-coloured CAD/CAM polycarbonate.
• Scope to prepare a new appliance/second device on a master digital cast, without the need to take further patient records.

The use of tooth-coloured CAD/CAM polycarbonate also offers aesthetic merit, while the mechanical properties of this material may allow stabilization splint appliances to be made to a minimum thickness of 0.3 mm. The latter would give the option of fitting opposing arch splints. For a case involving the rehabilitation of a severely worn dentition, where a full mouth rehabilitation is indicated with a planned increase in the VDO the use of separate arch splints allows further evaluation of the occlusal contours of the wax-up, on an arch-by-arch basis, especially when an increase in the height of the incisal pin by 4 mm or more is indicated.¹⁴ The use of separate splints in this way may also permit a ‘segment-by segment’ approach, enabling each arch to be restored separately, in a staged manner, with the presence of a splint at the unprepared arch, permitting the ongoing existence of a mutually protective occlusal scheme. The financial cost of care with this approach may, however, be prohibitive.

Techniques for the fabrication of a removable CAD/CAM polycarbonate stabilization splint for a case requiring an increase in the VDO have been described by Edelhoff et al.¹⁴ The protocol in summary involves, the preparation of a diagnostic wax-up, followed by an intra-oral mock-up. Scanning of the baseline casts and duplicate casts of the wax-up (diagnostic model) is subsequently carried out, followed by the positioning of the models into a virtual articulator. The cervical length/height of the splint is determined using information from the diagnostic model. Information from the wax-up is used to ascertain the static and dynamic occlusal features of the appliance, using the virtual articulator to correct any premature contacts that may have been introduced during the waxing-up stage. The appliance is then milled, followed by finishing and polishing to achieve a high lustre.

Summary and conclusion

Stabilization splints have a wide variety of applications in dentistry. It is beneficial for clinicians to be familiar with the indications, mode of construction and fitting of these devices.