Occlusal Guards Are the Unsung Heroes of Muscle Deprogramming in the Short Term

In 1983, Williamson and Lundquist¹ published a landmark electromyography (EMG) study that used a maxillary occlusal guard to evaluate the effect of two occlusal schemes on the temporalis and masseter (elevator) muscles. The two schemes were: canine-guidance that disarticulated all posterior tooth contacts during laterotrusive movements of the mandible and selective posterior tooth contacts (group function) during laterotrusive movements. Electrodes attached to the muscles recorded a reduction in EMG activity (ie, less muscle contraction) in patients with canine-guided occlusion as compared to patients with group function. The study established that when anterior guidance enables posterior teeth to disarticulate during working and nonworking mandibular excursions, elevator muscle contractions significantly decrease.

The investigators then designed permissive occlusal guards for their subjects. Subsequently, all elevator muscle contractions decreased bilaterally. This was attributed to the elimination of sensory feedback from interfering working and nonworking tooth contacts, which allowed the elevator muscles to move the mandible so that the condyles could slide posteriorly and up the slopes of the glenoid fossae for complete seating in their most anterior-superior (ie, centric relation [CR]) positions.

A 1987 study by Manns et al² determined the effects of excursive contacts on elevator muscle activity. When they compared EMG activity in patients exhibiting canine-guided occlusion with those displaying group function, they found that during both working and nonworking mandibular excursions, the masseter and temporalis muscles worked (ie, contracted) half as much in canine-guided occlusion as in group function. This was the result of eliminating contacts among posterior teeth in canine-guided occlusion.

Williamson and Lundquist¹ attributed this phenomenon solely to the elimination of posterior tooth contacts and not the contact of the canines themselves. This constituted a convincing case for designing canine guidance in occlusal guards for patients with myofascial pain. While canine guidance may be best for reducing elevator muscle activity, irreversible occlusion-changing procedures should be avoided in asymptomatic (healthy) patients.³

In 1999, Al-Quaran and Lyons⁴ found that soft occlusal guards tend to produce an increase in masseter muscle activity during maximum clenching; they recommended avoiding the use of soft appliances.

When they evaluated hard appliances, they discovered that the decrease in muscle activity was most pronounced in the temporalis muscles. Okeson⁵ reported that soft occlusal guards do not decrease nocturnal bruxism significantly. Eighty percent of his subjects experienced a ≥ 25% decrease in nocturnal muscle activity with a hard occlusal guard, whereas 70% of the participants who wore a soft guard experienced a ≥ 25% increase in nocturnal muscle activity. Soft occlusal guards were shown to increase nocturnal muscle activity in patients who were initially asymptomatic; therefore, they are contraindicated for use in symptomatic patients.

A 1989 investigation by Manns et al⁶ studied masseter muscle EMG activity when patients clenched on their own natural teeth and then on a hard occlusal guard. Increased muscle activity was observed when patients clenched on the occlusal guards. When the guards were altered to permit contact on only the six anterior teeth, muscle activity decreased by 40%.  When premolars and incisors were permitted to contact, a 20% decrease from maximum bite force was noted. Maximum muscle contractions were observed only when the molars were permitted to contact. The investigators concluded that muscle contractions in most people are dependent on the most posterior tooth contacts, and anterior guidance functions to prevent posterior contacts. Consequently, reducing posterior tooth contacts will decrease muscle contractions.

Discussion

A correctly designed and equilibrated hard, heat-polymerized acrylic resin occlusal guard manages bite forces by loading the temporomandibular joints with all teeth contacting simultaneously. Muscle activity and joint health depend on posterior teeth contacting and anterior guidance functioning to eliminate posterior tooth contacts during excursive movements of the mandible. A properly constructed and equilibrated appliance should have the following qualities:

• Canine-guided occlusion
• Pinpoint and equal intensity CR posterior tooth contacts with the opposing functional cusps with as many incisal edges as possible contacting the guard in CR (Figure 1)
• Smooth occlusal surface that will not provide any occlusal indentations into which opposing teeth can lock and effect heavy lateral or thrusting forces
• Shallow anterior ramp to ensure incisal guidance
• Should be stable (ie, not move) during CR closure and all eccentric movements

An occlusal guard with these design features should reduce elevator muscle activity. The smooth occlusal surface of the guard will eliminate sensory feedback from interfering tooth contacts, which will allow elevator muscles to move the mandible so that the condyles can seat in CR. With healthy condyles that can accept occlusal loading, a permissive occlusal guard that enables the condyles to seat during clenching should eliminate lateral pterygoid muscle resistance to the masseter and temporalis muscles and afford relief from myofascial pain. For this reason, occlusal guards are also known as “muscle deprogrammers.” As the temporalis and masseter muscles release their contractions (ie, become more relaxed), they are unable to exert the magnitude of potentially harmful forces as when posterior tooth interferences are present. For this reason, Dawson⁷ referred to these appliances as “permissive.”

The flaw with the muscle studies cited thus far is that none of the patients were studied long-term (ie, 3 months post-insertion of the guard). The role of altering occlusal guidance on muscle activity levels short-term was well documented in these studies; however, what happens long term appeared questionable until 2011, when Nanda et al⁸ designed an EMG study to assess the minimal time duration for using occlusal guards prior to restoring vertical dimension in patients with tooth attrition. EMG recordings were made at the following junctures:

1. During maximum voluntary clenching at pre-treatment prior to the delivery of the occlusal guard
2. Immediately at post-appliance insertion
3. After 1 week, 1 month, and 3 months

After the guards were inserted, EMG activity for both the masseter and temporalis muscles was reduced at rest and during clenching at both 1-week and 1-month. The highest EMG spikes in the graph, indicating elevated levels of muscle contraction, could be seen at pre-treatment. Immediately after the occlusal guards were inserted, the EMG spikes became shorter, indicating that muscle contractions decreased during maximum voluntary clenching.

At 1 week after insertion of the guards, muscle activity during clenching was reduced further. At 1 month, further decrease in muscle activity was noted during maximum clenching. So far, this study showed agreement with previous studies of shorter duration (in the range of immediate insertion and 1 to 6 weeks of use of an occlusal guard). However, by the third month, EMG activity spiked — it returned to pre-treatment levels and also greater than pretreatment levels. This was a novel finding.

A 2022 meta-analysis⁹ evaluated randomized clinical trials that compared two or more of the following 10 treatments for patients with temporomandibular disorder (TMD): counseling therapy, occlusal appliances, manual therapy, laser therapy, acupuncture, intramuscular injection of local anesthesia or botulinum toxin, muscle relaxants, hypnosis, ozone therapy, and placebo or no treatment.

In the short term (< 5 months), occlusal appliances ranked least effective of the four highest ranked treatments. At ≥ 6 months, occlusal appliances ranked third of the four most effective treatments. A 2023 study concluded that in the long-term (not defined by the authors), use of occlusal guards for pain reduction was inconsequential.¹⁰

Dao and Lavigne¹¹ cautioned against over-reliance on reports about the efficacy of occlusal guards as a therapy for reduction of myofascial pain and bruxism. Their data suggest that the decrease in myofascial pain might not be the result of treatment with an occlusal guard but rather the result of a placebo effect. The authors recommended using a hard occlusal appliance as a diagnostic tool to rule out pathology prior to fabricating definitive restorations. Patients with painful TMDs who also require prosthodontic rehabilitation should have treatment postponed until the condition has been resolved.³

Conclusion

For the first time, it was shown that muscle relaxation with an occlusal guard does not last long — after 3 months, muscle activity returns to its former elevated level and higher. The adaptive capacity of the elevator muscle fibers to modify their length has allowed dentists to restore occlusal vertical dimension (OVD) and teeth. Nanda et al⁸ recommended using an occlusal guard for 3 months prior to irreversible full-mouth rehabilitation to achieve muscle deprogramming, when the OVD will be re-established, and the teeth will be restored with full or partial coverage crowns.

An occlusal guard is a noninvasive way to assess a patient’s tolerance to the restored OVD while preventing further loss of tooth structure. It avoids preparing restorations when the elevator muscles are in a heightened state of contraction and when patients could exert higher — and harmful — bite forces that could lead to broken teeth and/​or provisional and definitive restorations. As dentists might not be able to prescribe a guard to relax the elevator muscles indefinitely, they must design an occlusion that survives their patient’s muscle contractions.

Several additional benefits could be derived from the use of occlusal guards. Kreiner et al¹² recommended prescribing an occlusal guard for patients exhibiting parafunctional activities when there is a need to protect opposing teeth and restorations from attrition, or when symptoms of myofascial pain exist. Porcelain is brittle by nature; crack propagation within the body of porcelain will occur as it is subjected to shear forces during parafunctional loading. With the increased use of ceramic dental restorations, opposing natural enamel will be worn over time while ceramic restorations will not. Occlusal guards could provide a safety net against the destructive capacity of porcelain against enamel.

Noncarious cervical lesions have been shown to develop in both posterior and anterior teeth as they encounter harmful interferences and flex during excursive movements — both in lateral and protrusive excursions.¹³ Occlusal guards designed to disarticulate posterior teeth during excursive movements could halt the progressive development of these destructive lesions.

According to Christensen,¹⁴ depending on the age of a general practice patient population and their types of restorations, as many as 30% could fall into the category of those who would benefit from using an occlusal guard.

References

1. Williamson EH, Lundquist DO. Anterior guidance: Its effect on electromyographic activity of the temporal and masseter muscles. J Prosthet Dent. 1983;49:816-823.
2. Manns A, Chan C, Miralles R. Influence of group function and canine guidance on electromyographic activity of elevator muscles. J Prosthet Dent. 1987;57(4):494-501.
3. Turp JC, Greene CS, Strub JR. Dental occlusion: a critical reflection on past, present and future concepts. J Oral Rehabil. 2008;35:446-453.
4. al-Quran F, Lyons M. The immediate effect of hard and soft splints on the EMG activity of the masseter and temporalis muscles. J Oral Rehabil. 1999;26:559-563.
5. Okeson JP. The effect of hard and soft occlusal splints on nocturnal bruxism. J Am Dent Assoc. 1987;114:788-791.
6. Manns A, Miralles R, Valdivia J, Bull R. Influence of variation in anteroposterior occlusal contacts on electromyographic activity. J Prosthet Dent. 1989;61:617-623.
7. Dawson PE. Occlusal Splints. In: Functional Occlusion from TMJ to Smile Design. St. Louis: Elsevier Mosby; 2007:380-382.
8. Nanda A, Jain V, Srivastava A. An electromyographic study to assess the minimal time duration for using the splint to raise the vertical dimension in patients with generalized attrition of teeth. Indian J Dent Research. 2011;22:303-308.
9. Al-Moraissi EA, Conti PCR, Alyahya A, Alkebsi K, Elsharkawy A, Christidis N. The hierarchy of different treatments for myogenous temporomandibular disorders: a systematic review and network meta-analysis of randomized clinical trials. Oral Maxillofac Surg. 2022;26:519-533.
10. Albagieh H, Alomran I, Binakresh A, et al. Occlusal splints-types and effectiveness in temporomandibular disorder management. Saudi Dent J. 2023;35:70-79.
11. Dao TT, Lavigne GI. Oral splints: the crutches for temporomandibular disorders and bruxism? Crit Rev Oral Biol Med. 1998;9:345-361.
12. Kreiner M, Betancor E, Clark GT. Occlusal stabilization appliances. J Am Dent Assoc. 2001;132:770-777.
13. Antonelli JR, Hottel TL, Brandt R, Scarbecz M. The role of occlusal loading in the pathogenesis of non-carious cervical lesions. Am J Dent. 2013;26:86-92.
14. Christensen GJ. Abnormal occlusal conditions: a forgotten part of dentistry. J Am Dent Assoc. 1995;126:1667-1668.

From Decisions in Dentistry. August/September 2024; 10(5):20-23