Long-term Effectiveness of Occlusal Splint Therapy Compared to Placebo in Patients with Chronic Temporomandibular Disorders

Abstract

Purpose

The aim of this study was to compare long-term effectiveness of stabilization splint (SS) with that of placebo splint (PS) in chronic TMD patients and to investigate differences in treatment outcomes based on diagnostic subgroups [disc displacement (DD)/myofascial pain (MP)].

Materials and Methods

Thirty-four female participants, diagnosed with chronic TMD, were classified in groups: one provided with SS and other with PS and were followed for six months. Treatment outcomes included spontaneous pain [visual analogue scale (VAS)], self-perceived quality of life (OHIP-14), pain-free maximal mouth opening (MCO), maximal mouth opening (MMO), level of perceived stress (PSS), characteristic pain intensity [graded chronic pain scale (GCPS)], and functional jaw limitation [jaw functional limitation scale (JFLS)].

Results

Baseline characteristics did not differ significantly between the two groups (p>0.05). After six months of treatment the changes in spontaneous pain and OHIP-14 scores differed significantly between treatment groups (p=0.004, p=0.02 respectively), with greater reduction in SS compared to the PS group. Pain-free maximal mouth opening did not change significantly over time, however MCO values differed significantly between the two treatment groups, with greater overall values in the SS compared to the PS group (p= 0.046), as well as between TMD subgroups, with greater overall values in MP compared to DD patients (p= 0.03). In the SS group, significant difference in JFLS categories was found between baseline and after 6 months of treatment in all except emotional and verbal expression (mastication p=0.00015; vertical jaw mobility p=0.00018). No such changes in JFLS categories were observed in the PS group.

Conclusions

During 6-month period, SS was more effective than PS in reducing spontaneous pain and improving self-perceived quality of life and functional limitations of the lower jaw. Moreover, significantely higher values of pain-free mouth opening were observed in patients treated with stabilization splint. While placebo might be partly responsible for improving the symptoms of TMD, it seems that itcannot maintain a continual long-term positive therapeutic effect.

Introduction

Temporomandibular disorders (TMDs) is a term covering neuromuscular and musculoskeletal conditions that affect masticatory muscles and temporomandibular joints (TMJ) as well as surrounding structures (1). The most common TMD diagnoses are myofascial pain and disc displacement (2, 3). Temporomandibular disorders can affect anyone, however women aged 20 – 50 years are most affected (4). The most common symptoms for which patients seek help are pain in orofacial region and limited lower jaw movements, which are often aggravated by function. In addition, patients are often worried about the sounds that appear in the TMJ (clicking or popping sounds, or crepitations) (5). The complexity of the etiopathophysiology of disorders and its effect on patients’ quality of life, as it is frequently stated in various research studies, requires thorough and careful therapeutic approach (1-5).

Treatment of TMD can be divided into non-invasive, reversible modalities, and options that are more invasive. There is an agreement among researchers that conservative and reversible therapies provide satisfactory results in reducing pain and improving jaw function, however no specific therapy has been proven to be more effective than the other (6). Majority of patients respond well to combination of behavioral and physical therapy approaches, but the most popular modality is certainly occlusal splint (7). Various types of splints, with different indications and functions, are used. However, the stabilization splint (SS), a full-arch hard acrylic device made in the position of centric relation (CR) is most frequently used type of device (8). The success of SS is attributed to various factors including changes in condylar position, elimination of occlusal interferences, establishment of stable occlusal relationship with uniform teeth contacts, changes in neuromuscular activity, and increase in vertical dimension of occlusion which probably contributes to the relaxation of the musculature and the relief of TMJ (7-9). It is not completely clear why any therapy is clinically more successful than no treatment at all, but researchers agree that the improvement of symptoms is probably due to combination of previously mentioned factors, and some of them consider placebo to be one of those factors too. Some studies are pointing out that SS does not appear to produce a better clinical outcome than a soft splint, a non-occluding palatal splint and physical therapy (7, 10), while others claim that hard stabilization appliance provides better clinical outcomes than placebo (11). The question remains whether the treatment success achieved with a SS is longer-lasting than the success attained with splint therapy that does not provide centric relation position and has a negligible influence on vertical dimension of occlusion.

The aim of this study was to compare a 6-month effectiveness of stabilization splint with that of placebo splint. The null-hypothesis was that there would be no difference between treatment groups in a 6-month treatment period.

Materials and Methods

The study was approved by the Ethics Committee of the School of Dental Medicine, University of Zagreb (01-PA-26-6/15, item 3.2). All participants were informed about the study protocol and provided with written informed consent. All experimental procedures were conducted in accordance with ethical standards of the Helsinki Declaration.

Participants

This randomized controlled clinical trial was carried out at the Department of Removable Prosthodontics, School of Dental Medicine, University of Zagreb from June 2016 to June 2019. The participants were recruited from patients seeking treatment for chronic TMD pain and/or limited lower jaw movements. In the research period, the only patients who met the inclusion criteria were women, therefore exclusively female participants were included in the study. The inclusion criteria were as follows: the report of chronic pain, lasting more than 6 months, spontaneous pain >30 mm on visual analogue scale (VAS), and diagnosis of myofascial pain (MP) or disc displacement (DD). Exclusion criteria were orofacial pain not related to TMD, degenerative joint disease, oral lesions, periodontal disease, systemic diseases, pregnancy, as well as previous active treatment for painful TMD. Patients with combined MP and DD were not considered for the study.

Sample size estimation

A statistical power analysis was performed for sample size estimation based on data from Michelotti et al. (12). The minimum difference in VAS and maximal comfortable mouth opening (MCO) scores between treatment groups was estimated to be 8.8 mm and 3.4 mm, respectively, with the standard deviation of 7.5 mm and 2.5 mm, respectively. With an alpha = .05 and power set at 80%, the projected sample size was approximately N =26 (13 per group).

Study protocol

Out of 38 female patients that met the inclusion criteria, 2 of them declined to participate in the study due to travel complications, thus 36 participants were included in the study. The randomization was performed using Microsoft Excel software after the codification of each patient. Two of the participants dropped out before they were provided with a splint. Eventually, 34 patients were randomized in SS group, and placebo splint (PS) group. The number of subjects was 19 in the SS group, and 15 in the PS group. Two of the patients in each treatment group withdrew before the completion of the therapy, thus 17 patients in the SS group and 13 patients in the PS group completed the 6-month treatment (Figure 1).

Figure 1.

Flowchart illustrating the selection and distribution of the participants into the study groups

At the baseline (T0) all patients were evaluated by the expert in TMD (IA). The patients were diagnosed using Diagnostic criteria for temporomandibular disorders (DC/TMD) (13). After the patients had been provided with a splint, they were followed during a 6-month period with follow-up appointments at 1st (T1), 3rd (T2) and 6th (T3) month. The follow-up evaluations (T1, T2, T3) were carried out by a clinician (EV) blinded for the type of treatment, the initial diagnosis and the pain intensity.

Treatment procedure

Both treatment groups were equally informed and counseled about their condition prior to splint therapy, in terms of explanation of the origin and prognosis of the disease.

The maxillary stabilization splint was fabricated on stone cast in ARTEX articulator. It was a hard acrylic splint (Resilit-S, Erkodent, Siemensstraße 3, 72285 Pfalzgrafenweiler, Germany), with a 2 mm thickness at the level of the first molar. The clinician (IA) adjusted the splint so that that the opposing teeth occluded uniformly and simultaneously with the occluding surface of the splint in centric relation. The same clinician adjusted the splint at follow-up appointments if it was needed.

The placebo splint was made of a thin thermoforming foil (Erkodent, 0.5 mm) fabricated on the patient’s maxillary stone cast. All contacts that interfered with maximal intercuspation have been removed. The increase in vertical dimension was less than 0.5 mm, thus providing negligible influence on occlusion and condylar position. (Figure 2 and 3).

Figure 2.

The specific design of the placebo splint adjusted to the upper stone cast

Figure 3.

Placebo splint in the mouth of the patient

The same dental technician made all splints. Patients in both treatment groups were instructed to wear their splints only during sleep.

Treatment outcomes

Changes in spontaneous pain were assessed by VAS, changes in oral health quality of life were assessed by the Oral health impact profile (OHIP-14), and the level of perceived stress by Perceived stress scale (PSS). Moreover, the study followed the changes in MCO for assessing pain-free mouth opening, maximal mouth opening (MMO) for assessing maximal mouth opening and Graded chronic pain scale (GCPS) for assessing characteristic pain intensity. All of the previously mentioned outcomes were measured at all time points. Jaw functional limitation scale (JFLS) scores for assessing functional limitations of the lower jaw were measured at baseline (T0) and at follow-up appointment at 6th month (T3). GCPS and JFLS questionnaires are part of DC/TMD protocol (13).

VAS pain

Spontaneous pain was evaluated with a 100 mm horizontal VAS scale. The left endpoint of the scale (0 mm) indicated “no pain” while right endpoint of the scale (100 mm) represented “worst pain imaginable” (14). The subjects with pain of 30 mm and more were included in the study.

MMO

Maximal mouth opening was defined as the maximum distance the participant could open their mouth regardless of the pain they felt, measured as the distance between the upper and lower central incisors.

MCO

Maximal comfortable mouth opening (pain-free maximal mouth opening) was measured as the distance between the upper and lower central incisors, and was defined as the maximum distance the participant could open their mouth without experiencing pain or discomfort (12).

PSS

Perceived stress scale is a 10-item questionnaire that evaluates subjective perceptions of stress over the previous month. Response options form a 5-point Likert scale: 0 =never, 1= almost never, 2 = sometimes, 3 = fairly often, 4 = very often. Possible scores ranged from 0–40, with higher scores indicating higher stress levels. The questionnaire was previously translated and validated by Hudek- Knežević etal. (15).

OHIP-14

The OHIP-14 questionnaire was used to evaluate how TMD-related pain and disability influence patients’ quality of life. Patients status was expressed throughout 14 questions, by choosing 1 of the 5 possible answers: 0 = never, 1 = hardly ever, 2 = sometimes, 3 = fairly often, and 4 = very often. Possible scores ranged from 0–56. The questionnaire, previously validated for the evaluation of TMD patients (16), was translated into and validated in Croatian (17).

GCPS

Graded chronic pain scale was used to evaluate two dimensions of chronic pain severity: pain intensity and pain-related disability. Subscale scores for pain intensity and disability are combined to calculate a chronic pain grade that allows classification of chronic pain patients into 5 categories: grades 0 (pain-free) to IV (high disability-severely limiting). All items are scored on a scale, with responses ranging from 0–10. Scores are computed and divided into 3 subscales: the characteristic pain intensity score calculated as the mean intensity ratings for reported current, worst, and average pain; the disability score is calculated as the mean rating for difficulty performing daily, social, and work activities; and the disability points score is derived from a combination of ranked categories of number of disability days and disability score (13, 18). In our study we evaluated only the changes related to characteristic pain intensity scores.

JFLS

Jaw functional limitation scale questionnaire is a reliable and valid form that assesses global limitations caused by TMD. The questionnaire consist of 52 items in groupations as follows: a) mastication (20 items), b) vertical jaw mobility (9 items), c) verbal and emotional expression (14 items), and miscellaneous (9 items) (19). In this study we followed the changes in 3 JFLS categories: mastication, vertical jaw mobility and verbal and emotional expression.

Statistical Analysis

Data analyses were performed using the Statistica 13.4.0 software package (1984-2018 TIBCO Software Inc.). The distribution of data was tested using the Shapiro-Wilk test. The baseline and 6-month follow-up data (JFLS scores, percentage change of MCO values and MMO values) were analyzed means of Student’s t-test. The changes in the means of measured variables (VAS, OHIP-14, PSS, GCPS, MCO and MMO scores) were analyzed using repeated measures analysis of variance (ANOVA) with time (baseline, 1st, 3rd, 6th month of therapy) as the within factor and diagnostic subgroups (MP and DD) and type of treatment (SS and PS) as the between factors. Bonferroni’s post hoc test was used to show where the differences were found. Eta squared (η2) was used to estimate the effect size. A value of p < 0.05 was considered statistically significant.

Results

Baseline characteristics did not differ significantly between the two treatment groups when observed in general, as well as when observed in each diagnostic subgroup (p>0.05). No significant age difference wasfound between both treatment groups (SS 38.89 ± 11.79; PS 32.66 ± 11.48; t=1.54, p=0.13) nor in both treatment groups when divided into diagnostic subgroups: MP subgroup (SS 42.25 ± 10.06; PS 36.37 ± 12.86 t=1.017, p=0.33), DD subgroup (SS 36.36 ± 12.88; PS 28.28 ± 8.81 t=1.45, p=0.16). Table 1 shows participants’ baseline data in both treatment groups.

Table 1. Demographics and baseline data of participants.

PSS scores did not differ significantly between treatment groups or the diagnostic subgroups (p>0.05), but they changed significantly over time (p=0.0049, effect size=0.11) (Figure 4).

Figure 4.

Changes in self-perceived stress (PSS) from baseline to 6th month of the therapy. SS stabilization splint; PS placebo splint; MP myofascial pain subgroup; DD disc displacement subgroup

VAS scores for spontaneous pain showed significant reduction over time (Wilks Lambda = 0.19, F=32.70; p=0.0001, effect size=0.56). Changes in spontaneous pain differed significantly between treatment groups, with greater reduction in the SS compared to the PS group (interaction time x treatment group; Wilks Lambda = 0.58, F=5.78; p=0.004, effect size=0.22) (Figure 5). The post hoc analysis showed that in the SS group the mean VAS values were significantly lower at the 1st, 3rd and 6th month of the treatment compared to the baseline (p=0.0007, p<0.0001 and p<0.0001 respectively), while in the PS group a significant difference in mean VAS values was found only between 6th month of treatment compared to the baseline (p=0.006). No statistical significance in VAS scores was present between TMD subgroups.

Figure 5.

Changes in spontaneous pain (VAS) from baseline to 6th month of the therapy. SS stabilization splint; PS placebo splint; MP myofascial pain subgroup; DD disc displacement subgroup

OHIP-14 scores also showed significant reduction over time (Wilks Lambda = 0.47, F=8.80; p=0.0004, effect size=0.29). Changes in OHIP-14 scores differed significantly between treatment groups, with reduced values only in the SS group during a 6-month period (interaction time x treatment group; Wilks Lambda = 0.62, F=4.32; p=0.02, effect size=0.15) (Figure 6). The post hoc analysis showed that in the SS group the mean OHIP-14 scores were significantly lower at the 3rd and 6th month of the treatment compared to the baseline (p=0.0009, p<0.0001 respectively), while in the PS group no significant difference in mean OHIP-14 scores was present between baseline and follow-up appointments. No statistical significance in OHIP-14 scores was found considering theTMD subgroups.

Figure 6.

Changes in health-related quality of life (OHIP-14) from baseline to 6th month of the therapy. SS stabilization splint; PS placebo splint; MP myofascial pain subgroup; DD disc displacement subgroup

MCO values did not change significantly over time, however pain-free maximal mouth opening differed significantly between the two treatment groups (F=4.37, p= 0.046) with greater overall values in the SS group (Figure 7). Also, a significant difference was found when comparing the MP group to the DD group with greater overall values in the MP group (F=5.42, p= 0.03). Significant difference in percentage change in MCO between the two treatment groups with greater increase in the SS group was found (SS 28.54%; PS 1.64%; t= 2.11 p= 0.043).

Figure 7.

Changes in maximal comfortable mouth opening (MCO) from baseline to 6th month of the therapy. SS stabilization splint; PS placebo splint; MP myofascial pain subgroup; DD disc displacement subgroup

MMO values did not differ significantly between the two treatment groups or the diagnostic subgroups (p>0.05) but they changed significantly over time (Wilks Lambda = 0.72, F=3.16; p=0.042, effect size = 0.12). Although the changes in MMO values did not differ significantly between the two treatment groups, a tendency of constant increase of MMO was present in the SS group (Figure 8). Also, a significant difference in percentage change in MMO between the two treatment groups with greater increase in the SS group was found (SS 23.82%; PS 0.73%; t= 2.32, p= 0.028).

Figure 8.

Changes in maximal mouth opening (MMO) from baseline to 6th month of the therapy. SS stabilization splint; PS placebo splint; MP myofascial pain subgroup; DD disc displacement subgroup

GCPS scores did not differ significantly between treatment groups or the diagnostic subgroups but decreased significantly over time (Wilks Lambda = 0.45, F=9.52; p=0.0003, effect size=0.35) (Figure 9).

Figure 9.

Changes in characteristic pain-intensity (GCPS) from baseline to 6th month of the therapy. SS stabilization splint; PS placebo splint; MP myofascial pain subgroup; DD disc displacement subgroup

All JFLS categories, except emotional and verbal expression, showed significant decrease between baseline measurements (T0) and the last time-point measurements (T3) in the SS group (mastication t=4.92, p=0.00015; vertical jaw mobility t=4.82, p=0.00018; emotional and verbal expression t=1.82, p=0.086). No such changes in JFLS categories were observed in the PS group (mastication t=0.24, p=0.81; vertical jaw mobility t=1.26, p=0.23; emotional and verbal expression t=-0.24, p=0.56) (Figure 10).

Figure 10.

Changes in JFLS categories between the baseline measurements (T0) and the last measurements (T3). Figure 10a. Changes in JFLS categories in stabilization splint group (SS). Figure 10b. Changes in JFLS categories in placebo splint group (PS).

Discussion

Studies that tried to compare the effect of placebo therapy with stabilization splint have sometimes yielded conflicting results. Some researchers attempted to equate the effect of stabilization with that of placebo devices encouraging the debate on the real value of the specific design of the stabiliztation splint. In these studies, the placebo splint is often a “nonsplint” designed as non-occluding device (20, 21).

In our research, starting with the assumption that patients may recognize non-occluding device as placebo, we took a slightly different approach. Since it is speculated that therapy value of SS lies in increasing of vertical dimension and changing of condylar position (22-25), we used very thin thermoforming foil that covered occlusal surfaces but did not provide occlusion in CR position. In addition, the effect on the vertical dimension wasn't notable, therefore when biting patients were in their habitual occlusion position vertically increased for less than 0.5 mm. With Michelotti et al. pointing out that counselling may be as successful modality as splint therapy (12), we informed both groups equally about the origin and prognosis of the disease, thus making the applied therapy the only diversity between groups.

Our results showed that changes in spontaneous muscle pain and oral health related quality of life differed significantly between patients treated with stabilization splint as compared to patients treated with placebo splint, with greater reduction through all 6 months found in the SS group. In addition, significant improvement in the SS group after 6th month of treatment compared to baseline was present in all JFLS categories, except emotional and verbal expression.

We may say that SS provided better continuous, long-term outcomes. PS performed very well in reduction of spontaneous pain in the short term, however reduction of OHIP-14 and JFLS scores was found only in the SS group across a 6-month period. It is interesting to note that graphic representation of PS performance during 6-month treatment in Figures 5, 6 and 9 resemble the graphic representation of SS but after 3rd month it is followed by either plateau or even mild deterioration towards the higher values. Regardless of the potentially negligible effect of the slightly increased vertical dimension of less than 0.5 mm, the design of the placebo device should be taken into consideration when interpreting these results since all the changes in vertical positioning of the lower jaw might affect the treatment outcomes. We may conclude that both SS and PS were effective in management of spontaneous pain. This could be attributed to prior education of patients on the disorder (12) or the fact that even a slight increase in vertical dimension can provide the muscle rearrangement leading to a relaxation of the elevator muscles and consequently, reduction of the pain. Significant differences between the treatment groups in improvement of pain and quality of life could be due to the fact that SS was thicker (providing a greater increase in vertical dimension of occlusion), as well as constructed to provide CR occlusion a nd specific condylar position, thus contributing to the relief of the TMJ (7, 9).

An interesting finding was that most of the mentioned parameters did not differ significantly between the TMD subgroups except the MCO with better results in the MP group. The reason for better improvement of the MCO results in the MP when comparing to the DD subgroup is probably the fact that in the DD group the biological barrier (displaced disc) is blocking the condyle without the possibility for restoring its position (26), thus limiting the full range of opening.

PSS scores did not differ significantly between treatment groups, although they changed significantly over time. The decreased PSS scores showed that psychological stress, considered the predisposing factor for TMD (27), was positively affected by treatment duration but had little to do with the type of the treatment.

Similar long-term effectiveness of SS was found by Ekberg et al. They monitored patients over 6 and 12 months and recommended the SS appliance for further use in TMD management (28). Moreover, Alajbeg et al. found that, when compared to placebo and amitriptyline therapy, SS showed a significantly greater change in the MCO (29). Still, in studies with the goal to compare TMD treatment options, patients have been monitored over different time periods, hence the inconsistent results can be attributed to different therapy duration. Furthermore, the design of occlusal or nonocclusal devices differ to a great extent and it is difficult to provide comparable results (20, 21, 28, 29).

The limitation of the present study lies in the fact that only female experiencing chronic, moderate to severe pain, whose response to the therapy may be different than in those with mild pain, were included in the study. However, the main advantage of the study was that we carefully selected and followed the subjects for a longer period of time with validated and widely accepted TMD diagnostic questionnaires and protocols, whereby our results showed that the specific design of the stabilization splint contributes to the treatment of TMD.

Conclusion

Due to significant improvement in spontaneous pain, health-related quality of life and functional limitations of the lower jaw in group treated with stabilization splint compared to placebo, we may conclude that stabilization splint showed better treatment effectiveness during a 6-month period. Moreover, significantely higher values of pain-free mouth opening were observed in patients treated with stabilization splint. Placebo might be partly responsible for improving spontaneous pain, probably even comparable with the effect of active therapy when observed in a short period of time, however, it seems that it cannot maintain a continual long-term positive therapeutic effect.