Exploring the Effects of Using an Oral Appliance to Reduce Movement Dysfunction in an Individual With Parkinson Disease: A Single-Subject Design Study : Journal of Neurologic Physical Therapy

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Exploring the Effects of Using an Oral Appliance to Reduce Movement Dysfunction in an Individual With Parkinson Disease: A Single-Subject Design Study

Lane, Hillary PT, DPT; Rose, Lindsey E. PT, DPT; Woodbrey, Megan PT, DPT; Arghavani, David DDS; Lawrence, Michael MS; Cavanaugh, James T. PT, PhD

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Journal of Neurologic Physical Therapy 41(1):p 52-58, January 2017. | DOI: 10.1097/NPT.0000000000000160



Movement dysfunction is a hallmark of Parkinson disease (PD). Gait and balance dysfunction, in particular, often become major barriers to mobility, potent risk factors for falls, and are frequently reported as detrimental to quality of life.1 Unfortunately, pharmacologic interventions used to treat PD symptoms have mixed effectiveness for improving gait and balance dysfunction.2 Exercise, in contrast, especially that targets specific gait and balance impairments, increasingly has been viewed as a necessary component of comprehensive PD intervention.3 Nonetheless, falls remain a major concern in the PD population,4,5 and accordingly, interventions designed to improve gait and balance remain a focus of contemporary PD research.6,7

Two small clinical reports, including a recent case report8 and an earlier case series,9 suggested that the use of an oral appliance originally designed for temporomandibular joint (TMJ) dysfunction, may also improve gait, balance, and grip strength in persons with PD.8,9 The articles were an extension of an older literature on TMJ dysfunction and pain that previously had described the positive, collateral effects of oral appliance wear on oral motor function, sleep, fatigue, strength, posture, and balance.10–12 The reports8,9 were the first to describe gait or balance outcomes associated with oral appliance wear in individuals with movement dysfunction due to a neurological condition. Importantly, the reports were limited by a lack of experimental control and relied on visual analysis of posture and gait as the primary means to describe intervention outcomes. Furthermore, they provided limited information regarding the design of oral appliances used.

The mechanism underlying the effect of oral appliance wear on human gait and balance remains a matter of speculation. Hypothetically, by restoring the proper vertical dimension of the maxillary-mandibular relationship, oral appliance wear reduces irritation of the auriculotemporal nerve arising from entrapment or compression associated with stress, trauma, jaw clenching, or teeth grinding.13 Reduced irritation, in turn, is thought to normalize sensory impulses traveling along the auriculotemporal nerve, a branch of the mandibular division of the trigeminal nerve, and its influence on postural reflexes and limb motor control mechanisms arising in the reticular formation.10,13

The primary aim of this exploratory research study was to address the design and measurement limitations of previous case reports by employing a single-subject design, 3-dimensional motion analysis, and a variety of measures to identify potential oral appliance-mediated changes in the gait, balance, and grip strength of an independently ambulatory individual with PD. Secondarily, we sought to examine the impact of routine oral appliance wear on the self-perceived health-related quality of life (HRQOL), gait, balance, and daily functioning of the participant. We hypothesized that any observable differences in movement dysfunction associated with oral appliance wear should be evident using quantitative measurement procedures. Finally, we sought to identify preliminary oral appliance specifications for vertical bite dimension that might produce a therapeutic effect on gait and balance.



The participant was a community-dwelling, English-speaking, 76-year-old Caucasian, man who approached the research team with anecdotal evidence suggesting the use of an oral mouthpiece for improving PD movement capability. Having been diagnosed with PD 4 years previously, the participant presented to the team with bilateral motor symptoms of rigidity and slight tremor, more so on his left side than the right, as well as axial rigidity and mildly decreased balance (modified Hohen and Yahr Stage = 2.5). His PD medication regimen included a variety of agents, producing a levodopa daily equivalent dosage of 1475 mg/d.14 He reported a previous medical history including long-term low back pain, spinal stenosis, and background tinnitus. Past surgical history included L3/L4 discectomy in 2003 and L1-L4 foraminectomy in 2007. He was independent with all activities of daily living and mobility, including driving, and participated in daily exercise.

Physical examination revealed that the participant's gross strength generally was 4/5 throughout the major muscle groups of the upper and lower extremities. His sensation was intact, except for bilateral impairments of light touch and sharp-dull discrimination in L3-S1 and L5-S1 spinal dermatomes, respectively. His mean gait speed, as measured on 3 trials of the 10-m walk test,15 was 1.6 m/s at a self-selected pace and 2.0 m/s at a fast pace. His 2-trial mean score on the Four Square Step Test (FSST)16 was 8.1 seconds. Although he denied recent falls, he scored an 18/28 on the Performance Oriented Mobility Assessment17 and scored 18/28 on the Mini-BESTest.18


Clinical, gait, balance, and grip strength data were collected over a 7-day period in the Motion Analysis Laboratory at the University of New England in Portland, Maine. To minimize the possibility of participant fatigue and to ensure that he remained in the “on-phase” of his PD medication cycle during testing, data collection was divided into 3 testing sessions limited to 1.5 hours each. Clinical data were collected during the first session. Gait, balance, and grip strength data were collected using a single-subject (A-B-A) design during the remaining sessions. Survey data regarding the self-perceived impact of routine oral appliance wear on HRQOL, gait, balance, and daily functioning were collected separately.

Gait and balance were assessed during the performance of FSST, serpentine walk, and tandem walk tasks. The tasks were chosen to represent a spectrum challenge that could be assessed in a relatively small amount of space using 3D motion analysis methodology. For each task, the baseline and withdrawal (A) phases were conducted without the appliance, whereas the intervention (B) phase was conducted with the appliance worn in place. Given the potential for participant fatigue, only 6 trials were conducted within each phase. The FSST and serpentine walk assessments were completed during the second session; the tandem walk and grip strength assessments were completed during the third session. The Institutional Review Board for the Protection of Human Subjects at the University of New England approved the study protocol, and the participant provided his informed consent.


Prior to data collection, the participant underwent a comprehensive dental evaluation for the sole purpose of fabricating a maxillary orthotic. Maxillary and mandibular alginate impressions were made using prefabricated perforated impression trays. The maxillary cast was mounted on a semiadjustable articulator (Denar Mark 320, Whipmix Corp, Lexington, Kentucky) using a facebow transfer record (Slidematic Facebow, Whipmix Corp, Lexington, Kentucky). The mandibular cast was mounted based on a centric relation interocclusal record that was made using polyvinyl siloxane (Blue Mousse, Parkell, New York). The resulting maxillary orthotic, which had a 3-mm increase in vertical bite dimension, was fabricated by a local dental laboratory. The appliance was delivered to the participant at a follow-up dental visit, when it was adjusted for even distribution of occlusal contacts in centric relation, posterior disclusion in protrusive and lateral mandibular movements, and proper fit. The participant was seen once for postdelivery adjustments of occlusion and the fit of the orthotic.


The FSST was conducted according to the standard protocol.16 For the serpentine walk task, the participant was required to walk in a “figure 8” pattern around 4 equally spaced cones spread over a 4.6-m distance. The tandem walk required the participant to walk heel to toe along a 3-m tapeline. For each of these tasks, 1-minute rest breaks were observed between each trial and 2-minute rest breaks were observed between phases. Grip strength of both the dominant and nondominant hands was recorded in pounds using a Jamar hand dynamometer (Model 5030J1, Sammons Preston Royolan, Bolingbrook, Illinois). For this task, rest breaks lasting 30 seconds were observed between each trial and rest breaks lasting 2 minutes were observed between phases.


During the FSST, serpentine walk, and tandem walk, the motion of each limb, trunk, and pelvis was tracked with 8 Oqus Series-3 cameras (Qualisys AB, Gothenburg, Sweden) using a sampling frequency of 120 Hz. Kinetic data were collected with 3 AMTI force plates (AMTI, Watertown, Massachusetts) using a sampling frequency of 1200 Hz. Clusters of retroreflective markers were placed on the pelvis over the posterior superior iliac spine and sacrum, thigh, lower leg, and forearms. Markers were also placed on the subject's bony landmarks: medial and lateral malleoli; first, third, and fifth metatarsal heads; upper and lower calcaneus; anterior superior iliac spines; acromion processes; medial and lateral wrist; medial and lateral epicondyles at the elbow and femur; deltoid tuberosity; occiput and approximately over the 10th thoracic and 7th cervical vertebra. The pelvis was constructed using a modified Helen-Hayes pelvis (CODA).19 Hip joints were determined with a regression formula20,21 and the knee, ankle, wrist, and elbow joints were defined as the midpoint between the respective medial and lateral markers. Segments were all allowed 6 degrees of freedom.

Kinetic and kinematic data collected during the FSST, serpentine, and tandem walk trials were used to generate a series of parameters (Table). The parameters had been decided upon a priori, based on hypothesized changes in motor coordination, gait, and balance during performance of the tasks with the oral appliance in place. Because of the aforementioned limitations in the previous literature and the exploratory nature of the study, we elected to treat all variables equally; none were considered as primary or secondary. Most of the parameters also seemed to the research team to be ones over which the participant would have limited conscious awareness or volitional control. Data were analyzed using Visual 3D (C-Motion, Inc, Germantown, Maryland).

Table. - Kinematic and Kinetic Parameters Recorded During Each Trial
Parameter Task Description
Limb stance time FSST, tandem Time (s) in single-limb stance; time (s) in double-limb stance
Foot clearance FSST Average maximum height (m) of center of gravity of the foot from floor
Center of mass trajectory FSST, tandem, serpentine Position of pelvis COG relative to laboratory during trial
Braking force FSST Force (N) on the body while slowing down during direction changes
Movement time FSST, tandem Overall time (s) to complete task
Trunk orientation in space FSST, tandem, serpentine Lateral lean of trunk (degrees) relative to floor
Arm position Tandem Distance (m) of each wrist to ipsilateral hip
Abbreviations: COG, center of gravity; FSST, Four Square Step Test.

Self-perceived HRQOL was assessed using the Parkinson Disease Questionnaire (PDQ-39)22 before and after 1 month of wearing the oral appliance during daily activities. The self-administered scale includes 39 items comprising 8 domains (ie, mobility, activities of daily living, emotions, stigma, social support, cognition, communication, and body discomfort). Raw scores are transformed on to a 0 (perfect health) to 100-point (worst health) scale. The reliability, validity, and sensitivity to change of the PDQ-39 have been established in community-dwelling individuals with PD.22 A minimally important difference value of 1.6 has been reported previously for the overall score.23

Following 1-month of wearing the oral appliance, the participant also rated changes in his gait, balance, strength, and activities of daily living function using a standard Global Rate of Change (GRC) scale.24 The GRC questions (eg, “To what extent did wearing the mouthpiece improve your standing balance during activities of daily living?” or “To what extent did wearing the mouthpiece impact the ease of your movement during walking at home and in the community?”) were designed specifically for the study and were based on an 11-point scale. Scores between 0 and −5 indicated that the mouthpiece made standing balance or ease of movement worse, whereas scores between 0 and +5 indicated the mouthpiece made each experience better. A change score of at least 2 points on an 11-point GRC scale has been recommended previously as an indicator of clinically meaningful change.24

Data Analysis

Visual inspection of FSST, serpentine walk, tandem walk, and grip strength data were conducted by plotting trial values for each parameter using Microsoft Excel (Microsoft Corporation, Redmond, Washington). Mean values were calculated within baseline, intervention, and withdrawal phases. Celeration lines were constructed for each phase, their slopes calculated, and the 2-standard deviation (SD) band method was used to help evaluate the significance of changes in movement capability during the intervention phase.25 A visual analysis was also conducted of marker trajectories for the participant's extremities, trunk, and pelvis center of gravity (COG) during each trial. PDQ-39 change scores and GRC ratings in response to survey questions were compared with the previously published minimal clinically significant change values.


Four Square Step Test

The participant completed the FSST more quickly during the intervention phase (mean ± SD = 8.03 ± 0.24 seconds) compared with the baseline (9.76 ± 0.44 seconds) and withdrawal (9.74 ± 0.82 seconds) phases. The change during intervention was considered significant because it exceeded 2 SDs of the baseline level (Figure 1). There were no visible between-phase differences in foot clearance from the floor or braking force values. The mean ± SD trajectory of the participant's pelvic COG is depicted in Supplemental Digital Content 2 (https://links.lww.com/JNPT/A156). The tracing shows a path traveled more closely toward the center of the FSST area during the intervention phase. Left lateral trunk lean was less (ie, the participant stood more upright) during the intervention trials, in excess of 2 SDs of the mean baseline and withdrawal trials (Figure 2).

Figure 1.:
Four Square Step Test—completion time (seconds). A, B, and A represent the baseline, intervention, and withdrawal phases, respectively. The shaded area represents 2 standard deviations above and below mean baseline and withdrawal phases. Celeration line slopes: ABaseline = −0.01, B = −0.01, AWithdrawal = 0.69.
Figure 2.:
Four Square Step Test—amount of trunk lean to the left from a vertical axis (centimeter) throughout each trial. A, B, and A represent the baseline, intervention, and withdrawal phases, respectively. The shaded area represents 2 standard deviations above and below mean baseline and withdrawal phases. Celeration line slopes: ABaseline = −1.29, B = 0.35, AWithdrawal = −0.33.

Serpentine Walk

There were no observable between-phase changes in trunk orientation or arm range of motion during the serpentine walk task. In addition, the mean ± SD trajectory of the participant's pelvic COG was consistent across phases.

Tandem Walk

The participant completed the tandem walk more quickly during the intervention phase (9.15 ± 0.74 seconds) compared with the baseline (11.21 ± 0.90 seconds) and withdrawal (11.70 ± 3.21 seconds) phases (Figure 3); however, the difference was not significant. Similarly, there was a nonsignificant difference in double-limb support time during intervention (mean ± SD = 0.32 ± 0.23 seconds) compared with baseline (0.43 ± 0.32 seconds) or withdrawal (0.48 ± 0.39 seconds) phases (Figure 4). The mean trajectory of the participant's pelvic COG reveals a path traveled more closely to the line while wearing the mouthpiece than when not wearing it (see Supplemental Digital Content 3, https://links.lww.com/JNPT/A157). In addition, he held his arms generally closer to his sides during intervention, as indicated by a significant decrease in arm height relative to the hip (Figure 5). Overall, the participant appeared more stable during the tandem walk when wearing the oral appliance (see Supplemental Digital Content 4, https://links.lww.com/JNPT/A158).

Figure 3.:
Tandem walk task completion time (seconds). A, B, and A represent the baseline, intervention, and withdrawal phases, respectively. The shaded area represents 2 standard deviations above and below mean baseline and withdrawal phases. Celeration line slopes: ABaseline = 0.57, B = 0.13, AWithdrawal = 1.48.
Figure 4.:
Tandem walk task—double-limb support time (seconds). A, B, and A represent the baseline, intervention, and withdrawal phases, respectively. The shaded area represents 2 standard deviations above and below mean baseline and withdrawal phases. Celeration line slopes ABaseline = 0.005, B = 0.005, AWithdrawal = −0.007.
Figure 5.:
Tandem walk task—height of the right wrist from the hip (meter). A, B, and A represent the baseline, intervention, and withdrawal phases, respectively. The shaded area represents 2 standard deviations above and below mean baseline and withdrawal phases. Celeration line slopes: ABaseline = 0.005, B = 0.02, AWithdrawal = −0.05. Positive numbers indicate that the wrist was held lateral to the hip joint; negative numbers indicate that the wrist was held medial to the hip joint.

Grip Strength

The participant demonstrated increased grip strength while wearing the oral appliance, but the magnitude of the effect was slightly different between left and right sides. With his more impaired left hand, he produced a mean ± SD of 37.2 ±1.9 lb of force while wearing the oral appliance compared with a mean of 29.2 ± 3.4 lb at baseline and 35.2 ± 3.1 lb at withdrawal. The difference between A and B phases was marginally significant (Figure 6). With the right hand, however, the intervention appeared to have a smaller, nonsignificant effect (40.0 ± 3.0 lb of force during intervention; 35.33 ± 3.3 lb during baseline; 38.5 ± 3.3 lb during withdrawal; Figure 7).

Figure 6.:
Grip strength (lb) results for the left hand. A, B, and A represent the baseline, intervention, and withdrawal phases, respectively. The shaded area represents 2 standard deviations above and below mean baseline and withdrawal phases. Celeration line slopes: ABaseline = 0.67, B = 0.33, AWithdrawal = −1.00.
Figure 7.:
Grip strength (lb) results for the right hand. A, B, and A represent the baseline, intervention, and withdrawal phases, respectively. The shaded area represents 2 standard deviations above and below mean baseline and withdrawal phases. Celeration line slopes: ABaseline = −1.0, B = −0.5, AWithdrawal = 0.

Survey Data

Self-perceived HRQOL improved from baseline (PDQ-39 score = 22.03) to follow-up (PDQ-39 score = 19.48). The 2.55-point decrease in overall score exceeded a previously established minimally important difference value of 1.6.23 The change in overall score appeared to have been driven primarily by decreases in emotional well-being (Δ = −8.33), communication (Δ = −8.33), and stigma (Δ = −6.25) domain scores, all of which also exceeded their respective minimally important difference values.23

The participant reported that he wore the oral appliance almost daily and that it did not inhibit his daily routine. He reported no adverse events. Based on his GRC scale ratings after 1 month of use, he perceived improvements in his ease of movement in the community (+3), ease of movement while performing activities of daily living (+4), and standing balance while performing activities of daily living (+4). The change scores exceeded the previously recommended minimally important change value of 2 points.24 He reported no change regarding the impact of oral appliance wear on strength while completing daily activities.


The purpose of this study was to explore the potential impact of wearing an oral appliance designed to increase vertical bite dimension on the movement capability and quality of life of an individual with mid-stage PD. Our results revealed that in association with wearing an oral appliance that created a 3-mm increase in vertical bite dimension, the participant appeared to display relatively modest task-specific improvements in speed of movement (Figure 1), postural stability (Figure 5; Supplemental Digital Content 4), upright posture (Figure 2), the accuracy of whole body movement trajectory in space (Supplemental Digital Content 2 and 3), and strength in his more impaired upper extremity (Figure 6). Separately, the participant reported an improvement in HRQOL, balance, and ease of movement after wearing the appliance for 1 month. To our knowledge this was the first study that attempted to systematically quantify the effects that had previously been described in 2 clinical reports.

Many of the observed changes with oral appliance wear were likely to have been interrelated. The increased speed of performance may have explained the decreased double support time (FSST only) and may have resulted from his more accurate trajectory in space. The more accurate trajectory and improvements in postural stability may have been influenced by his more upright posture. Improved force production capability, if occurring throughout the entire left side, may have influenced speed and posture. Although our measures of walking speed did not include a standard timed walk test, the walking speed for the 3-m tandem walk test equates to 0.27 m/s at baseline, and 0.33 m/s in the intervention phase. This 0.06-m/s difference, although not statistically significant, met the anchor-based criteria for a moderate meaningful change in walking speed for persons with PD.26

Not all of the movement parameters included in the study revealed evidence of reduced movement dysfunction during device wear. In particular, trunk rotation and lateral lean, arm swing range of motion, and pelvic COG trajectory collected during the serpentine walk task, as well as braking forces and time spent in single- or double-limb support during the FSST, were noncontributory. Possible explanations for the negative findings included a lack of parameter sensitivity to change or that the tasks may not have been challenging enough to emphasize the participant's deficits. Alternatively, the participant's clinical presentation (ie, relatively unimpaired gait speed combined with observable balance impairment) may have influenced which measures did nor did not improve with intervention. It is also plausible that the increased vertical bite dimension created by the oral appliance may not have been sufficient to produce a change in some parameters.

Like the motion analysis data, the survey data also suggested a positive effect of appliance wear. The PDQ-39 revealed improvements in self-perceived emotional well-being, communication, and stigma. Interestingly, the PDQ-39 did not reveal the self-perceived reduction in movement dysfunction that was captured by the GRC items. Such differences are likely to be related to differences in the constructs of each measure and their responsiveness to change.

Hypothetical Mechanism

What remains unknown is the underlying causal explanation for the observed changes. Clearly, despite the researchers' attempt to select variables over which he would have limited control, the participant could simply have exerted greater effort when wearing the device; that is, the observed changes may have resulted from his own expectations, or perhaps his awareness of the researchers' expectations, for improvement. More speculatively, however, and in line with the underlying hypothesis supporting the use of an oral appliance reducing movement dysfunction,10,13 the changes in his performance may have resulted from neurophysiological changes induced by using the oral appliance to restore a more proper vertical bite dimension.

Although the details of the neurophysiological hypothesis are beyond the scope of this article, it is based on known neuroanatomical projections from trigeminal nuclei to the nucleus raphe of the reticular formation. Efferent functions of the reticular formation influence postural reflexes and righting reactions and play a critical role in phasic movement and muscle tone maintenance.10 A small number of animal and human studies suggest that afferent stimulation of trigeminal nuclei can influence postural and limb movement activity. For example, stimulation of TMJ tissues can trigger postural reflexes in rats.27 Similarly, several human studies in the dental literature describe the maxillary-mandibular relationship (ie, vertical bite dimension) and its positive or negative influence on posture.9,11,28,29

For persons with PD, the prevalence of altered vertical bite dimension has not been directly investigated and remains generally unknown. However, there is indirect, albeit nominal, evidence suggesting that at least some individuals with PD experience orofacial muscular control problems that contribute in part to diminished dental health.30,31 By extension, therefore, one might presume that orofacial muscular control problems logically could affect the maxillary-mandibular relationship, and therefore, alter the afferent input of the trigeminal nerve on the reticular formation.

To be clear, the neurophysiological hypothesis does not explicitly state that posture and gait problems in person with PD are the exclusive result of altered vertical bite dimension. At best, the hypothesis suggests that altered vertical bite dimension might be contributory. Previous, small sized, uncontrolled clinical reports provided the only, albeit weak, evidence of improved gait and balance resulting from the use of an oral appliance to restore vertical bite dimension.10,13 Our exploratory study, which was designed to be modestly more objective and better controlled than the previous studies, simply added another piece of tenuous evidence to the small body of literature that describes the hypothesis and provides an example of its predicted effect. Much more remains to be known before the hypothesis can be considered to be compelling.


Inherent limitations are known to occur in single-subject design research. First, study results may be extrapolated only to patients with characteristics similar to those of the participant. Unlike most individuals with PD, the participant in this study had preconceived notions about the benefits of the oral appliance. His expectations may have spuriously influenced the kinematic results and biased his perceptions about the ease and benefit of wearing the appliance on a daily basis. Second, the internal validity of the study was limited by the inability to blind the patient while using the oral appliance. Third, there may have been task performance carryover effects between the baseline and intervention phases. Participant performance within some phases did not appear to be stable, suggesting possible learning or fatigue confounders. Finally, the order of tasks may have influenced participant performance.

Future Directions

Our study adds to a small body of preliminary, descriptive evidence that suggests a benefit to wearing an oral appliance for persons with PD who experience gait and balance dysfunction. Subsequent research will be needed to determine the potential effect of oral appliance wear in a much larger sample of affected individuals, across various stages of disease severity, and either alone or in combination with gait and balance training. The proposed hypothetical mechanism underlying the observed changes warrants scientific scrutiny. Further studies that seek to identify primary outcome variables with relatively large effect sizes or that examine kinematic measures beyond the scope of this study would be beneficial in establishing the validity and generalizability of the intervention. Finally, assuming that stronger evidence begins to emerge, evaluating the differential effects of various appliance thicknesses on movement capability would be helpful in determining optimal prescription parameters.


This study provided quantitative, descriptive evidence regarding the potential effect of wearing an oral appliance on the movement capability of an individual with mid-stage PD. Improvements were noted in some aspects of movement, postural control, and HRQOL. Although the results of the study are promising, future research with a larger sample is necessary to improve upon the current knowledge of the effectiveness of this intervention.


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balance; gait; intervention; physical therapy

Supplemental Digital Content

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