Effects of Hard Gum Chewing on Facial Soft Tissue Morphology

Facial morphology is influenced by multiple functional factors, including masticatory muscle activity. Although hard gum chewing has been promoted as a method to improve facial definition and jawline appearance, there is limited scientific evidence demonstrating whether intentional chewing can produce measurable changes in facial soft tissues.

The purpose of this prospective pilot clinical trial is to evaluate the effect of intentional hard gum chewing on facial morphology using three-dimensional stereophotogrammetry (3dMD). Twenty healthy adults aged 20-40 years will be enrolled. Participants will complete a standardized hard-gum chewing protocol every other day for 4 weeks. Each chewing session will last approximately 16 minutes and will follow a predefined protocol designed to provide a consistent masticatory stimulus.

Three-dimensional facial images will be obtained at baseline (T0), immediately after the intervention period (T1), and after a 4-5-week washout period (T2). Facial changes will be assessed through surface registration techniques, heatmap analysis, regional displacement measurements, and linear and angular soft-tissue measurements. Participants will also complete questionnaires regarding muscle fatigue, soreness, oral behaviors, and temporomandibular symptoms.

The primary objective is to determine whether intentional hard gum chewing produces measurable changes in facial morphology. Secondary objectives include evaluating participant-reported outcomes and monitoring for adverse events or symptoms suggestive of temporomandibular disorders.

Because this is a pilot study, the results will be used to estimate effect sizes and feasibility for future larger clinical trials investigating the relationship between masticatory activity and facial soft-tissue adaptation.

• Hard Gum Chewing (Behavioral): Participants will chew a standardized hard chewing gum (Jawliner®, hard hardness level) every other day for 4 weeks. Each session will consist of two 8-minute chewing blocks following a standardized protocol with controlled chewing pace, progressive force application, and alternating chewing sides. Total chewing time per session will be approximately 16 minutes.

Primary outcomes

• Changes in Facial Morphology Assessed by 3dMD Stereophotogrammetry [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Quantification of facial morphology changes using three-dimensional stereophotogrammetry, including regional surface distance maps (heatmaps), linear and angular measurements, and global and regional RMS deviation values.

Secondary outcomes

• Soft-Tissue Surface Topography Changes [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Evaluation of localized hypertonia and volumetric shifts using three-dimensional heatmap analysis and regional surface displacement measurements.
• Change in Body Weight [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Body weight will be measured in kilograms using a calibrated digital scale to monitor changes in body weight that could influence facial soft-tissue morphology.
• Body Height [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Standing height will be measured in centimeters using a Scale. Height will be used together with body weight to characterize participants' body size and, when applicable, to calculate body mass index.
• Participant-Reported Muscle Fatigue, Soreness, and Facial Definition Perception [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Self-reported outcomes related to muscle fatigue, soreness, chewing effort, and perceived facial definition collected through study questionnaires.
• Diagnostic Criteria for Temporomandibular Disorders (DC/TMD) Symptom Questionnaire [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Assessment of temporomandibular disorder symptoms using the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD) Axis I Symptom Questionnaire. This questionnaire is part of the standardized DC/TMD diagnostic protocol and evaluates jaw pain, headache attributed to TMD, joint noises, jaw locking, and functional symptoms. Responses are recorded as presence or absence of symptoms and do not generate a total score. Presence/absence of symptoms (Yes/No). This questionnaire does not generate a total score.
• Oral Behaviors [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Assessment of oral parafunctional behaviors using the Oral Behaviors Checklist (OBC), one of the DC/TMD Axis II instruments. The questionnaire consists of 21 items assessing awake and sleep-related oral behaviors. A total score ranging from 0 to 84 will be calculated, with higher scores indicating more frequent oral parafunctional behaviors.
• Patient Health Questionnaire-4 (PHQ-4) [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Assessment of psychological distress using the Patient Health Questionnaire-4 (PHQ-4), included as part of the DC/TMD Axis II psychosocial assessment. Total scores range from 0 to 12, with higher scores indicating greater psychological distress.
• Jaw Functional Limitation Scale-8 (JFLS-8) [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Assessment of jaw functional limitation using the Jaw Functional Limitation Scale-8 (JFLS-8), a DC/TMD Axis II instrument. The questionnaire evaluates limitations in mastication, jaw mobility, and verbal and emotional expression. Scores range from 0 to 10, with higher scores indicating greater functional limitation.
• Patient Health Questionnaire-9 (PHQ-9) [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Assessment of depressive symptoms using the Patient Health Questionnaire-9 (PHQ-9), administered as part of the DC/TMD Axis II psychosocial evaluation. Total scores range from 0 to 27, with higher scores indicating greater depression severity.
• Jaw Functional Limitation Scale-20 (JFLS-20) [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Assessment of jaw functional limitation using the Jaw Functional Limitation Scale-20 (JFLS-20), a DC/TMD Axis II instrument. Scores range from 0 to 10, with higher scores indicating greater functional limitation.
• Generalized Anxiety Disorder-7 (GAD-7) [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Assessment of anxiety symptoms using the Generalized Anxiety Disorder-7 (GAD-7), administered as part of the DC/TMD Axis II psychosocial evaluation. Total scores range from 0 to 21, with higher scores indicating greater anxiety severity.
• Patient Health Questionnaire-15 (PHQ-15) [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Assessment of somatic symptom burden using the Patient Health Questionnaire-15 (PHQ-15), administered as part of the DC/TMD Axis II psychosocial evaluation. Total scores range from 0 to 30, with higher scores indicating greater somatic symptom severity.
• Diagnostic Criteria for Temporomandibular Disorders (DC/TMD) Clinical Examination [Baseline (T0), Post-Intervention (T1, 4-5 weeks), and Washout (T2, 8-9 weeks)]: Clinical examination performed according to the Diagnostic Criteria for Temporomandibular Disorders (DC/TMD) Axis I Examination Protocol, including mandibular range of motion (mm), pain on palpation of the temporomandibular joints and masticatory muscles, pain during mandibular movements, joint sounds, and diagnostic clinical findings. Continuous variables will be reported in millimeters, and categorical findings will be reported as present or absent.

Sex: All, ages: 20 Years to 40 Years

Inclusion Criteria:

* Healthy, non-smoking adults.
* Age 20 to 40 years.
* Presence of a full dentition from central incisor to first molar in all four quadrants, excluding second molars.
* Stable occlusion with no ongoing orthodontic treatment or myofunctional therapy.
* Ability and willingness to comply with the prescribed hard-gum chewing regimen for 4 weeks and attend all study visits.
* Ability to provide written informed consent.

Exclusion Criteria:

* Age younger than 20 years or older than 40 years.
* Evident facial asymmetry from a frontal view.
* Signs or symptoms of temporomandibular disorders (TMD) according to the Diagnostic Criteria for TMD (DC/TMD).
* Less than the normal range of condylar motion during maximum opening and closing.
* Parafunctional habits such as bruxism with pain, clenching, or habitual gum chewing for more than 1 hour per day.
* History of major craniofacial trauma, orthognathic surgery, or congenital craniofacial anomalies.
* Ongoing orthodontic treatment or recent removal of fixed appliances (<6 months).
* Significant malocclusion or missing teeth in the anterior or premolar-first molar region that may affect mastication.
* Systemic diseases or medications affecting muscle or bone metabolism.
* Previous use of botulinum toxin (Botox) within the last 6 months or anticipated use during the study period.
* Pregnancy or lactation.
* Allergy or intolerance to the chewing gum ingredients.

• Results publication: Tzakis MG, Kiliaridis S, Carlsson GE. Effect of a fatigue test and chewing training on masticatory muscles. J Oral Rehabil. 1994 Jan;21(1):33-45. doi: 10.1111/j.1365-2842.1994.tb01122.x. PMID 8133387
• Results publication: Tzakis MG, Kiliaridis S, Carlsson GE. Effect of chewing training on masticatory efficiency. Acta Odontol Scand. 1989 Dec;47(6):355-60. doi: 10.3109/00016358909004803. PMID 2609943
• Results publication: Kiliaridis S, Tzakis MG, Carlsson GE. Effects of fatigue and chewing training on maximal bite force and endurance. Am J Orthod Dentofacial Orthop. 1995 Apr;107(4):372-8. doi: 10.1016/s0889-5406(95)70089-7. PMID 7709901
• Results publication: Shirai M, Kawai N, Hichijo N, Watanabe M, Mori H, Mitsui SN, Yasue A, Tanaka E. Effects of gum chewing exercise on maximum bite force according to facial morphology. Clin Exp Dent Res. 2018 Feb 22;4(2):48-51. doi: 10.1002/cre2.102. eCollection 2018 Apr. PMID 29744215

• Flavio Uribe, Principal investigator, UConn School of dental medicine
• Thais Fernandes Poleti, PhD, Principal investigator, UConn School of dental medicine