Effects of Hard Gum Chewing on Facial Soft Tissue Morphology

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.