Facial volume reduction and masticatory function after masseter botulinum toxin injection: a prospective study

Abstract

Background

Botulinum toxin type A (BTX-A) is widely used for masseter reduction to improve lower facial contour. While its esthetic effects are well established, the potential impact on masticatory function remains unclear, particularly in healthy individuals. This study aimed to evaluate changes in facial volume, patient satisfaction, and perceived chewing function following BTX-A injection.

Methods

A quasi-experimental clinical trial was conducted in 24 healthy adults who received bilateral masseter injections of BTX-A (50 units total). Lower face volume was assessed using 3D stereophotogrammetry at baseline and at 4, 8, and 12 weeks post-injection. Patient satisfaction was measured using the FACE-Q™ questionnaire, and masticatory function was assessed with the Chewing Function Questionnaire (CFQ) and visual analog scale (VAS). Friedman and Wilcoxon signed-rank tests were used for statistical analysis.

Results

Significant reductions in lower face volume were observed at all follow-up time points (p < 0.05), with the greatest change occurring by week 12. Satisfaction scores with facial appearance and treatment outcome significantly increased. Masticatory function remained stable, with no significant changes in CFQ or VAS scores. Participants reported no perceived decline in chewing ability throughout the study period.

Conclusion

Our results suggest that botulinum toxin injection into the masseter muscle may reduce lower face volume and improve patient satisfaction without compromising perceived masticatory function in healthy adults. These findings provide preliminary evidence for the esthetic application of BTX-A and may contribute to future investigations into its potential roles in areas such as bruxism management and prosthodontic planning.

Trial registration

: TCTR20240617001. Registered on June 17th, 2024.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12903-025-06990-4.

Introduction

Botulinum toxin type A (BTX-A) is widely used for lower facial contouring by reducing masseter volume, particularly in individuals with masseter hypertrophy [1–3]. Through temporary chemodenervation, BTX-A induces progressive masseter atrophy and soft tissue reduction [4]. This non-surgical approach has gained popularity, especially among East Asian populations seeking facial slimming [5].

While esthetic outcomes of BTX-A are well established [6, 7], increasing attention has been paid to its potential effects on oral function. Some clinicians have expressed concern that reduced masseter activity may compromise masticatory function or occlusal balance [8–10]. Conversely, BTX-A has been investigated in dental contexts for managing bruxism [11] and temporomandibular disorders [12], and occasionally discussed as an adjunct in oral rehabilitation, although supporting evidence remains limited.

Despite its expanding use, studies simultaneously assessing esthetic and functional outcomes are scarce, particularly in healthy individuals treated for cosmetic reasons [10]. Moreover, many previous investigations have relied on subjective assessment or visual inspection [13, 14], limiting objectivity and reproducibility. Integrating validated measures such as three-dimensional (3D) stereophotogrammetry for volume analysis [15, 16] and patient-reported instruments like the FACE-Q questionnaire [17, 18] may provide a more comprehensive evaluation of both physical changes and patient satisfaction. However, studies combining these methods remain limited.

This study therefore aimed to evaluate both esthetic and functional outcomes of masseter BTX-A injection in healthy adults. We hypothesized that BTX-A would reduce lower face volume while maintaining perceived masticatory function and enhancing patient satisfaction.

Materials and methods

Study design

This quasi-experimental clinical trial was conducted at Mahidol Dental Hospital, Bangkok, Thailand, between July and October 2024. The study received ethical approval from the institutional review board (IRB No. MU-DT/PY-IRB 2024/024.1505) and was registered in the Thai Clinical Trial Registry (TCTR20240617001). All participants provided written informed consent before enrollment. This study adhered to the TREND (Transparent Reporting of Evaluations with Non-randomized Designs) statement. The completed TREND checklist is provided as supplementary material [19].

Study participants and sample size calculation

Eligible participants were Thai-speaking adults aged 20 years or older who had not received botulinum toxin injections in the masseter muscle during the past year. Individuals with neuromuscular disorders, allergies to botulinum toxin, pregnancy, or active infection at the injection site were excluded. Based on a previous study reporting lower face volume changes after bilateral masseter injections of 25 units per side [20], the sample size was calculated using G*Power (version 3.1.9), with α = 0.05 and power = 80%. This calculation indicated a minimum of 20 participants, and we recruited 24 to account for potential attrition.

Botulinum toxin injection procedure

Each participant received bilateral injections of 50 units of Nabota^® botulinum toxin (25 units per side), administered with a standardized 3-point technique by a single experienced maxillofacial surgeon. The toxin was diluted with 4 mL of saline (yielding 5 units per 0.1 mL) and delivered using a 1-mL syringe under aseptic conditions. Participants were monitored for 30 min after the injection to observe any immediate adverse reactions.

Patient satisfaction assessment

Patient satisfaction was measured using the FACE-Q™ questionnaire (Thai version). Permission to use the tool was granted by the original developers [21, 22]. The questionnaire was translated from English into Thai using standard forward–backward translation, followed by cognitive debriefing to ensure conceptual accuracy. Reliability testing confirmed a Kappa coefficient ≥ 0.70. The questionnaire included two domains: satisfaction with the lower face and jawline, assessed at baseline and all follow-up visits, and satisfaction with the overall treatment outcome, assessed only at follow-up. Responses were rated on a 4-point Likert scale and converted to a 0–100 scale, with higher scores indicating greater satisfaction.

Data collection and evaluation

Participants completed assessments at baseline (T0) and at 4 (T1), 8 (T2), and 12 (T3) weeks post-injection. Each visit followed a standardized sequence: completion of the FACE-Q questionnaire, standardized facial photography, 3D facial scanning, and subsequent volume analysis. To ensure methodological consistency and minimize variability, all procedures—including questionnaire administration, photography, scanning, and analysis—were performed by a single trained researcher. Because 3D volume analysis required serial image superimposition, blinding to time points was not feasible. Questionnaires were always completed prior to photography and scanning.

Facial volume assessment

Three-dimensional facial scans were obtained using the RAYFace scanner (RFS100, Ray Co., Korea), a stereophotogrammetry system. Images were processed using Geomagic Control X software. Serial scans were superimposed using a best-fit alignment to calculate volume changes. The region of interest was delineated by four anatomical reference planes: (1) a horizontal plane through the bilateral exocanthion (Ex–Ex′), (2) an upper boundary plane through the bilateral alare (Al–Al′), (3) a middle plane through the bilateral cheilion (Ch–Ch′), and (4) a lower boundary plane defined by the menton (Me), bilateral tragus (Tra–Tra′), and gonion (Go–Go′). The region was projected perpendicularly from the mid-sagittal plane. These planes and landmarks are illustrated in Fig. 1.

Fig. 1.

Three-dimensional facial imaging showing 11 anatomical reference points and 4 reference planes used for lower face volume assessment. The landmarks include exocanthion (Ex, Ex′), alare (Al, Al′), cheilion (Ch, Ch′), menton (Me), tragus (Tra, Tra′), and gonion (Go, Go′). Volume was calculated in the defined region projected perpendicularly from the mid-sagittal plane

Masticatory function assessment

Self-reported masticatory performance was assessed using two validated tools: a Thai-modified version of the 6-item Chewing Function Questionnaire (CFQ) [23, 24], and a visual analog scale (VAS) [8]. The CFQ measured perceived chewing difficulty across different food types, scored on a 5-point Likert scale with total scores ranging from 0 (worst) to 30 (best), where higher scores indicated better chewing performance. The VAS asked participants to rate their overall chewing ability from 0 (unable to chew) to 10 (normal chewing), with higher scores indicating better function. Both assessments were conducted at T0, T1, T2, and T3.

Data analysis

All data were analyzed using SPSS version 25. Descriptive statistics were used to summarize participant characteristics and outcome measures. Changes across time points were analyzed using Friedman and Wilcoxon signed-rank tests. A p-value < 0.05 was considered statistically significant. No missing data were recorded throughout the study.

Results

Participant characteristics

A total of 24 participants were enrolled in the study, comprising 18 females (75%) and 6 males (25%). The mean age was 37.38 ± 9.04 years (range 26–56). Body mass index (BMI) was evenly distributed, with 12 participants in the normal range (18.5–24.9) and 12 in the pre-obesity/obesity range (≥ 25). Most participants (79.2%) had no prior experience with botulinum toxin injections in the lower face. These demographic data are summarized in Table 1.

Table 1.

Demographic characteristics of participants (N = 24)

Variables	Total n (%)
Gender
Female	18 (75)
Male	6 (25)
Age (mean ± SD)	37.38 ± 9.04
≤ 35	12 (50)
> 35	12 (50)
BMI*
Normal	12 (50)
Pre-obesity/obesity	12 (50)
Botulinum toxin experience (Lower face area)
No	19 (79.2)
Yes	5 (20.8)

^*BMI Body Mass Index

Lower face volume reduction

Three-dimensional facial imaging analysis demonstrated a statistically significant reduction in lower face volume following botulinum toxin injection into the masseter muscle. The median volume significantly decreased from baseline (T0) to 4 (T1), 8 (T2), and 12 (T3) weeks post-injection on both the left and right sides, as well as in combined measurements (Table 2). On the left side, the median differences were − 2,489 mm³, −3,632 mm³, and − 2,731 mm³ at T1, T2, and T3 compared with T0 (p < 0.05; Table 3). The right side showed greater decreases, with median differences of −3,114 mm³, −6,790 mm³, and − 6,253 mm³ at T1, T2, and T3, respectively (p < 0.05; Table 3), with the largest change observed at 8 weeks. When both sides were combined, the overall lower face volume was reduced by −10,540 mm³ at 4 weeks, −7,597 mm³ at 8 weeks, and − 13,458 mm³ at 12 weeks (p < 0.05; Table 3).

Table 2.

The median volume (mm³) of 3D facial imaging analysis at baseline (T0), 4 (T1), 8 (T2), and 12 (T3) weeks after the botulinum toxin injection

Area	Volume [mm3, median (q1-q3)]				p-value*
	T0	T1	T2	T3
Left side	235,364 (219,078–269,013)	232,875 (214,548 − 263,442)	231,732 (215,273 − 261,103)	232,633 (213,779 − 263,688)	0.002
Right side	238,900 (223,246–263,735)	235,786 (218,858 − 257,902)	232,110 (219,339 − 257,633)	232,647 (219,831 − 257,853)	< 0.001
Both sides	481,030 (442,907 − 529,899)	470,490 (439,458–521,047)	473,433 (434,174–514,191)	467,572 (439,800 − 515,133)	< 0.001

*Friedman’s statistical analysis and the significant level was taken at p < 0.05

Table 3.

The median difference (mm³) of the lower face volume after the botulinum toxin injection between each time point (T0, T1, T2, T3)

Area	Volume difference (mm3, median)				p-value*
		T1	T2	T3
Left side	T0	−2,489	−3,632	−2,731	< 0.05
	T1	-	−1,143	−242
	T2	-	-	901
Right side	T0	−3,114	−6,790	−6,253	< 0.01
	T1	-	−3,676	−3,139
	T2	-	-	537
Both sides	T0	−10,540	−7,597	−13,458	< 0.005
	T1	-	2,943	−2,918
	T2	-	-	−5,861

^*p-values represent pairwise comparisons between T0 and subsequent time points (T1, T2, T3). Since all were significant, the least significant p-value is presented for each region. Comparisons among T1, T2, and T3 were not significant and are not shown

Subgroup analysis by participant characteristics

Numerical differences in lower face volume reduction were observed across participant subgroups. Male participants exhibited a greater median reduction of 10,587 mm³ (IQR: 3,807–17,975) compared to females (2,926 mm³; IQR: 976–7,618). Participants aged ≤ 35 years showed a greater median reduction of 6,917 mm³ (IQR: 1,128–14,806) than those > 35 years (2,926 mm³; IQR: 1,337–6,434). For BMI, individuals with normal BMI (18.5–24.9) demonstrated a greater reduction (6,293 mm³; IQR: 2,154–11,010) than those with overweight or obesity (2,773 mm³; IQR: 737–7,562). Participants with prior botulinum toxin experience showed a reduction of 6,047 mm³ (IQR: 1,506–9,270), while those without previous exposure showed 5,292 mm³ (IQR: 950–8,093). None of these subgroup differences reached statistical significance. As this analysis was exploratory and underpowered for between-group comparisons, the findings should be interpreted with caution.

Patient satisfaction

All participants completed the FACE-Q assessments at baseline and follow-up visits. Satisfaction with the lower face and jawline significantly improved, with median scores (IQR) increasing from 34.0 (34.0-50.5) at baseline (T0) to 66.0 (66.0-76.5) at 12 weeks (T3), representing a 32% increase (P < 0.001; Fig. 2). Satisfaction with the overall treatment outcome was also favorable, with 59% of participants reporting positive perceptions at week 12.

Fig. 2.

FACE-Q satisfaction scores for lower face and jawline, and overall treatment outcome at baseline (T0) and 4 (T1), 8 (T2), and 12 (T3) weeks after injection. Scores range from 0 to 100, with higher values indicating greater satisfaction

Masticatory function

Self-reported chewing ability remained stable across all time points. The median CFQ score (IQR) declined slightly from 27.5 (23.25-30.00) at baseline to 24.0 (21.25-30.00) at week 12, while VAS scores showed no change, remaining at a median of 10. The non-parametric analysis revealed no statistically significant differences (Fig. 3). Notably, participants consistently reported normal chewing ability and did not perceive functional limitations at any time point, suggesting that botulinum toxin injection did not perceptibly impair masticatory function. Item-level responses from the CFQ (Q1–Q6) are shown in Fig. 4, where only minor fluctuations were observed without a meaningful pattern of decline.

Fig. 3.

Self-reported chewing ability measured by the Visual Analog Scale (VAS; range: 0–10) and Chewing Function Questionnaire (CFQ; range: 0–30) at baseline (T0), and at 4 (T1), 8 (T2), and 12 (T3) weeks after botulinum toxin injection. Higher scores indicate better function. No statistically significant changes were observed over time

Fig. 4.

Median scores for each item of the Chewing Function Questionnaire (CFQ; Q1–Q6) across all time points. CFQ items evaluate chewing difficulty for various food textures on a 5-point scale (0 = most difficulty, 5 = no difficulty). Scores remained stable, with no significant changes across items or time points

Discussion

This study found that botulinum toxin injection into the masseter muscle significantly reduced lower face volume, with the most prominent change observed at 12 weeks post-treatment. Patient satisfaction with facial appearance and overall treatment outcome improved substantially, while participants consistently reported normal chewing ability across all follow-up points. Together, these findings support both the esthetic effectiveness and functional safety of the procedure in healthy individuals.

A key strength of this study lies in its integration of validated subjective and objective outcome measures. The use of high-precision 3D facial scanning allowed for accurate and reproducible quantification of volume changes—offering a methodological advantage over earlier studies that relied on calipers or visual assessment [20, 25, 26]. In parallel, patient-reported outcomes collected via the FACE-Q provided insight into participants’ perceived treatment benefits [27, 28], further underscoring the relevance of botulinum toxin for facial contouring.

Unlike many previous studies in facial esthetics, which primarily focused on morphological changes such as masseter thickness reduction assessed by ultrasound or CT [1, 2, 29], this research also evaluated perceived masticatory function, which is particularly relevant in the context of dental applications. For example, Park et al. demonstrated significant reduction in masseter thickness for up to three months, Kim et al. reported favorable esthetic outcomes in a large Asian cohort, and Choe et al. highlighted dose-dependent decreases in muscle mass. However, these studies did not assess functional outcomes. Botulinum toxin is increasingly used by dentists not only for esthetic enhancement but also in the management of conditions such as bruxism [30, 31] and temporomandibular disorders [32]. In some clinical settings, it has also been used as a supportive measure during oral rehabilitation, although this application has not been formally studied. In these scenarios, weakening of the masseter muscle is often expected to reduce occlusal force or parafunctional stress. However, our findings suggest that while volume reduction was clearly achieved, participants—who were functionally healthy—did not perceive any notable decline in chewing ability. This highlights a potential disconnect between anatomical change and functional perception, suggesting that the functional impact of botulinum toxin may be minimal in individuals without baseline dysfunction.

These findings contrast with previous studies involving clinical populations with bruxism, which have shown significant reductions in muscle activity and pain following botulinum toxin treatment [33, 34]. Such studies often used objective neuromuscular assessments like electromyography (EMG) or bite force testing [35, 36], whereas our study relied on subjective self-report measures in healthy individuals. The difference in study populations and outcome measures may account for the apparent discrepancy and highlights the need to interpret functional effects of botulinum toxin within the clinical context of its use.

This study had several strengths, including the use of a standardized injection technique, calibrated data collection, and validated outcome measures. However, certain limitations should be acknowledged. The sample size, while adequate for primary outcomes, limited the robustness of subgroup analyses. A placebo or sham control was not included due to ethical concerns, which may restrict causal inference and generalizability. All measurements were performed by a single trained researcher, but intra-rater reliability (e.g., ICC) was not tested, limiting formal confirmation of consistency and introducing potential error. In addition, blinding was not feasible during 3D analysis because scans required superimposition and alignment. Although these factors may introduce bias, all datasets were anonymized and standardized alignment protocols were applied to mitigate error. The 12-week follow-up may not capture long-term effects, and objective functional tests such as bite force or EMG were not included. Future research should address longer-term functional adaptation and examine whether masseter reduction impacts prosthodontic or occlusal outcomes in clinical populations.

Conclusion

Our results suggest that botulinum toxin injection into the masseter muscle may reduce lower face volume and improve patient satisfaction without compromising perceived masticatory function. The combined use of 3D scanning and validated patient-reported outcomes supports these findings. While participants consistently reported normal chewing ability, functional effects may be minimal in healthy individuals. These results may inform dental applications such as bruxism management and prosthodontic planning, though larger studies with objective functional tests and longer follow-up are needed.

Abbreviations

BTX-A

Botulinum toxin type A

3D

Three-dimensional

CFQ

Chewing Function Questionnaire

VAS

Visual Analog Scale

ROI

Region of interest

IRB

Institutional Review Board

IQR

Interquartile range

EMG

Electromyography

BMI

Body Mass Index

TREND

Transparent Reporting of Evaluations with Non-randomized Designs

TCTR

Thai Clinical Trials Registry

Authors’ contributions

N.S. and C.T. contributed to the conception, study design, data collection, data analysis, and manuscript preparation. K.A. contributed to the conception, study design, data collection, and manuscript preparation. All authors read and approved the final manuscript.

Funding

Open access funding provided by Mahidol University. This study received no external funding.

Data availability

The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethical approval and consent to participate

The study protocol was approved by the Institutional Review Board of Mahidol University (IRB No. MU-DT/PY-IRB 2024/024.1505). All participants provided written informed consent prior to study enrollment. This study was conducted in accordance with the Declaration of Helsinki.

Consent for publication

Written informed consent was obtained from the participant for the publication of clinical images (Fig. 1). A copy of the signed consent form is available for editorial review upon request.

Competing interests

The authors declare no competing interests.