The nasolabial fold (NLFs) is the keystone of the smiling mechanism [1]. However, except for one case report [2], there have been no reports of smile changes after correction of deep NLFs. Kang et al. [2] suggested that the combination treatment using polydioxanone (PDO) threads and hyaluronic acid (HA) filler had resulted in a natural and charming smile. De Maio [3] explained that suppression of laughter movement accompanied by both myo-modulation and increased firmness of the NLF due to volume restoration was the mechanism by which this was achieved.

The authors performed this combination treatment in 11 patients to confirm its effect on the shape of the smile through morphological analysis. To evaluate its effect, the smile width and inter-labial gap was measured in before and after photographs of the patients [4,5], and the smile index ratio was calculated to compare the smiles pre- and post-operatively [6].

Patient selection

From November 2021 to December 2021, we performed the combination treatment on 14 female patients without a previous history of cosmetic treatment for midface rejuvenation at our hospital. Each patient was given the opportunity to review the manuscript and consented to its publication. We received the patient’s consent form about publishing all photographic materials.

Procedure materials

PDO thread in the form of a mesh tube (Essorb; PineBM Co, Ltd, Daejeon, Korea) and HA filler (Polyderm; PineBM Co, Ltd) were used in the combination treatment.

Procedure for the combination treatment

Thirty minutes after injecting local anesthesia (2% lidocaine solution with epinephrine [1:100,000]) at the entry site and modified Klein’s tumescent solution at the NLF area, the PDO thread was sequentially placed in the lateral folded part (LFP), medial depressed part (MDP), and central creased part (CCP) of the NLF, and the HA filler was then injected sequentially in the same manner using the Sequential Autologous Fat Injection technique [2,7].

Entry sites were created on both sides of the proximal NLF using a sharp needle (18-gauge). After PDO thread insertion, the filler was injected retrogradely using a blunt canular (20-gauge, 5.0 cm in length). After placement of these materials, a 3M tape was applied to the recipient areas for immobilization and prevention of swelling and further bleeding.

Measurement of parameters

To evaluate changes in the smile index in the frontal view, the distance from outer commissure to outer commissure during a fully expanded smile (the inter-commissural distance) and that between the most inferior portion of the tubercle of the upper lip and the deepest midline point on the superior margin of the lower lip (the inter-labial gap) were measured by a non-medical researcher in Adobe Photoshop 3 (Adobe, San Jose, CA, USA) months after the procedure (Fig. 1). The patients provided written consent for the publication of all photographic materials.

Figure 1. Measurement of the parameters of the frontal smile. The inter-commissural width (A) and inter-labial gap (B) of the patients were measured for the analysis of the smile index on Adobe Photoshop.

Statistical analysis

The smile index (inter-commissural distance/inter-labial gap) was calculated from the inter-commissural distance and inter-labial gap length measured before and after the procedure in the 11 patients. Means, standard deviations, and correlations of the obtained values were calculated. All data were analyzed using Excel software (Microsoft, Redmond, WA, USA). Statistical significance was set at p<0.05.

As shown by the statistical values in Table 1, 2, the inter-commissural distance and inter-labial gap length of all patients decreased, and the smile index increased.

Table 1 . Changes and variations in the inter-commissural distance, inter-labial gap, and smile index after correction of nasolabial fold using a combination of polydioxanone thread and hyaluronic acid filler.

No. of case	Age (yr)	No. of thread			Amount of filler (ml)			Smile Style			Intercommissural distance (mm)			Interlabial gap (mm)			Smile index (ratio)
		Rt	Lt		Rt	Lt		Before	3 months after		Preoperative	Postoperative		Preoperative	Postoperative		Preoperative	Postoperative
1	67	5	5		2	2		Commissure	Commissure		37.83	31.66		4.48	3.22		8.44	9.83
2	46	5	6		1.2	2		Commissure	Commissure		37.39	37.21		15.31	14.00		2.44	2.66
3	47	6	6		1.9	2.7		Commissure	Commissure		43.38	40.18		18.49	17.08		2.35	2.35
4	52	6	6		1.4	1.2		Commissure	Commissure		47.08	44.77		15.31	13.26		3.08	3.38
5	52	6	6		1.3	2.1		Commissure	Commissure		40.52	39.61		8.68	7.98		4.67	4.96
6	55	6	6		2.5	3		Cuspid	Cuspid		41.97	41.16		17.92	17.36		2.34	2.37
7	47	5	6		1.4	0.8		Commissure	Commissure		41.51	39.92		13.82	13.07		3.00	3.05
8	55	5	5		1.3	2.3		Cuspid	Cuspid		40.99	38.63		24.65	20.17		1.66	1.92
9	56	8	6		3.6	2.6		Cuspid	Cuspid		41.06	39.97		16.80	15.68		2.44	2.55
10	57	6	5		1.6	1.6		Commissure	Commissure		42.50	38.61		4.29	3.17		9.91	12.18
11	60	6	6		1.8	0.8		Commissure	Commissure		40.47	38.72		13.82	12.51		2.93	3.10
Average	54	5.82	5.73		1.82	1.92					41.34	39.13		13.96	12.5		3.93	4.4
SD	6.25	0.87	0.47		0.71	0.75					2.62	3.14		6.11	5.57		2.72	3.40

Rt, right; Lt, left; SD, standard deviation..

Table 2 . Descriptive statistics of the inter-commissural distance, inter-labial gap, and smile index after correction of nasolabial fold using a combination of polydioxanone thread and hyaluronic acid filler.

Materials & measure	No. of case	Minimum value	Maximum value	Average	Standard deviation	Variance
No. of thread (Rt)	11	5.00	8.00	5.82	0.87	0.76
No. of thread (Lt)	11	5.00	6.00	5.73	0.47	0.22
Filler (Rt) (ml)	11	1.20	3.60	1.82	0.71	0.5
Filler (Lt) (ml)	11	0.80	3.00	1.92	0.75	0.56
Age (yr)	11	46.00	67.00	54	6.25	39
Smile style 3 months after	11	1.00	3.00	1.55	0.93	0.87
Intercommissural distance before (mm)	11	37.39	47.08	41.34	2.62	6.84
Intercommissural distance 3 months after (mm)	11	31.66	44.77	39.13	3.14	9.88
Interlabial distance before (mm)	11	4.29	24.65	13.96	6.11	37.35
Interlabial distance 3 months after (mm)	11	3.17	20.17	12.5	5.57	31.03
Smile index before (ratio)	11	1.66	9.91	3.93	2.72	7.39
Smile index 3 months after (ratio)	11	1.92	12.18	4.4	3.40	11.58

Rt, right; Lt, left..

However, in the correlation analysis (shown in Table 3), a decrease in inter-commissural distance correlated with an increase in the smile index (r=-0.684, p=0.020), but a decrease in the inter-labial gap and an increase in the smile index were not significantly correlated (r=0.061, p=0.858).

Table 3 . Correlation between the decrease in the inter-commissural distance or inter-labial distance and increase in the smile index.

Meaurement	Interlabial gap	Intercommissural distance	Smile index
Interlabial gap (after 3 months - before)
(r)	1	0.138	0.061
p-value (both sides)		0.686	0.858
No. of case	11	11	11
Intercommissural distance (After 3 months - before)
(r)	0.138	1	0.684*
p-value (both sides)	0.686		0.020
No. of case	11	11	11
Smile index (after 3 months - before)
(r)	0.061	0.684*	1
p-value (both sides)	0.858	0.020
No. of case	11	11	11

(r), Coefficient of correlation..

*p-values <0.05 are considered significant..

The average age of the 11 female patients was 54±6.25 years. Among them, eight patients had the commissure smile type (72.7%) and three had the cuspid smile type (27.3%). The average number of inserted PDO threads was 5.82±0.87 and 5.73±0.47 in the right and left, respectively. The volume of injected filler was 1.82±0.71 ml and 1.92±0.75 ml in the right and left, respectively.

Morphological change

In all cases, the LFP unfolded, the MDP was augmented, and the CCP disappeared. There was great and natural improvement in the deep NLF without significant lateral bulging. Instead, the mid-facial volume over the area of the mid-facial furrow increased, and eventually, it appeared raised (Fig. 2-5).

Figure 2. Photograph of case 2 before (A) and 3 months (B) after the procedure taken at the fully expanded frontal smile stage. Polydioxanone mesh tube thread and hyaluronic acid filler were inserted into the deep nasolabial folds of a 46-year-old female who has a commissure smile type. The deep nasolabial folds and perioral wrinkles greatly improved. Her smile also became younger and more charming in appearance due to the increased mid-facial volume, outstanding restriction in the movement of the corner of her mouth (upward and outward) (A), and decreased gingival exposure from 4 mm (maxillary type 2) to 2 mm (maxillary type 1).

Figure 3. Photograph of case 4 before (A) and 3 months (B) after the procedure taken at the fully expanded frontal smile stage. Polydioxanone mesh tube threads and hyaluronic acid filler were inserted into the deep nasolabial folds of a 52-year-old female who has a commissure smile type. The deep nasolabial folds and perioral wrinkles greatly improved. Her mouth looks smaller and younger due to the restricted movement of the corners of her mouth caused by the volume restoration in the nasolabial fold and sub-malar region. Additionally, decreased exposure of her maxillary teeth and increased thickness of the upper and lower lips are observed.

Figure 4. Photograph of case 6 before (A) and 3 months (B) after the procedure taken at the fully expanded frontal smile stage. Polydioxanone mesh tube threads and hyaluronic acid filler were inserted into the deep nasolabial folds of a 55-year-old female with a cuspid smile type. There was great improvement in the deep nasolabial folds, decreased exposure of the maxillary teeth, and thinning of the upper lips. A charming smile with natural contour is seen (B). The right mandibular teeth are exposed (B).

Figure 5. Photo of case 9 before (A) and 3 months (B) after the procedure taken at the fully expanded smile stage. Polydioxanone mesh tube thread and hyaluronic acid filler were inserted into the deep nasolabial folds of a 56-year-old female with a cuspid smile type. The nasolabial folds and perioral wrinkles greatly improved. Despite the decreased inter-commissural distance, the maxillary teeth are less exposed, and the mandibular teeth are more exposed (B).

The perioral wrinkles and Marionette lines also significantly improved, although the thread and filler was not placed there (Fig. 2-4).

In the commissure smile type, the corners of the mouth were more affected by the restricted zygomatic muscles, and decreased exposure of the maxillary teeth was observed (Fig. 2, 3). In contrast, in the cuspid smile type, the levator labii muscles were more affected and the maxillary teeth were less exposed. Interestingly, increased exposure of the mandibular teeth was observed, which may be due to the counteraction of the restricted contraction of the levator labii muscles (Fig. 4, 5). Like these results, a similar tendency was observed in the other cases (1, 2, 4, 6, 7, 9, and 10). However, the degree of exposure was different depending on the type of smile or the restriction of the lip elevator muscles by the combination treatment.

Statistical change

The inter-commissural distance decreased from 41.34±2.62 to 39.13±3.14 mm and the inter-labial gap decreased from 13.96±6.11 to 12.5±5.57 mm; however, the smile index increased from 3.93±2.71 to 4.53±3.40 (Table 1).

The NLF is a natural feature of the face, and it tends to deepen and lengthen with age. Most patients are concerned about these features of aging because they are exaggerated during smiling and appear relatively earlier than those of other parts of the face. There are several procedures for improving deep NLF, among which the simple and common method of multi-layered filler injection is the most popularly used [8]. However, there is a limit to how perfectly deep NLF can be corrected using the filler in a gel form because it does not strengthen the NLF foundation by increasing the firmness of the NLF, which ultimately supports the midface as well. Therefore, Kang et al. [2] introduced a technique that combined PDO thread insertion and HA filler injection. They reported a case that demonstrated softening of the NLF without migration and a change in the smile, with a younger look.

In this study, the authors performed the combination treatment in 11 patients, and they obtained the same results with that of the previous study. The previous report was the first to show that the shape of the smile can be changed by the correction of deep NLF; moreover, the NLF is generally regarded as the keystone of the smiling mechanism [1].

With aging, the NLF deepens, lengthens, and sags. Sagging is thought to occur due to volume loss in the NLF and midface due to aging and excessive facial movements. Sagging ultimately appears as folding of parts of the NLF. Therefore, the weakening of the foundation of the NLF may eventually cause mid-facial sagging as well. Therefore, the authors divided the NLF into three parts based on morphological differences (the LFP, MDP, and CCP) because there is a definite difference in thickness between the three parts of the NLF [2]. The technical purpose of this treatment was to make the folded part unfolded, the depressed part augmented, and the central crease to disappear. Ultimately, this was performed to achieve the same NLF thickness from the deep fat layer to the superficial fat layer, including the skin. The authors expected that strengthening the NLF foundation using a combination of PDO thread and HA filler would lead to both volume restoration and support of the midface. However, the authors unexpectedly discovered a change in smiles.

All patients were highly satisfied with the softening of the NLF, and they all had a younger looking and charming smile. This result was the same for both commissure and cuspid smile types. However, the change in the shape of the smile differed according to the smile type or degree of volume correction of the combination treatment (even with the same type of smile) (Fig. 2-5). The volume deficiencies and weakened foundation due to the deep NLF was restored by this combination treatment; the shape of the smile may be changed by restoring the volume and firmness of the NLF. This can be explained by both the movement restriction of the lip elevator muscles and the decreased tone of the lip levator muscles [9,10].

Ackerman and Ackerman [6] defined the smile index and suggested that the lower the smile index, the less youthful the smile appears.

In this study, inter-commissural width and inter-labial gap decreased by 2.21% and 1.47%, respectively, in all cases, regardless of smile type, and the smile index increased by 0.46%. An increase in the smile index indicates a younger look; therefore, these measurements showed results consistent with the appearance of a smile with a smaller, younger, and more charming look after the procedure.

As shown in Table 3, in the correlation analysis, a decrease in the inter-labial distance was correlated with an increase in the smile index (r=-0.684, p=0.020), but a decrease in the inter-commissural distance and an increase in the smile index were not significantly correlated (r=0.061, p=0.858).

In conclusion, our results show morphologically and statistically that improving the deep NLF using combination treatment to restore the soft tissue volume and increase the foundation produces a smile shape that looks younger and more charming.

The authors have nothing to disclose.