Intracorporeal evaluation of hyaluronic acid fillers with varied rheological properties and correlations with aesthetic outcomes

Abstract

Background

Understanding the differences in soft tissue filler rheology and how these properties can impact clinical results is a fundamental concepts for any injector. This study aimed to assess the tissue integration characteristics of hyaluronic acid (HA) fillers manufactured with different technologies (Non‐Animal Stabilized HA [HA‐N] or Optimal Balance Technology [HA‐O]) using ultra‐high‐frequency ultrasound.

Methods

Twelve female participants with mild‐to‐moderate midface volume loss and temporal hollowing were enrolled and treated with HA‐N and/or HA‐O. Participants were seen at five visits (screening/baseline [treatment], and Weeks 1 [optional touch‐up], 4, 6, and 8 [follow‐up visits]). Ultrasound was used to evaluate the degree of product integration.

Results

On ultrasound, HA‐N presented with distinct borders, minimal tissue integration, and a capacity to displace tissues. Conversely, HA‐O tended to spread horizontally within the same tissue plane and integrated within tissues. The volumizing capacity of the HA‐O fillers was dependent on particle size.

Conclusion

HA‐N is suited for deep injections in areas such as the upper lateral cheek and under the muscle of the temporal region when a lifting effect is desired; HA‐O is best suited for subcutaneous injections, in areas of dynamic movement or for patients with thin skin; and can be injected subcutaneously or supraperiosteally when a volumizing effect is desired.

Abbreviations

HA

hyaluronic acid

HA‐L

Restylane Lyft

HA‐N

non‐animal stabilized hyaluronic acid

HA‐O

Optimal Balance Technology hyaluronic acid

HA‐R

Restylane Refyne

HA‐V

Restylane Volyme

1. INTRODUCTION

In the field of aesthetics, there is a growing interest in non‐invasive facial rejuvenation procedures. This is exemplified by an approximate 1.6 million increase in soft tissue filler treatments between the years 2010 and 2020. ¹ , ² , ³ This rising demand can be partly attributed to the growing aging population and social norms that favor a youthful appearance. ⁴ With increased age, the face begins to lose fat deposits, bone resorption occurs, and the skin becomes thinner in areas such as the midface (i.e., cheeks) and temporal regions. ⁵ , ⁶ , ⁷ , ⁸ , ⁹ In response to this growing demand for soft tissue fillers, medical device manufacturers are continuously developing new products or revising current ones to produce products that can be used for an array of indications. Hyaluronic acid (HA)‐based fillers are the most widely used aesthetic injectable products, compared to those containing calcium hydroxyapatite, poly‐L‐lactic acid, and polymethyl‐methacrylate microspheres. HA fillers are manufactured to have specific and varied biophysical features including but not limited to: the elasticity modulus [G‐prime (G’)], which measures how much the product can recover after compression and is used as an indication of gel strength/firmness; ¹⁰ and the viscosity modulus [G‐double‐prime (G’’)], which measures the inability of the product to recover from compression. The G’’/G’ cross‐over point (i.e., the point where G’ and G’’ have the same value or the strain value for the G‐prime [xStrain]) is used as an indication of gel flexibility. ¹⁰ , ¹¹ , ¹² , ¹³ , ¹⁴ , ¹⁵

As one of the most popular soft tissue fillers on the market, Restylane (Galderma, Uppsala, Sweden) has a variety of formulations, rheological properties, and indications. ¹⁰ , ¹⁵ , ¹⁶ The two manufacturing technologies used to create this family of fillers are Non‐Animal Stabilized Hyaluronic Acid (NASHA™ [HA‐N]) and Optimal Balance Technology (OBT™ [HA‐O]; referred to as XpresHAn in the United States)]. During the manufacturing process, HA‐N gels are sieved through filters of different diameters to produce gels of various particle sizes, making them adaptable to specific clinical indications. This process creates crosslinked HA which is then suspended in non‐crosslinked HA gel to decrease the required extrusion force needed during injections. Fillers manufactured with HA‐O technology differ in their degrees of crosslinking, as well as particle size. Thicker or thinner fillers are obtained by varying particle size and firmer or softer fillers are obtained by varying crosslinking. ¹⁷ , ¹⁸ , ¹⁹ HA‐N technology results in firmer products with large G’ and small xStrain values, making these fillers optimal for lifting areas of the face. ²⁰ , ²¹ Conversely, gels manufactured with HA‐O technology are marked by lower G’ and higher xStrain values, creating flexible products that integrate well with tissues and are thereby optimal for areas with dynamic movement, thin skin, or superficial injections. ¹⁰ , ¹⁵ , ²⁰ , ²²

Research has demonstrated that different rheological parameters have a significant impact on the tissue integration capabilities of fillers. ²³ For instance, products with low G’ and high xStrain values generally integrate better with surrounding soft tissues and are therefore ideally suited for more superficial planes of injection due to their ability to compress and expand with surrounding tissues during animation and expression. ¹¹ , ²² Conversely, firm products that do not readily integrate with surrounding tissues are marked by large G’ and small xStrain values. These fillers integrate in a bolus‐like fashion, making these products less dynamic. This gives firm products the ability to provide significant tissue lift, rather than movement. ¹¹ Thus, it is essential for injectors to determine the optimal product selection, based on multiple patient and filler factors, to obtain the desired treatment outcome. ²³ Herein, we investigated the tissue integration characteristics of three HA fillers for use in the upper and midface (i.e., the temporal regions and cheeks, respectively). The fillers used in this study included Restylane Lyft (HA‐L), which is manufactured using HA‐N technology, in addition to Restylane Volyme (HA‐V) and Restylane Refyne (HA‐R), which are manufactured using HA‐O technology.

2. MATERIALS AND METHODS

2.1. Ethical considerations

This study was conducted according to the ethical principles outlined in the Declaration of Helsinki, the International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use (ICH) Tripartite Guidelines and with adherence to applicable laws and regulations regarding human trials in Canada. ²⁴ , ²⁵ Prior to any participant enrollment, study documentation including informed consent forms and the protocol was reviewed and fully approved by an independent ethics review board (Canadian SHIELD Ethic Review Board, #2022‐01‐004).

2.2. Study sample

A total of 12 female participants (24 hemifaces), 18 years of age or older, with temporal and midfacial volume loss were enrolled. This sample size was selected in accordance with the recommended sample size for pilot studies. ²⁶ The degree of temporal hollowing and midfacial volume loss was assessed by the Galderma Temple Volume Deficit Scale (GTVDS), and the upper‐ and lower cheek fullness scales (Updated versions), respectively. ²⁷ , ²⁸ At Baseline, all participants required a GTVDS between 1 (mild hollowing) and 3 (severe hollowing). The eligibility criteria for this study are outlined in Table 1.

TABLE 1.

Eligibility criteria.

Inclusion criteria	Exclusion criteria
1. At the time of consent, female participants 18 years of age and older	1. Participants with a GTVDS score of 0 at Baseline
2. Participants with established temporal hollowing and/or midface contour deficits	2. Participants with an upper Midface aesthetic scale score of 0 at Baseline
3. Participants with a GTVDS score between 1 to 3 at Baseline	3. Participants with a lower Midface aesthetic scale score of 4 at Baseline
4. Participants with at least an upper Midface aesthetic scale score between 1 to 3 at Baseline. There is no limitation to the minimum lower Midface aesthetic scale score at Baseline, but the maximum value must not surpass 3	4. Current pregnancy or lactation (sexually active women of childbearing age had to agree to use medically acceptable methods of contraception for the duration of the study [e.g., oral contraceptives, condoms, intrauterine device, shot/injection, patch])
5. Accepted the obligation not to receive any other facial procedures throughout the study duration	5. Hypersensitivity to Restylane products, HA fillers or amide local anesthetics
6. Accepted the obligation not to receive any other facial procedures throughout the study duration	6. Participants presenting with porphyria or any other liver diseases
7. No previous facial fillers in the temporal region for 12 months prior to this study	7. Inability to comply with follow‐up and abstain from facial injections during the study period
8. At screening, no presence of fillers in the areas under assessment	8. Heavy smokers, classified as smoking more than 12 cigarettes per day
9. Capable of providing informed consent	9. History of severe or multiple allergies manifested by anaphylaxis, since drug allergies might preclude optimal management of complications
	10. Previous tissue revitalization therapy in the treatment area within 6 months before treatment with laser or light, mesotherapy, radiofrequency, ultrasound, cryotherapy, chemical peeling, or dermabrasion
	11. Previous facial surgery, including liposuction
	12. Lifetime history of permanent implants in the treatment region
	13. Lifetime history of semi‐permanent dermal fillers in the treatment region
	14. History or presence of any disease or lesion near or at the treatment area, including inflammation, active or chronic infection, including in the mouth, dental, head and neck region
	15. Facial psoriasis, eczema, acne, rosacea, perioral dermatitis, herpes zoster or any other facial condition that may increase the risk of cutaneous penetration of infective agents
	16. Scars, deformities, piercing, or tattoos in the treatment areas
	17. Facial cancer or precancer (e.g., actinic keratosis)
	18. History of radiation therapy in the treatment area
	19. History of bleeding disorders, or treatment with thrombolytics, anticoagulants, or inhibitors of platelet aggregation (e.g., Aspirin or other non‐steroid anti‐inflammatory drugs [NSAIDs]), within 2 weeks before treatment
	20. Participants with immune disorders such as systemic lupus erythematosus, rheumatoid arthritis, mixed connective tissue disease, and Hashimoto's thyroiditis, or participants using immunosuppressants
	21. Participants with a tendency to form hypertrophic or keloid scars, or any other healing disorders
	22. Participants with known hypersensitivity to lidocaine or agents structurally related to amide‐type local anesthetics (e.g., certain anti‐arrhythmics)
	23. Participants administered dental block or topical administration of lidocaine within 2 weeks of treatment
	24. Participants with epilepsy, impaired cardiac conduction, severely impaired hepatic function or severe renal dysfunction
	25. Current remote infections (e.g., urinary tract, sinuses, intestinal tract, oral cavity)
	26. Planned dental procedures during the 2‐week period before and after filler treatments, including teeth cleaning, tooth extraction and gum grafts
	27. Planned COVID‐19 vaccinations during the 2‐week period before and after filler treatments
	28. History of other treatment/procedure that, in the treating investigator's opinion, would interfere with the study injections and/or study assessments or exposes the participant to undue risk by study participation

2.3. Procedures

Participants were seen at five visits over a span of 8 weeks, including screening/Baseline (visit 1) and Weeks 1 ± 2 days (visit 2), 4 ± 3 days (visit 3), 6 ± 3 days (visit 4) and 8 ± 3 days (visit 5). The duration of the follow‐up period was satisfactory to meet the primary endpoint of the study (assess the tissue integration of each product family), as such, a longer follow‐up duration was not required. All participants were treated in the bilateral temporal regions and midface (lateral or anteromedial cheeks) at Baseline. Any participant not presenting with optimal correction 1 week after the Baseline treatment was eligible for a touch‐up. At each visit, various efficacy and safety measurements were collected.

Following informed consent, each participant was screened in accordance with the inclusion and exclusion criteria (Table 1) and only those who met all the eligibility criteria were considered for enrollment. Medical history and a list of all concomitant medications/procedures were then recorded. Females of childbearing potential were required to provide a negative urinary pregnancy test prior to all treatments. At all visits, standardized 2‐and 3‐dimensional imaging (Cannon, EOS Rebel T2i, Canfield Scientific Vectra XT) was taken. Participants also underwent ultrasonography to assess the degree of product integration (Clarius HD₃ Portable Ultrasound Machine, L20). All ultrasound assessments were performed by a Blinded Evaluator (BE), to whom treatment allocation was concealed. The Pinch and Slide Tests were conducted by the BE, prior to any study treatment as well as all follow‐up visits. To perform the Pinch and Slide Tests, the area of injection was identified using a skin marking pencil and photographed. ²³ The BE then pinched the skin to accumulate the yielding skin and pulled laterally using normal tension. The volume of pinched skin was measured using a caliper (Electronic Digital Caliper, MENTOR^® MemoryGel™ Xtra). This procedure was performed before and after treatment administration to demonstrate less excursion of tissues given the improved support. Additionally, at weeks 1, 4, 6 and 8, participants were asked to complete a subjective 5‐point satisfaction questionnaire (very satisfied, satisfied, neutral, dissatisfied, very dissatisfied). Participants were also instructed to complete an at‐home AE journal that was collected at the following visit. The AE journal outlined possible side effects associated with injection, including bruising, swelling, redness, pain/tenderness, lump/bump formation, and itching. If touch‐up treatment was required, as assessed by the treating investigator, a second AE journal was provided to the participant.

Following initial injection, if participants were deemed to not have achieved optimal correction, as assessed by the treating investigator, a touch‐up was administered following a negative urinary pregnancy test for childbearing females. Participant‐reported or physician‐observed AEs were recorded at each visit. Based on participant imagery for each visit, the treating investigator assessed the participants using the GTVDS, Midface aesthetic scales (assess each side of the face separately), and a 5‐point Global Aesthetic Improvement Scale (GAIS) with ratings from “very much improved” to “worse” following injection. ²⁸ , ²⁹

2.4. Injection techniques

The study product was administered by authorized personnel trained in the appropriate injection techniques, in accordance with local legislation. All injections were performed by the principal/treating investigator (A.N), who has extensive injection experience. Participants received the same product on both sides of the face, and at both treatments (if applicable). The volume of product used was at the Principal Investigators’ discretion to achieve optimal correction, although a minimum of 0.5 mL to a maximum of 2 mL of product per side was used at each treatment session. Before beginning injections, the treatment area was cleansed with an antiseptic and allowed to dry. To ensure patient safety, aspiration was completed before injection of the product in the temporal region to reduce the risk of injecting into a blood vessel.

2.5. Statistical methods

Analyses were performed on the data of all participants who received treatment and whose endpoints were available at Week 8. Continuous variables were summarized in tables and included the number of participants, hemifaces, mean, standard deviation, median, min, max, and range. All categorical variables were presented in tables as frequencies and percentages. All evaluations were considered in the assessment of product safety and efficacy.

3. RESULTS

3.1. Demographics and participant retention

A total of 12 female participants (24 hemifaces) with temporal and midfacial volume loss were enrolled. All participants received treatment in the bilateral temporal regions as well as the bilateral cheeks. The corresponding treatment assignment was determined by the treating investigator and reflected a marriage between the rheological characteristics of the product and the participant's tissue coverage (e.g., thin or thick). The average age of participants was 56.75 years (SD: 9.36; Range: 41 to 69). Most participants self‐identified as being Caucasian (10/12. 83.33%), while the remaining identified as Arabic/West Asian (2/12, 16.66%). The average body mass index was 22.92 (SD: 2.90). There were no participants who dropped out of the study. There were four missed visits (4/60; 6.66%); two in Week 6 and two in Week 8.

3.2. Treatment

Topical anesthetics and ice were not used before injection, as the products were manufactured with the addition of lidocaine. At Baseline, all participants (12/12; 100%) received treatment, but only 7/12 (58.33%) required touch‐up at Week 2. Treatment information, including the type of device used (needle or cannula), injection technique, and depth of product placement is summarized below and detailed in Table 2. There were no cases of positive aspiration. All participants remained on site for 10 min following treatment, to observe for AEs.

TABLE 2.

Treatment information.

Treatment area	Group	Volume a [Mean (SD)]	Device	Technique	Depth	n
Temporal region	HA‐V	1.87 (0.71)	27G Needle	Bolus	Supra‐periosteum	4
	HA‐R	0.77 (0.08)	25G Cannula	Fanning	Subcutaneous	4
	HA‐L	0.92 (0.07)	27G Needle	Bolus	Supra‐periosteum	4
Midface	HA‐V	1.13 (0.16)	27G Needle	Bolus	Subcutaneous and supra‐periosteum	6
	HA‐L	1.00 (0.18)	27G Needle	Bolus	Supra‐periosteum	6

^a Average unilateral volume (mL) injected (Baseline + Week 2).

Abbreviations: HA‐L, Restylane Lyft; HA‐V, Restylane Volyme; HA‐R, Restylane Refyne; SD, standard deviation.

3.3. Efficacy evaluations

3.3.1. Ultrasound

Select images captured during the ultrasound evaluations are depicted in Figures 1, 2, 3, 4, 5. HA fillers were easily recognizable as hypoechoic (anechoic) regions of either boluses or linear threaded deposits and often generated posterior artifacts. With distinct borders and an absence of intermixed hyperechoic collagen fibers, HA‐N formed localized hypoechoic lobular or spindle‐shaped spaces within the tissue, displayed minimal tissue integration over time, and a capacity to lift superior tissues (Figures 3 and 5). As evidenced in Figure 3, HA‐N injected into the supraperiosteal layer pushed against the dense fibrous structures of the dermis. Conversely, HA‐O tended to spread horizontally within the same tissue plane, giving rise to an elongated configuration (Figures 1 and 4). HA‐O also integrated within tissues, as evidenced by the presence of hyperechoic collagen fibers within the HA, a blurring of the HA border, and a decrease in the size of the measurable HA globule over time (Figures 2 and 4).

FIGURE 1.

Tissue integration of Restylane Refyne in the temporal regions over time. Ultrasound images depicting 1 mL of Restylane Refyne in the temporal regions over time (Baseline [A], Week 4 [B], Week 6 [C]). Filler was placed in the subcutaneous layer using a fanning technique.

FIGURE 2.

Tissue integration of Restylane Volyme in the temporal regions over time. Ultrasound images depicting 1.5 mL of Restylane Volyme in the temporal regions over time (Baseline [A], Week 4 [B], Week 8 [C]). Filler was placed in the supraperiosteal and periosteal layers using a bolus technique.

FIGURE 3.

Tissue integration of Restylane Lyft in the temporal regions over time. Ultrasound images depicting 1.0 mL of Restylane Lyft in the temporal regions over time (Week 2 [A], Week 8 [B]). Filler was placed in the supraperiosteal layer using a bolus technique.

FIGURE 4.

Tissue integration of Restylane Volyme in the midface region over time. Ultrasound images depicting 1.0 mL of Restylane Volyme in the midface (cheek) region over time (Week 4 [A], Week 8 [B]). Filler was placed in the supraperiosteal layer using a bolus technique.

FIGURE 5.

Tissue integration of Restylane Lyft in the midface region over time. Ultrasound images depicting 1.5 mL of Restylane Lyft in the midface (cheek) region (Week 2 [A], Week 6 [B]). Filler was placed in the supraperiosteal layer using a bolus technique.

3.3.2. Global aesthetic improvement scale

At all follow‐up visits, 91.66% (11/12) participants at least improved [improved (score = 1), much improved (score = 2), or very much improved (score = 3)] in their global aesthetic appearance. There was one participant whose appearance remained essentially the same as the original condition (score = 0), at all follow‐up visits. There were no participants whose appearance worsened from the original condition.

3.3.3. Galderma temple volume deficit scale

A chi‐square test of independence showed that there was a significant association between visit and GTVDS scores, on both the right (χ² [12, N = 56] = 29.04, p = 0.004) and left sides (χ² [12, N = 56] = 30.03, p = 0.003). GTVDS scores were more likely to be lower at follow‐up visits, compared to Baseline. Maximal results were observed in Week 4 (Figure 6).

FIGURE 6.

Participant scores on the Galderma Temple Volume Deficit Scale (GTVDS). Displayed is the GTVDS frequency distribution per visit. Left Temporal Region (A), Right Temporal Region (B).

3.3.4. The upper‐ and lower cheek fullness scales

Following treatment, most participants displayed at least a one‐point improvement in the fullness of their upper (Week 1 [9/12, 75.00%], Week 4 [10/12, 83.33%], Week 6 [8/10, 80.00%], and Week 8 [9/10, 90.00%]) and lower cheeks (Week 1 [7/12, 58.33%], Week 4 [11/12, 91.66%], Week 6 [8/10, 80.00%], and Week 8 [9/10, 90.00%; Figure 7]).

FIGURE 7.

Frequency distributions per visit, for participant scores on the Cheek Fullness Scales. Displayed is the frequency distribution of the Cheek Fullness Scales per visit. Upper (A)—and Lower (B) Cheek region.

3.3.5. The Pinch and Slide Tests

A multivariate general linear model was used to determine if the mean values for the Pinch and Slide Tests significantly differed based on the visit. A Dunnett's T3 (equal variances not assumed) post hoc test revealed that the results of the Pinch test were significantly higher at Weeks 2 and 4, compared to Baseline; and the results of the Slide test were significantly reduced since Baseline, at Weeks 4 (right side only), 6, and 8 (Table 3; p ≤ 0.05).

TABLE 3.

Results of the pinch and slide tests.

Test	Location	Visit	Mean (mm)	Std. Error	95% confidence interval
					Lower bound	Upper bound
Pinch	Right midface	Baseline	11.83	1.76	8.28	15.37
		Week 2	17.50a	1.76	13.95	21.04
		Week 4	17.72a	1.84	14.02	21.43
		Week 6	15.72	1.84	12.02	19.43
		Week 8	13.81	1.84	10.11	17.52
	Left midface	Baseline	12.08	1.82	8.42	15.74
		Week 2	17.83a	1.82	14.17	21.49
		Week 4	18.54a	1.90	14.72	22.36
		Week 6	16.63	1.90	12.81	20.46
		Week 8	14.36	1.90	10.54	18.18
Slide	Right midface	Baseline	10.66	0.75	9.16	12.17
		Week 2	8.33	0.75	6.82	9.83
		Week 4	7.27a	0.78	5.70	8.84
		Week 6	5.00a	0.78	3.42	6.57
		Week 8	5.00a	0.78	3.42	6.57
	Left midface	Baseline	9.66	0.82	8.00	11.32
		Week 2	9.08	0.82	7.42	10.74
		Week 4	7.54	0.86	5.81	9.28
		Week 6	5.90a	0.86	4.17	7.64
		Week 8	5.54a	0.86	3.81	7.28

^a The mean difference is significant at the 0.05 level.

3.3.6. Participant satisfaction

Subjects receiving treatment in both areas (cheeks and temples) reported a high level of satisfaction following treatment (Table 4). Overall, 90% of subjects treated in the midface region (cheeks) and 80% of subjects treated in the temple region, reported being at least “satisfied,” “very much satisfied” and “extremely satisfied” at Week 8. Following optimal correction (Week 2) all subjects were at least “neutral” to “extremely satisfied” with regard to study treatment with no subject reporting being “dissatisfied.”

TABLE 4.

Visit × satisfaction crosstabulation with the treatment of the temporal region. ^a

Temporal region
		Dissatisfied	Neutral	Satisfied	Very much satisfied	Extremely satisfied	N
Visit	Week 2	1 (8.33%)	6 (50.00%)	1 (8.33%)	1 (8.33%)	3 (25.00%)	12
	Week 4	0 (0.00%)	6 (50.00%)	2 (16.67%)	1 (8.33%)	3 (25.00%)	12
	Week 6	0 (0.00%)	5 (50.00%)	1 (10.00%)	0 (0.00%)	4 (40.00%)	10
	Week 8	0 (0.00%)	2 (20.00%)	3 (30.00%)	1 (10.00%)	4 (40.00%)	10

^a Participants were asked to rate their overall level of satisfaction with the treatment of the temporal region.

3.4. Safety evaluations

3.4.1. Adverse events

There was one treatment‐emergent AE observed by the treating injector within ten minutes following injection (i.e., mild bruising on the left midface). The AE did not require treatment and resolved on its own. Participant‐reported AEs, which were recorded in a daily diary, are summarized in Table 5.

TABLE 5.

Visit × satisfaction crosstabulation with the treatment of the midfacial region. ^a

Midface
		Dissatisfied	Neutral	Satisfied	Very much satisfied	Extremely satisfied	N
Visit	Week 2	2 (16.67%)	4 (33.33%)	1 (8.33%)	2 (16.67%)	3 (25.00%)	12
	Week 4	0 (0.00%)	3 (25.00%)	4 (33.33%)	2 (16.67%)	3 (25.00%)	12
	Week 6	0 (0.00%)	2 (20.00%)	2 (20.00%)	2 (20.00%)	4 (40.00%)	10
	Week 8	0 (0.00%)	1 (10.00%)	3 (30.00%)	2 (20.00%)	4 (40.00%)	10

^a Participants were asked to rate their overall level of satisfaction with the treatment of the midface region.

4. DISCUSSION

High‐frequency ultrasound has been used since the late 1990s to visualize underlying skin structures, and recent technological advancements have contributed to a growing list of clinical applications. ³⁰ For example, ultrasound has been used to measure skin thickness and visualize HA fillers in vivo. ²³ Several studies have also characterized soft tissue fillers by their physicochemical properties. ¹⁷ , ¹⁹ , ³¹ , ³² , ³³ However, few publications describe attempts to correlate differences in these properties to variability in the degree of tissue integration and resulting aesthetic outcomes. ¹⁸ , ¹⁹ As one way to differentiate fillers is based on their manufacturing technology, this study aimed to investigate whether there were any differences in the intracorporeal integration properties of various fillers that can be related to their respective manufacturing methods. The selected fillers all share the same HA concentration (i.e., 20 mg/mL) but vary in their degree of cross‐linking and particle size, resulting in a range of HA gels that can be used to address individual needs and provide personalized treatments. ³⁴ More specifically, HA fillers manufactured using the HA‐N technology are firm gels with a high G’ and thereby provide distinct lifting capacity and tissue support. Conversely, HA fillers manufactured with the HA‐O technology are characterized by soft and flexible gels with varying degrees of cross‐linking, thereby providing capacity for contouring and maintenance of natural expression when used in areas of dynamic movement. ³⁵ Although previous ultrasonographic studies have demonstrated that HA fillers can be reliably identified, few studies have investigated the differential distribution patterns of specific HA fillers. Therefore, in this study, we report observations using ultra‐high‐frequency ultrasound to compare the distribution patterns of three HA fillers in the upper and middle face (i.e., temporal regions and cheeks).

Upon reviewing the images captured by ultra‐high‐frequency ultrasound, HA fillers were easily recognizable as hypoechoic (anechoic) regions of either bolus or linear threaded deposits which often generated posterior artifacts. With distinct borders and an absence of intermixed hyperechoic collagen fibers, HA‐N deposited in a bolus formed localized hypoechoic lobular or spindle‐shaped spaces within the tissue displayed minimal tissue integration over time, and a capacity to lift superior tissues (Figures 3 and 5). As evidenced in Figure 3, HA‐N injected into the supraperiosteal layer pushed against the dense fibrous structures of the dermis. Conversely, HA‐O, whether deposited using a bolus or fanning technique tended to spread horizontally within the same tissue plane, giving rise to an elongated configuration (Figures 1 and 4). HA‐O is also integrated within tissues, as evidenced by the presence of hyperechoic collagen fibers within the HA, a blurring of the HA border, and a decrease in the size of the measurable HA globule over time (Figures 2 and 4). While interpreting the above ultrasonographic findings, it is important to take into consideration the influence of shear‐stress and compression/stretching parameters that are exerted on HA gels in vivo. Once injected into the skin, fillers are subject to various mechanical stresses (e.g., facial animations, external forces) that can result in either lateral shearing (i.e., linear gliding) and/or torsion (i.e., rotation) on a plane. Together, these parameters are referred to as the shear‐stress parameters. Other deformations of the filler can include compression and/or stretching of the implant. These parameters have been extensively described in the literature, ³¹ , ³⁶ with a detailed review and exemplary cases provided by Pierre et al. ³² Briefly, deformations of filler caused by shear stress occur when force is applied along the surface of the implanted material, in which case, the dimensions of the filler remain intact but its shape change. Alternatively, when force is applied perpendicularly by stretching or compressing along an axis, the filler's shape is retained, but its dimensions are changed as the material deforms in one direction. While multiple types of deformation can occur simultaneously, under certain circumstances and within specific facial areas, one type of deformation may be more dominant than the other. ³² Moreover, given a filler's rheologic properties (e.g., G’, xStrain, Tan), it may be more susceptible to different types of deformation and thus, it can be assumed that treatment effectiveness will be impacted in a related way. Indeed, we observed correlations between rheological properties, ultrasound assessments and clinical outcomes. For example, with low G’ and high xStrain values, HA‐R was the most flexible of the fillers under evaluation and provided a smoothing effect when injected subcutaneously or in thin skin. On ultrasound, it was most susceptible to lateral spread and compression and had a high degree of tissue integration. With a high G’ and low xStrain, HA‐L provided a significant lifting effect in treatment areas. On ultrasound, it was most resistant to lateral spread and compression and had a low degree of tissue integration. HA‐V has a large xStrain value and was therefore susceptible to lateral shear; however, given its higher G’ value among the HA‐O fillers and increased particle size, it displayed some resistance to compression and was capable of volumizing treatment areas (Table 6).

TABLE 6.

Participant‐reported adverse events following treatment.

		Adverse events [Count (n), frequency (%) and duration (range)]
Group	Visit/Treatment area	Bruising	Swelling	Erythema	Pain	Skin sensations	Bumpiness	n
HA‐L	Baseline—Midface	3 (42.86%) 1–9 days	5 (71.43%) 1–9 days	1 (14.23%) 1 day	6 (85.71%) 2–9 days	2 (28.57%) 1–8 days	4 (57.14%) 4–9 days	7
	Week 2 – Midface	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	1 (100.00%) 5 days	0 (0.00%) 0 days	1 (100.00%) 12 days	1
	Baseline—Temporal region	0 (0.00%) 0 days	1 (25.00%) 3 days	0 (0.00%) 0 days	2 (50.00%) 1–2 days	1 (25.00%) 2 days	1 (25.00%) 1 day	4
	Week 2 – Temporal region	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0
HA‐V	Baseline—Midface	3 (60.00%) 2–7 days	4 (80.00%) 3–8 days	2 (40.00%) 1–4 days	5 (100.00%) 3–9 days	2 (40.00%) 2–5 days	4 (80.00%) 2–9 days	5
	Week 2 – Midface	1 (33.33%) 14 days	1 (33.33%) 1 days	0 (0.00%) 0	2 (66.67%) 4–6 days	2 (66.67%) 1–4 days	2 (66.67%) 6–9 days	3
	Baseline—Temporal region	1 (25.00%) 5 days	2 (50.00%) 1–6 days	0 (0.00%) 0 days	2 (50.00%) 2–7 days	1 (25.00%) 1 day	1 (25.00%) 2 days	4
	Week 2 – Temporal region	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	1 (50.00%) 2 days	0 (0.00%) 0 days	0 (0.00%) 0 days	2
HA‐R	Baseline—Midface	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0
	Week 2 – Midface	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0
	Baseline—Temporal region	2 (50.00%) 2 days	4 (100.00%) 6–9 days	1 (25.00%) 1 day	4 (100.00%) 2–8 days	0 (0.00%) 0 days	3 (75.00%) 4–8 days	4
	Week 2 – Temporal region	0 (0.00%) 0 days	1 (100.00%) 3 days	0 (0.00%) 0 days	0 (0.00%) 0 days	0 (0.00%) 0 days	1 (100.00%) 3 days	1

Abbreviations: HA‐L, Restylane Lyft; HA‐R, Restylane Refyne; HA‐V, Restylane Volyme.

Based on the results of this study, it can be concluded that HA‐L is suited for deep injections in areas such as the upper lateral cheek and under the muscle of the temporal region when a lifting effect is desired. ¹⁴ , ²² HA‐R is best suited for more superficial injections, in areas of dynamic movement (i.e., medial cheek), or in participants with thin skin, when a smoothing effect is desired. ¹⁴ , ²⁰ , ²² HA‐V can be injected subcutaneously or supraperiosteally and can provide significant volumization. Figures 8 and 9 demonstrate the lifting and volumizing effects of these products in exemplary participant images. Considering the results of this study, injectors may be able to personalize treatments by selecting fillers with appropriate rheology given a patient's unique anatomy and considerations of the desired outcomes (Table 7).

FIGURE 8.

Treatment case #1. Clinical images of a 43‐year‐old female at Baseline (A), Week 4 (B), and Week 6 (C) following treatment with 2.1 mL (right side = 1 mL + left side = 1.1 mL) of Restylane Refyne in the temporal regions and 1.0 mL (right side = 0.7 mL + left side = 0.3 mL) of Restylane Volyme in the midface (cheek) region.

FIGURE 9.

Treatment case #2. Clinical images of a 63‐year‐old female at Baseline (A), Week 4 (B), and Week 8 (C) following treatment with 3.0 mL (right side = 1.5 mL + left side = 1.5 mL) of Restylane Volyme in the temporal regions.

TABLE 7.

Product descriptions, ultrasonographic characteristics, and observed clinical outcomes.

Product	Manufacturing technology	Particle size [MA (µm)]	Degree of crosslinking	Rheology			Ultrasonographic characteristics	Related deformation parameters	Observed clinical outcomes	Ideal indications
				G’	xStrain	Tan (δ)
HA‐V	HA‐O	229.3	3	239	930	0.209	When injected into the subcutaneous or supraperiosteal layers, HA‐V provided significant volume. It tended to spread laterally within the same tissue plane, giving rise to an elongated configuration (Figure 4). HA‐V integrated within tissues, as evidenced by the presence of hyperechoic collagen fibers within the HA, a blurring of the HA border, and a decrease in the size of the measurable HA globule over time (Figures 2 and 4).	Susceptible to lateral shear, minimal resistance to compression.	Significant volume increases in treatment areas.	For subcutaneous or supraperiosteal injection in areas of dynamic movement.
HA‐R	HA‐O	138.9	2	116	1442	0.431	With the lowest G’ value of the Restylane family, HA‐R did not provide a significant lifting nor volumizing effect. With the highest Tan value of the Restylane family, HA‐R displayed the greatest degree of tissue integration.	Susceptible to lateral shear and compression.	Injections into the subcutaneous tissue gave a smooth effect, especially in thin‐skinned participants.	For injection in superficial areas or thin skin.
HA‐L	HA‐N	368.9		977	17	0.203	With distinct borders and an absence of intermixed hyperechoic collagen fibers, HA‐N formed localized hypoechoic lobular or spindle‐shaped spaces within the tissue, displayed minimal tissue integration over time, and a capacity to lift superior tissues (Figures 3 and 5).	High resistance to compression and lateral shear.	Significant lifting effect in treatment areas.	For supraperiosteal injection or thick skin.

Abbreviations: HA‐L, Restylane Lyft; HA‐N, Non‐Animal Stabilized Hyaluronic Acid; HA‐R, Restylane Refyne; HA‐V, Restylane Volyme; HA‐O, Optimal Balance Technology hyaluronic acid.

CONFLICT OF INTEREST STATEMENT

Andreas Nikolis is or has been a consultant, speaker, and trainer for Galderma (Lausanne, Switzerland), Prollenium (Montréal, QC), Merz (Frankfurt, Germany) and Allergan (Dublin, Ireland). Steven Weiner is or has been a consultant, speaker, and trainer for Galderma. The other authors state no conflict of interest.

PHOTOGRAPHIC CONSENT

The exemplary participants displayed in this manuscript consented to the publication of their full‐face photographs.

Nikolis A, Enright KM, Cotofana S, Nguyen Q, Raco L, Weiner S. Intracorporeal evaluation of hyaluronic acid fillers with varied rheological properties and correlations with aesthetic outcomes. Skin Res Technol. 2024;30:e13838. 10.1111/srt.13838

DATA AVAILABILITY STATEMENT

The data are available from the corresponding author upon reasonable request.