Anatomical proposal for hyaluronic acid filler injection in subzygomatic arch depression: A dual‐plane injection technique

A subzygomatic arch depression creates a bulky face outline. To smoothen these depressions and correct facial contours, hyaluronic acid filler injection methods are frequently used. However, the complexity of the subzygomatic region make it difficult for practitioners to effectively volume the region. The conventional injection of single layer injection has limitations of lack in volume addition and unwanted undulations and spreading. The anatomical factors were reviewed with ultrasonography, three‐dimensional photogrammetric analysis, and cadaver dissection. In this anatomical study, the present knowledge on localizing filler injection with a more precisely demarcated dual‐plane injection was suggested. This study presents novel anatomical findings related to the injection of hyaluronic acid filler injection in the subzygomatic arch depression.


| INTRODUCTION
Facial aging, marked by facial sagging and volume loss, especially loss of subcutaneous and deep compartment fasts, produces subzygomatic arch depression (Rewari et al., 2020;Yi, Lee, Hur, Seo, & Kim, 2022). It does not only result in aging and fatigue, but also causes a protrusion of the zygomatic arch and zygoma. These changes result in the individual appearing older than they are or giving a negative facial expression.
Hyaluronic acid (HA) filler injection is one of the most popular cosmetic treatments due to advantages such as lower risks and shorter time as compared to surgery, and it is less invasive with a satisfactory filling effect (Yi, Lee, Hur, Bae, & Kim, 2022). HA, filler treatments for subzygomatic arch depression are recommended for those who desire a plump face with a less pronounced zygoma. Filling the volume of the subzygomatic arch depression is difficult due to facial structures such as the superficial muscular aponeurotic system (SMAS) and parotidomasseteric facial. These taut facial structures are hurdles when injecting a massive filler to correct the lateral facial contour and may lead to unwanted undulations and spreading.
Previous studies suggested that the filler injection plane should be at the subcutaneous fatty layer, which is above the SMAS (Seo, 2021). However, injecting in a single plane above the SMAS has limitations such as superficial bumpy shaping and a lack of volume addition (Kim, 2016). Clinically, injecting deeply into the SMAS also has the advantage of having a bulk correction of the subzygomatic Kyu-Ho Yi and Wook Oh contributed equally to this study. arch depression, which avoids the grotesque effect of superficial bumpy shaping. One of the reasons for subzygomatic arch depression is the atrophy of the deep fat layer, and filler injection into the deep layer is necessary in cases of severe depression.
Another issue is the puncture point, wherein previous studies suggested that the entry point of the cannula was indicated as the anterior part of the cheek in the vertical line drawn from the lateral canthus. Clinically, making a hole anteriorly is not recommended for patients due to aesthetic issues.
One crucial concept that can aid the prevention of patient dissatisfaction after HA filler injection, but is often overlooked by injectors, is the understanding of the anatomical structures of subzygomatic areas. Therefore, the study anatomically evaluated a novel method for filling up a subzygomatic arch depression with a dual plane using a posterior approach.

| MATERIALS AND METHODS
Three cadavers (87 males, 84 males, and 98 females) with subzygomatic arch depression were enrolled to perform the HA filler injection. Ultrasonography was performed in real-time in cadavers. The three-dimensional (3D) photogrammetric analysis with the 3D scanner (Morpheus Co., Ltd., Seongnam, Korea) was performed before and after the procedure to compare facial volume changes. Each cadaver was scanned five times with patents at the frontal, 45 , and 90 oblique views at 60 cm from the face under the same brightness. After reconstructing the 3D facial image by merging three scanned images, the entire scanned image was automatically reoriented.
This study was conducted in compliance with the principles of the Declaration of Helsinki. Informed consent was obtained from the patients who underwent the procedure and families of the cadavers prior to the procedure and dissections. Patients were excluded from this study if they were pregnant, had a history of drug allergy, other serious medical conditions, or had surgical or nonsurgical treatment of the facial area (including HA fillers) within the previous 6 months. All prospective patients received a sufficient explanation of the purpose of the study and protocols, and patients were free to withdraw from the treatment and research at any time. After the anatomical cadaver experiment, two clinical patients underwent the dual plane injection technique for filling up subzygomatic depression. In addition, we assessed patient satisfaction using a self-report questionnaire on their last visit. The questionnaire assessed if the patients were "very satisfied," "satisfied," "disappointed," or "very disappointed" with the procedure.

| CADAVERIC EXPERIMENT
HA filler mixed with green dye was injected according to a predefined guideline. A needle (18 G) puncture was done 2 cm anterior to the tragus as an entry point for the cannula insertion (23 G) ( Figure 1). In this study, a Lorient filler (Joonghun Pharmaceutical Company) was used, specifically 2 mL of number 6 (hard-type) and 1 mL of number 2 (soft-type) for the deep and superficial planes, respectively.
The dual-plane technique was conducted based on the location of the SMAS, with the superficial plane comprising the subcutaneous fatty layer, and the deep plane comprising the sub-SMAS fatty layer.
The dual-plane technique is a multistep treatment wherein the deep and superficial regions are volumized consecutively. The two planes should be injected separately with different types of HA fillers to avoid spread from one plane to the other. The HA filler injection should proceed to create a smooth gradation from the most sunken area below the zygomatic arch to the middle of the masseter muscle (three separate points), which is done in a retrograde manner and layer-by-layer (Figure 1).
For injections into the deep plane, manual spreading is not needed, and refraining from rubbing the filler can prevent projection.
In contrast, soft manual shaping was conducted in the superficial plane, and spreading the filler can improve its capacity for expansion and smoothing the contour (Salti & Rauso, 2015).
Following this, the subzygomatic area was dissected layer-bylayer to reveal the HA filler. The skin and SMAS layers were removed to observe the superficial and deep plane injections, respectively.
F I G U R E 1 Entry point of filler injection in the sunken cheek at 2 cm anterior to the tragus. The schematic image of the dual-plane HA filler injection in the transverse plane, which is 1 cm below the inferior margin of the zygomatic arch. HAF, hyaluronic acid filler; Ma, masseter muscle; Pg, parotid gland; PMF, parotidomasseteric fascia; SCF, subcutaneous fatty layer; SMAS, superficial muscular aponeurotic system; SSF, sub-SMAS fatty layer.

| RESULTS
The cadaver experiment revealed that excessive localized injection into the deep plane of the SMAS, as observed by real-time ultrasonography, resulted in most of the filler shifting toward the anterior part of the masseter muscle, causing filler volume loss. According to the ultrasonography and dissection results, the ideal location for the filler injections was at a dual plane, below and above the SMAS layer (Figures 2 and 3). According to the photographs and 3D evaluations, the lateral profile of the face improved the general effect of depression. Detailed information on ultrasonographic guided injection of HLA fillers at the superficial and deep planes can be found in Videos S1 and S2.

| DISCUSSION
Facial aging is characterized by wrinkles, sagging skin, and a decrease in the amount of fat and elasticity in the face, particularly in the cheeks and surrounding tissue, which usually results in subzygomatic arch depression (Rewari et al., 2020). Wrinkles can be treated using botulinum neurotoxin injections, however, volume loss caused by the aging process is primarily treated with dermal filler injections (Yi et al., 2021;Yi, Lee, Hu, & Kim, 2022a, 2022bYi, Lee, Kim, Yoon, et al., 2022;Yi, Lee, Lee, Hu, et al., 2022). Subzygomatic arch depression may result in an appearance of premature aging and fatigue resulting from zygomatic arch and zygoma protrusion. HA, filler treatments for subzygomatic arch depression are recommended for people who wish to have a plump face with a less pronounced zygoma.
The subzygomatic region consists of the skin, a subcutaneous fatty layer, the SMAS, the sub-SMAS fatty layer, parotidomasseteric fascia, and the masseter muscle (Figure 4). Previously, subzygomatic arch depression rejuvenation was performed by a single-layer filler injection into the subcutaneous fatty layer above the SMAS.
However, we approached this problem by considering two possible layers of injection; the subcutaneous (superficial plane) and sub-SMAS (deep plane) fatty layers. The ideal injection regions for subzygomatic arch depression are the most sunken areas below the zygomatic arch to the middle of the masseter muscle.
F I G U R E 2 Ultrasonography study of the subzygomatic arch depression in (A) schematic image, (B) before injecting the HA filler, and (C) after injecting the HA filler in dual-plane. The red marked fascia and red arrow represent the superficial muscular aponeurotic system while green is the parotidomasseteric fascia. D, deep plane; DTF, deep temporal fascia; HAF, hyaluronic acid filler; Ma, masseter muscle; PG, parotid gland; PMF, parotidomasseteric fascia; S, superficial plane; SMAS, superficial muscular aponeurotic system; Tp, temporalis muscle; Za, zygomatic arch.
F I G U R E 3 Anatomical study of the subzygomatic arch depression hyaluronic acid filler injection in the (A) superficial and (B) deep layers of the cadaver.
The SMAS layer is can be distinctly visualized using real-time ultrasonography. When large volumes of filler are injected in a single plane in the sub-SMAS fatty layer, the spilling effect occurs wherein the filler particles shift toward the premasseteric region. In contrast, high-volume injection in the subcutaneous fatty layer is accompanied by irregularities and inferior migration. Therefore, a dual-plane injection should be conducted.
The filler injection should advance as a smooth-out gradation from the most sunken area below the zygomatic arch to the middle of the masseter muscle. We also recommend using a cannula due to the wide target area of subzygomatic arch depression and to avoid vascular and neural damage. The insertion point of the treatment area should be 2 cm anterior to the tragus to avoid the superficial temporal artery and pretragal area, which has a risk of vascular damage. This is also important due to esthetic and patient preferences, rather than conventional anterior cheek puncture. After injecting into the deep plane, massage is not needed; hence, refraining from touching the hard-type filler can avoid loosening its capacity for projection. Meanwhile, a moderate massage may help to spread the filler to improve its capacity for expansion and smoothing the contour in the superficial plane.

| Case 1
A 27-year-old woman presented with severe subzygomatic arch depression. A preliminary clinical photograph showed a prominent sunken area under the subzygomatic region, especially on the right side ( Figure 5A). The HA filler injection was performed using the dualplane injection technique ( Figure 5B). On Day 8 after the injection, bruising, and swelling had subsided. The 3D photogrammetric comparison was conducted and the difference in height was 4.5 mm (right) and 3.7 mm (left; Figure 5C). This volunteer responded as "satisfied" in the patient satisfaction questionnaire.

| Case 2
A 32-year-old woman complained of a negative appearance due to subzygomatic arch depression. A preliminary clinical photograph demonstrated a prominent sunken area under the subzygomatic region.
To improve subzygomatic arch depression, the HA filler injection was performed using the dual-plane injection technique. On Day 7, after the injection, bruising, and swelling had subsided. The subzygomatic arch depression had greatly improved, and the patient was very satisfied. In both cases, there were no significant side effects, such as infection or prolonged swelling.

| CONCLUSION
In the subzygomatic area, the complex convexity with tight facial layers makes it difficult for practitioners to smoothen the lateral facial contour. This study extensively analyzed HA filler injections with cadavers and patients to provide anatomical guidelines for amendment in facial contouring. The filling of the subzygomatic depression could be maximized by injecting a hard-and soft-type HA filler into the sub-SMAS (deep plane) and subcutaneous (superficial plane) fatty F I G U R E 4 Coronal section of the face and lateral compartment of the facial layers. DDTF, deep layer of deep temporal fascia; PMF, parotidomasseteric fascia; SDTF, superficial layer of deep temporal fascia; SMAS, superficial muscular aponeurotic system; STF, superficial temporal fascia.

SUPPORTING INFORMATION
Additional supporting information can be found online in the Supporting Information section at the end of this article. F I G U R E 5 Patient with subzygomatic arch depression (A) before and (B) after treatment with HA filler injection. Photogrammetric 3D analysis of the volume difference after hyaluronic acid filler injection in a patient with subzygomatic arch depression (C).