Soft Tissue Augmentation

Soft Tissue Augmentation

Samantha L. Schneider, MD

Hema Sundaram, MD

M. Laurin Council, MD

Aesthetic medicine has continued to grow in the United States and worldwide. Patients are increasingly interested in minimally invasive procedures such as soft tissue augmentation with fillers, which has been shown to improve patients’ psychosocial states.1 In 2011, Americans spent $10.4 billion on elective surgical and nonsurgical procedures,1 with soft tissue fillers representing the second most common minimally invasive procedure after botulinum toxin.2 Physician administrations of soft tissue fillers have more than doubled from 1.3 million in 2007 to 2.7 million in 2017,3 and the number of treatments performed annually has grown by over 300% from 2000 to 2017.4 All of these statistics demonstrate the importance of fillers in the practice of dermatology.

rheology and biophysical propertiesRHEOLOGY AND BIOPHYSICAL PROPERTIES

In order to choose the appropriate soft tissue filler products to address a particular patient’s needs, it is important to understand the products’ physical properties. Rheology is the study of how materials react and deform under mechanical stress, which ultimately helps physicians understand the rationale for the behavior of certain soft tissue fillers.7 These include many of the fillers available for aesthetic use today, particularly hyaluronic acid (HA) and calcium hydroxylapatite (CaHA). There are four main parameters to consider—G′, Viscosity, G*, and tanδ, as well as a product’s cohesivity.

G′ (elastic modulus) is a measurement of the product’s elastic properties, which quantifies the ability of the filler to recover its initial shape when faced with applied force.7 Another way to explain G′ is that it represents the fillers’ ability to resist deformation once placed within the skin.8 G′ is typically related to the amount of hyaluronic acid cross-linking present in a product, with more cross-linking leading to a higher G′ value.9 Higher G′ products aid in lifting and volumizing and are particularly useful for subcutaneous implantation in areas with higher levels of muscular activity as they are more effective at resisting deformation.10,11 Higher G′ products can be of value in areas such as the nasolabial folds, upper cheeks, nasal dorsum, and chin where these products provide outward projection.12 Soft fillers, with lower G′, are less resistant to deformation and may provide a softer feel in thinner-skinned, mobile areas such as tear troughs and lips.12 G″ (viscous modulus) is a measurement of a filler’s viscosity.10 Viscosity, which can also be measured as complex viscosity, determines how spreadable the product will be in the tissue, as well as its resistance to flow (extrusion force) during the injection. Higher viscosity products stay localized to the site of injection, whereas lower viscosity products are more likely to spread.9,10 G* (complex modulus) measures viscoelasticity of the filler product. It is a measure of how much applied energy is stored versus how much is dissipated.13 Fillers with higher G* represent harder fillers, which should be injected more deeply.13 Tanδ is the ratio of a product’s viscosity to its elasticity, which describes the filler’s balance between fluidity (related to viscosity) and elasticity.13 Finally, cohesivity describes the ability of the solid and fluid phases of the filler gel to remain together.14,15 For hyaluronic acid fillers, the affinity between the solid and fluid phases may contribute to three-dimensional tissue expansion rather than outward tissue projection alone.12 Keeping these concepts in mind can allow the physician to select the most appropriate product for a given injection location and patient goal.


There are many options available on the market today for soft tissue augmentation including hyaluronic acid, poly-L-lactic acid (PLLA), CaHA, and permanent fillers including poly-methymethacrylate (PMMA). Table 3.1 provides a comprehensive overview of products available on the US market and their FDA indications.16,17,18,19

Hyaluronic Acid

Hyaluronic acid (HA) products were first approved for use in the United States in 2003.6 Glycosaminoglycans occur naturally in the body to provide scaffolding and volume. With age and increasing exposure to ultraviolet radiation, a patient’s natural HA decreases. There is also loss of all hard and soft tissues, including fat, prompting clinicians to supplement with soft tissue facial fillers.20 A patient’s decreasing facial volume due to loss of bone, fat, and other subcutaneous and cutaneous components results in the accentuation of facial lines and hollows.

HA fillers restore volume themselves and can potentially upregulate synthesis of new collagen and elastin. Volumizing with HA fillers predominantly relies upon their intrinsic ability for hygroscopic uptake of up to 1000 times their molecular weight in water, allowing hydration to augment tissue volume.20 They are polysaccharides composed of alternating residues of monosaccharides Dglucuronic acid and N-acetyl-Dglucosamine.6,7 Streptococcal bacteria fermentation processes are used to develop most HA fillers.6 Because HA fillers do not have any protein components, typically there is little or no immunologic reaction when they are injected into patients.20,21

Non-crosslinked HA has a short half-life of in the skin approximately 1 to 2 days.20 HA fillers have varying levels of crosslinking to slow their degradation.20 Several commercially available products use a 1,4-butanediol diglycidyl ether (BDDE) cross-linking agent.20

Understanding of the properties of various HA fillers allows the provider to select the most appropriate product for each patient. In order to create the scaffolding network that leads to volume, many products have a combination of low molecular weight and high molecular weight HA. Products with a higher concentration of HA have a stiffer consistency and have been associated with increased duration in the tissue. Higher molecular weight HA allows for greater lift. Less cross-linked products are easier to inject into the patient but may be more quickly degraded, although newer methods of HA manufacture allow a decrease in cross-linking while preserving durability. As previously discussed, HA fillers rely upon hygroscopic water update to create maximal volume. Patient can therefore develop increased volume over the days and weeks following filler injection.20 This is important to consider during the in-office procedure so as not to overvolumize patients.

HA products are highly versatile and can be used in most anatomic locations. They are highly malleable, and clinicians sometimes mold filler immediately after injection to optimize the appearance and minimize nodules.20 Softer, less viscous HA products are useful in the lips, tear troughs, and superficial wrinkles, whereas harder, more viscous HA products are injected into areas of deep volume loss such as the cheeks and temples.7,13 Most HA products are FDA-approved for correction of nasolabial folds. Because of the large variety of products, many providers develop a multifaceted, patient-centric approach where various products are used in layers to provide the best cosmetic outcomes. Using this technique, higher G′, more viscous products are placed deeper for maximal volume achievement, whereas fine lines are addressed with lower G′, less viscous products more superficially.12 HA lasts anywhere from 6 to 18 months, or longer, depending on the product and the location of implantation.6,7,22 HA products also have the advantage of being reversible with varying doses of hyaluronidase.23,24,25,26

Poly-L-Lactic Acid

PLLA was first developed by French chemists in the 1950s5 and ultimately approved by the FDA in 2004.6,27 It is a synthetic polymer that comes in powder form and is reconstituted using sterile water 48 to 72 hours before injection. Once injected, the particles settle into
the subcutaneous tissue and the water is absorbed by the body.7,28 The particles in the subcutis are treated as small foreign bodies similar to PMMA, which induces fibrosis and collagenesis.5,6,7,28 It is important to keep this in mind as the injections are being performed because the patient will lose some volume initially as the water is resorbed and then will gain more volume over the following several months with collagenesis.7,28 Techniques to avoid clogging the needle while injecting the PLLA suspension include using a 25-gauge needle or a wider cannula for injection, reconstituting the particles with larger volumes than recommended by the package insert, storage of the reconstituted product for 48 to 72 hours before injection, and thorough remixing of the product suspension just prior to injection.7,28

PLLA is injected into the subcutaneous tissue.7 It was first approved by the FDA for HIV lipoatrophy where it has shown improvement in skin thickness, confidence, self-perception and quality of life, and decreased anxiety and depression.5,6,29 For the cosmetic patient, PLLA is most frequently used in the cheeks but can be used in other facial areas.6 After FDA approval for HIV lipoatrophy, the FDA indications of PLLA were expanded to include shallow to deep nasolabial fold rhytids and other facial wrinkles that are amenable to a deep dermal grid pattern of injection.27 Patients typically require three injections spaced 4 to 8 weeks apart.5,7,28 The durability of results from PLLA is, on average, 18 to 24 months,5,6,28,29 though some report durability up to 3 years.29

PLLA has the potential side effect of delayed-onset nodules, which is why many clinicians consider this product to be contraindicated in the lips and tear troughs.5,6 The incidence of nodules has decreased significantly with the use of larger reconstitution volumes, reconstitution 48 to 72 hours ahead of injection, and mixing well prior to injection.29 In addition, some clinicians recommend that patients massage the injected areas five times daily for 5 days to prevent nodule formation.5,7,28,30 Other clinicians recommend massage only on the day of injection.28 No objective evidence exists to support massage as an effective method of preventing nodules.30

Calcium Hydroxylapatite

CaHA, a natural component of bone, is now also used as a commercial filler.5,7 Similar to PMMA and PLLA, CaHA is a 30% concentration of small spherules (approximately 25-45 µm diameter) of synthetic CaHA in a neutral gel matrix composed of sodium carboxymethylcellulose which is resorbed after injection.6,7 After the gel is resorbed, the microspherules remain in the tissue and stimulate collagen formation.5,6 Over time, the body breaks down the microspherules into calcium and phosphate, which are excreted by the body.5,6

CaHA is injected into the deep subcutaneous tissue,7 typically with a 27-gauge needle or blunt cannula.6,31 CaHA is now FDA-approved with and without 0.3% lidocaine for use in patients with HIV lipoatrophy6,32,33,34 as well as for moderate to several facial wrinkles and folds (such as the nasolabial folds) that would permit subdermal injection.33 In 2015, CaHA became the first FDA-approved filler for the dorsal hands.5,6,33,35 For the cosmetic patient, it can be used for the chin, mandibular line, nasolabial folds, marionette lines and midface as well as for atrophic scars. Mixing CaHA with lidocaine suspension and/or saline, to reduce its G’ and viscosity and make it softer and more spreadable, is preferred by some when injecting the hands, face or other areas. CaHA can form subcutaneous nodules and is thus avoided in the lips and tear troughs. Since there has been no reversal agent, nodules have been addressed by saline or lidocaine dispersion or by excision.5,6,7,10 Robinson reported in a proof-of-concept study that intralesional sodium thiosulfate, topical sodium metabisulfite under occlusion, or a combination of the two could potentially
dissolve CaHA filler in cadaveric porcine skin samples.36 Rullan et al. applied these concepts to two patient cases and illustrated the utility of intralesional sodium thiosulfate to dissolve CaHA nodules in vivo.37 The durability of results from CaHA is typically 1 to 2 years depending on the site injected.5,6,7,32 CaHA has a radio-opaque appearance on x-ray and CT images because it is composed of a constituent of the bone.7

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Jun 9, 2022 | Posted by in Aesthetic plastic surgery | Comments Off on Soft Tissue Augmentation
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