While the importance of this property is still evolving, it has been reported that hyaluronic acid filler products with high cohesive properties demonstrate a greater extent of integration intradermally.15,17 One suggested rationale for this observation may be that products with high cohesion, which are also low G′ (softer gels), may facilitate an ability to deform and squeeze more easily into smaller compartments in the tissue in comparison with a firmer product.14 For now, the absence of a standard evaluation method for cohesion limits the scientific community’s ability to advance our understanding of this property.
In vivo, a combination of two types of forces acts on implanted hyaluronic acid filler products: (1) lateral shear or torsion forces and (2) stretch/compression forces (Fig. 6).40 The degree to which these forces act on the product depends on several factors such as the plane of injection (i.e., superficial versus deep) and the anatomical location (i.e., tear trough, malar cheek, perioral region). Although product indications and instructions for use are important for characterizing a product’s commercial identity, the skills necessary to create an aesthetic effect mean that much of the product’s actual use is in the hands of the injector. Summarized in Tables 2 through 6 are the author-recommended injection planes for a variety of anatomical locations.
Additional factors to keep in mind regarding clinical outcomes also include skin quality (e.g., laxity) and degree of correction needed. These variables will potentially differ significantly between patients, and their specific nature will determine not only what degrees of gel strength and firmness are appropriate but also whether a targeted or distributed placement of gel is most effective. The contributing authors agree that for patients with thinner skin, where product palpability/visibility is an important consideration, products with lower G′ values are generally most appropriate. Lower G′ products are softer and more easily distributed in the tissue. Although not among the lowest G′ products, VYCVOLL, XPRESRD, and NASHSLK would still be considered appropriate for thinner skin and would still be able to achieve lift and projection with a natural appearance because the products are soft enough to distribute well in the tissue.
In general, products with higher G′ values are firmer, are indicated for deeper planes of injection, and support a greater degree of correction, whereas lower G′ (softer) products are indicated for more superficial planes of injection and less severe corrections. The higher G′ products are also best for corrective needs where deep, targeted product deposition and less distribution are necessary to achieve lift and projection. For areas such as the malar cheek, chin, and jawline, where the product can be placed against the bone for projection, a higher G′ product will provide a greater advantage over a lower G′ product because it will have greater resistance to the compressive forces inherent in the deeper injection plane. Although lower G′ products are generally indicated for more superficial planes of injection (or areas with less corrective need), they can still be used in deeper planes to achieve a clinical effect, but larger volumes will be required than with a higher G′ product. Alternatively, lower G′ products may be layered on top of higher G′ products.
For optimal corrective results of mild tear troughs in patients with thinner or transparent skin, the authors use a lower G′ product (e.g., VYCVOLB or XPRESRR) because it distributes and integrates well; however, for deeper tear troughs, a higher G′ product with greater lift capacity such as NASHR is an ideal option. When effective lift and projection of the malar cheek are desired in patients who have adequately thick skin and subcutaneous tissue quality, VYCVOLU and NASHLYF are optimal choices. Regardless of skin quality, intermediate G′ products are a more effective solution for the nasolabial folds and in the marionette/melomental region where product visibility on facial animation may be a concern. The intermediate G′ products VYCVOLL and XPRESRD are also good options for areas of “facial animation” and for support and contouring in areas such as the midface.
The correction of perioral rhytides (barcode lines) presents a unique structural challenge, as perioral rhytides are prone to dynamic stress in an area that may not have adequate skin thickness. A higher G′ product is desirable for this challenge. However, placing a firm product deep enough beneath the wrinkle to avoid product palpability/visibility will often not correct the “defect” but only give more anterior (visible) projection to the rhytide. In the authors’ experience, NASHASIL, in small aliquot doses, is a good option for superficial and mid-dermis injections for correction of “stiff” perioral rhytides and fine oral commissures (that do not correct when the skin is stretched). Alternatively, if the rhytides are easily effaced when the skin is stretched, a lower G′ product (softer gel) such as CPMBB, VYCVOLB, or XPRESRR may be a suitable alternative.
Until enough clinical experience is gained, differentiating products by their rheologic and physicochemical properties may serve as a useful way to select which products are most suitable for a given clinical need. Among the variety of parameters used to differentiate products, G′ (elastic modulus) seems to be the most widely used and perhaps is the most logical, as it represents the product’s predominant rheologic property. Although physicochemical properties are also valuable means for product differentiation, the lack of standard measurement techniques among different researchers remains an obstacle for true comparison between products. The ability to find trends between a product’s rheologic and physicochemical parameters appears to be strongest among products of similar concentrations and those produced by the same technology, but not between manufacturing technologies.
Although there is a wide body of literature describing how such data can be used to characterize different hyaluronic acid products, there are very few studies that correlate in vitro measurements with in vivo performance. There are potentially many different properties that impact product characteristics, and future studies may help to correlate product properties with clinical experiences. Ultimately, there are no substitutes for all the technical nuances learned through practical experience. In the absence of data, author discussions that provide practical experience with specific product attributes and techniques as they relate to clinical performance are tremendously valuable. The data and discussion topics presented here represent a source of practical information intended to educate and assist clinicians and injectors in selecting the products best suited to the needs of each patient. Some author recommendations may include off-label use of products.
The authors acknowledge Q-Med AB/Galderma, Uppsala, Sweden, for providing the data for all products presented. The authors thank Åke Öhrlund and Per Winlöf (Q-Med/Galderma) for technical expertise and scientific input; and Alessandra Nogueira, M.D., and Lynette Arlati, M.S.N., F.N.P.-B.C. (Galderma Laboratories, L.P., Fort Worth, Texas), for sharing their clinical expertise in the preparation of this article.
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