Although only recently introduced, chemically-modified hyaluronic acid dermal fillers have gained widespread acceptance as “redefining” dermal fillers in the fields of dermatology and cosmetic facial surgery. Although hyaluronic acid-based dermal fillers have a low overall incidence of long term side effects, occasional adverse outcomes, ranging from chronic lymphoplasmacytic inflammatory reactions to classic foreign body-type granulomatous reactions have been documented. These long-term adverse events are reviewed.
Keywords:
hyaluronic acid, Restylane®, Hylaform®, injectable dermal filler, foreign body reaction, granuloma
A major goal of cosmetic surgeons, dermatologists and pharmaceutical companies has been the development of biocompatible materials with prolonged clinical longevity for use as esthetic facial soft tissue augmentation agents. Ideal properties of soft tissue fillers include biocompatibility (low risk of foreign body-type reaction), reasonable clinical appearance and duration, ease of use and minimal tendency to migrate to distant sites.
Currently available materials can be broadly subcategorized as nonbiodegradable (permanent) or biodegradable (temporary). The biodegradable materials can be further subdivided into those of intermediate or long duration. Examples of permanent materials include liquid silicone (eg, Silikon®), solid silicon particles in suspension (eg, Bioplastique®), polymethyl methacrylate microspheres with bovine collagen (eg, Artecoll®), acrylic hydrogel particles with unmodified hyaluronic acid (eg, Dermalive®), calcium hydoxylapatite (eg, Radiesse®, formerly called Radiance), and various polyacrylamide gel formulations (eg, Aquamid®). Many of these permanent fillers are associated with a definite risk of delayed foreign body-type reactions (Vargas-Machuca et al 2006). Biodegradable materials include polylactic acid microspheres (eg, New-Fill®), allogeneic human collagen from tissue culture (eg, CosmoDerm®), autologous fat and collagen grafts, and xenogeneic material such as bovine collagen (eg, Zyderm®) and glutaraldehyde-treated bovine collagen (eg, Zyplast®). Until recently, bovine collagen was regarded as the “gold standard” in facial soft tissue augmentation. However, the use of bovine collagen is associated with a 3%–5% risk of delayed hypersensitivity reactions (Lowe et al 2001), necessitating double skin testing prior to treatment (Narins et al 2003).
To date, no universally applicable dermal filler has been developed, although manufacturers of hyaluronic acid-based products claim that their products are close to fulfilling many of the requirements of an ideal tissue augmentation agent. Based on the rapid acceptance of these materials by both clinicians and patients, it would appear that many practitioners believe that there may be some validity to these claims. Hyaluronic acid-based temporary dermal fillers are being employed with increasing frequency for the treatment of facial skin lines and for lip augmentation procedures (Carruthers and Carruthers 2003). It is estimated that in the United States in 2004 alone, 878,000 patients were treated with hyaluronic acid-based fillers (Matarasso et al 2006), both animal source (Hylaform, Biomatrix Inc., Ridgefield, NJ; a hyaluronic acid extract derived from rooster combs) and nonanimal source (Restylane®, Q-Medical Corporation, Uppsala, Sweden; a cross-linked hyaluronic acid injectable filler produced from bacteria by microbiologic engineering techniques).
As stated by Walker (2006), although the aging process itself has been extensive researched, “there is a paucity of data and peer-reviewed papers on human responses to interventions in aging,” much of which involves “replacement therapy”. This paper will review the potential long-term side effects associated with the use of cross-linked hyaluronic acid injectable fillers as replacement therapy in dermatology/plastic surgery.
Glycosaminoglycans (GAGs), also referred to as mucopolysaccharides, are large negatively-charged unbranched polymers composed of pairs of repeating sugar units, one of which is an amino sugar. The main GAGs include chondriotin-4-sulphate, chondroitin-6-sulfate, dermatan sulfate, heparan sulfate, keratan sulfate, heparin, and hyaluronic acid. Of these, hyaluronic acid (hyaluronan), consisting of repeating units of the monosaccharide D-glucuronic acid and the amino sugar N-acetyl-D-glucosamine linked together via alternating beta-1,4 and beta-1,3 glycosidic bonds, is the most prevalent. It has been estimated that the average 60 kg human body contains 12 g of hyaluronic acid (Matarasso et al 2006). Polymers of hyaluronan can range in size from 4,000 to 20,000,000 Daltons in vivo.
In the extracellular matrix (ECM), hyaluronic acid is produced predominantly by fibroblasts through a complex of cytoplasmic proteins on the plasma membrane called hyaluronan synthases and is extruded through the plasma membrane into the extracellular space. Hyaluronan is ultimately degraded in a step-wise manner by a family of hyaluronidase enzymes (Stern 2003). The first step in the process is the cleavage of high molecular weight hyaluronic acid to 20 kDa fragments. These fragments are further degraded into smaller units, primarily tetrasaccharides. These different-sized hyaluronic acid molecules have varying biological effects.
HA is a major component of the ECM of the dermis, where it is a major contributor to the formation a resilient gel-like ground substance that resists compressive forces.
The traditional view of hyaluronic acid as simply being an inert “filler” material has recently been questioned (Stern 2003). Hyaluronic acid contributes to tissue hydrodynamics by creating space for the movement of cells. Hyaluronic acid is believed to regulate the diffusion of nutrients, metabolites, and hormones between cells, and stimulates fibroblast proliferation, migration and collagen production. Hyaluronic acid also regulates cell proliferation and motility by regulating cell/cell and cell/matrix interactions through the cell membrane receptor CD44 (Stern 2003).
The high molecular weight hyaluronic acid molecules have anti-inflammatory, anti-angiogenic, and immunosuppressive properties, whereas the 20 kDa fragments and the very low molecular weight hyaluronic acid degradation products stimulate the synthesis of new blood vessels, inflammatory cytokines and induce inflammatory responses in macrophages and dendritic cells secondary to infection and tissue injury.
Hyaluronic acid is present in high concentrations in embryonic tissue and in malignant neoplasms. Overexpression of CD44 has been linked to the growth of a number of malignant neoplasms, and in some cancers, hyaluronan levels correlate with poor prognosis. Hyaluronic acid likely contributes to tumor growth via its interaction with the CD44 receptor (Hill et al 2006).
The widespread tissue distribution of hyaluronic acid, being a major component of tissue ranging from skin and cartilage to vitreous humor, accounts in part for the potential of hyaluronic acid as a universal soft tissue replacement material. Another benefit of hyaluronic acid is that, in contrast to collagen, its chemical structure is reportedly identical across different species (Richter et al 1979). Therefore, the risk of imunogenicity to hyaluronic acid-derived products is believed to be low. In addition, as a result of its water-binding affinity, hyaluronic acid forms a high viscosity hydrated polymer that purportedly maintains much of its volume by binding additional molecules of water as it degrades.
The first hyaluronic acid-based biomedical product, Healon, was developed as an ophthalmic-surgical aid for use in various anterior segment procedures, as a vitreous replacement after vitrectomy and in retinal detachment surgery. Hyaluronic acid has also been used in the treatment of osteoarthritis of the knee as a joint fluid supplement, typically administered by injection into the knee joint.
The process of aging is characterized by loss of resiliency and atrophy of underlying adipose tissue. A progressive reduction in hyaluronic acid content has been described in aging skin (Ghersetich et al 1994), leading to the suggestion that variations of hyaluronic acid levels in the dermis may account for some of the changes seen in aged skin, including decreased turgidity, wrinkling, and decreased elasticity. In view of these observations and the fact that hyaluronic acid is a major contributor to the extracellular matrix of the dermis, hyaluronic acid-derived products have been extensively investigated as injectable dermal supplements. The principal drawback with purified hyaluronic acid dermal filler is the short half-life of hyaluronic acid in the dermis, estimated at 24–48 hours. In order to increase the longevity of hyaluronic acid to the point that it is practical for clinical use, pharmaceutical companies have developed longer lasting hyaluronic acid formulations through chemical cross-linking.
Hyaluronic acid-derived dermal fillers have been available in Europe since 1996 (Andre et al 2005). The first chemically-modified hyaluronic acid filler approved by the US Food and Drug Administration (FDA) for the correction of moderate to severe wrinkles and skin folds was Restylane, approved in December 2003. Chemically modified hyaluronic acid formulations currently available in the United States include the nonanimal source hyaluronic acid (NASHA) materials: Restylane, Restylane Fine Line, Restylane Perlane, Restylane SubQ, Juvederm (Inamed Corporation, Santa Barbara, California) and Captique (Inamed Corporation, Santa Barbara, California). The FDA has also approved a line of animal-derived hyaluronic acid products: Hylaform Regular, Hylaform Fine and Hylaform Plus. Differences between these formulations include hyaluronic acid source, particle size, degree of cross-linking, and concentration of hyaluronic acid. Hylaform is cross-linked by glutaraldehyde vinyl sulfone, whereas Restylane is stabilized by treatment with 1,4-butandiol diglycidylether. The concentration of hyaluronic acid in Restylane is approximately four-fold that in Hylaform; 20 mg/ml versus 5.5 mg/ml (Patel et al 2006).
In addition to the correction of moderate to severe wrinkles and skin folds, hyaluronic acid-based dermal fillers are also widely used for the correction of scars and for lip augmentation. Intradermal injection of hyaluronic acid is contraindicated in patients with autoimmune disorders, on immunosuppressive therapy, in patients with active herpetic lesions, and in patients with acneiform lesions.
Because of their limited half-life, most patients require at least twice yearly injections of chemically-modified hyaluronic acid products. Some (Hamra 2006) have argued that, from a long-term economic perspective, patients may be better served by conventional plastic surgery in certain clinical scenarios.
Treatment options for patients presenting with delayed hypersensitivity reactions range from simple observation to local treatment with topical corticosteroids or intralesional injection of corticosteroids. Lesions that fail to resolve with conservative therapy may benefit from treatment with systemic therapy with corticosteroids, although in some case surgical intervention may ultimately be required. Intralesional administration of hyaluronidase has also been successfully employed to resolve nodular lesions (Soparkar and Patrinely 2005). Recently, etanercept, an inhibitor of tumor necrosis factor-alpha activity, has been used to resolve lesions in patients with silicone granulomas (Desai et al 2006), although its use in treating patients presenting with delayed hypersensitivity reactions to hyaluronic acid has not been documented to date.
Injection of any foreign material into the dermis triggers an inflammatory response; essentially a protective response intended to eliminate the initial cause of cell injury as well as to repair any tissue damaged as a result of the original insult. The initial phase, lasting from several hours to days, is characterized by an acute inflammatory process localized to the injection site. The hallmark of acute inflammation involves circulatory changes leading to the release of soluble mediators of inflammation, and chemotaxis of neutrophils to the site of injury. If these areas of acute inflammation fail to resolve and become walled off, abscess formation ensues. By definition, an abscess is defined as an acute inflammatory process characterized by a localized collection of dead and dying neutrophils surrounding a foreign agent or organism.
Long term tissue reactions to any unresorbed foreign material are generally characterized by a chronic inflammatory reaction, consisting of an infiltration of mononuclear cells (eg, macrophages, lymphocytes and plasma cells) and attempts at healing characterized by the formation of a collagen capsule and/or granulation tissue. Many late-occurring adverse reactions to injectable dermal agents most likely represent localized foreign body-type granulomatous inflammation. Granulomatous inflammation represents a specific type of chronic inflammation caused by antigens that evoke cell-mediated hypersensitivity (eg, tuberculosis) or agents that persist at site of inflammation (eg, foreign bodies). The histologic presentation of granulomatous inflammation is composed of modified macrophages, termed epithelioid macrophages, which can occasionally fuse to form “foreign body-type” multinucleated giant cells, as well as T-lymphocytes, occasional plasma cells, and a proliferation of fibroblasts and capillaries. The epithelioid macrophages are typically found adherent to the implant surface, contributing to the formation of a soft tissue capsule.
In the dermatologic literature, the term “sterile abscess” is broadly used to describe a nodular lesion that has formed as a result of foreign bodies and/or injected medications that have not been totally absorbed. In many cases, these “sterile abscesses” represent a chronic inflammatory granulomatous reaction. From a cosmetic point of view, surgical treatment to evacuate the contents of the “sterile abscess” may be required to reduce the likelihood of indurated scar-like tissue forming (Lowe et al 2005).
The exact cause of these chronic inflammatory reactions following the injection of chemically-modified hyaluronic acid fillers has been debated. The literature in this area makes extensive reference to the fact that, because hyaluronic acid is identical across species, these products are not recognized as foreign by the body and therefore should not trigger any long-term inflammatory response. However, as reviewed in this manuscript, it is apparent that although the risk of developing a clinically evident host reaction is limited, numerous cases have nevertheless been documented. The most widely accepted view is that these late side effects are related to contamination by residual bacterial and or avian proteins from the production process. These products do indeed contain trace amounts of hyaluronin-associated protein and, in the case of Restylane, streptococcus equi-derived bacterial antigens (Andre 2004). The documented decrease in the incidence of late adverse reactions following improvements in the Restylane manufacturing process, resulting in a substantial decrease in protein contaminants, has been widely cited in support of this theory (Friedman et al 2002).
Another possible explanation may lie in the chemical cross-linking process that is used to increase the clinical half-life of these products compared to that of native hyaluronic acid. While remnants of the chemical agents used in the cross-linking (“stabilizing”) process could trigger a delayed inflammatory reaction, it is more likely that the chemical cross-linking process itself, which reportedly modifies the structure by 0.6%–1.0%, (Andre 2004), introduces an immunogenic potential by inducing changes in the three dimensional structure of these molecules. The breakdown products of these chemically-modified hyaluronic acids could also be a cause of the immunologic response. Alternatively, these breakdown products could themselves be further metabolized to other immunogenic compounds (Coleman 2005).
The possibility that some of these long-term side effects could be in part technique-related, for example related to local deposition of excessive tissue volumes of hyaluronic acid, cannot be entirely discounted.
Of interest is the observation that, in vivo, hyaluronic acid is depolymerized into lower molecular weight fragments by enzymatic digestion. Even more intriguing are recent findings that hyaluronic acid and its degradation products are important regulators of dendritic cells and macrophages (Termeer et al 2003). Hyaluronic acid degradation products are potent activators of macrophages through the CD44 cell surface receptors (Leonhardt et al 2005), while dendritic antigen-presenting cells and T-cells appear to regulate the synthesis and degradation of hyaluronic acid directly by synthesizing hyaluronic acid synthetases and hyaluronidases. Although only speculation, it is plausible that changes in the balance between high molecular weight hyaluronic acid and its degradation products in a local microenvironment, induced by the injection of chemically-modified hyaluronic acid particles (with altered degradation kinetics) could lead to altered T-cell and/or macrophage activation and hence granuloma formation in a certain subset of patients.
Although chemically-modified hyaluronic acid dermal fillers, both animal and nonanimal source, have a very low incidence of long term side effects, patients need to be informed of the potential risk of foreign body reactions to these injectable agents.
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