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In Vitro Analysis of the Degradation of Calcium Hydroxylapatite Dermal Filler

A Proof-of-Concept Study

Robinson, Deanne M., FAAD, MD*,†

doi: 10.1097/DSS.0000000000001683
Original Article

BACKGROUND A dissolving agent for calcium hydroxylapatite (CaHA, Radiesse) soft-tissue filler would be of value should inadvertent intravascular injection, vascular compromise, nodule formation, or overcorrection occur.

METHODS In a prospective, single-center, proof-of-concept study, 12 cadaveric porcine skin samples were injected with CaHA (0.4–0.8 mL). Samples were then randomized to intralesional injection of 0.2-mL sodium thiosulfate (STS, 12.5 g/50 mL); 1 to 2 g of topical sodium metabisulfite (SMB, 25% SMB in 120-mL gel) applied with occlusion, or both intralesional STS and topical SMB. Control samples were not treated after CaHA injection. A 4-mm punch biopsy was obtained 24 hours after treatment, and tissue sections were stained with hematoxylin and eosin and prepared for light microscopy. A board-certified dermatopathologist estimated the amount of CaHA present in each sample.

RESULTS Intralesional STS alone or combined with topical SMB completely dissolved CaHA in the porcine skin samples. Topical SMB treatment reduced, but did not entirely clear CaHA from the tissue samples. The control samples contained easily identifiable CaHA microspheres.

CONCLUSION This proof-of-concept study illustrates the potential reversibility of CaHA filler with intralesional STS, topical SMB, and the combination of both agents. Larger, in vivo, studies are now warranted to provide further insight.

*Modern Dermatology, Westport, Connecticut;

Yale New Haven Hospital, New Haven, Connecticut

Address correspondence and reprint requests to: Deanne M. Robinson, FAAD, MD, 761 Main Avenue, Suite 102, Norwalk, CT 06851,

Supported by D.M. Robinson is a paid Merz consultant. Editorial assistance was provided by Jenny Grice.

The author has indicated no significant interest with commercial supporters.

Calcium hydroxylapatite (CaHA, Radiesse; Merz North America, Inc., Raleigh, NC) received Food and Drug Administration (FDA) approval for aesthetic use in 2006 for the correction of moderate-to-severe facial wrinkles and folds and for correcting facial fat loss in people with HIV. In 2015, it received FDA approval for improving the appearance of the back of the hand due to volume loss. It has been extensively studied in the aesthetic field where more than 3,000 patients worldwide have received CaHA in controlled clinical trials.1 Calcium hydroxylapatite also has a long history of use as an implant material in plastic and reconstructive surgery including dentistry; orthopedics; ear, nose, and throat surgery; and in the treatment of stress urinary incontinence.2–6

Calcium hydroxylapatite provides immediate correction with a 1:1 ratio of CaHA to tissue volume. As the sodium carboxymethyl cellulose gel dissipates, the CaHA microspheres induce neocollagenesis by fibroblast activation forming a lattice around which new collagen forms, allowing for the growth and integration of tissue.7–9 In this manner, the patient's own new collagen production contributes to the treatment effects.10,11 The slowly biodegrading CaHA microspheres and the neocollagenesis that they induce contribute to the longevity of CaHA,12 with treatment effects lasting at least 12 months,13,14 and up to 30 months when used for the correction of nasolabial folds.14

When dermal filler injections are performed by experienced, well-trained practitioners, adverse events occur in less than 1% of patients, and most of these are minor and transient.15 A recent literature review of the safety and complications of CaHA reported since its approval for aesthetic use confirms its excellent safety profile.16 However, as treatment indications expand and the number of treatments performed increases, so do the number and spectrum of adverse events. When more serious complications occur, they can cause patients considerable suffering and leave them with longlasting or permanent functional and aesthetic deficits.17

An effective dissolving agent may be valuable for providers concerned about inadvertent intravascular injection, vascular compromise, nodule formation, or overcorrection; until now, such a tool has only been available for hyaluronic acid (HA) dermal fillers in the form of hyaluronidase.18 The search for an effective dissolving agent for CaHA has turned to products that are effective for disorders of calcium deposition, such as sodium thiosulfate (STS) and sodium metabisulfite (SMB). Sodium thiosulfate has been shown to reduce the formation of calcium kidney stones when taken orally in patients with recurrent calcium urolithiasis,19 and intralesional STS has proved effective at resolving the deep cutaneous lesions of uremic calcifying arteriopathy (calciphylaxis) in patients with chronic kidney disease.20 In 2 case studies, both intralesional STS and topical SMB have proved effective at reducing lesions associated with calcinosis cutis nodules.21,22

The current proof-of-concept study was conducted to determine whether intralesional STS and topical SMB may also help to dissolve CaHA in porcine skin samples and thus offer potential as agents to reverse the effects of CaHA filler.

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For this prospective, single-center, proof-of-concept study, porcine skin was collected from animals slaughtered in accordance with US Department of Agriculture guidance. No experimental procedures were performed on live animals.

Twelve porcine skin samples were obtained from the shoulder of a single, large, female white pig (aged approximately 1 year) within 15 minutes of sacrifice and preserved in formalin. Samples were injected subdermally with CaHA using a 25-G needle with bolus volumes ranging from 0.4 to 0.8 mL. Injection sites were identified with a permanent marker pen. The samples were then randomized to 1 of 3 treatments: topical SMB (formulated in petrolatum jelly: ∼1–2 g applied topically, 25% SMB in 120-mL gel) application with occlusion; intralesional injection of ∼0.2-mL STS (concentration 12.5 g/50 mL), or both topical and intralesional treatments as described above. The SMB and STS treatments were performed approximately 1 hour after CaHA injection. In the control group, samples were not treated with STS or SMB after CaHA injection. After 24 hours, 4-mm punch biopsies of the treated areas were taken for histologic processing and evaluation. Tissue samples were fixed by direct immersion in a 10% formaldehyde solution before processing for light microscopy by dehydration, embedding in paraffin, and sectioning. The 5-μm thick sections were stained with hematoxylin and eosin to evaluate the presence, absence, or degradation of CaHA. Sections were studied by light microscopy at magnifications of ×4, ×10, and ×40. A board-certified dermatopathologist who was blinded to control or treatment reviewed all specimens to estimate the amount of CaHA remaining in each sample.

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In the control porcine skin samples injected with CaHA dermal filler alone, the CaHA microspheres were clearly visible in the subdermis (Figure 1A). By contrast, histologic analysis of skin samples treated with CaHA and immediate intralesional STS, with or without topical SMB, revealed no remaining CaHA microspheres 24 hours after the initial injections (Figure 1B, C). Intralesional STS alone was as effective as the STS and topical SMB combination (Table 1). In CaHA skin samples treated with topical SMB alone, the amount of CaHA was reduced by approximately 50% (Table 1), but some microspheres were still visible (Figure 1D). Histologically, no apparent evidence of tissue injury in the area of intralesional STS or topical SMB was observed.

Figure 1

Figure 1



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This proof-of-concept study in a porcine model demonstrated that intralesional STS, either alone or in combination with topical SMB, was able to dissolve previously injected CaHA microspheres. The STS was injected immediately after CaHA, and on histologic analysis of the treated area 24 hours later, no visible CaHA microspheres were observed. Topical SMB was also associated with CaHA degradation, but to a much lesser extent with approximately 50% of the CaHA remaining after topical treatment alone.

The findings raise the possibility that intralesional STS could be used to dissolve CaHA dermal filler when used for aesthetic purposes. Although correct injection technique, knowledge of anatomy, and filler selection mitigate the risk of many complications, errors can still occur. For example, CaHA is indicated for subdermal implantation, and too superficial placement or injection into highly mobile areas may lead to visibility of the opaque material and nodularity.23 Other events such as vascular compromise are unpredictable and although rare can happen with any dermal filler even with the most experienced injector.

The availability of an agent capable of reversing the effects of a filler in the advent of an injection error or subsequent adverse effect is a valuable tool for physicians. Indeed, HA reversibility has been described as the single most important factor driving the widespread use of HA fillers24 where the availability of hyaluronidase can be used to manage undesirable outcomes and serious complications.18 Although the current study was conducted in a porcine model, previous research has demonstrated the feasibility of STS use for dissolving calcium salt deposits in humans.21,22,25 In a recent study, intralesional STS at the same concentration as this study (12.5 g/50 mL) was used to dissolve localized cutaneous calcinosis nodules in a patient with lupus panniculitis.21 The STS was injected into individual calcified lesions using 0.5 mL/cm and injecting every 5 to 10 mm.21 After 1 to 2 treatments per lesion, the lesions softened and decreased in size before completely resolving.

An ideal agent for dissolving a dermal filler would be well tolerated with minimal adverse effects (systemic or local). Sodium thiosulfate is on the World Health Association's model list of essential medicines as an intravenous agent for the treatment of cyanide poisoning, with which it binds to form a nontoxic thiocyanate, and as a topical agent (15% solution) for certain fungal infections.26 It is also used intravenously at median doses of 25 to 50 g over 1 hour for the treatment of calcific uremic arteriolopathy.27 High-dose intravenous administration can cause hypotension, gastrointestinal upset, and anion gap metabolic acidosis and therefore requires close monitoring,28 but no adverse effects have been reported at the substantially lower intralesional doses used to dissolve individual calcified lesions.21,22,25,28 Sodium thiosulfate is classified by the FDA as “generally recognized as safe,” and there are no known contraindications.

The exact mode of action of STS has not been confirmed, but is likely to be multifactorial. Proposed mechanisms include chelation of calcium into a calcium thiosulfate salt whose solubility is 250- to 100,000-fold higher than CaHA,29 and induction of acidosis by modulation of local pH thereby increasing mineral solubility (the electrostatic bonds between calcium and phosphate ions are weakened by the reaction between hydrogen and phosphate ions).29,30 Dissolution of CaHA by either mechanism releases calcium and phosphate ions, which are safely removed through the body's normal physiological excretory processes.

This proof-of-concept study confirms the feasibility of using intralesional STS to dissolve CaHA, but is subject to several limitations, namely the experimental nature of the study and the inability to extract clinical conclusions from a cadaveric porcine specimen. Ongoing studies are being conducted to provide further insight into the potential of STS for this role. These will aim to address a number of unanswered questions including the optimal STS concentration for intralesional injection, the minimum amount of STS required relative to the amount of CaHA to be dissolved, STS efficacy when administered immediately after CaHA injection as well as longer term (e.g., for the treatment of established nodules), time to onset of action (a near immediate effect is required in the event of a vascular occlusion), and ability of STS to pass through an intact arterial wall to dissolve a CaHA emboli, as has been demonstrated for hyaluronidase in in vitro models of intact facial arteries containing HA emboli.31 Case studies will also be documented to investigate the safety of intralesional STS injections.

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The results of this study strongly support the potential of intralesional STS for dissolving injected CaHA and are important and positive additions to the body of literature establishing the safety and efficacy of CaHA for aesthetic use.

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