Introduction

Physicians and patients continue to strive for the ‘ideal filler’, which would offer consistent, sustained results that remain natural and free of complications over time and that would be biocompatible, safe, cost-effective, and versatile. Injected properly, highly purified liquid injectable silicone (HPLIS) meets the majority of these criteria. Although use of injectable silicone has historically been met with controversy, when more modern HPLIS is properly injected in small amounts with the microdroplet technique with repeat treatments spaced weeks apart, the physician may routinely achieve optimal and enduring correction of scars, rhytides, and facial atrophy.

HPLIS may be much less forgiving than temporary fillers and is a potential liability when it is injected incorrectly, results in undesired augmentation, or serves as a nidus for inflammation and infection. Therefore, experience and precise technique are important to achieve good outcomes. Physicians should use HPLIS only after extensive training in proper technique and patient selection. Candidates for treatment should have clear treatment objectives and understand that multiple treatment sessions may be required to achieve optimal correction. Patients who desire immediate correction or are uncertain of their treatment goals are better treated with shorter duration, temporary fillers rather than HPLIS.

Basic science

Silicon (Si) is a relatively inert element that is essential to humans in small amounts, and is second only to oxygen as the most abundant element of the Earth’s crust. ‘Silicone’ describes a group of synthetic polymers containing elemental silicon. Polymers in the silicone family may exist in solid, liquid, and gel states, with various chemical, physical, and thermal properties. Silicone polymers also vary with regard to purity, sterility, and biocompatibility. Although various silicone polymers are employed for medical use, polydimethylsiloxane is the liquid injectable silicone used for soft tissue augmentation. The molecular structure of this colorless, odorless, non-volatile oil consists of repeating dimethylsiloxane units with terminal trimethylsiloxane ends.

The viscosity of a given liquid silicone product is dependent upon the mean number and chain length of the dimethylsiloxane subunits within the polymer, with longer chain molecules conferring a higher viscosity. Viscosity is measured in centistokes (cs), where 1 cs equals the viscosity of water. Current HPLIS is either 1000 cs (similar to the viscosity of honey) or 5000 cs.

Silicones have not been found to be carcinogenic, and have demonstrated ‘an enviable record of safety’ according to a 1998 National Science Panel investigating silicone implants reported by Diamond and colleagues. HPLIS is not altered in vivo, although small amounts may be phagocytized and enter the reticuloendothelial system.

Mechanism of action

HPLIS first creates immediate volume enhancement by a direct space-filling effect. Additional filling occurs by the deposition of new collagen via fibroplasia. After injection, a localized inflammatory reaction ensues consisting of neutrophil migration and some degree of macrophage phagocytic activity, with fibroblasts depositing a thin-walled collagen capsule around the silicone microdroplet. This capsule effectively anchors the microdroplet in place and prevents migration. Several filler products, both temporary and permanent, were reported by (DHJ) in 2009 to induce collagen fibroplasia as a partial mechanism of action for aesthetic improvement.

Rather than attempting to reach optimal correction in one session, fibroplastic fillers require that smaller amounts of product be injected over several sessions spaced 1–2 months apart or longer. This avoids overcorrection by allowing the fibroplastic process adequate time to occur prior to subsequent treatment sessions.

Controversy

The past several decades have witnessed notable debate regarding the safety of liquid injectable silicone, with both critics and advocates arguing their positions based largely on anecdotal data rather than rigorous trials. The true number of patients treated with liquid silicone available prior to 1990 who have historically experienced treatment success versus significant complications is simply unknown.

A further difficulty in historically analyzing the safety of ‘silicone’ as an augmenting agent is that, apart from the modern, US Food and Drug Administration (FDA)-approved products available since the 1990s, an unknown number of products claiming to be silicone have likely been adulterated, impure, or non-silicone substances altogether. Although highly purified 1000 and 5000 cs products intended for injection into the human body were FDA approved for human use in the 1990s, various substances masquerading as ‘silicone’ have been injected for the past 60 years, at times with significant complications, as reported by Delage et al, Baselga & Pujol and Rapaport et al. Even products labeled as ‘medical-grade’ silicone have not historically been regulated or authenticated. A 1989 analysis by Parel of six ‘medical-grade’ silicone oils commonly used for injection revealed six different products of variable viscosity, each with significant amounts of elemental impurities and low-molecular-weight adulterants that can produce inflammatory and granulomatous reactions.

Critics argue that liquid silicone is an inherently unpredictable implant, fraught with potential complications. Several anecdotal reports of complications such as cellulitis, nodules, granulomatous reactions, and migration have been described by the groups above, although variables such as product purity, volume, and injection technique could not be established with certainty. Furthermore, complications were reported by Rapaport et al to occur as long as 36 years after treatment. Migration of product to other areas of the body may occur when large boluses of liquid silicone are injected, but this has never been reported when using the microdroplet technique, as in the studies by Duffy in 2005 and Price and colleagues in 2006.

Advocates posit that HPLIS is extremely safe and beneficial when three rules of injection are strictly followed: (1) use only FDA-approved products intended for injection into the human body, (2) exclusively employ the microdroplet technique, and (3) strictly follow a protocol utilizing limited volumes injected over multiple sessions spaced monthly or longer.

Several authors have published excellent safety records of longer term follow-up on patients treated with liquid silicone. Wallace and colleagues reported long-term follow-up over 41 years using liquid silicone as a soft tissue substitute for plantar fat loss in over 1500 patients, with 25 000 recorded silicone injections; they found that the host response to injections consisted of a ‘banal and stable fibrous tissue formation’. Other authors including Jones et al, Orentreich & Leone, and Hevia have published multiple reports of their extensive and successful experience with liquid silicone, and reiterate that the three principles of product purity, appropriate technique, and proper protocol are imperative for success. Duffy, who has written extensively on the subject, gathers that liquid injectable silicone (LIS) has been used for soft tissue augmentation worldwide for at least 40 years, and hypothetically in at least 200 000 patients in the USA. He cautions that, although pure liquid silicone may be a superior filler for the permanent correction of certain defects, physicians who use it must realize that its misuse, or the use of other materials masquerading as pure silicone, have created ‘a pervasive climate of distrust and a veritable minefield of extraordinarily unpleasant medico-legal possibilities’. Such perceptions reiterate the importance of ongoing trials as they replace anecdotal reports with more rigorously obtained data. Despite 60 years of use, only within the past decade have well-designed trials begun with the newer generation of standardized, highly purified products injected according to strict protocol. These studies have so far demonstrated an excellent profile of safety and efficacy. Collection of ongoing objective data and longer term follow-up are necessary to provide clarity into the true risks and benefits of soft tissue augmentation with modern HPLIS.

Indications and patient selection

Although there are currently no FDA-approved cosmetic indications for HPLIS, it may be legally injected off-label for the augmentation of HIV-associated facial lipoatrophy, nasolabial folds, labiomental folds, mid-malar depressions, lip atrophy, hemifacial atrophy, acne scarring, other atrophic scarring, age-related atrophy of the hands, corns and calluses of the feet, and healed diabetic neuropathic foot ulcers (see the studies by Orentreich, Balkin, and Fulton et al) (Figs 10.1–10.4). HPLIS is specifically contraindicated for injection into the breast, eyelids, or bound-down scars or injection into an actively inflamed site. Its safety has not been studied in pregnant women. It should not be injected into patients with chronic bacterial sinusitis, dental caries, other active bacterial infection, or in those who may be predisposed to trauma in the treated area. Additionally, HPLIS is not a substitute for surgical lifting, chemical or laser resurfacing, dermabrasion, or treatment of dynamic rhytides with botulinum toxin. The ideal patient is one with appropriate insight into the permanent and off-label nature of LIS, a realistic attitude regarding achievable results, in good physical health, and compliant with recommendations. Patients seeking immediate correction or temporary augmentation should be treated with temporary fillers. Serious consideration by both the patient and physician must be given to the longevity of results obtained with HPLIS. Although permanent fillers may be preferred to temporary fillers owing to their longevity, one must consider the possibility that personal and societal aesthetic goals may change over time. Furthermore, an undesirable result will be unlikely to diminish with time and may be difficult to correct.