Injectable Silicone: Can It Mitigate Plantar Pedal Pressure?

By Stephanie C. Wu, DPM, MS

Ambulation exposes the foot to a collaboration of focal pressure and repetitive stress, and ground reaction forces generated in response to weightbearing activities are the commonly responsible stressors.1
The portion of the foot in contact with the ground varies during the stance phase of gait. Accordingly, the site of ground reaction force application varies, generally progressing from the heel at first contact to the hallux at toe-off.2
These forces contain vertical, anteroposterior and mediolateral components. However, the vertical force is much greater than the other two.3 Vertical forces can damage healthy tissue through compression and deformation. The mechanical stress generated by the anteroposterior and mediolateral components of ground reaction force are shear forces, which stretch and tear tissue.

The magnitude of ground reaction forces also varies over the course of the stance phase. Typically, the heel is in contact with the ground the first 64 percent of the contact phase while the forefoot is in contact the last 59 percent.1 The amplitude follows a bimodal pattern with force initially rising at heel strike, decreasing as the foot rolls forward and then ascending a second time during toe-off.
The midfoot therefore experiences significantly less pressure forces than the heel and forefoot due to variations in the site of application and the magnitude of ground reaction forces. Consequently, the heel and forefoot are subjected to greater pressure, making these two areas susceptible to increased wear and tear.
Anatomically, the body accommodates this by providing plantar fat pads in these locations to assist with shock absorption. However, progressive loss of fat pad occurs due to structural anomalies (such as plantarflexed metatarsals and limited joint mobility); pathologic conditions (such as collagen vascular disease and diabetes); and normal age-related changes during the course of a lifetime.
In situations in which the patient is sensate, the resulting skin on bone situation is extremely painful, especially with ambulation. This severely affects the patient’s activities of daily living and quality of life.
In situations with insensate patients, continued normal ambulation can wear a hole in their skin, akin to normal patients wearing holes in their socks. Fat pad atrophy, regardless of the cause, is often associated with substantial emotional, physical, productivity and financial losses.4-6

Understanding The Impact Of Fat Pad Atrophy And How Silicone Injection May Be Beneficial
It is well documented that plantar pressure is directly proportional to plantar tissue thickness.7,8 Historically, physicians have treated corns and calluses with a myriad of palliative measures and more recently by surgical intervention.
Fat pad atrophy is common among people with collagen vascular disease and diabetes, particularly in the forefoot.5,9 The loss of fatty tissue has been noted to be the fundamental mechanism associated with pressure related foot disorders.

Consequently, augmenting this high-risk area with an inert, viscoelastically robust substance such as silicone has the potential benefit to mitigate pressure. The commonly used unit for viscosity of fluids is centistokes (cs), with water having a centistokes value of 1. Silicone fluid used to replace soft tissue has a viscosity of 350 cs and one can easily inject this with a 25- to 27-gauge needle and a standard syringe.
The rationale for considering injectable silicone in the foot is that, regardless of the causes leading to increased digital or plantar pressure, there is an associated loss of subcutaneous fatty tissue. The development of essentially inert silicone fluids has provided the potential of augmenting the body’s own soft tissue via injection.
There is an injection procedure that allows physicians to implant silicone fluid to form a stable subdermal cushion between skin and weightbearing bone. The internal pad eliminates or reduces pain, and frequency of care for most patients. In cases of insensitive feet, this internal pad reduces the incidence of pressure ulcers.10
PodiaSil (Therapeutic Silicone Technologies) is a 350-cs injectable silicone that has been approved for marketing in Europe for the prevention of diabetic foot ulcers.10 Though previous studies suggest significant improvement in soft tissue thickness and subsequent profound reduction in plantar pressure, availability, production and medical/ legal factors have reduced the potential for widespread implementation.
Previous reports suggest a positive therapeutic use of liquid silicone injections in the foot to replace fat padding at callus sites, corns and localized painful areas.5,10-12 To date, physicians have given more than 20,000 injections with anecdotal support of reduced callus formation, reduced diabetic foot ulcer recurrence and relief of localized pressure-related foot pain in non-diabetic patients.9-12
Only minimal side effects have been reported. Unfortunately, this anecdotal evidence has been limited to a few centers.

What The Studies Reveal On Treatment Efficacy
Balkin injected silicone beneath corns and calluses in 1,585 patients, and gathered surgical and postmortem specimens for histologic analysis.10 Two pathology labs that studied the specimens noted no inflammation, infection, allergy or granulomas. Long-term clinical follow-up also found no evidence of significant adverse responses. Balkin concluded that medical fluid silicone appears to be safe, effective and stable biomaterial for treating weightbearing loss of plantar fat.10

Van Schie, et al., investigated the effectiveness of liquid silicone injections in the diabetic foot to reduce risk factors for ulceration in a randomized double-blind, placebo-controlled trial.5 A total of 28 diabetic neuropathic patients without peripheral vascular disease were randomized to active treatment with six injections of 0.2 mL liquid silicone in the plantar surface of the foot or the placebo treatment with an equal volume of saline.
Researchers noted no significant differences regarding age or neuropathy status between the two groups. All injections were under the metatarsal heads at sites of calluses or high pressures. The study authors measured barefoot plantar pressures and plantar tissue thickness under the metatarsal heads at baseline and at three, six and 12 months after the first injection.
Patients who received silicone treatment had significantly increased plantar tissue thickness at injection sites in comparison with the placebo group (1.8 versus 0.1 mm) and correspondingly had significantly decreased plantar pressures (-232 versus -25 kPa) at three months. There were similar results at six and 12 months.5
Physicians did note a trend toward reduced callus formation in the silicone-treated group in comparison with no callus reduction in the placebo group. The results of this study further confirm the efficacy of plantar silicone injections in reducing recognized risk factors associated with diabetic foot ulceration.
At a two-year follow up, researchers noted in the silicone group that the plantar tissue thickness, which had initially increased by an average of 1.6+/-0.9 mm at 12 months, remained increased at 24 months.9 However, the study authors found that the reduced peak plantar pressure in the silicone group at 12 months (-165.0+/-253.5 kPa) was lower at 24 months.
The reduction in the pressure time integral (PTI) in the silicone group did not reach significance at 12 months (-0.71+/-1.17 kPa/s). Although the PTI returned to baseline at 24 months for the silicone group, it was significantly increased in the placebo group (0.64+/-0.37 kPa/s).
This suggests that silicone may still exhibit some pressure-reducing properties after 24 months.9 The results indicate the cushioning properties of injected silicone are reduced at 24 months after the injection. This finding suggests that booster injections may be required in certain patients.
Studies are warranted to determine silicone’s efficacy to mitigate or prevent pressure ulcers because it appears to induce a relatively stable soft tissue prosthesis between skin and bone, and reduce the frequency of insensitive ulcer recurrence in the foot.12

What You Should Know About Fluid Migration
The migration of this silicone fluid remains the most significant, single adverse response ever since Balkin first reported silicone foot injection fluid drifts.11 Despite findings that even relatively small amounts of silicone can migrate and, in rare instances, require surgical excision, it has long been assumed that fluid migration was due to over-injection.
One may see such movement as a thick silicone skin tag proximal to weightbearing metatarsal heads. At times, there may be a fine keratotic leading edge. These skin tags are essentially asymptomatic on bearing weight or direct palpation.
In one long-term study, from 1964 to 1995, 1,350 patients (986 female and 364 male patients with a mean age of 60.8 years) received silicone injections. The author of the study mostly injected over the soles but also performed silicone implantation in the lesser toes, the hallux, heels and bases of the first and fifth metatarsals.
Among this group, 885 patients received plantar injections beneath 1,879 metatarsal heads. Of these patients, 17 (1.92 percent) developed a soft to firm mass of migrant fibrous silicone tissue over the dorsum at 21 sites. Four patients had a single migratory site bilaterally. The study author noted that these complications occurred only with weightbearing and were painless upon firm palpation. (Balkin had reported this unusual response previously.13)
Balkin found that the earliest post-injection appearance occurred at 15 months and the latest at 13 years with an average post-injection appearance of five years. Four out of the 885 patients (0.45 percent) experienced sufficient discomfort in shoegear to warrant surgical removal, which was uneventful and without recidivation.

Migration from beneath a first or fifth metatarsal head tends to travel proximal medial or proximal lateral respectively. In all instances in which silicone migrated from plantar to dorsal, it followed implants beneath a second, third, or fourth metatarsal head. It is unknown why this type of migration was not observed or detected in earlier cases when larger amounts were injected. The 17 cases reported here received total amounts ranging from 0.4 to 4.1 mL (with a mean of 1.46 mL). Of the 17 patients with dorsal migration, 16 (94 percent) were women.
Aside from the lymphatic role in transporting silicone droplets, altered biomechanics induced by women’s shoes, which considerably increase forefoot pressure, appear to be contributory. Silicone implanted at metatarsal heads two, three or four may migrate distally or proximally as can a natural fat pad under weightbearing conditions.
Regardless of fluid migration, in most patients, the originally injected calluses remained improved or resolved, indicating that a further reduction of injected silicone might be desirable. Considering the inordinate forces to which feet are subjected, it may be impossible to prevent migration in every case.
In the study, the greatest amount of fluid injected into a patient at a single plantar site was 17.8 mL. This massive amount was 10 to 15 times greater than is currently suggested for a callus and is remarkable for its size and appearance. Yet over a 30-year course, it has remained asymptomatic.
Silicone migration following lesser toe implantation can also occur. Similar to plantar migration, such movement is infrequent and rarely symptomatic. Though silicone injections for a corn may make the toe appear fuller, there are no inflammation characteristics such as heat, redness, swelling or pain.
In several hundred treated small toes, a need for surgical excision of migrant silicone due to discomfort was rare (less than 0.5 percent). However, even in these rare cases, as with plantar migration, the original painful keratosis often resolved.

A Closer Look At The Histopathology Findings
The morphologic cellular responses and end fate of silicone have also been analyzed microscopically. In Balkin’s study, he noted 33 surgical biopsies and 124 postmortem specimens from 32 patients.10 Of these, 58 were digital and 66 were plantar. The earliest post-injection tissue examined was one month and the oldest was 29 years. Regardless of the time since injection, each specimen showed the presence of silicone.
Balkin noted the fluid was well retained at the deposit site by two essentially non-inflammatory tissue responses – histiocytosis and fibrosis.10 Histiocytes phagocytise foreign matter are part of the body’s scavenger system. The silicone is engulfed and retained within the histiocyte cell body as countless microscopic droplets.
The second key reaction to silicone fluid is that it stimulates the production of collagen fibers. The newly formed mesh of fibrous tissue acts like a web to further entrap and retain silicone fluid where it is deposited. Microscopic findings also show that numerous droplets envelop microneural and microvascular structures.
With thickening skin and encircling nerves with this resilient fibrous silicone coating, there is decreased neural impingement by the bone. This reduces stress and pain. Similar encircling of tiny blood vessels at pressure points appears to spare or protect vascularity via this cushioning mechanism.
This benefits patients who have neuropathic skin that suffers from pressure due to unrecognized callus or tight shoes. These patients are less likely to shift body weight as opposed to patients with intact nerves. Accordingly, these longer periods of unrelieved stress, when standing, walking or at rest, can diminish or stop local circulation.
Lymphocytes, eosinophils, fibroblasts or plasma cells are characteristic of chronic inflammation but one rarely sees these in silicone-injected tissue. Post-mortem specimen gathering has also afforded an opportunity to study inguinal nodes in 11 patients, including four in whom other lymph node systems were studied, as well as all major viscera.
Although the body does not reject silicone fluid, microscopic droplets are transported into the groin lymph nodes without clinical signs or symptoms. Other deep nodal systems and viscera revealed no silicone. Histopathologic findings suggest that injecting medical-quality silicone into the foot is a safe procedure.10

In Conclusion
Chemical and biomedical engineering advances have provided the healthcare industry with implantable polymeric biomaterials capable of repairing or replacing body parts. One such polymer, silicone fluid, can augment soft tissue and be remarkably well retained.
With regard to the lower extremity, this means that a quick, outpatient, injectable procedure can control or eliminate corns and calluses. When it comes to patients with diabetic neuropathy, this procedure can help eliminate the formation of ulcers.
The debate over the legitimacy of silicone as a safe tool for soft tissue augmentation has spanned well over half a century. Proponents concede that injections of questionable purity and/or of massive quantities have produced unfavorable outcomes. They assert that there are very few problems with “injectable-grade” silicone when experienced physicians perform the procedures.
Despite these claims, the literature is replete with disastrous outcomes following silicone fluid injection, often many years after the initial treatment.14 Unfortunately, as recently as 2006, reports in The New England Journal of Medicine and The New York Times failed to distinguish between the use of medical grade silicone injected by physicians trained in the microdroplet technique and the use of large volumes of industrial grade products injected by unlicensed or unskilled practitioners.15
Several independent evaluations and studies have found injected silicone fluid to be safe and effective as a facial dermal filler and as a soft tissue substitute for treating pressure induced foot disorders. Constant long-term patient review and extensive microscopic analysis have found no serious complications. No tumors or systemic responses were noted. Additionally, injected silicone does not impair healing nor impede venous or arterial circulation.
All drugs and medical devices have some degree of risk, and silicone foot injections are no different. The current level of scientific evidence supporting weightbearing pain relief far outweighs the risk of painless fluid migration or the rare need for surgical removal. For patients with diabetes, the fluid can prevent insensitive digital or plantar ulceration. With this capability, there is the extraordinary further potential of preventing toe, foot or leg amputation.
By breaking the chain of events of increased pressure leading to ulceration, physicians may be able to dramatically mitigate disastrous diabetic foot complications and their social and economic costs.
Further confirmation of these most favorable findings through official investigation, followed by silicone approval and appropriate use, could herald a new and exciting era in the history of foot care.

Dr. Wu is an Assistant Professor in the Department of Surgery at the Dr. William M. Scholl College of Podiatric Medicine at the Rosalind Franklin University of Medicine and Science in Chicago. She is the Director for Educational Affairs and Outreach at the Center for Lower Extremity Ambulatory Research (CLEAR) in Chicago.

Dr. Wu is also the Director of the National Center of Limb Salvage (NCLP) at the Advocate Lutheran General Hospital in Park Ridge, Ill.

Dr. Wu dedicates this article to Sol Balkin, DPM. She says Dr. Balkin, best known for his lifetime of research into the use of silicone in the feet, “was a warm
and compassionate practitioner who epitomized the highest values that a podiatrist could possess.”

For further reading, see “Tissue Volumizing: Can We Create An Internal Orthotic?” in the October 2005 issue of Podiatry Today, “A Guide To Offloading The Diabetic Foot” in the September 2005 issue, or “How To Manage Heel Ulcers In Patients With Diabetes” in the March 2005 issue.

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