Silicone– Polydimethylsiloxane (PDMS)
Silicone (Silastic) is one of the most maligned and hence, most studied biomaterial used in medicine today. It’s a synthetic non-resorbable polymer consisting of repeating ‘Siloxane‘ monomers (Silicon, oxygen, and a saturated hydrocarbon side group). Poly–dimethyl siloxane [-(CH3)2Si-O-]n where methyl forms the side chain is used in most medical applications.
Depending on the length (n) and degree of cross-linking of the PDMA chains, the mechanical properties of silicone can change to fluid (short, straight chains- <30 monomers), gel (lightly cross-linked chains), or rubber (greater degree of cross-linking– > 3000 monomers).
Advantages of Silicone
- Highly biocompatible- inert, nontoxic, nonallergic
- Resistant to resorption
- Easily to sterilize
- Safe- has no proven association with systemic diseases like autoimmune connective tissue disease, neurologic disease, or breast cancer- anaplastic large cell lymphoma (ALCL).
Disadvantages of Silicone
- Nonporous- hence, not incorporated–> hard fibrous encapsulation– sometimes painful and disfiguring.
- Smoothness- prone to extrusion, displacement– leads to removal.
- Silicone rubber has a low tensile strength and may tear easily- eg. in finger joint implants. 💡
- Silicone gel in silicone rubber shell can diffuse out (gel bleed- thus altering its physical property), or rupture out- implant failure– which may be difficult to remove from soft tissue (Inflammatory reaction- not proven). Hence the shell is made of fully polymerized
silicone with an amorphous (noncrystalline) silica filler added for strength.
- Fluid silicone (low molecular weight) as injectable soft-tissue filler led to severe tissue
reaction and migration of the silicone and hence not used.
- Implants and expanders are Expensive.
Uses of Silicone
1. Tissue expansion
2. Breast augmentation
Most breast implants are Silicon gel encased in a ‘silicone elastomer shell‘– they can be classified based on:
a. Surface– Smooth or Textured (less capsule formation, less mobile).
b. Shape– Round or Teardrop (more natural shape).
c. Projection/Profile– low, moderate, moderate plus, high, ultra high.
3. Chin, nasal, malar augmentation
4. Penile and Testicular implants
5. Ear, Finger, other Facial Prostheses
These prostheses can be either osseointegrated or tightly fitting which are custom-made, shaped and colored to match the anatomy of the structure it’s replacing, as closely as possible. They mainly supplement aesthetically, but can also have a minimal function.
6. Hypertrophic scar treatment, Orbital floor reconstruction
Silicone Gel Sheets (SGS) are used for the above. The mechanism by which it works for a hypertrophic scar is mentioned below.
7. Hand surgery
- Small joint arthroplasty
- Flexor tendon replacement (Hunter’s tendon rod)
- Bone block spacers
Mechanism of action in Hypertrophic Scar
The mechanism of action of Silicon Gel Sheet (SGS) in the treatment of hypertrophic scar is not well understood and the following have been proposed:
- The therapeutic effect is not due to pressure, but rather due to wound hydration by decreasing the water vapor transmission rate to nearly half that of normal skin, causing a buildup of moisture on the skin surface under the silicone sheet. Hydration is most likely responsible for the decreased capillary activity, reduced hyperemia, and reduced collagen deposition.
- Altered hydration is thought to cause electrostatic changes that influence collagen deposition and remodeling within the scar.
- The static electricity generated by friction has also been proposed as a plausible reason for silicone’s anti-scarring effect.
Problems with SGS in scar management
- Skin maceration under the sheet
- Persistent pruritus
- Skin breakdown, skin rash under the SGS and the adhesive tape used to hold the SGS
- Foul smell from the gel
- Poor durability of the Silicone sheet
- A poor response of the scar to treatment
- Poor patient compliance
- Neligan’s Plastic Surgery- Vol 1- Principles- 4Ed (2017)
- Textbook of Plastic, Recon, Aesthetic Surgery- Vol 1- Principles and Advances in Plastic Surgery- Agrawal, Bhattacharya- 1Ed (2017)