Silastic Prostheses Overview

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Silastic Prostheses: Overview

What Is Silastic?

Silastic is a registered trade name for medical-grade silicone rubber (polydimethylsiloxane elastomer) originally developed by Dow Corning. In surgical practice, "Silastic" and "silicone" implants are used interchangeably. These prostheses are flexible, inert, non-porous devices that do not allow fibrous tissue ingrowth, which makes them repositionable and replaceable. Their key property is elasticity - they deform under load and recover shape, making them suitable as flexible spacers rather than true load-bearing joint replacements.

Core Principle: Spacer Arthroplasty

Unlike metal or ceramic total joint replacements, a silicone/Silastic implant does not resurface bone or replicate cartilage mechanics. Instead, it acts as a spacer between two bones, allowing motion without painful bony contact. Over time, a fibrous pseudocapsule forms around the implant that contributes to joint stability. Long-term studies have confirmed that virtually all silicone implants fracture at some point, yet most continue to provide satisfactory pain relief and preserved motion even after fracture due to this surrounding encapsulation.

"Rather than a true replacement of the joint, the silicone implant acts as a spacer between the two bones adjacent to the joint. This allows for motion without bony contact that would produce pain. Long-term studies have shown that all implants fracture over time, but usually continue to preserve motion and pain relief."

Schwartz's Principles of Surgery, 11th Ed.

Types and Sites of Use

1. Metacarpophalangeal (MCP) Joint Arthroplasty (Most Common)

The Swanson Silastic MCP implant is the most widely used and best-studied Silastic prosthesis.

Indications:

Rheumatoid arthritis with severe ulnar drift deformity
Marked flexion contractures
MCP joint pain with radiographic destruction
Failed conservative treatment

"Silicone implants are the most common prosthetic for MCP arthroplasty and are generally implanted in patients with severe decreased arc of motion, marked flexion contractures, MCP joint pain with radiographic abnormalities, and severe ulnar drift."

Harrison's Principles of Internal Medicine, 22nd Ed.

Technique (Swanson):

Dorsal approach to the MCP joint
Resection of the metacarpal head
Medullary canal preparation in both metacarpal and proximal phalanx
Implant sizing by trial (largest that fits without tension)
Collateral ligament reconstruction and centralisation of extensor tendons
Postoperative dynamic splinting and hand therapy

Outcomes:

Good pain relief in the majority
Arc of motion typically 0-45 degrees postoperatively
Normal motion is not expected

2. Proximal Interphalangeal (PIP) Joint Arthroplasty

Key features:

Silicone arthroplasty provides pain relief but without significant improvement in motion
Produces better results in traumatic arthritis than in rheumatoid arthritis
High fracture rates reported with PIP silicone implants, yet clinical outcomes may remain reasonable
Alternative technique: volar plate interposition arthroplasty (technically more demanding but durable)

Surgical approach: Dorsal, with longitudinal incision through the central tendon; proximal phalangeal head resection; canal preparation in both phalanges.

"Patients should be clearly informed that arthroplasty procedures are done for reduction or elimination of pain and not for improvement in motion or strength. Moreover, joint motion may be decreased after the procedure."

Campbell's Operative Orthopaedics, 15th Ed. 2026

3. Thumb Metacarpophalangeal Joint

Used when joint stability cannot be restored adequately by soft-tissue alone
Silicone implant arthroplasty can provide satisfactory function "despite the possibility of implant breakage, dislocation, and particulate synovitis"
If restoration of joint stability is doubtful, arthrodesis is more predictable

4. Trapeziometacarpal (CMC1 / Basal Thumb) Joint

Indications: Rheumatoid or osteoarthritis not responding to conservative treatment, subluxated joint with synovitis and osteophytes, positive grind test.

Key points:

Ligament and capsule reconstruction is mandatory postoperatively
Cast immobilisation for 6 weeks to prevent subluxation
Complications: subluxation 5-20%, dislocation 0-19%, silicone synovitis 50%
Various modifications of the flexible silicone implant have been tried; outcomes do not differ significantly enough to recommend any particular device
Most surgeons now prefer non-implant (ligament reconstruction with tendon interposition - LRTI) techniques

"Complications include implant subluxation (5% to 20%) or dislocation (0% to 19%) and silicone synovitis (50%). Relief of pain has been excellent in most instances."

Campbell's Operative Orthopaedics, 15th Ed. 2026

5. Wrist Arthroplasty

Swanson also developed a silicone rubber intramedullary stemmed flexible hinge implant for the wrist (radiocarpal joint)
Used in rheumatoid arthritis with pan-carpal destruction
High rates of loosening, subsidence, and silicone synovitis led to its largely being abandoned in favour of total wrist arthroplasty (TWA) or wrist arthrodesis
Titanium grommets were added to protect the stems from metal-edge wear

6. Elbow / Radial Head

Silastic radial head prostheses (Swanson) were widely used in the 1970s-1980s for radial head fractures and rheumatoid arthritis
Complications of silicone synovitis, capitellar wear, and implant fragmentation led to replacement by metallic radial head prostheses, which are now standard
The Silastic radial head is now largely of historical interest

7. Hallux (First MTP Joint) - Foot

Earlier silicone-based implants for first metatarsophalangeal (MTP) arthroplasty (hallux rigidus, hallux valgus) showed:

Unacceptably high rates of implant failure and loosening
Silicone synovitis from polymeric silicone particle wear causing bone erosion

"Silicone arthroplasty can result in a heavy synovitis with destruction of the joint."

Miller's Review of Orthopaedics, 9th Ed.

Current recommendation: Metallic or silicone implant arthroplasty is not recommended for the first MTP joint. Arthrodesis (MTP fusion) is the gold standard, with newer synthetic cartilage (PVA hydrogel) implants under investigation as motion-preserving alternatives in selected patients.

Complications

Complication	Details
Implant fracture	Near-universal with long-term follow-up; fibrous encapsulation often preserves function
Silicone synovitis	Foreign-body reaction to silicone microparticles; causes progressive bone erosion, joint destruction, and pain. Incidence up to 50% at CMC1. Requires implant removal + synovectomy
Silicone lymphadenopathy	Silicone particle migration to regional lymph nodes; described with finger joint prostheses
Subluxation / dislocation	5-20% subluxation, 0-19% dislocation (CMC1 data)
Infection	Low but serious; requires implant removal
Stiffness / limited arc of motion	Expected outcome - not a true failure
Bone erosion at implant stems	Especially wrist and radial head applications

When to Use vs. Avoid

Prefer Silastic Arthroplasty	Prefer Arthrodesis
Rheumatoid arthritis MCP joints (multi-digit)	Single MTP joint disease
Low-demand patients needing motion preservation	Younger, high-demand patients
PIP traumatic arthritis	Instability-dominated deformity
Bilateral hand involvement	Failed prior implant arthroplasty

Historical Context and Current Status

Silastic prostheses were pioneered by Alfred Swanson (Grand Rapids, Michigan) from the 1960s onwards. The Swanson flexible hinge implant remains the benchmark for MCP arthroplasty in rheumatoid arthritis. At other sites, silicone implants have largely been replaced:

Radial head - replaced by modular metallic prostheses
Wrist - replaced by total wrist arthroplasty or arthrodesis
First MTP - replaced by arthrodesis; PVA hydrogel spacers under investigation
CMC1 - non-implant LRTI techniques preferred by most centres

For the MCP and PIP joints of the fingers, Swanson Silastic arthroplasty remains an established and widely used option, particularly in rheumatoid arthritis, with over 40 years of published long-term data supporting its safety and efficacy.

Sources:

Campbell's Operative Orthopaedics, 15th Ed. 2026 - MCP Arthroplasty, Thumb MCP/CMC sections, Hallux Rigidus
Schwartz's Principles of Surgery, 11th Ed. - Small Joints section
Harrison's Principles of Internal Medicine, 22nd Ed. - Surgery in RA
Miller's Review of Orthopaedics, 9th Ed. - First MTP arthroplasty
Firestein & Kelley's Textbook of Rheumatology - First MTP arthroplasty in RA

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