Biomaterials in Plastic Surgery- with Mnemonics

💡 Study Tip ➜ This is a long nagging topic, but ‘Biomaterials in Plastic Surgery’ is a major question and all the alloplastic implants bolden in the classification have been asked in various exams and are described in detail under the ‘Selected Biomaterials’ section.

Definition

“Biomaterials”/ alloplastic implants are synthetic and naturally occurring materials used to replace, reconstruct, or augment the tissue, organ or function of the human body.

History

Implants have been used for ages in the form of gold, silver and even seashells mostly as dental implants. Most of the modern ‘biomaterials’ developed in the last 7 decades. 

The first implantable biomaterial made was an artificial lens developed by the British ophthalmologist, Harold Ridley in 1949 after he observed that splinters of canopy plastic  (made of polymethyl-methacrylate) unintentionally implanted in the eyes of fighter pilots who had been shot at by the enemy, seemed to heal without any adverse effect. [💡 MNEMONIC ➜ Hard old Riddle]

In 1952, Per-Ingvar Bránemark, a Swedish orthopedic surgeon, noted ‘osseointegration‘ between titanium and bone and coined the term. [💡 MNEMONIC ➜ Brain Mark]

Choosing an Implant

Ideal Implant (10 points)

Properties of an ideal implant were described by Cumberland and Scales in 1952-53 for hernia repair and are still fundamentally applicable.

1. Biocompatible (Chemically inert: No foreign-body inflammatory reaction, No allergic hypersensitivity reaction, Nontoxic)
2. Noncarcinogenic- ⭐Oppenheimer effect: smooth-surfaced implants of any material induce tumor over time in experimental animals.
3. Mechanically reliable (Good tensile strength, Resistant to resorption, corrosion, and deformation)
4. Malleable (into desired shape and form)
5. Good tissue incorporation (Allows collagen ingrowth)
6. Minimal Wound complications
7. Tolerates Infected environment
8. Freely available and affordable
9. No interference with Radiographic Imaging (Radiolucent)
10. Easy to Sterilize.

💡 MNEMONIC ➜ Car Bio M²echanics incorporate WIFI-S.

Advantages of Implants (6 points)

Advantages of implants over autologous tissue are:

1. No Donor site morbidity
2. Reduced operative time (No Donor site harvest)
3. No resorption over time (Doesn’t Vanish)
4. Less Disease transmission
5. Unlimited quantity
6. Prefabricated (Tailor-made to the patient)

💡 MNEMONIC ➜ No Donor2 for Diseases- Vanished! Just Unlimited Tailors!!

Classifications

Implants have been classified based on:

1. Physical State
   1. Fluid- injectable silicone, collagen, tissue adhesives
   2. Solid- metals, polymers, ceramics etc.
2. Physical Form
   1. Solid -or- porous/ meshed (meshed- tissue incorporation- eg. Polypropylene mesh)
   2. Smooth -or- rough (smooth- encapsulation- eg. Hunter’s silicone rod)
3. Origin
   1. Natural
   2. Synthetic
   3. Semisynthetic / Hybrid
4. Resorbability
   1. Resorbable (all natural, some synthetic implants- mentioned below)
   2. Nonresorbable
5. Composition
   1. Metals
      • Stainless steel (Fe-Cr-Ni)- Craniofacial implants, Dental wires, arch bars.
      • Titanium– Craniofacial implants (also see, Osseointegrated implants)
      • Vitallium (Co-Cr-Mo)- Craniofacial implants (but replaced by Titanium now)
      • Gold, Platinum- Eyelid weights (Facial palsy)
   2. Polymers
      • Nondegradable (Nonresorbable)- Poly…
        • …dimethylsiloxane (PDMS- Silicone)
        • …tetrafluoroethylene (PTFE– Teflon, Gore-Tex)
        • …methyl methacrylate (PMMA– Bone Cement)
        • …ester (Mersilene mesh, Dacron vascular graft)
        • …ethylene (Medpor)
        • …propylene/prolene (Marlex mesh, Prolene suture/ mesh)
        • …amide (Biofibre hair implant)
        • …urethrane (transparent- Tegaderm, Opsite)
      • Biodegradable (Resorbable plates, mesh, scaffold) – Poly…
        • …glycolate (PGA- Dexon, Vicryl)
        • …lactate (PLA)-caprolactone (PCL- delayed resorbable)
        • …lactide-co-glycolide (PLGA- delayed resorbable)
   3. Ceramics
      1. Bioinert- Alumina, Zirconia, Carbon
      2. Bioactive- Bioglass, Glass
      3. Bioresorbable- HydroxyapatiteTricalcium phosphate
   4. Tissue adhesive / Super Glue
      • Fibrin glue (earliest)
      • Cyanoacrylate (Butyl-, Octyl-)
      • Platelet gels (from PRP)
   5. Biological Materials & Biomaterials
      • Skin Substitutes
      • Integra
      • Epicel (cultured epidermal autografts)
        • Dermagraft
        • Apligraf
   6. Bioprosthetic Material
      • Small intestinal submucosa
      • Acellular dermal matrix (ADM)- Human (AlloDerm), Porcine
      • Others: Bovine Bone, Pericardium, Fetal dermis
6. Future Biomaterials
   • Modulate their environment to produce a tissue-specific response.
   • ‘Sense’ their surroundings and change their biochemical/mechanical properties in response to the needs of the environment.

The Ultimate Biomaterial would have tissue-specific properties- individualized to the exact biological, chemical, and functional needs of the reconstruction.

Complications

1. Infection- Implant being a foreign body is prone to infection especially the porous alloplastic implants. Perioperative antibiotic use, avoidance of implant handling and air exposure before insertion, use of antibiotic-impregnated PMMA (bone cement) will help decrease infection rate. 
2. Toxicity- The response of host cells to implants or the substances leached from the implant can cause local cellular or even systemic toxicity.
3. Carcinogenicity– The phenomenon of tumors produced by implanted solid materials in experimental animals is termed “solid state carcinogenesis” or “Oppenheimer’s effect.” Still to be proved in humans.
4. Hypersensitivity Reactions- It’s uncommon as most alloplastic materials are too large to undergo intracellular processing by macrophages and subsequent presentation to the immune system. However, degradation or breakdown products resulting from corrosion or leaching from an implanted material may cause immunogenicity.
5. Implant Exposure and Extrusion- Infection followed by skin necrosis or pressure necrosis and implant extrusion can be avoided by preventing infection as mentioned above and also tension on incision closure site.
   
6. Device Failure- It can be minimized by appropriate use of the implant within the parameters and indications for use outlined by the manufacturer.
   
7. Visibility and Palpability- It can be minimized by good surgical technique. Surface contour and texturing, material hardness and other design parameters can be useful in camouflaging an implant.

Selected Biomaterials used in Plastic Surgery

1. Titanium (Ti)

Pure Ti is being used since 1940. Since the introduction of its alloy (Ti – 6% Al – 4%Va) in the 1980s, titanium has replaced other alloys used for medical implants though, in plastic surgery when malleability is required like for Orbital mesh- pure titanium is used. 

Properties of pure Ti and Advantages: 

1. Stronger (10 x bone)
2. Lighter
3. Have higher resistance to corrosion, cause less inflammation
4. Has less stiffness. Hence,
   • malleable (more with pure titanium)
   • has less “stress shielding” (localized osteopenia secondary to the implant protecting the bone from normal loading)
5. Has less than 0.5% iron, hence,
   • they do not set off metal detectors, and
   • they do not create a significant artifact on CT or MRI
6. Uniquely, form chemical bonds with the surrounding mineralized bone (unlike the typical fibrous tissue forming between the implant and bone)- used to create osseointegrated implants. 

Uses: As plates and screws for rigid craniofacial reconstruction and mesh for orbital floor and wall reconstruction.

2. Silicone- Polydimethylsiloxane (PDMS)

Described in a separate post here: Silicone

3. Polytetrafluoroethylene (PTFE)- Teflon, Gore-Tex

Teflon was accidentally invented by Roy Plunkett in 1938 while he was trying to develop a refrigerant. It consists of a carbon backbone with fluorine side chains. It has been used extensively form coating the frying pans to hiking boots.

Expanded microporous PTFE (ePTFE or Gore-Tex) was created by Bob Gore in 1969 when he rapidly stretched PTFE and has many plastic surgical uses.

Advantages of Gore-Tex:

1. Very stable chemically
2. Cannot be crosslinked (flexible)
3. nonadherent surface
4. When made with a pore size of 10–30 µm (microporous)- it allows some limited tissue ingrowth
5. Infection rate- very low.

Use of Gore-Tex: 

1. Vascular Grafts
2. Mesh for chest and abdominal wall reconstruction (nonadhesive properties- reduce hernia repair site adhesion – Dual-surfaced mesh)
3. Implants for facial augmentation (lip, chin, nasal, malar, forehead)
4. Static sling in facial palsy.

4. Polymethyl methacrylate (PMMA)- Bone Cement

PMMA is a high-molecular-weight polymer (acrylic resin) that may be either heat or cold cured. 

1. Heat-cured MMA are pre-formed/pre-fabricated rigid implants (e.g joint implants)
2. Cold-cured implants are molded in the operating room by adding liquid MMA the monomer to powdered Methyl MMA, which then forms a paste. The polymerization process is an exothermic reaction that takes approximately 10 minutes outside the body. Saline irrigation is needed to cool the surrounding tissues during the curing process to avoid local tissue damage.

Advantages:

1. Inexpensive
2. Surgical manipulation easy
3. Density similar to bone
4. Radiolucency
5. Good long-term biocompatibility.

Problems:

1. Heat produced can damage surrounding tissues
2. Increased infection risk- Reduced by keeping the edge of the implant >1cm away from the incision.
3. Extrusion
4. A mechanical failure at the Bone-polymer interface.

Uses:

PMMA is commonly used in Orthopedics to secure a joint replacement implant to the bone and Dentistry for dental plates. In Plastic Surgery, it’s used for:

1. Cranial bone reconstruction
2. Forehead augmentation
3. Chest wall reconstruction with Polypropylene mesh and prefabricated PMMA ribs- used to reconstruct the large chest wall defects after resection of chest wall tumors
4. A suitable carrier for sustained antibiotic release in the site of infection- eg. Gentamicin impregnated beads with flaps for defects with osteomyelitis.
5. Temporary spacers for lower limb large bony defects.
6. HTR-PMMA – A new composite- porous and negatively charged- Stimulates bony ingrowth.

5. Polyethylene (Medpor)

Polyethylene consists of a carbon backbone with hydrogen side chains (ethylene).

Types:

• low, high, and ultrahigh
• solid and porous

The most commonly used medically- high-density polyethylene (HOPE) or Medpor.

Advantages:

1. Allows bony and soft tissue ingrowth (porous)
2. Its sheets and blocks can prefabricated or carved intraoperatively for individual patients.

Problems:

1. Difficult to place in soft tissue as it’s stiffer than ePTFE and the large pores adhere to the tissue
2. If removal is required, it’s difficult due to tissue ingrowth
3. Can’t be identified clearly even in CT- hence, implant displacement difficult to diagnose.

Uses:

1. Cranioplasty
2. Facial implants– chin, nasal, malar, mandibular angle augmentation
3. Orbital floor reconstruction (alone or with Titanium mesh embedded)

Ultra-high-density molecular weight polyethylene (UHMWPE) is used in Orthopaedic procedures.

6. Polypropylene (Marlex, Prolene)

Polyprolene, or polypropylene, has a carbon backbone and side chains of hydrogen and methyl groups.

It’s available as monofilament knitted meshes (Marlex) and suture materials (Prolene).

Advantages:

1. Very high tensile strength
2. Low foreign body reaction
3. Knitted mesh- porous- strong fibrovascular incorporation.

Problems:

1. Mesh can erode through adjacent soft tissues over time. Hence, it shouldn’t be used in pelvic organ prolapse, esp. when close to the vaginal wall.
2. Direct placement over abdominal viscera can cause dense adhesions, fistula, and erosion.
3. Risk of infection on the surface of the mesh present.
4. Reoperation through the mesh or removal is difficult due to the generalized fibrotic adhesion to adjacent intraperitoneal viscera due to scarring.

Uses:

1. Mesh– Abdominal wall and chest reconstruction, pelvic organ prolapse (with caution- see below) repair.
2. Suture– Thicker- for suturing under tension (eg. Abdominal wall); thinner- facial suturing and vessel anastomosis.
3. Patellar tendon repairs after total knee arthroplasty (Mesh)

7. Biodegradable polymers

Most biodegradation begins through a chemical reaction such as hydrolysis or oxidation and involves some sort of biological process (e.g., enzymatic or cellular process) to eliminate the material completely.

The material itself, as well as all the breakdown products of the material, must be biocompatible. Most of these are α-hydroxy acids, specifically poly(lactic acid) (PLA), poly(glycolic acid) (PGA), poly(caprolactone) (PCL) and combinations, or copolymers of PLA-PGA, poly(lactic-co-glycolic acid) (PLGA). These polymers degrade through hydrolysis, ending in lactic, glycolic acid, or caproic acid.

Altering the ratios of lactic to glycolic acid or adding carbon fibers or other polymers can modify the rate of degradation. ↑ lactic acid →  ↓ rate of degradation.  💡 

Hence, Rate of degradation:  PGA (weeks to months)> PLA > PLGA (9-15 months)> PCL (2-3 years).

PGA and PLA are crystalline and have high tensile strength while PCL has low tensile strength and high elongation at breakage

 💡 Vicryl is 92% PGA and 8% PLA.

Advantages:

1. No need for removal later
2. In Pediatric craniofacial surgery- with growth- No risk of intracranial migration of implants or growth restriction.
3. Doesn’t interfere with postoperative radiographic imaging or oncologic follow-up

Uses:

1. Vicryl resorbable sutures
2. Biodegradable mesh for use in abdominal wall reconstruction- It helps contain the viscera initially and subsequently resorbs, creating an iatrogenic hernia that is repaired at a later stage. 
3. Plates and screws for craniofacial or hand applications-  structural integrity remains intact for 8 weeks to allow for bony healing. PGA is good for lower limb orthopedic fixing.
4. As a scaffold in ’tissue engineering’– for skin substitutes, cartilage etc. 
5. Polycaprolactone (PCL) is used to plug cranial Burr holes.
6. 3D reconstruction of the skull model using PLA used for craniofacial mock surgery and intraoperatively.
7. Midface tissue augmentation due to wasting caused by Protease inhibitors. (Sculptura)

8. Fibrin tissue adhesives

Fibrin sealants were the first tissue adhesives used medically. They consist of: fibrinogen and thrombin with a small amount of factor XIII and calcium that catalyze the reaction and form polymerized fibrin.

Preparation:

Screened donors provide pooled human plasma as the source of the two ingredients. The commercially available product also contains an antifibrinolytic to decrease degradation and bovine-derived aprotinin, which acts as a stabilizer. To prevent disease transmission, the products undergo heat pasteurization and ultrafiltration. It must be refrigerated and requires approximately 20 minutes preparing. The two bottles need to be placed in a special warmer. Once heated, the dual syringe delivery system is used to mix the two ingredients immediately before they are applied.

Delivery mechanisms:

1. A straight blunt-tipped needle- Simplest.
2. Sprayer/mister applicators- Produce the thinnest layer of polymerized fibrin to the wound- strongest adhesive properties.
3. Applicators for endoscopic use.

• Strength of the glue- Concentration of fibrinogen
• Rate of polymerization– Concentration of thrombin. (Thus, in applications where the tissues need to be manipulated (e.g., a large flap), a lower concentration of thrombin should be used.)

9. Cyanoacrylate glue

Earlier- butyl, currently- octyl-cyanoacrylates (CA) polymerize to glue the superficial layer of the skin. Octyl-cyanoacrylates polymerizes when it comes in contact with the moisture present in the air.

Problems with Butyl-cyanoacrylates:

1. Have short chains- rapid breakdown– wound dehiscence.
2. Breakdown products (formaldehyde, cyanoacetate)- a severe inflammatory reaction if the glue penetrates the skin.

Advantages of Octyl-cyanoacrylates:

1. Have longer side chains– a stronger and longer-lasting polymer.

Uses:

1. Skin closure- of the superficial layer of the skin, where it’s applied. (So, it’s important that the deeper layers are well approximated with tension-free sutures.)

 💡 The outcomes of traditional suturing and octyl-2-cyanoacrylate were found to be equivalent.

10. Skin substitutes

Various biological materials and biomaterials [Note: they are not the same- visit the link to know the difference] have been used to cover large raw areas and defects. For classifications and details, visit ➜ Skin substitutes.

Bibliography

1. Neligan’s Plastic Surgery- Vol 1- Principles- 4Ed (2017)
2. Textbook of Plastic, Recon, Aesthetic Surgery- Vol 1- Principles and Advances in Plastic Surgery- Agrawal, Bhattacharya- 1Ed (2017)
3. Plastic Surgery Secrets Plus- Weinzweig- 2Ed (2010)