Biopolymers in orthopedics

Polymers for use as biomaterials in orthopedics, in addition to the demand for biocompatibility and non-toxicity, must have appropriate degrees of thermoplasticity, strength, crystallinity, degradation characteristics and hydrophilicity . Following are the main polymers used as biomaterials in orthopaedic and other applcations.

Poly-L-Lactic Acid (PLLA).  Polymer-based absorbable implants were first used in the early 1960s when American Cyanamid developed Dexon, a polyglycol material that was used as a resorbable suturing material. It was commercialized by Davis and Geck in 1970. When blended with polylactic acid (PLA), polyglycol forms a copolymer structure that breaks down gradually in the presence of bodily fluids through hydrolysis. The main resorbable medical grade polymer in current use is Poly-L-Lactic Acid (PLLA). It is more hydrophobic than PLA or PGA and maintains its structure in the body for longer; it is used in the manufacture of interference screws, soft tissue anchors, urological stents, tacks and staples.

Polymethylmethacrylate (PMMA).  This is the most commonly used orthopedic cement, used primarily to anchor hip stems in hip arthroplasty operations. It is also commonly used in the treatment vertebral compression fractures.

Polytetrafluoroethylene (PTFE).  PTFE was discovered in 1938 by chemists at DuPont, but was not marketed until after World War II. It is a fluorinated carbon with a high molecular, partly crystalline structure, resistant to virtually all chemicals. It offers an extremely wide working temperature range, from -200 to +300 °C. Its surface is adhesion-resistant due to shielding of the carbon chain by fluorine atoms.

A major use of PTFE is to make the prosthesis for the Anterior Cruciate Ligament (ACL) repair procedure. The ACL has considerable strength and modulus due to an aligned type I collagen network that bears great loads while undergoing little deformation. However, while the ACL’s mechanical properties increase during development, they begin to deteriorate with age and may therefore need to be augmented by prosthesis.

PTFE is also used in graft augmentation devices to protect biological grafts. Its intended use is to be a temporary load-bearing device and it does not require long-term performance capability. Apart from its use in graft augmentation, PTFE is also used in microporous hydrophobic membranes (MHM) that are used in products such as vented blood warmers, in-line suction filters and vented suction containers.

Polyurethane The Polymer Technology Group produces polyurethane bionate, used in applications that have a potential mode of degradation such as pacemaker leads; also as base polymers for surface modification, known as surface modifying end groups (SMEs). SMEs can permanently modify surface properties, such as blood compatibility, abrasion resistance, coefficient of friction, and resistance to degradation in implants.

Polyvinyl chloride (PVC). Vinyl has proved to be one of the most successful modern synthetic materials; it is a polymer formed by chlorine (about 57 percent by weight), carbon and hydrogen. It is long-lasting and safe in production, use and disposal. Typical uses for vinyl in the healthcare field include blood and IV bags, dialysis tubing, catheters, labware, pressure monitoring tubing, breathing tubes and inhalation masks. Vinyl is durable, sterilizable, non-breakable and cost-effective.

Polydimethylsiloxane (PDMS or silicone).  Silicones are synthetic polymers with a linear repeating silicon-oxygen backbone. However, organic groups attached directly to the silicon atoms by carbon-silicon bonds prevent formation of the 3D network found in silica.. Silicone is used in a variety of fields such as medicines, food processing, and a wide range of medical devices as well as putty and sealants. Silicone oil is commonly used as a lubricant in syringes and blood giving sets. Silicones are used during surgery to repair retinal detachment. Silicones are also used for breast prosthesis and in topical applications.

Polyester.  Polyethylene terephthalate (PET)—linear and aromatic polyester—was first manufactured by DuPont in the late 1940s. It is still known by the original trade name of Dacron. Current medical applications of PET include implantable sutures, surgical mesh, vascular grafts, sewing cuffs for heart valves, and components for percutaneous access devices.

PET sutures have been used in the medical field for half a century and are especially valuable for critical procedures, where strength and stable performance over a long duration is necessary. Woven PET is used in surgical meshes for abdominal wall repair and in applications requiring surgical “patching.”

Synthetic vascular prostheses made of woven as well as knitted PET are used in the repair of the thoracic aorta, ruptured abdominal aortic aneurysms, and to replace iliac, femoral, and popliteal vessels. PET is also used as a sewing cuff around the circumference of the heart valves to promote tissue ingrowth and to provide a surface to suture the valve to the surrounding tissue. Percutaneous tunneled catheters also use PET cuff to stabilize catheter location and minimize bacterial migration through the skin.

Polymer Biomaterials Used in Orthopaedics

 

Polymer Type

Orthopaedic Application

PLLA, PGA, PLA

Soft Tissue Anchors, Screws, Staples

PMMA

Bone Cement

Polyurethane

Facial Prostheses

PDMS

Bones and Joints

Nylons

Joints

 

Source: MedMarket Diligence, LLC; Report #M625, "Emerging Trends, Technologies and Opportunities in the Markets for Orthopedic Biomaterials, Worldwide."

 

Bioresorbable Polymers

There is an increasing demand for biodegradable or bioresorbable fixation implants for use in procedures such as shoulder reconstruction, small joint fixation, meniscal repair and cruciate ligament fixation . The total number of such procedures in the USA is estimated to be more than 250,000 each year. The biodegradable polymers used to meet this demand include polyglycolide (PGA), polyglycolide-co-lactide, polylactic acid (PLA), and poly-L-lactic acid (PLLA).
 

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