New biomedical materials are expanding orthopaedic design options. PEEK textiles, for example, which already are used in other therapeutic areas, now are proving their potential in the orthopaedic field.
Novel uses of new and existing materials can be applied in musculoskeletal procedures ranging from bone grafting, fusion and motion-preserving spinal repair to mending of cartilage, joints, ligaments, tendons and other soft tissues. Materials such as biomedical textiles offer more choice and flexibility in design than many traditional materials and allow manufacturers to expand their portfolios with biologically based minimally invasive products. Even treatments and devices that typically have been engineered with metals and hard plastics now are shifting towards soft implants and fixation methods.
Because of biomedical fabric versatility, such as compressibility and ability to transform shape at the delivery site, textiles now are being considered in procedures such as the internal stabilisation of a long bone fracture, annular repair and the dynamic stabilisation of the spine.¹
Developments in tissue engineering such as hybrid biologic/synthetic osteoconductive composite scaffolds, bone morphogenic proteins (BMP), demineralised bone matrices (DBM), stem cells and gene therapy are among the leading-edge treatment options for musculoskeletal repairs.2
|PEEK textiles can be used in general surgery applications when engineered with warp knit technologies.|
Orthopaedic market drivers
A shift in population demographics, including a growing elderly population and higher obesity rates, affects joint, back and extremities device development. Active patients increasingly consult with surgeons about treatment options and minimally invasive surgical techniques that return them to health in a shorter time than traditional larger-scale surgical procedures. Spinal devices, in particular, are being re-engineered with flexible and compliant fabric structures to minimise the loss of natural movement.
In response to these needs, the global orthobiologics market is estimated to more than double from US$4.3 billion in 2009 to US$9.6 billion by 2016.3 The competitive orthopaedic soft tissue repair market also is expected to grow from US$920 million to US$1.6 billion in the same time period.4
The emergence of biologically and synthetically derived biomimetic or orthobiologic materials has resulted in their increasing adoption by orthopaedic device designers as they develop products to correct a wide range of deformities, with decreased recurrence rates. Furthermore, the use of soft fixation anchors with similar profiles could provide a significant technological improvement. Such soft implants give rise to a host of benefits, including flexibility and shape-transformable designs, a smaller implant size and microporous construction that potentially can benefit the biologic healing process by acting as scaffolds or conduits.
All-textile or all-suture designs used to create soft anchors permit easier arthroscopic implant methods, and minimise biomechanical concerns associated with rigid bone anchors. Because such soft anchors can be positioned in closer proximity to each other, they provide increased surface area contact between the repaired tissue and bone; in other words, a more biomechanically beneficial fixation. This design approach can be applicable for low load-sharing applications such as labral tears, meniscus and hip capsules.
A bifurcate braid PEEK textile structure can be used in orthopaedic applications because of the fabric’s unique architecture.
The diversity of implantable textiles presents an opportunity for advanced design of devices for fixation and anchoring of soft tissues, enabling device manufacturers to continue to transform the future of sports medicine and arthroscopic treatments.
Achieving outcomes that tend towards more controlled tissue regeneration and favourable biomechanical properties is also a current area of focus in the field. Soft implants designed with polyetheretherketone (PEEK)-based fibres can assist in promoting these properties by incorporating a variety of forming methods, including three-dimensional weaving, to create highly ordered and specialised scaffolds that exhibit directionally oriented ingrowth properties.
Biomechanical properties of soft tissue and bone within the joints represent another area of active study—optimising repair of tears that may occur during joint reconstruction. Scaffolds with areas targeted to mimic the biomechanical properties of native tissues such as bone, tendon and muscle have the ability to bridge tissues of differing properties.
Textiles constructed from absorbable and bioactive polymers are ideal in tissue engineering and orthobiologic applications because they offer short-term tissue support while the body repairs itself and promote long-term biologic integration.
The functional flexibility, strength and elasticity of implantable textile structures can help preserve a patient’s natural range of motion, a crucial aspect of orthopaedic patient recovery. Procedures such as internal stabilisation of a long bone fracture, annular repair and spine stabilisation are areas of continued focus for PEEK textiles. The higher modulus of PEEK allows the material to better handle the extreme loading conditions associated with the body’s natural movement than the more traditional polyethylene terephthalate (PET). Another property of PEEK that lends itself well to spinal tethers and spinal stabilisation is its high fatigue life; it can last a long time within the body without wearing down. This combination of properties makes PEEK a resourceful material for use in sports medicine devices that require highly stressed loading conditions and resistance to mechanical degradation in applications such as rotator cuff technologies and various joint, tendon and ligament repair therapies.
New uses for PEEK
The recent introduction of motion-preserving technologies such as artificial disc replacement, interspinous spacers and dynamic stabilisation technologies has provided surgeons with alternatives to fusion for some patients.
|PEEK fibres can be woven, knitted or braided into textile structures and used for a wide range of medical applications.|
For instance, some bone graft delivery devices consist of woven open-pore bags that are filled with bone chips and anchored to the spine to aid in the spinal fusion procedure for stabilisation. Spacer applications can vary from a textile spacer designed to replace the nucleus to spacers that relieve pressure between vertebrae. Tethers can be used to secure the spine while maintaining flexibility during scoliosis realignment. A newer opportunity is the use of textile rigging to secure the spine by wrapping around the lamina instead of aggressive drilling of the bone.5
PEEK traditionally has been used in the rigid components of a medical device. As one of the most biostable plastics on the market with broad performance characteristics, PEEK is already a widely used alternative to titanium-based implants, especially within orthopaedics and spinal fusion. The material exhibits several favourable properties, such as strength, stiffness, fatigue resistance, biocompatibility, thermal stability, radiolucency and similarity to bone with respect to mechanical properties.
PEEK resin in fibre form, however, is a new platform for device designers to consider. Advanced polymers in fibre form offer therapeutic innovation options well suited to address industry pressures in the areas of patient demographics, surgeon and patient expectations, healthcare policy developments and cost control.
From a materials standpoint, biocompatible polymers offer some key advantages over metal- and ceramic-based implants:
Design freedom with PEEK fibre
Fibre-based approaches yield a number of advantages for next-generation PEEK-based components. Because PEEK polymer is conducive to textile-forming processes, PEEK fibre components have the potential to yield the maximum amount of design freedom for custom orthopaedic devices. Advantages include:
The process for engineering biomedical textiles using PEEK involves extruding the polymer into a fibre form, then creating patterns using traditional textile-forming techniques and incorporating these into devices or device components.
Three traditional textile-forming technologies—knitting, weaving and braiding—are in use with PEEK today. Each has flexible mechanical and physical properties that lend themselves to specific structural geometries. As such, textile engineers can leverage fabric design flexibility and properties for the desired therapeutic outcome.
Fabrics can also contain varying orientations in the geometry to affect porosity in eliciting tissue ingrowth in certain areas, while serving as a tissue barrier in others. Polymers, metals and emerging biologic material filaments can be formed into an ordered composite fabric structure to attain the desired device form and function, performance characteristics and biological response.
Knitted textiles are created by interlocking loops of yarn in either a weft (transverse stitching) or warp (longitudinal stitching) pattern. This technology creates flat, broad or tubular structures that are highly conformable and can be compressed and inserted into small cavities via a cannula or trocar. Knitting is particularly appropriate for yielding rapid ingrowth properties.
Another useful forming technology, weaving, involves two independent systems of fibre interlaced at right angles in an over/under perpendicular pattern. Weaving allows control over textile pore size and density while creating unique structures such as tapered, tubular, flat, buttonhole and near-net shapes. Woven fabrics are dimensionally stable, strong and highly versatile for device designers.
Braided textiles involve interweaving three or more yarns in a diagonally overlapping pattern to produce a variety of shapes and near net-shape structures. Braids are generally flexible, porous and kink-resistant when guided through and around anatomical pathways. They can be designed to yield varying densities and have the ability to compact into catheters and expand dimensionally as required.
In addition to these popular technologies, three dimensional (3-D) weaving is an increasingly used technique in biomedical textile engineering. It combines materials in various thicknesses and densities to create durability, flexibility and long-term wearability. 3-D weaving can be used in creating orthobiologic scaffolds to promote tissue growth and specifically direct that growth so that, for instance, one of the materials absorbs over time, disconnecting from the nonabsorbable material. An additional created structure is nonwoven, formed from fibres and filaments; this structure is highly absorbent and often is used in wound care.
Benefits of a complete and transparent supply chain
While the confluence of pressures—including cost containment, quality integrity and speed to market—continue to build, the importance of a reliable supply chain has become apparent. With device designer needs at the forefront, Solvay Specialty Polymers and Secant Medical have joined forces to address that need. The companies have formed a supply chain for Solvay’s Zeniva PEEK that allows medical device companies to develop custom implantable fabrics made from Zeniva.
Biomedical textiles present an exciting design option with a multitude of variables that can provide flexibility and choice to enable device designers to advance orthobiologic innovation. Zeniva’s history of reliability and biocompatibility in rigid format components bodes well for designers considering its use in fibre components. The established reputation of PEEK as the material base, combined with textile-forming technologies, expands the applications, features and benefits available. The convergence of these technologies, along with the growing cost-competitiveness of the PEEK market, provides a new path to the polymer’s adoption within the orthopaedic segment and a new direction in device design.
1. S. O’Reilly, “The Hot Trend in Spine: Motion Preservation Mania,” Medtech Insight, 9, 6 (2007).
2. Windhover Information, Medtech Insights, Tissue Engineering and Cell Transplantation: Technologies, Opportunities, and Evolving Markets in the U.S., (2007).
3. Companies and Markets, Orthobiologics Market to 2016 – Alternative to Surgery and Superior Outcomes are Driving Wider Adoption of Orthobiologics (2011), available from Internet: www.companiesandmarkets.com/Market-Report/orthobiologics-market-to-2016-....
4. Bharatbook, U.S. Market for Orthopaedic Soft Tissue Medicine 2010, (2010), available from Internet: www.prlog.org/10523151-us- market-for-orthopaedic-soft-tissue-and-sports-medicine-2010-by-bharatbook.html.
5. Zimmer.com, Universal Clamp Fixation System, in Zimmer.com Home Page [online], available from Internet: www.zimmer.com/web/enUS/pdf/Universal_Clamp_Spinal_Fixation_System.pdf