FEATURE
FEA and orthopaedics
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The bioresorbable materials most commonly used in today’s orthopaedics include polylactic acid (PLA); polyglycolic acid (PGA); the L–isotope form of PLA: PLLA; the copolymer of PLA and PGA: PLGA; and the DL–isotope form of PLA: PDLLA.1,2,3 The degradation process of these materials in the body involves two steps: hydrolysis followed by metabolization.4 The process of hydrolysis involves the breaking of the polymer chains within the implant material to produce by-products that include lactic acid and glycolic acid single molecules. These degradation products are metabolized in the liver and produce carbon dioxide as a by-product, which the body can eliminate.1
The rate of material degradation in the body is governed by many factors, including sterilization techniques, manufacturing process, cystallinity of the material, polymer bonds, and implant geometry.4
In the area of high-strength fracture fixation, PLLA is favoured by product specialists because of its slow rate of complete resorption into the body. The racemic mixture (mixture of isomers) consisting of PDLLA also shows characteristics that could be utilized in high-strength situations. However, PLLA’s semicrystalline structure provides much higher strength in the material than PDLLA’s amorphous structure, thus PLLA is the preferred material for use in fracture fixation devices (FFDs).2
Material characteristics of PLLA
To date, PLLA has not had sufficiently high strength characteristics for use in the fixation of larger fractures such as the humerus and femur. Much of the referenced PLLA research has focused on veterinary applications using rabbits.6,7 For injuries such as ligament damage and skeletal fractures, PLLA interference screws and plates have been used successfully to fixate and heal tissue and bone.
Anterior cruciate ligament reconstructive surgery currently uses titanium, steel, or degradable interference screws to secure the graft within the femur and tibia. The metallic screws will remain in the patient’s knee for the rest of his/her life. This can potentially cause problems including stress shielding (the weakening of healing bone resulting from excessive rigid fixation for prolonged periods of time) and micro-motion (the weakening and fragmenting of surrounding bone as a result of small movements of the implant within the bone) of the screw causing the graft to fail. Metallic screws are also nonconducive to tissue regrowth that facilitates complete healing.8 This has given rise to the increased use of the bioresorbable interference screws in this application area. Bioresorbable screws will promote and foster the growth of the surrounding bone tissue, as well as limit stress shielding and micro-motion.
In many applications, PLLA can perform a function similar to that of a traditional metallic-based device. However, the unique properties of PLLA implants may provide the clinician with a level of surgical versatility that is not found with titanium or stainless-steel devices.9
Advantages of PLLA include:
- reduced risk of stress shielding
- postoperative diagnostic imaging free from the obstruction or artefact caused by metallic implant materials
- elimination of the need for secondary surgery to remove metallic implants
- better growth of tissue around the implant that will gradually dissipate load to the healing tissue or bone.
Engineering a novel bioresorbable
Studies have been performed using extruded fibres of PLLA with highly orientated polymer chains alongside mechanical properties of a typical bulk polymer of PLLA. Using the Q800 DMA analyzer (TA Instruments; New Castle, DE, USA) to obtain the properties of the fibres and running degradation studies, a full mechanical analysis of the fibres was conducted. With this information, techniques such as finite-element analysis (FEA) and computer-aided design were used to model and analyze specific geometries and orientations of the fibres within the model.
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The PLLA material used in the studies described here was a bulk polymer, which therefore has no high-strength characteristics (Figure 1). This can lead to plastic deformation of implants; for example, deformation of screw heads after the application of low forces. Thus, insertion forces must be dissipated over a large area to prevent the screws from failing.
To strengthen the final device, fibres within the PLLA material are being considered. The PLLA fibres are long and continuous through the length of the implant. This gives the material far greater strength characteristics and a similar degradation rate compared with the bulk polymer.10
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Figure 2 [7]: PLLA fibre under scanning electron microscope (SEM). Magnification: 365 times.
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Figure 3 [8]: PLLA fibre, internal structure under SEM. Magnification: 646 times.
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) [9] |
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Figure 4 [9]: Degrading PLLA fibre after 20 weeks. Magnification: 360 times.
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Figure 5 [10]: Accelerated degradation of PLLA fibre in buffered saline solution at 37°C for 12 weeks. Magnification: 355 times.
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A range of fibre and matrix geometries can be considered to produce the desired material characteristics for the specific device application. For example, a helix formation of fibres surrounded by a PLLA matrix has been investigated for a high-strength bone screw. The helix can be put under compression on insertion, thus dissipating the forces from the insertion torques to the fibres. To produce the highly orientated structure that is needed within the fibre, a different manufacturing process is required from that used for the matrix, which inherently means the fibres degrade slower than the bulk material. The manufacturing process involved in making the fibres, which consists of extrusion and rolling to orientate the polymer chains, produces high strength and high orientation within the fibres. This manufacturing process can be controlled and therefore used as a parameter with which to modify the final device’s degradation rate. An example of a fibre used in this type of application is shown in Figures 2 and 3. The degradation of these PLLA fibres can be accurately modelled by accelerated testing (Figure 6). PLLA fibres were exposed for 20 weeks at 20°–24°C and a relative humidity (RH) of 35%. An accelerated degraded sample was exposed to buffered saline solution (100% RH) at 37°C for 12 weeks. Tested fibres are shown in Figures 4 and 5. Accelerated testing can significantly speed up the design of the bioresorbable devices.
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Figure 6 [11]: Plot of accelerated degradation time/real-time degradation for PLLA fibres.
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The relationship between the accelerated degradation, real-time degradation, and FEA studies and the parameterized manufacturing process could give control over the degradation time. Geometry and implant position in the body also contribute to the degradation, and therefore these factors also need to be considered in the accelerated testing. All these factors are capable of being quantified with further research, thus bioresorbable implants are potentially capable of being engineered to exact requirements.
Next steps
Current research into this area is in its initial stages. Validation of accelerated testing and the manufacturing of the fibres will have to be completed before all data from the studies and manufacturing process can be used as an accurate prediction tool for real-time degradation in vivo. A higher level of understanding of bioresorbable implants must be achieved before the replacement of metallic implants such as screws and plates for femur fixation is at a low enough risk to patients and surgeons. The natural progression for FFDs is the use of engineered devices that are designed for each individual by controlling all contributing factors and the parameters mentioned above, and providing favourable tissue and bone recovery in relation to the metallic counterparts.
The next stage for bioresorbable implants is for them to be used in high-strength requirement situations such as femur fixation. This should be achievable with further research and improvements to the specific material manufacturing process.
The future is bioresorbable
With the development of new manufacturing techniques, bioresorbable implants that can be applied to high-strength situations can now be considered possible. With advantages over the current metallic designs, it seems likely that all fracture fixation implants could be constructed from bioresorbable materials. Further developments in the fields of bioresorbable materials and implant design could lead to novel methods of healing bone and tissue trauma.
There is scope for customizing parameters within FFDs, which could lead to individual implants being produced for the patient. Bioresorbable materials have a bright future in the world of implantables. They are the favoured material to work with by a growing number of surgeons because of the postoperative advantages and potential for customization. The advantages for the patient include faster and more stable tissue and/or bone regrowth leading to a more complete and preferred therapy.
References
1. F Harvey, “A Safe and Effective Material Poly–L–Lactide Acid in Surgery [12],” (August 2007).
2. D Farrar, “Business Briefing: Medical Device Manufacturing and Technology,” Smith & Nephew (2005) www.touchbriefings.com [13].
3. CC Chu, “Biodegradable Polymeric Biomaterials: An Overview,” LD Bronzino, ed., The Biomechanical Engineering Handbook, CRC Press Inc., Boca Raton, FL, USA (1995).
4. N Willcox and S Roberts, “Delayed Biodegradation of a Meniscal Screw,” Arthroscopy—The Journal of Arthroscopic and Related Surgery 20, 6, 20–22 (2004).
5. I Dos Santos et al., “A Route to High Radioactivity H-3 Labelled PLA Polymers,” Polymer International 48, 4, 283–287 (1999).
6. Y Shikinami et al., “The Complete Process of Bioresorption and Bone Replacement Using Devices Made of Forged Composites of Raw Hydroxyapatite Particles/poly L-lactide (F-u-HA/PLLA),” Biomaterials 26, 27, 5542–5551 (2005).
7. E Hochuli-Vieira et al., “Rigid Internal Fixation with Titanium versus Bioresorbable Miniplates in the Repair of Mandibular Fractures in Rabbits,” Int. J. Oral Max Surg., 34, 2, 167–173 (2005).
8. VK Ganesh et al., “Biomechanics of Bone-Fracture Fixation by Stiffness-Graded Plates in Comparison with Stainless-Steel Plates [14],” BioMedical Engineering Online, (2005).
9. S Ghosh et al., “The Double Porogen Approach as a New Technique for the Fabrication of Interconnected poly (L-lactic acid) and Starch-Based Biodegradable Scaffolds,” Journal of Materials Science—Materials in Medicine 18, 2, 185–193 (2007).
10. R Inaiet et al., “Structure and Properties of Electrospun PLLA Single Nanofibres,” Nanotechnology 16, 2, 208–213 (2005).
Mark Berry, MEng, is a health care engineer at Cambridge Consultants Ltd., Science Park, Milton Rd., Cambridge CB4 0DW, UK. He can be reached by e-mail at mark.berry@cambridgeconsultants.co.uk [15].