Resorbable polymers offer flexible options for implants.
|The flexible traits of resorbable polymers enable them to be used in many applications.
Materials designed to disintegrate in the body have made a valuable contribution to improving medicine. They were first introduced for use in wound closure more than 40 years ago. Today, resorbable medical devices are used in all areas of healthcare, including orthopaedics. Thanks to the safety and versatility of their core components, resorbable polymers are expected to have a significant effect on modern medicine. Resorbable polymers have properties that can be tailored to an increasing range of specific applications. Built upon tried and tested technology, these materials provide flexible yet reliable solutions that can benefit patients and physicians.
Implants composed of resorbable polymers play an important role in orthopaedic surgery. A range of devices that are absorbed by the body over time offer many benefits compared to nonresorbable polymer or metallic equivalents. They provide temporary functionality support and can bridge a gap in the healing process without the need for follow-up surgery to remove the device.
For example, resorbable polymers can be used as both a temporary support and a scaffold for the growth of new tissue, because the polymers gradually break down and are replaced by new tissue. They are flexible and can be tailored to specific applications because the degradation time and mechanical properties of devices can be optimized. They also enable better postoperative imaging than metallic equivalents, because they are radiolucent.
Because resorbable polymers can be impregnated with drugs, devices composed of these polymers can be adapted to perform secondary functions such as controlled drug delivery. Once inside the body, the polymer releases compounds to prevent secondary complications, such as a bacterial infection. Examples in orthopaedics are Ciprofloxacin-releasing fixation screws and pins, which have yielded positive clinical results and are being developed further by Bioretec.1 Future applications of resorbable polymers in tissue regeneration are of growing interest and are expected to provide exciting new options in medical therapies.2
Lactide-based polymers are the macromolecules most commonly used in resorbable medical device manufacturing. They include synthetic polyesters such as polylactide and copolymers with glycolide and ε-caprolactone. This class of materials has tunable properties that include degradation time and mechanical strength.
The technology was originally introduced in wound closure. Commercially available products based on lactide- and glycolide-based polymers have been on the market for more than 40 years.3 They have an established track record of safe and effective use in numerous applications including wound closure, orthopaedic treatments, controlled drug-delivery systems, and vascular closure devices. They were first introduced for use in orthopaedic implants in the 1980s and have proven in vivo biocompatibility and biological safety. The polymers degrade by hydrolysis and are eliminated by the human body through natural pathways via the Krebs cycle. In the Krebs cycle, the hydrolysis products are converted to water and carbon dioxide.
The first resorbable orthopaedic implants were used as fracture fixation devices by Pentti Rokkanen, MD, in 1984.4 Since then, numerous products have been developed and commercialized for a range of orthopaedic applications, including within sports medicine, trauma, and spinal implants. Currently available resorbable devices for orthopaedic applications include pins, screws, plates, rods, tacks, and suture anchors.
The objective of placing a resorbable implant is to temporarily fulfill the functionality of the defective tissue. During the healing phase, the tissue restores its initial functionality while the functionality of the implant gradually decreases. The strength of the device and its degradation must be coordinated precisely with the healing process. The implant needs to retain its strength for the minimum required length of the healing process. The mechanical strength of the implant combined with the increasing strength of the healing tissue must also allow the tissue to serve its function. The degradation time of the implant must therefore be finely adapted to the tissue type and the individual clinical application.
Tunable properties, such as mechanical strength and degradation time, make lactide-based polymers remarkably versatile in these applications. However, an in-depth knowledge of their structure, properties, and the process of their degradation in vivo is required. With extensive knowledge and expertise in polymer science, one can create and supply materials that perform precisely as required in many different clinical scenarios.
|Some anterior cruciate ligament screws are made with resorbable polymers.
Within the body, the implant first absorbs water from the surrounding tissue. This process allows the polymers to gradually break down into smaller fragments, which are consequently broken down into lactic acid and eventually released from the body via the Krebs cycle. The rate at which the polymers degrade is related to their hydrophilicity, among other factors. The rate can be altered to slow or speed this important part of the degradation process. The more hydrophilic a polymer, the quicker it will degrade. For example, polyglycolide degrades faster compared to polylactide.
Another factor that influences the rate of degradation is the molecular weight of the original polymer—its chain length. The longer the original chain length, the more time it takes to completely break down.
Finally, the crystallinity of the polymer (whether amorphous or arranged in a crystal form) influences the degradation rate. Highly structured or semicrystalline polymers degrade more slowly than their amorphous equivalents. Semicrystalline polymers are preferred for the manufacture of a device that must fulfill a mechanical property, whereas amorphous polymers allow faster degradation and are the polymer of choice for drug-release systems.
The mechanical properties of lactide-based polymers, such as the strength of an implant and the properties at body temperature, can be modified by using various monomer building blocks. A range of polymers, from relatively rigid to very flexible, can be created.
The composition of the resorbable polymer plays a vital role in the performance of the finished device, particularly when it comes to the degradation time and mechanical strength. The performance of devices made from resorbable polymers can be tuned by tailoring the molecular weight, crystallinity, and hydrophilicity of the polymer. However, additional factors are of equal importance. These include processing and manufacturing methods, device design, and the site of implantation. These factors are interconnected in developing and controlling the performance of a resorbable device. Developing effective, high-performance devices requires an interdisciplinary approach involving polymer chemists and engineers with device and clinical experts.
As people gain understanding, awareness, and familiarity with resorbable devices, the desire to apply new, innovative products based on this unique family of materials continues to grow. The global market for medical devices made from lactide- and glycolide-based polymers is estimated to be worth more than $1 billion.5 With developments underway and new applications on the horizon, the positive effects of these versatile molecules on modern orthopaedics is set to further increase.
1. TJ Makinen, et al., “Efficacy of Bioabsorbable Antibiotic Containing Bone Screw in the Prevention of Biomaterial Related Infection Due to Staphlyococcus Aurous,” Bone 36 (2005): 292–299.
2. AG Mikos,et al., Polymer 35 (1994): 1068–1077.
3. EE Schmitt and RA Polistina, 1967, Surgical sutures, U.S. Patent 3297033, American Cynamid Company.
4. P Rokkanen, et al., The Lancet (1985): 1422–1424.
5. DD Jamiolkowski and EJ Dormier, “An Introduction to Biomaterials,” CRC Press (2006): 139–160.
Robert Van Tuil is marketing director at Purac Biomaterials (Gorinchem, The Netherlands). Carolien Geurts is a marketing specialist at the company.
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