Resorbable materials have found a home in several key clinical areas within orthopaedics. Implants that integrate such materials offer benefits over traditional static metal implants, especially in function and strength, but their use requires careful consideration from manufacturers during every stage of product development.
Polymer Implant Complications
- Material degrades too quickly, causing premature implant failure.
- Toxic/acidic degradation
- Material degrades too slowly, causing late-stage tissue reactions.
“It’s definitely a delicate balance,” said Elizabeth Perepezko, biomaterials research scientist of global fixation and antinfective technologies at Biomet Inc. Perepezko shared her expertise in working with resorbable materials at the 2012 OrthoTec conference
First, let’s focus on the positives of polymeric implants. They provide temporary fixation and avoid stress shielding (by allowing the mechanical function to gradually transfer back to the intertissue). The end result is an implant that doesn’t need to be removed at a later point, which cuts down on surgeries.
Polymeric implants give surgeons more flexibility in the OR. Due to the glass transition temperature, polymeric implants can be easily manipulated using standard heating methods. “You can contour and modify it to the patient’s anatomy in the OR rather than having it premade and preplanned ahead of the surgery,” said Perepezko.
When designing implants, patient comfort is paramount, and polymeric implants offer permanent palpability. This characteristic is especially important in craniofacial applications. “[When] you can look at temperature changes with metallic [implants] in the jaw, you can have some discomfort in the patient in going from a warmer to colder rooms—they can have nerve discomfort,” said Perepezko.
Now, here are the challenges. Polymers degrade by hydrolysis, and several factors affect degradation behavior. Modifying the composition of the polymer chemistry can influence the degradation rate, as can polymer processing, and where the implant is placed in the body. “There’s evidence to show that resorbable implants of the same composition, implanted above the neck and the lower extremities can have different degradation profiles,” said Perepezko. It’s important to consider what kind of circulation or metabolism is in the surrounding tissue. For example, bone and soft tissue have different water content. For patients with compromised circulation (such as diabetics), the use of resorbable implants can be risky. Manufacturers also need to think about whether the degradation of the implant will negatively affect the healing of surrounding tissue.
The manufacturing process itself presents problems to resorbable materials, because polymers are sensitive to heat, and thermal degradation is a challenge in standard polymer processing. “Controlling that and making sure that you’re not accelerating the degradation rate with processing is important,” said Perepezko. During polymer processing, injection molding and extrusion are aggressive methods that can have a high influence on the degradation properties of the material. To avoid shear damage, keep heat low.
Moisture levels must be carefully controlled during sterilization, packaging, and storage conditions. Gamma sterilization can accelerate the degradation profile, because the radiation can shorten the polymer chains. “You can try to build a cushion into your material so that whatever you’re losing in the gamma cycle is getting you to the point where you want to be, or you can use other methods like gas plasma or ethylene oxide,” said Perepezko. “But those two methods add moisture to the cycle, so now you have to consider what kind of moisture damage would affect your overall degradation profile.” Maintaining this balance can be one of the most crucial stages in the process.
Finally, manufacturers need to make sure the packaging and storage conditions are controlled, because the materials aren’t inherently stable. Being able to control moisture long-term is critical.
Maria Fontanazza is managing editor at UBM Canon. Follow her on Twitter: @MariaFontanazza.
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