Why a Polymer Is Critical to Successful Orthopedic Device Performance

When nations worldwide suffered the effects of COVID-19 and the resultant global lockdown, there was an understandable drop in demand for orthopedic devices. This drop was only temporary, however, since the number of U.S. adults with osteoarthritis is expected to rise by 49% by 2050 and those experiencing arthritis-related physical challenges will swell by 52% by 2040.¹

According to the United Nations report, “World Population Ageing,” there were 703 million people worldwide aged 65 years or older in 2019. This number is projected to double to 1.5 billion by 2050.² There is a tremendous opportunity for manufacturers that can develop more innovative, safe, and user-centric devices, but the supply chain is key to maximizing this opportunity.

One polymer material, ultra-high molecular weight polyethylene (UHMW-PE), is uniquely suited to meet the high standards for biocompatibility and flexibility required in manufacturing orthopedic implants. This polymer has been used since the early 1960s and has demonstrated multiple attributes, including long-term performance, exceptional quality, and compliance with regulatory agencies, based on ASTM F648 and ISO 5834-1/-2, to become the gold standard solution for orthopedic surgical implant technologies.

Patient safety is always the most important attribute in medical devices and, therefore, biocompatibility and a safe toxicity profile are essential requirements. For orthopedic devices, mechanical properties such as impact strength and wear resistance, combined with the implant’s durability, are fundamental to ongoing performance and patient satisfaction. Consider the average weight for men is around 200 pounds (91kg) and weight for women is 171 pounds (77.5kg),³ hip and knee implants in particular must be able to bear a great deal of weight, movement, and stress.

Widely recognized as the best material for tribological applications, UHMW-PE negates the risk of wear particles leaching, even in high-impact scenarios such as hips and knees, to ensure there is a low risk of osteolysis or loosening of the implant in the bone.

Part of the material’s success is due to its crosslinking, which is achieved by ionizing radiation; the process forms crosslinks in the amorphous portion of the polymer. Used since the mid-1990s, this improved material has been shown to significantly reduce wear rates in comparison with conventional UHMW-PE,^4-6 extending the implant’s life for up to 30 years.

Besides the wear resistance profile for manufacturers and patients, UHMW-PE also offers advantages for surgeons. During the past couple of decades, devices were produced in extremely rigid metals or ceramics, which meant these “hard-on-hard” implants required surgeons to be incredibly precise in positioning hip replacement devices. This was essential to avoid excessive wear on the implant’s edges and poor patient outcomes such as “squeaking hips.” The squeaking hip is a phenomenon unique to total hip replacements using hard-on-hard bearings. With all modern knee replacements now using UHWM-PE, there is no equivalent “squeaking knee.”

UHMW-PE devices are much more forgiving, allowing for additional flexibility in positioning by the surgeon. As a result, UHMW-PE devices are the predominant choice for all replacements today. It is also perhaps in part due to the evolution in these devices that knee arthroplasty and hip replacements are among the most successful and frequent operations in the world today.⁷

As with all medical devices and their components, there are challenges to overcome. One of those challenges is to maintain material purity and sterility in its raw state from its point of origin (the polymer provider), through manufacturing (converters, OEMs), and distribution (healthcare facilities and the patient).

One of the most significant challenges with UHMW-PE is that it cannot be injection molded, which impacts manufacturing speed and cost. Its incompatibility with injection molding is due to the extremely high molecular weight of the polymer. This is its defining characteristic in terms of wear resistance and low friction, but also its greatest challenge from a processing perspective. That said, UHMW-PE has been clinically accepted since the 1960s, and the processing challenges are widely accepted, with manufacturing operations optimized to mitigate cost, risk, and time.

Although crosslinked UHMW-PE is the gold standard raw material for orthopedic devices, innovation has not ceased in this space.

Vitamin E—specifically alpha-tocopherol—is a natural substance that already exists in the human body and has been used as a stabilizer since 2009 for orthopedic implants made from UHMW-PE.

Vitamin E is beneficial because most implants are treated by modern irradiation techniques for sterilization and/or performance improvement. These processes can accelerate in vivo oxidation, shelf-aging of the implants, and degradation that leads to reduced mechanical properties, including decreased wear resistance. Alpha-tocopherol also prevents oxidation of the UHMW-PE material.

Studies have shown that vitamin E-containing, irradiated UHMW-PE demonstrated wear resistance superior to conventional UHMW-PE. In addition, it showed the vitamin E-containing, irradiated UHMW-PE’s wear rate was not affected by the larger femoral head size. When compared to irradiated and melted UHMW-PE, the vitamin E-containing, irradiated UHMW-PE demonstrated improved mechanical strength and fatigue crack propagation resistance.⁸

As previously discussed, there are some prerequisites regarding the supply of polymers for orthopedic devices—with purity and sterility topping the list. While UHMW-PE is the only credible polymer option, there are several producers of this material, and selecting the right supplier is fundamental to successfully commercializing devices.

OEMs should be looking for partners with a proven track record in supplying high volumes of UHMW-PE, with high, consistent standards of quality and processability in every batch, so the risks associated with any outsourced supply chain arrangement are mitigated.

Experience, reputation, and trust related to the supplier’s values and behaviors should also be carefully considered. In a truly customer-centric organization, teams are built around customers to ensure peers talk with peers so a common language is maintained and a common goal is achieved at every stage, from R&D and production to commercial functions.

While the material is a given for all the reasons outlined in this column, the choice of partner is not. UHMW-PE is uncompromising in its quality, and OEM program leaders should expect the same level of quality from their suppliers. 

References

1. bit.ly/odtorthoinsights092201
2. bit.ly/odtorthoinsights092202
3. bit.ly/odtorthoinsights092203
4. Nivbrant B, Roerhl S, Hewitt B, Li M. In vivo wear and migration of high cross linked poly cups: A RSA study; 49th Annual Orthopaedic Research Society; New Orleans, LA. 2003. [Google Scholar]
5. Martell JM, Incavo SJ. Clinical performance of a highly crosslinked polyethylene at two years in total hip arthroplasty; A randomized prospective trial; 49th Annual Orthopaedic Research Society; New Orleans, LA. 2003. [PubMed] [Google Scholar]
6. Digas G, Karrholm J, Malchau H, Bragdon CR, Herberts P, Thanner J, Estok DM, Plank GR, Harris WH. RSA evaluation of wear of conventional versus cross-linked polyethylene acetabular components in vivo; 49th Annual Meeting Orthopaedic Research Society; New Orleans, LA. 2003. [Google Scholar]
7. bit.ly/odtorthoinsights092204
8. bit.ly/odtorthoinsights092205

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Dr. Rainer Walkenhorst is senior manager, T&I Engineered Materials, at Celanese. With a Ph.D. in polymer physical chemistry, Rainer joined Celanese more than 15 years ago and has become a key industry leader with global responsibility for UHMW-PE, including new product development, new application development, and customer support. Before joining Celanese, Rainer worked across R&D, technical marketing, and product management.