In the 1990s, it was generally considered that the lifetime of artificial joints was approximately 15 years. When the product reaches the end of its life, it is necessary to perform revision surgery to remove the artificial joint and replace it. Thus, revision surgery imposes a physical, mental, and economic burden on the patient. Therefore, it is necessary to determine ways to improve the longevity of artificial joints to avoid performing revision surgeries.

Artificial joints are composed of metal and polymer materials. Although there are various causes for revision surgery, deformation and the wear and tear of the polymer-bearing component is considered the most critical cause.

To support body weight and perform the movement of walking, medical devices need to be flexible as they are physically subject to very complex and severe conditions. In addition, it is a harsh environment for polyethylene because of the constant contact with biological components such as blood, synovial fluid, and lipids. Furthermore, they must simultaneously be biocompatible.

We believe that the development of biomaterials that can endure these conditions will greatly contribute to the development of artificial joints that can be used over a lifetime; therefore, we have begun the development of such materials.

First, together with Kyoto University, we began analyzing polyethylene that had been used in vivo for a long period but unfortunately had to be removed. We studied the types of deformation and abrasion that had occurred and analyzed its chemical state. Thus, it was clarified that polyethylene causes oxidative fatigue in the harsh in vivo environments.

To suppress oxidative fatigue, we focused on "vitamin E." As vitamin E is also used in pharmaceuticals and supplements, it is an antioxidant that can be safely used with pre-established biological safety. In addition, it has heat resistance that can withstand heat during the molding process, and is not water-soluble; therefore, it does not dissolve excessively in the body.

When we evaluated polyethylene with vitamin E, we found that it exhibited an extremely high antioxidant capacity.
While it was clear that vitamin E's antioxidant capacity was a contributing factor, it was found that added vitamin E improved moldability, and dramatically reduced fusion defects (voids that occur during molding).

We have repeatedly conducted various tests to apply this material, with its excellent antioxidant capacity, to the knee products. After meeting the requisite standards in many tests, including tensile, wear, and impact fatigue tests, knee simulator wear tests were performed.

When a human walks, the knee joint is exposed to extremely severe loading and sliding conditions. Loads of three and five times the body weight is applied during normal walking and when climbing stairs, respectively; along with simultaneous flexion/extension, internal/external rotation, and forward/backward/internal/external parallel movements. When the foot leaves the ground, a phenomenon called lift-off occurs, where the tibia and femur separate, and an impact force is applied when the foot lands. In this complex condition, the material we developed reduced wear to two-thirds, in comparison with conventional materials.

After confirming its biological safety, a clinical trial was conducted at Chiba University and its affiliated hospitals, and the marketing approval was obtained in 2009. The world's first material "Blend-E" implant for artificial knee joints was launched. By August 2022, it had been used in over 39,000 surgeries.

Having successfully commercialized this material for knee joints, we embarked on new developments to apply it to hip joints. The hip joint has a ball-and-socket structure and bones are in constant contact with each other hip. Compared with the knee joint, the hip joint has a larger contact area, is more constrained, and slides in multiple directions. In other words, although knee and hip joints are used in vivo, their geometries and motions are completely different, therefore, Blend-E could not be applied as is.

Therefore, we focused on cross-linking process. By applying the cross-link treatment, it was expected that the strength of polyethylene would be increased, and its resistance to a multidirectional sliding motion would be improved. However, we faced some problems, such as the destruction of the chemical structure of vitamin E by irradiation during the treatment and vitamin E inhibition. By adapting the irradiation method and subsequent heat treatment during the cross-linking process, we succeeded in achieving cross-linking while maintaining the antioxidant capacity of vitamin E.

In 2012, we obtained marketing approval for the hip replacement products "Blend-E XL THA Liner" and have subsequently launched it.

• Blend-E Japanese medical device manufacturing and marketing approval number: 22100BZX00882000
• Blend-E XL THA liner Japanese medical device manufacturing and marketing approval number: 22500BZX00125000

Both products are expected to have excellent clinical performance as artificial joint materials that exhibit excellent mechanical and chemical properties. Although it is important that the material be naturally wear-resistant, it is extremely difficult to completely eliminate abrasion in a complex in vivo environment. In addition, it has been reported that the submicron-sized wear debris generated during abrasion, causes an immune response, leading to osteolysis. However, recent studies have shown that abrasion debris from polyethylene with vitamin E is less likely to cause osteolysis than that without vitamin E.

Thus, the Blend-E series is expected to serve as a useful material that will bring us closer to the realization of artificial joints that can be used over a lifetime.