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The Past, Present, and Future of Hip Joint Replacements

Posted on: 04/21/2010
The first Baby Boomers are turning 65 next year (2011). A Baby Boomer is someone born in the United States between 1946 and 1964. Some of those folks already have hip or knee joint replacements. Many more will be candidates for total hip replacement in the next 10 years. Chances are the type of hip replacement they receive will be very different from what their parents or even grandparents might have been given.

That's because we are now on the third-generation of hip replacement implants. That means the implants have changed in major ways three times since they were first introduced more than 40 years ago. Changes in implant materials, surface, and component parts (e.g., liners, sockets, femoral head and stem) are the subject of this review article.

In some ways, today's surgeons face even bigger challenges than those early surgeons. The rate of implant failure requiring another operation remains fairly high. Patients are getting implants at an earlier age and wearing them out faster in part because of greater activity levels than ever before.

Other risk factors for implant failure include patient problems such as being overweight and surgical factors (e.g., failure to balance muscles properly, improper placement of the implant, surgeon inexperience). And implant materials, bearing surfaces, toughness, and wear mechanisms can contribute to implant wear and tear and ultimate failure.

This last factor (important bearing surface) is the topic of today's article. Types of surfaces, their advantages and disadvantages, and future alternatives are all presented for your consideration in making a choice about your preferred type of hip implant.

Let's start with the metal-on-polyethylene implant. This is probably the most commonly used hip implant. The socket has a plastic liner. The round head of the femur that fits into the socket is metal with a metal stem that sets down inside the femoral shaft to hold it in place. It's the least costly of all the types available. It goes in easily and doesn't have to be set into the bone exactly-so to have a good result. But there are some problems. Most notably, it wears out faster than other types and isn't as stable. Older adults who are fairly inactive are the best candidates for the metal-on-polyethylene implant type.

Next comes the ceramic-on-polyethylene. The plastic liner is the same as the one used in the metal-on-polyethylene implant. The difference here is in the material used for the stem and femoral head: ceramic instead of metal. Ceramic is hard and scratch resistant. That's important in keeping wear debris out of the joint. There is a risk that the ceramic will crack or fracture and it doesn't hold up as good as the ceramic-on-ceramic implants.

Those ceramic-on-ceramic surfaces have the lowest friction, roughness, and biologic reactivity. The surface is resistant to wear and tear so this type is used most often for younger, more active patients. And, of course, it can be used with anyone who has metal sensitivities. With both surfaces made of ceramic, there's an even greater risk of implant fracture. Pieces of the ceramic can get imbedded in the joint capsule making it difficult to remove the broken implant. The two other disadvantages of ceramic-on-ceramic are the cost (most expensive) and the fact that these implants can squeak loudly enough to be heard.

The last choice with an equal number of pros and cons is the metal-on-metal implant. This type of implant does create tiny flecks of metal that enter the blood, urine, and organs and to which patients may react. This type of implant is not advised for women who can get pregnant or anyone with kidney disease. The implant is the most stable and gives more athletic adults greater freedom to run, jump, and participate in sports activities.

The authors provide an in-depth review of all the research, study, and subsequent changes that have been made over the years to come up with each of these implant types. New technologies have been developed to improve strength, stability, wear rates, and implant sizes. In the process, scientists have been able to improve hardness, lubrication, and scratch resistance while lowering friction.

Thinner shells combined with an ability to polish itself have made it possible to reduce wear and prolong the life of the implant. The self-polishing feature makes it possible for scatches that develop to smooth away with movement. Manufacturers have been able to create an implant with a larger femoral head to increase joint stability while preserving mobility (motion). The risk of hip dislocation is lower with a larger femoral head size.

There isn't a perfect implant that can be used for everyone. Each type has its own advantages and disadvantages. The surgeon tries to make the best choice for each individual based on age, size, activity level, and sensitivity to materials (ceramic, plastic, metal). In the future, we can expect to see new surface coatings developed that will reduce debris while extending the life of the implant. Long-term studies 20 to 30 years after implantation will give us more feedback on what works best for which patients.

Karim Elsharkawy, et al. Evolution of Bearing Surfaces in Total Hip Arthroplasty: A Review. In Current Orthopaedic Practice. March/April 2010. Vol. 21. No. 2. Pp. 198-208.

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