Longevity of Total Hip Replacements

Total hip replacement (THR) for painful arthritis is one of the most successful operations in modern surgery. Pain relief is almost universal and greater mobility and increased function are expected in the vast majority of patients. One unresolved issue that is of utmost importance to those considering this surgery is the durability of the hip replacement—that is, how long will it last?. Three general questions must be explored in order to understand this issue: 1. How long will a hip replacement function before revision (re-operation) surgery is necessary?; 2. Why will the hip replacement ultimately fail?; and 3. Is there a way to extend the life of contemporary total hip replacements? Research to answer these questions has been one focus of the adult reconstruction service of Orthopedic Associates of Portland.

How long will a hip replacement function before revision surgery is necessary?

In a presentation at the Harvard Medical School sponsored course, "Total Hip Replacement: The Dawn of a New Era" in September 1999, Dr. McGrory presented the results of total hip replacements preformed by Dr. Omar D. Crothers with a minimum 10 year follow-up. At a median follow-up of 13.2 years, 74 % of the total hip replacements studied had not been revised, and the majority of these were functioning well. On follow-up radiographs, there was significant wearing of the plastic (ultra-high molecular weight polyethylene) portion of the hip replacement in 13 %.

Radiograph of a hip replacement showing that the femoral head (ball) has worn into the plastic lining of the acetabulum (cup). Note the dark area above the cup which shows bone loss due to a reaction to the small plastic particles produced.

Of the 26 % that underwent revision surgery, over one-half had surgery to insert a new plastic component of the hip replacement.

This study is valuable because it one of the longest follow-up studies for contemporary type prostheses. Previous studies are not as useful (even though the follow-up time may be longer) because they look at hip prostheses that are no longer in use. The study shows that the major limiting aspect of contemporary hip replacement surgery is the prosthesis durability, particularly the plastic liner of the socket.

Why will a total hip replacement ultimately fail?

In order to clarify the reason for the failures of the hip replacements discussed above, a retrieval analysis of the worn plastic inserts removed at surgery was undertaken.

Dr. McGrory of the Maine Joint Replacement Center, in conjunction with Massachusetts General Hospital researchers Orhun Muratoglu, Lamia Mounib, Charles R. Bragdon, Murali Jasty, and William H. Harris carefully examined the metal backed acetabular liners retrieved at revision surgery. The components were analyzed under reflected light for cracks in the plastic, and 75 % of the specimens were microtomed parallel to the surface of the extended lip of the liner. A total of 20-30 serial sections of 80 um thickness were collected, and a portion of each liner was selected for oxidation analysis.

In a poster presentation at the 24th Annual Meeting of the Society for Biomaterials in San Diego, California, the authors showed their results in an abstract entitled " Anisotropic Oxidation and Radial Cracks in Retrieved Acetabular Components." They found that all components exhibited a focal distribution of multiple, grossly visible radial cracks superiorly and posteriorly in the rim of the plastic liner. The infra-red analysis of the four quadrants studied showed on average 50% higher oxidation indices in the regions with radial cracks.

These findings allow insight into the failure of the polyethylene-bearing surface of contemporary total hip replacements. The prosthesis surfaces are at higher risk of failure if the plastic has oxidized, because oxidized polyethylene is weaker and more brittle. The location of the weak portion of the socket liners was also important.

This study was the first to demonstrate that the oxidation known to weaken plastic liners may be increased by stress, and is not necessarily uniform throughout the component. Such information confirms the need to sterilize and store plastic components in inert or vacuum environments if the durability of the plastic is to be improved.

Based on information in these and similar studies, it has become clear that the current limiting factor in the durability and longevity of total hip replacements is the "wearing out" of the plastic portion of the articulation of the ball and cup.

Is there a way to extend the life of contemporary total hip replacements?

Several new and pending products aim to address this very question. A ceramic-on-ceramic (refers to the ball and socket surfaces) type of hip replacement is undergoing pre-FDA approval studies in the United States, and has been in use for some time in Europe. In contrast to the traditional metal on plastic articulation, ceramic surfaces are said to have better wetability and toughness. Ceramic components, however, are brittle and can fracture more easily that other materials. Another alternative, a metal-on-metal type of hip replacement, was recently approved by the Food and Drug Administration (FDA). The first such surgery at Maine Medical Center with this type of articulation was performed in March of this year. Precisely machined metal on metal surfaces have shown minimal wear in European studies, but concerns remain about metal ion release into the patient's body. One of the most exciting potential advances in articulation technology, however, is crosslinking of the ultra-high molecular weight polyethylene that has been used since total hip replacement surgery was introduced to the United States in the late 1960s.

Crosslinking is a process in which polyethylene molecules are bonded together to result in a stronger material, substantially improving the material's wear resistance.

Schematic drawing showing how radiation is used to "crosslink" the polyethylene chains of the plastic lining of the acetabulum (cup) resulting in a more durable product.

It has been reported that the amount of crosslinking of polyethylene is directly related to its wear performance, and significant reductions in polyethylene wear have been shown in joint simulation studies with highly crosslinked polyethylene components.

Graph demonstrating a 90 % reduction in wear (measured by weight loss of the plastic) at 5 million cycles (approximately 5 years normal wear) based on hip simulator studies.

Such a material should lead to much better resistance to wear, fewer polyethylene particles, less osteolysis (bone loss around the hip because of the body's reaction to the particles) and less frequent implant loosening. The best method to crosslink the polyethylene, and to what degree, are unknown. Further, relevant long-term clinical studies are lacking for this new re-engineered product.

One such study is currently underway at Maine Medical Center. Dr. McGrory is a co-investigator in a multi-center, blinded, prospective, randomized study comparing one type of highly crosslinked polyethylene (the "melt-irradiated" type) with conventional polyethylene. Patients who enroll in the study will be followed carefully for a decade or more, and radiographs will be analyzed with three-dimensional digital technology to compare wear rates. The Maine Medical Center Internal Review Board has approved the study, which hopes to prove or disprove the superiority of this type of polyethylene.

Conclusion

Two recent projects supported by the Joint Replacement Center at Orthopedic Associates of Portland add to our knowledge of the expected longevity of total hip replacements and the primary cause of their failure. A third, ongoing study will prospectively assess crosslinking of the plastic portion of the artificial hip. This is a recent innovation that has the potential of significantly extending the life of hip replacements.