Over the past 2 decades it has become clear that the major stumbling block to prolonged survival of hip replacements is the wear of the bearing surfaces (area where the ball articulates or joins with the cup). Wear is defined as the progressive shedding of minute amounts of material from the implant. Wear particles cause bone resorption and granuloma tissue production. They may cause pain and, ultimately, loosening of the prosthesis. The parts can also wear through, destroying the portion of the prosthesis that supports the bearing surface. This is not surprising in that the forces of 4 to 7 times of body weight are transmitted millions of times every year through each hip joint

          The traditional bearing surfaces used in hip replacement surgery were metal and plastic. Although the very first hip replacement surgeries used varied bearing materials, a polished metal ball and high molecular weight polyethylene cup became the standard for the first three decades that this surgery has been offered. Over the last ten years sterilization and manufacturing techniques have improved, but metal (cobalt-chrome) on UHMWPE remains the standard bearing surfaces of choice for hip replacement surgery. Alternative bearings are those that are considered highly wear-resistant and are an “alternative” to conventional polyethylene.

Biomaterial advances have allowed experimentation with new bearing surfaces, and there are now several different options when hip replacement surgery is considered. The exact “best” option is unknown, and the choice of bearing surface is controversial. The main reason for this uncertainty is that the results of laboratory testing of the various surfaces do not always translate to similar findings when the new materials are used in people. Many of the contemporary “new” materials have been tried in their firsat generation forms in other countries or at other times in the history of hip replacement. The newest technologies and techniques do not have enough follow-up to know which, if any, will be better than the current gold standard. At OAP, clinical follow-up studies in both ceramics and crosslinked polyethylene are ongoing.

Surface choices can be broken down into hard-on-soft and hard-on-hard bearings. In general, hard-on-soft bearings demonstrate more linear wear for the same time period as hard-on-hard bearings, but are likely less susceptible to catastrophic failure. Hard-on-soft couples include not only cobalt-chrome on polyethylene, but also ceramic or ceramic like materials on polyethylene. Hard-on-hard surfaces demonstrate less linear wear in most cases but are likely more sensitive to failure due to surgical technique (eg component positioning, third body debris, etc). Hard on hard bearings include metal-on-metal and ceramic-on-ceramic. Transition forces between prosthesis and bone may be elevated in hard-on-hard couples, as there is no dampening effect of a soft material. Micro-separation, a newly discussed concept where the ball and socket separate slightly during the swing phase of gait, may change the expected wear profile of hard-on-hard surfaces.

Crosslinked UHMWPE Crosslinked UHMWPE was FDA approved in 2000 and several varieties of this category of product exist.  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 methods to crosslink the polyethylene, and to what degree, are unknown. Further, relevant long-term clinical studies are lacking for this newly re-engineered product. Two older, small studies (from Japan and England), in which a form of crosslinking was used, are available for review. These studies both showed decreased wear of the crosslinked plastic after the first year in use, and the decrease was about 75%. A more recent study from Sweden using RSA analysis confirmed a 50% reduction in wear when crosslinked HMWPE was compared with contemporary polyethylene from the same company. The Maine Joint Replacement Center at OAP is participating 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. At two years, we have found decreased wear with the crosslinked polyethylene, but because of plastic deformation known as creep, the differences are not yet statistically significant.

Metal-on-Metal. This combination has been FDA approved and is offered by several companies. Although metal-on-metal couples were originally introduced in the 1960s, a poor understanding of optimal design characteristics and limitations of the manufacturing process caused poor results. Second-generation metal-on-metal products were developed to address problems such as loosening of the hip replacement, high frictional torques and seizing of the articulation. Important design characteristics appear to be use of a hard, high-carbon wrought alloy; the goals of polar contact, optimal clearance, and maximal spericity; and extremely low surface roughness. One potential concern in this type of hip replacement is the metal-metal acetabular connection, where the CoCr / Ti junction may be difficult to disassociate or where fretting corrosion could occur. Despite a lower volume of wear associated with metal-on-metal implants, the particles that are produced are very small, possibly resulting in a larger number of particles compared with metal-on-polyethylene couples. This may be of some concern because the full biological response to metal particles or ions is currently unknown. A major negative issue with contemporary metal-on-metal couples is a question of metal toxicity and carcinogenesis. Elevated urine and blood levels of the metals that make up the prosthesis have been measured, and ongoing research will confirm the safety of this combination. This issue seems to be most important in patients with poorly functioning kidneys, as the metal ions can build up in the blood. Although “seizing” of the implant has been reported, this is uncommon.

  Ceramic-on-Ceramic. This combination was FDA approved for general use in the USA in 2003 and is offered by several companies. Alumina ceramics appear to be most appropriate for hard-on-hard bearings. In contrast to traditional metal-on-plastic articulation, ceramic surfaces are said to have better wetability and toughness. Second-generation ceramic-on-ceramic products were developed to address problems such as chipping and breaking, and today’s components are the result of many improvements in ceramic technology. There is a decrease in prevalence of component fractures; better “proof testing” and quality control; reduction in grain boundaries with an increase in material strength and toughness; less impurities which act as potential stress risers; and a better understanding of taper design and tolerance. The major negative issue with contemporary ceramic-on-ceramic couples is a question of wear characteristics in the body and the possibility of component impingement. Some ceramic components that have been retrieved at re-do (or revision) surgery have demonstrated “striping” of the femoral head. This could be due to microseparation during normal walking, and in at least one study, increased the wear of the couple.  

Chart comparing laboratory wear results of various alternative bearing surfaces used in total hip replacement

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