|Hip Prosthesis Designs
Since Sir Charnley performed the first modern hip replacements in the 1960's, there have been continual advancements in the evolution of the hip replacement prostheses themselves. Some of these changes have involved the materials used for the femoral stems and cups, the materials used for the ball bearing surfaces, whether or not cement is utilized, and most recently, modular designs that allow different components to be put together at the time of surgery to "custom build" a prosthesis to the right length and offset
Decision To Select The Implant
There are many factors that the surgeon must consider when choosing an implant. First, there are many different options today, including cemented versus noncemented components, modular versus monoblock (single piece) designs, and several different choices for bearing surfaces. Second, there are advantages and disadvantages to each of these choices, and the rest of this chapter presents the most common pros and cons of each design choice. Third, it is a fact of life that cost is increasingly becoming a factor in this aspect of the surgery, and in many places around the country, surgeons are at least somewhat limited by hospitals (and in some regions, limited quite severely) to what choices of implants can be considered. In some cases, the newest and best prostheses may cost more than the hospital will be reimbursed by insurance or Medicare, and the hospital may have policies in place to limit the use of such implants.
In general, operating rooms may often allow orthopaedic surgeons who perform a large volume of joint replacement surgeries more latitude because of the numbers of patients in which the hospital does not take losses. Surgeons performing a small number of joint replacements each year (once a month or less) may not have that flexibility at the community hospital.
Cemented And Noncemented Hips
The parts of a typical total hip prosthesis. There is usually a metal acetabular shell (socket), a liner, femoral head (ball), & stem.
In the early days, all hip replacements were secured into the bone with cement. This is still the case for most knee replacements, for mechanical reasons that will be discussed in the knee section. The cement itself has changed very little, and polymethylmethacrylate (PMMA) cement is utilized in all sorts of orthopaedic applications where cement may be needed. In actuality, it functions more as a grout than true cement, filling in the porous spaces around a prosthesis.
The cement offers the advantage of initial strength. At the time the patient leaves the operating room, it is as strong as it will ever be. This is also the downside, given that cemented components eventually loosen much like a cobblestone in a cemented walkway. Although cement was initially used for securing all components into bone, over the years noncemented designs and porous coatings have evolved that outperform cemented replacements in active patients with stronger bones. However, there are still surgeons who prefer and advocate cemented hip replacements, and nearly all surgeons will on occasion use cement for very poor quality bone, such as performing a partial hip replacement for a hip fracture in an elderly patient with very osteoporotic bone.
Another advantage of cement is the the ability to mix antibiotics into it, like vancomycin or gentamicin. These antibiotics leach out over a period of months. Sometimes antibiotic-impregnated cement beads or spacers are placed temporarily into a joint in order to fight deep infection for weeks or months.
Besides concerns over mechanical loosening, however, cement can greatly complicate revision surgeries in which the interior of the bone has to be scraped or drilled extensively to remove old cement. The cement itself can also cause serious complications during surgery as it sets, sometimes dropping blood pressure dramatically. While not usually a problem, this can be of significant concern to both the surgeon and anesthesiologist if it occurs during surgery. Noncemented components use porous surfaces that are designed to allow bone to grow into the metal. These provide a very strong interface after a period of months. Once the prosthesis is solidly fixed, it is quite secure. Some implants are coated with hydroxyappetite, a chemical that further promotes bone formation.
Introduction to Bearing Surfaces
The ball and socket comprise the bearing surface. The bearing surface can be made from a different material than the stem and socket components. This allows the surgeon to have some flexibility in using the best material for the stem and socket (such as titanium, which is structurally strong but does not work well as a bearing) and the best material for the bearing itself (such as cobalt chrome or ceramics).
There are a number of different materials now used for the bearings, but the most common are metal on plastic (e.g., a cobalt chrome ball on a polyethylene liner), metal on metal (both the ball and the liner are made from cobalt chrome), ceramic on ceramic (both the ball and the liner are made out of a very tough, lowfriction ceramic material such as alumina oxide), or hybrid materials (such as cobalt chrome ball with a zirconium oxide surface against a plastic liner). There are distinct advantages and disadvantages to each.
Stability vs. Wear in Bearing Selection
Stability vs. wear. Larger diameter bearings have a better range of motion (and therefore more stability and less risk of dislocation), but the increased contact surface area causes increased bearing wear.
One principal concept is the trade-off between stability and wear. A larger diameter ball is more stable and has a greater range of motion before the stem impinges against the side of the socket. This makes it more difficult to dislocate and improves a patient's range of motion. However, as the diameter of the ball increases, the surface area of the bearing increases as well, and the bearing wears out more quickly.
For many years, the optimum bearing size was thought to be 28 mm, providing a good balance between stability and wear against a plastic (polyethylene) liner. However, as better materials were engineered - particularly ceramic and metal on metal hip designs - it has become feasible to use larger bearings to improve range of motion.
Bearing Surfaces - Cobalt Chrome on Polyethylene (Metal on Plastic)
The initial ball bearing surfaces utilized metal balls (of different types) on plastic liners, which is still common today in most places. Some early designs in the 1960's and 1970's tried industrial coating materials like Teflon, which seemed like a good idea, but turned out not to work very well as the Teflon wore away very quickly.
The most common type of bearing surface in use today is a cobalt chrome ball (femoral head) that moves against a polyethylene (plastic) liner inside the hip socket. This is also the least expensive type of material that is available. It does not typically last as long as ceramic on ceramic or metal on metal ball bearings, but works fine for a patient with a life expectancy of 15 years or so. It may be used for younger patients because of some of its other advantages (see below), but it is usually explained to them that they will likely need revision surgery in 10 to 20 years to at least replace the liner as the plastic portion wears away.
Advantages of metal on plastic bearings include the low cost, essentially zero risk of fracture (as compared to ceramics), minimal metal ion accumulation (as opposed to metal or metal bearings), and relatively rare occurrence of problems with clicking or audible noises. Polyethylene liners also can come with raised lips on one side or constrained designs, which help to prevent dislocation at the cost of range of motion in a hip in which the metal socket is imperfectly aligned or when the patient may have trouble following instructions (e.g., early dementia or noncompliance).
The principal disadvantage of metal on plastic is that the metal head wears away the plastic liner over time. Small particles of plastic accumulate within the joint, and the white blood cells in the body try to swallow and dissolve these particles. However, when the white blood cells are unsuccessful at digesting these artificial particles, the white blood cells may burst, releasing the chemicals (enzymes) used to dissolve foreign bodies and bacteria into the bone around the prosthesis. Over time, this causes large cysts to form and areas of loosening occur around the prosthesis, called osteolysis, that frequently leads to a revision surgery at some point as the hip replacement becomes loose.
In the 1990's there were some new discoveries that led to better plastic liners. The sterilization process used then included gamma irradiation in air (bathing the components in radiation to sterilize them). After some time, it was discovered that this process caused the plastic to become brittle after a few years (the reason was because air molecules integrated into the plastic), causing some plastic liners sterilized in this fashion to not last as long as liners used prior to or after that time period. As a result, major changes were made in the industry, and since the late 1990's plastic (polyethylene) liners have been manufactured differently in order to provide a significantly longer lifespan. But it will likely be another 10 or 15 years before we truly know how much longer these materials last than the 10-15 year lifespan of previous plastic liners.
Bearing Surfaces - Metal on Metal
A metal bearing on a metal surface will never break, and it will wear out much more slowly than metal on a plastic liner, usually lasting for decades. Because of the slow volumetric wear rate, a larger diameter ball can be used than with metal on plastic liners, which gives greater stability and range of motion.
Additionally, the wear particles that do form in a metal on metal surface are very small metallic particles, which are much smaller than the plastic particles and do not cause as much of the process of osteolysis like the plastic particles do.
However, we do know that metal on metal bearings generate higher levels of heavy metal ions in the body. These accumulate in the lymph nodes, liver, spleen, and other places in the body. There has been at least the theoretical concern that high levels of these heavy metal ions might cause health problems later, possibly even some types of cancers. However, this debate continues today after many years of such concern, and most surgeons agree that there is little evidence so far of detrimental problems from this in the tens of thousands of patients who have received such implants.
The metal ion accumulation is the reason that most surgeons recommend against metal on metal surfaces for patients with kidney disease (because it is chiefly responsible for eliminating metals from the bloodstream) or in women who may potentially still have children.
We caution our patients about these potential concerns, but balancing this risk for younger, active patients is the known risks that occur with large revision surgery in 10 to 15 years that will be more likely with metal on plastic bearings. Some types of implants, such as hip resurfacing implants, are only available in metal on metal bearings because these are the only materials that will hold up in the design over time. (Ceramic resurfacings still face engineering problems for use in resurfacings because the socket component would have to be made out of all ceramic and risk fracturing with the thin dimensions required.)
Ceramic on Ceramic
Ceramics have been around in hip replacement applications for about 30 years, but they have only recently begun to receive popularity in the U.S. over the past 10 years.
These ceramics are high performance, highly polished bearings that are usually made from alumina oxide or zirconium oxide. They have a very long life, with the lowest friction and wear of any of the bearings. Additionally, what little wear particles are generated are usually biologically inert (think of sand), and do not cause problems with metal ion accumulation or osteolysis as is seen with plastics.
However, there are some disadvantages to ceramics. They can very rarely fracture, although the reported rates for the current generation of ceramics is one in thousands. Most of the fractures that we have seen have occurred as the result of a trauma that was significant enough to have broken a bone or because of design problems in which the stem of the femur impinges against the ceramic liner over time. Ceramic bearings can also sometimes cause audible noises (probably about 1 in 400), such as squeaking or clicking.
Ceramics are also very expensive compared to metal on plastic, and many hospitals across the country may force their surgeons to use the less expensive implants when the hospital would receive less payment for the implant costs (typically this happens with Medicare or Medicaid, which reimburses the hospitals - and physicians - only a fraction of what private insurance or payers will cover). Increased public awareness and pressure for Medicare and Medicaid to cover the cost of these more expensive implants is needed if those patients want to have access to the higher performance implants.
Hybrid Materials (OxiniumTM)
Some newer materials have been available in recent years that offer unique characteristics. One such material that we frequently use is OxiniumTM, which is essentially a metal (cobalt chrome) head that has a ceramic surface (zirconium oxide). This is a patented material developed by one of the larger orthopaedic implant companies, Smith & Nephew. The metal head undergoes a special treatment process to form a ceramic layer, so that the general idea is that the final bearing surface has the benefits of both ceramics and cobalt chrome: the ceramic surface has very low friction and wear without metal ion accumulation, and the cobalt chrome ball cannot fracture as ceramics rarely can.
This is a relatively new, expensive material, and it has been in widespread use since the late 1990's. The data so far after the first decade of use are promising, and the laboratory studies show that these materials last a very long time in simulators. In our practice, we typically will select this material for someone who needs better wear characteristics than are seen with cobalt chrome on polyethylene but may not (for various reasons) be a good candidate for a ceramic on ceramic bearing surface. It is also useful for patients with a metal sensitivity or allergy, since it generates very few metal particles or ions as the bearing wears.
OxiniumTM is of particular interest in knee replacements, as is discussed in that section. Ceramic bearings are not usually a feasible option in most knee replacements, and we have found this material to be very useful in knee replacement prostheses.
Modular Hip Replacement Designs
A newer development in recent years has been the movement towards implants that have interchangeable, modular parts rather than a single piece design that comes in several sizes. Some modular designs allow different sized components for the femoral stem, the femoral neck, and the femoral head (ball) in order to achieve hundreds or even thousands of different combinations to try to achieve the best reconstruction possible. In this way, the surgeon can increase the offset without making the leg longer, change the angle that the device sits within the socket (anteversion or retroversion), or reconstruct a difficult and distorted joint such as a dysplastic hip. Smaller incisions can also often be used as the prosthesis is assembled in place, somewhat like a ship in a bottle.
A potential downside to modular components is the question of whether or not they may fail at the modular junctions, whereas a single piece design may be less likely to break. However, modular designs have been continually evolving, and it appears likely that more and more hip replacements in the future will be modular to allow greater flexibility in reconstructing patients' individual anatomy.
This chapter is by no means an exhaustive list of the different factors used in implant selection, but it acquaints you with the major design differences and the general benefits and drawbacks of each.
The surgeon also considers a number of other factors in implant selection as well. The geometry of the femur varies considerably from patient to patient; some patients have a narrow "champagne glass" shape to the femoral canal, and others have a straight cylindrical shape. There are prostheses that fit each of these and other geometries. Some patients have either a valgus or varus femoral neck, meaning that there is an excessively large or small angle to the neck-shaft relationship and implant changes have to be made accordingly.
Not infrequently, one leg is already longer or shorter than the other, and the surgeon needs to plan on how to try to correct that problem at the time of surgery if possible. Some femurs have a significant bow to them or even old fractures, and these require additional strategies to make the artificial joint fit down the canal (such as cutting and re-aligning the femur to make it straighter).
The acetabular socket can be misshapen or too shallow (as in hip dysplasia) and will require additional reconstruction to solidly accept the metal cup. Some sockets are too deep (called acetabular protrusio), and reaming the socket will likely result in a hole through the pelvis that will need to be dealt with structurally. Some sockets have very poor bone and need an implant that allows for extra screws to be used for supplemental fixation. Some patients have large cysts present on their x-rays that may be large enough to plan on grafting and filling at the time of surgery.
Some bones have cysts or thinned areas that will need to be examined and possibly grafted. If areas of bone are significantly weakened, strut grafts need to be wired or cabled around the shaft for additional support.
Some patients have already had previous surgeries or old fractures, which can greatly add to the complexity of the case if the surgeon needs to plan for old scar tissue and the removal of old hardware such as pins, screws, or plates.
Some implants are cast, and others are forged. Some stems are made of titanium and others may be made from cobalt chrome or other alloys. Some stems have a very low modulus of elasticity, which means they do not bend very much, and can have mechanical implications. Some stems have a clothespin design at the bottom to allow for greater flexibility in the implant and less thigh pain. Some designs are too stiff or too solid, resulting in stress shielding, a process which causes bone to absorb and disappear over time because it is not adequately loaded enough.
In the end, there is a long list of factors that can play a role in the implant selection, but your surgeon is trained to consider the pros and cons of each and uses these factors to reach a decision for which implant to use.
Please remember the information on this site is for educational purposes only and should not be used to make a decision on a condition or a procedure. All decisions should be made in conjunction with your surgeon and your primary care provider.