During a hip replacement, the head of the femur is removed. A metal stem is inserted and a ball fitted on top of the stem to replace the head of the femur. A metal liner or cup is then placed in the acetabulum and may be fixed with 2 or 3 screws. Into this is placed a plastic, metal or ceramic liner. The ball then fits into the cup, creating the new hip joint.
The great majority of hip replacement implants are fixed bearing, meaning that there is a stem that inserts into the femur, a ball that replaces the head of the femur or top of the thigh bone, and a shell that is fixed and lines the hip socket (acetabulum). One manufacturer though now makes a mobile-bearing hip implant as well.
The major differences between implants are materials (metal, polyethylene and ceramic) and fixation. In terms of fixation, your surgeon will anchor the hip implant components to bone using either cement fixation or fixation by ‘bone ingrowth’ (cementless fixation) – or a hybrid of the two.
A hybrid total hip replacement implant has one component, which could be either cup or stem, inserted without cement. The other component is inserted with cement. Both types have a solid track record for stability and limited problems in the long-term.
Although most hip implants are a fixed-bearing design, some are of a mobile-bearing hip implant design which features an insert that is designed to fit snugly into the acetabular shell yet can move a little as the hip moves.
The hip joint then has two points of articulation or movement, essentially a ball within a synthetic socket, which resides within the natural hip socket. The liner is not secured into the acetabular shell but loose-fitted so it can glide smoothly between the ball and the shell.
Cemented Fixation
Cemented fixation uses an acrylic polymer called polymethylmethacrylate (PMMA) to affix the prosthesis to the remaining bone surface.
When inserting a prosthesis into bone, both the bone and the cement must lock together in order to make the insertion last. Bone cement is not glue and it doesn’t stick to anything. It simply acts as a filler between the bone and the implant. In order to be dependable, it has to extrude into the honeycomb structure of the internal bone.
Cement is a durable and hard substance, much like metal. Bone, of course, is organic. It can be weak in small particles and easily fragmented. If the cement sits atop of the honeycomb inside of the bone, the pressure from the cement and/or the prosthesis can cause these somewhat delicate spicules to break off and die back, leaving a space between the bone and the cement. This is referred to as loosening.
In order to prevent this, a technique is employed whereby the cement is forced into the spaces between the bony spicules. When the cement sets three things result:
To do this, the surgeon will flush out all the fat, blood and bone debris from that honeycomb with a special surgical power washer called a pulse lavage. Then the area is thoroughly dried before the bone cement is inserted under pressure.
The cemented bond is generally very durable and reliable with a considerable and successful history of usage. A patient with a cemented total hip replacement can put his/her full bodyweight on the limb and walk without support almost immediately after surgery. Patients can usually walk without support within three to six weeks post-surgery.
Complications are relatively unusual. Most patients with cemented implants experience no problems post-surgery.
Cementless Fixation
Within the last several decades, new implant designs have been approved that allow attachment to the bone without cement. These implants are generally larger and longer than those affixed with cement.
Most cementless hip implants are textured or have a porous surface coating around much of the implant that allows new bone to grow into the surface of the implant. Because they depend on new bone growth for stability, cementless implants require a longer healing time than cemented replacements (generally about eight weeks before walking without support for some surgeons; others may permit immediate weight bearing) and are not recommended for those patients with osteoporosis.
How do you determine the kind of implant that is best for you? Talk with your surgeon about which implant will work best for your body and lifestyle. He or she will take those factors into account as well as his or her experience and preference with a particular implant design. The final choice of implant will be based upon health factors specific to you as well as your surgeon’s experience, education and expertise with the specific manufacturer’s products. Be sure to talk with your surgeon about which implant type he or she intends to use in your hip replacement and the reasons for the specific implant chosen.
Although the decision is ultimately your surgeon’s, you are well-advised to have a working understanding of the available options in order to understand the recommendations your surgeon will make, as well as the reasons for your recovery and discharge instructions.
There are a large number of hip implant devices on the market. Each manufacturer has different models but each style falls into one of four basic material categories:
These category names reference the materials used for the implant bearings. The stem and ball fit into and articulate against the cup or acetabulum. Each component can be made of one of several materials.
There is no consensus in the orthopedic community regarding the single “best” bearing or material. The choice generally comes down to your surgeon’s preference. Each surgeon has innumerable reasons for choosing one device over another, including his or her personal experience as well as tool and implantation method preferences.
Metal-on-plastic (polyethylene) is the longest tried and tested bearing. The convex femoral stem is constructed of metal (usually a cobalt chrome alloy) and the concave cup liner is made of a plastic called polyethylene.
This combination has been in use in various forms since some of the earliest hip replacements back in 1960 (when it was called the Low Friction Arthroplasty (LFA)). Some years later, the make-up of the polyethylene was improved. The current plastic used in hip replacement implants is referred to as Ultra Highly Cross-Linked PolyEthylene (UHXLPE) or Ultra High Molecular Weight PolyEthylene (UHMWPE), a very stable and reliable plastic material with greatly reduced risk for wear.
Because of its durability and performance, Metal-on-Polyethylene has been the leading artificial hip component material chosen by surgeons since hip replacement surgeries were first been performed. It is also the least expensive bearing.
All implants shed debris as they wear. Over time, the body may see polyethylene wear particles as invaders or a source of infection. As the body starts to attack them, this leads to osteolysis, a “dissolving of the bone”, which may result in having to replace the implant (known as revision).
As noted above, technological advances have reduced the risk of wear in Metal-on-Polyethylene implants. They wear at a rate of about 0.1 millimeters each year. The other materials, metal and ceramic, being more modern developments, already have high wear resistance built in.
Ceramic is the 21st century answer to hip replacement as it is both hard and durable, it wears minimally and the material is widely deemed to have no toxic or side effects in the human body. Hip implants can be constructed as ceramic-on-UHMPE or ceramic-on-ceramic.
If you are a very active individual or a relatively young patient, your surgeon may prescribe an all-ceramic hip joint. Ceramic-on-Ceramic is a good combination with longevity and reliability.
In these hip joints, the traditional metal ball and polyethylene liner are replaced by a high-strength ceramic bearing that has the reputation for ultra low wear performance. Clinical studies, monitored by the FDA and begun in 1998, have demonstrated excellent performance although it should be noted that ceramic has been used in hip replacements for many years prior to that.
All-ceramic hip joints have been used in Europe since the 1980s but have only more recently received the FDA’s approval for marketing in the United States.
There was a history of two issues with ceramic hips: catastrophic shattering and squeaking. Shattering was more of an issue in the 1980’s and 1990’s but the product has been substantially improved since then, essentially eradicating the shattering problem. Squeaking, however, remains a bit of a problem for a few patients. Often the noises abate over time but sometimes they don’t. If the squeaking is intolerable, a revision may be necessary.
Ceramic is the hardest implant material used in the body, and has the lowest wear rate of all, to almost immeasurable amounts (1000 times less than Metal-on-polyethylene, about 0.0001 millimeters each year). Consequently, there is usually no inflammation or bone loss, nor systemic distribution of wear products in the body. New ceramics offer improved strength and more versatile sizing options.
Ceramic-on-UHMWPE (Ultra High Molecular Weight PolyEthylene) is a good combination of two very reliable materials. Ceramic heads are harder than metal and are the most scratch-resistant implant material. The hard, ultra-smooth surface can greatly reduce the wear rate on the polyethylene bearing. The potential wear rate for this type of implant is less than Metal-on-Polyethylene.
Ceramic-on-Polyethylene is more expensive than Metal-on-Polyethylene, but less than Ceramic-on-Ceramic. In the past, there had been incidents of fractures in ceramic components, but newer, stronger ceramics have resulted in considerable reduction of fracture rates (0.01%) compared to the original, more brittle ceramics.
Some ceramic-on-polyethylene implants utilize a vitamin E-stabilized, highly crosslinked polyethylene bearing material. Vitamin E, a natural antioxidant, is expected to improve the longevity of the implant bearings used in total joint replacements. In laboratory testing, these liners have demonstrated 95-99% less wear than some other highly crosslinked polyethylene liners.
Ceramic-on-Polyethylene implants have a potential wear at a rate of about 0.05 millimeters each year, i.e. 50% less than Metal-on-Polyethylene. The newer, highly crosslinked polyethylene liners have shown potential wear rates as little as 0.01 millimeters each year.
Only a qualified orthopedic surgeon can determine which implant system is best for your unique situation. There are many factors your surgeon uses when recommending hip implants. Be sure to ask questions and share any concerns you might have regarding specific implants with your surgeon early in the process.
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