What is Hip Replacement Implant ?
Hip replacements are among the most common orthopedic procedures. When a hip replacement is performed, the arthritic, damaged hip joint is removed. The ball-and-socket hip joint is then replaced with an artificial implant. The materials used in the implant depend on several factors, including the age of the patient, the activity level of the patient, and the surgeon's preference.
Below are brief descriptions of some of the most commonly used hip replacement implants. Not all implants are options for all patients. These are general statements about the different implants; if you have specific questions about a particular implant you must discuss this with your doctor
Metal and Plastic Implant
The metal and plastic implants are the most commonly used hip replacement implants. Both the ball and the socket of the hip joint are replaced with a metal prosthesis, and a plastic spacer is placed in between.
The metals used include titanium, stainless steel, and cobalt chrome. The plastic is called polyethylene. The implant is secured to the bone by one of two methods; it is either press-fit or cemented into place. In the press-fit method, the implant is fit snuggly into the bone, and new bone forms around the implant to secure it in position. When an implant is cemented, a special bone cement is used to secure the prosthesis in position.
Metal-on-metal implants use similar materials, but there is no plastic piece inserted between. Metal-on-metal implants do not wear out as quickly as the metal and plastic materials. The metal and plastic implants wear at a rate of about 0.1 millimeters each year. Metal-on-metal implants wear at a rate of about 0.01 millimeters each year, about 10 times less than metal and plastic. Despite the low wear rates, it is not known that metal-on-metal implants will last longer. There are also concerns about the wear debris that is generated from the metal-on-metal implants. Metal ions are released into the blood, and these metal ions can be detected throughout the body. The concentration of these metal ions increases over time. There are no data to show that these metal ions lead to increased rates of cancer or disease, but no one knows for sure.
Ceramic-on-ceramic implants are designed to be the most resistant to wear of all available hip replacement implants. They wear even less than the metal-on-metal implants. Ceramics are more scratch resistant and smoother than any of these other implant materials. Unfortunately, there are also problems with ceramic implants. Again, there is no long-term data available on how well these implants work over time. Also there are concerns that these ceramic implants can break inside the body.
Metal and Highly Crosslinked Polyethylene
One of the more commonly used implants are new types of plastic that are designed to be more resistant to wearing out. These so-called highly crosslinked plastics are manufactured in a way that they wear out less quickly than the traditional plastics. These new implants have only been available for a few years, so whether or not they do work better than traditional plastic implants, we will not know for quite some time.
Cemented Total Hip Replacement
Over the past 40 years, there have been many improvements in both the materials and the methods used to hold the femoral and acetabular components in place. Today, the most commonly used bone cement is an acrylic polymer called polymethylmethacrylate (PMMA).
A patient with a cemented total hip replacement can put full weight on the limb and walk without support almost immediately after surgery, resulting in a faster rehabilitation. Although cemented implants have a long and distinguished track record of success, they are not ideal for everyone.
Cemented fixation relies on a stable interface between the prosthesis and the cement and a solid mechanical bond between the cement and the bone. Today's metal alloy stems rarely break, but they can occasionally loosen. Two processes, one mechanical and one biological, can contribute to loosening.
In the femoral component, cracks (fatigue fractures) in the cement that occur over time can cause the prosthetic stem to loosen and become unstable. This occurs more often with patients who are very active or very heavy. The action of the metal ball against the polyethylene cup of the acetabular component creates polyethylene wear debris. The cement or polyethylene debris particles generated can then trigger a biologic response that further contributes to loosening of the implant and sometime to loss of bone around the implant.
The microscopic debris particles are absorbed by cells around the joint and initiate an inflammatory response from the body, which tries to remove them. This inflammatory response can also cause cells to remove bits of bone around the implant, a condition called osteolysis. As the bone weakens, the instability increases. Bone loss can occur around both the acetabulum and the femur, progressing from the edges of the implant.
Despite these recognized failure mechanisms, the bond between cement and bone is generally very durable and reliable. Cemented total hip replacement is more commonly recommended for older patients, for patients with conditions such as rheumatoid arthritis, and for younger patients with compromised health or poor bone quality and density. These patients are less likely to put stresses on the cement that could lead to fatigue fractures.
Cementless Total Hip Replacement
In the 1980s, new implant designs were introduced to attach directly to bone without the use of cement. In general, these designs are larger and longer than those used with cement.
They also have a surface topography that is conducive to attracting new bone growth. Most are textured or have a surface coating around much of the implant so that the new bone actually grows 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.The orthopaedic surgeon must be very precise in preparing the femur for a cementless impact. The implant channel must match the shape of the implant itself very closely. New bone growth cannot bridge gaps larger than 1 mm to 2 mm.
Your surgeon may recommend a period of protected weight-bearing (using crutches or a walker) to give the bone time to attach itself to the implant. This protected weight bearing helps to ensure there is no movement between the implant and bone so a durable connection can be established. Cementless femoral components tend to be much larger at the top, with more of a wedge shape. This design enables the strong surface (cortex) of the bone and the dense, hard spongy (cancellous) bone just below it to provide support.
The acetabular component of a cementless total hip replacement also has a coated or textured surface to encourage bone growth into the surface. Depending on the design, these components may also use screws through the cup or spikes, pegs, or fins around the rim to help hold the implant in place until the new bone forms. Usually these components have a metal outer shell and a polyethylene liner.
Initially, it was hoped that cementless total hip replacement would eliminate the problem of bone resorption or stem loosening caused by cement failure. Although certain cementless stem designs have excellent long-term outcomes, cementless stems can loosen if a strong bond between bone and stem is not achieved.
Patients with large cementless stems may also experience a higher incidence of mild thigh pain. Likewise, polyethylene wear, particulate debris, and the resulting osteolysis (dissolution of bone) remain problems in both cemented and uncemented designs. Improvements in the wear characteristics of newer polyethylene and the advent of hard bearings (metal-on-metal or ceramic) may help resolve some of these problems in the future.
Although some orthopaedic surgeons are now using cementless devices for all patients, cementless total hip replacement is most often recommended for younger, more active patients and patients with good bone quality where bone ingrowth into the components can be predictably achieved. Individuals with juvenile inflammatory arthritis may also be candidates, even though the disease may restrict their activities.
Hybrid Total Hip Replacement
A hybrid total hip replacement has one component, usually the acetabular socket, inserted without cement, and the other component, usually the femoral stem, inserted with cement. This technique was introduced in the early 1980s, so long-term results are just now being measured. A hybrid hip takes advantage of the excellent track records of cementless hip sockets and cemented stems.
Partial Hip Replacements
If only one part of the joint is damaged or diseased, a partial hip replacement may be recommended. In most instances, the acetabulum is left intact and the head of the femur is replaced, using components similar to those used in a total hip replacement. The most common form of partial hip replacement is called a bipolar prosthesis.
A newer technique for hip replacement that has recently emerged is called hip resurfacing. In this procedure, the socket is replaced similar to a total hip replacement. The femur, however, is covered or "resurfaced" with a hemispherical component. This fits over the head of the femur and spares the bone of the femoral head and the femoral neck. It is fixed to the femur with cement around the femoral head and has a short stem that passes into the femoral neck.
Hip resurfacing is an emerging procedure, most commonly performed in younger patients. It is too early to assess the long-term success of this procedure.
What Type of Hip Replacement Implant is Best ?
There are many orthopedic manufacturing companies that produce different implants used in hip replacement surgery. Most of these companies make several different hip replacement prostheses.
Is there a way to determine which hip replacement implant is best?
Asking orthopedic surgeons what hip replacement is best is like going to a car show and asking people which car is best. You'll find lots of different answers, and each person thinks they've got the right one.
One of the most important aspects of hip replacement surgery is determining the proper implant to be used in the operation. However, people disagree on what criteria are most important to selecting the best hip replacement implant. Do you use the implant with the newest design, or the implant with the longest track record
Should a surgeon use implants suggested or asked for by their patients ?
Orthopedic supply companies have begun to advertise directly to consumers. The advertisements might make you believe that one implant has been shown to be better than others. This has been seen recently with ads for ceramic hip replacements.
The truth is that no one knows what the "best" implant is. The ideal hip replacement implant will allow for normal activities, normal motion, and last the patient's lifetime. The only way to know if these goals will be met is to use the implant in people doing normal activities, and follow their results for decades. Therefore, implants designed recently do not have long-term track records that prove their longevity.
Many people credit Sir John Charnley, a British orthopaedist, with performing the first modern total hip replacement. His innovations included combining a metal stem and ball with a plastic shell and using a methacrylate cement to hold the devices in place.
Today, the stem portions of most hip implants are made of titanium- or cobalt/chromium-based alloys. They come in different shapes and some have porous surfaces to allow for bone ingrowth.
Cobalt/chromium-based alloys or ceramic materials (aluminum oxide or zirconium oxide) are used in making the ball portions, which are polished smooth to allow easy rotation within the prosthetic socket. The acetabular socket can be made of metal, ultra-high molecular-weight polyethylene, or a combination of polyethylene backed by metal. All together, these components weigh between 14 and 18 ounces, depending on the size needed.
All the materials used in a total hip replacement have four characteristics in common : -
- They are biocompatible. They can function in the body without creating either a local or a systemic rejection response.
- They are resistant to corrosion, degradation, and wear. Therefore, they will retain their strength and shape for a long time. Resistance to wear is particularly significant in maintaining proper joint function and preventing the further destruction of bone caused by particulate debris generated as the implant parts move against each other.
- They have mechanical properties that duplicate the structures they are intended to replace. For example, they are strong enough to withstand weight-bearing loads, flexible enough to bear stress without breaking, and able to move smoothly against each other as required.
- They meet the highest standards. These high standards extend to fabrication and quality control at a reasonable cost.
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