Total hip joint replacement enables hundreds of thousands of people to live fuller, more active lives. Using metal alloys, high-grade plastics and polymers, orthopaedic surgeons can replace a painful, fractured or disfunctional joint with a highly functional, long-lasting prosthesis. Over the past half-century, there have been many advances in the design, construction and implantation of artificial hip joints, resulting in a high percentage of successful long-term outcomes.
Implant design
The hip joint is called a ball-and-socket joint because the spherical head of the thighbone (femur) moves inside the cup-shaped hollow socket of the pelvis. To duplicate this action, a total hip replacement implant has three parts: the stem, which fits into the femur and provides stability; the ball, which replaces the spherical head of the femur and the cup, which replaces the worn-out hip socket. Each part comes in various sizes in order to accommodate various body sizes and types. In some designs, the stem and ball are one piece; other designs are modular, allowing for additional customization. Several manufacturers make hip implants. The brand used by your doctor or hospital depends on many factors, including your needs (based on your age, weight, bone quality, activity level and health), the doctor's experience and familiarity with the device, and the cost and performance record of the implant. These are issues you may wish to discuss with your doctor.
Implant construction
Today, the stem portions of most hip implants are made of strong but light-weight metal alloys, which come in different shapes and degrees of roughness. Other alloys or ceramic materials are used in making the ball portions, which are polished smooth to allow easy rotation within the prosthetic socket. The socket can be made of metal, ultrahigh 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:
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Implant insertion
During a total hip replacement procedure, the surgeon will take a number of measurements to ensure proper prosthesis selection, limb length and hip rotation. After making the incision, the surgeon works between the large hip muscles to gain access to the joint. The femur is pushed out of the socket, exposing the joint cavity. The deteriorated femoral head is removed and the socket is prepared by cleaning and enlarging with circular reamers of gradually increasing size. The new shell is implanted securely within the prepared hemispherical socket. The plastic inner portion of the implant is placed within the metal shell and fixed into place.
Next, the femur is prepared to receive the stem. The hollow center portion of the bone is cleaned and enlarged, creating a cavity that matches the shape of the implant stem. The top end of the femur is planed and smoothed so the stem can be inserted flush with the bone surface. If the ball is a separate piece, the proper size is selected and attached. Finally, the ball is seated within the cup so the joint is properly aligned and the incision is closed.
Hip replacements may be "cemented," "cementless" or "hybrid," depending on the type of fixation used to hold the implant in place. Although there are certain general guidelines, each case is individual and your surgeon will evaluate your situation carefully before making any decisions. Do not hesitate to ask which type of implant will be used in your situation and why that choice is appropriate for you.
Cemented hip replacements
Over the past 40 years, there have been many improvements in both the materials and the methods used to hold the femoral and socket components in place. Today, the most commonly used bone cement is an acrylic polymer. With this method, 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 successful track record, they are not ideal for everyone.
Cemented fixation relies on a stable interface between the prosthesis and the cement, as well as 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.
1. 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 is more often the case with patients who are very active or very heavy. In addition, the action of the metal ball against the polyethylene cup of the socket component may produce microscopically small debris. This debris can then trigger a biologic response that further contributes to loosening of the implant and sometime to loss of bone around the implant.
2. The debris particles from the metal device 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 socket and the femur, progressing from the edges of the implant.
The bond between cement and bone, however, is generally very durable and reliable. Cemented replacement, therefore, is more commonly recommended for patients over age 60, 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.
Replacement without cement
In the 1980s, new implants were designed that attach directly to bone without the use of cement. In general, these implants are larger and longer than those used with cement. They also have a surface 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, thus attaching it more firmly. Because they depend on new bone growth for stability, cementless implants require a longer healing time than cemented replacements.
The 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 1mm to 2 mm. For 6- to 12-weeks, it will be necessary to use crutches or a walker to give the bone time to attach itself to the implant. This cautionary measure 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 of the bone and the dense, hard spongy bone just below it to provide support. The socket component of a cementless 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. The pelvis is prepared for a cementless socket component using a process similar to those employed in a cemented procedure. The intimate contact between the component and bone is crucial to permit inward, stabilizing bone growth.
Initially, it was hoped that the cementless procedure 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. Wear, debris, and inflammation 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 surgeons are now using cementless devices for all patients, the cementless procedure is most often recommended for younger (under 50 years of age), more active patients and patients with good bone quality where the desired bone growth 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 replacements
A hybrid replacement has one component, usually the 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 cases, the socket is left intact and the head of the femur is replaced, using a component similar to those employed in a total hip replacement. Another option uses a half-surface device, made of a cobalt/chromium alloy. This device resembles a half circle and fits over the head of the femur, thus sparing the bone of the femoral head. It is fixed to the femur with cement around the femoral head and has a short stem that passes into the femoral neck.
Longevity and outcomes
Hip replacement operations are highly successful in relieving pain and restoring movement. However, the ongoing problems with wear and debris may eventually necessitate further surgery, including replacing the prosthesis. Men and patients who weight more than 165 pounds have higher rates of failure. The chance of a hip replacement lasting 20 years is about 80 percent.
The source for this information is the American Academy of Orthopaedic Surgeons.