The Hip Replacement Booklet

Everything You Need to Know, if You’re Considering Hip Replacement

By Bert J Thomas, MD

 Background Info on Hip Replacement

About Adult Hip Pain

Hip pain can make it very difficult to perform simple daily activities. It can take a person that is normally a "go getter" and reduce their activity to slowly walking from place to place. The pain is usually mother nature's way of saying, "there is a problem in this area." The pain could be caused from trauma, arthritis, or any other possible form of disease process.

Severe arthritis of the hip can cause constant pain and can make sitting or walking very difficult. Many improvements in hip surgery have enabled people to function with less pain and better mobility. There are two types of components used for joint replacement: One is held in place by cementing the component to the bone. The other has a porous metal or chemical coating that does not require cement. Both the condition of the bones and patient age are considered important factors when deciding which type of component will be better suited. Cementless components are most commonly used in younger, more active patients with abundant, hard bone.

Hip replacements have been done for more than 30 years in the United States. Replacements have been very successful in restoring movement and reducing pain. Both cemented and cementless components can loosen over time and surgery may be repeated for patients who need their first replacement removed.

Your main objective is to have your hip examined by an orthopaedic surgeon. An orthopaedic surgeon has specialized training in evaluating a possible problem. The process of evaluation is usually a simple office visit and an X-ray.

The modern Hip Replacement was invented in 1962 by Sir John Charnley, an orthopedic surgeon working in a small country hospital in England. His work has been one of the great triumphs of Twentieth Century surgery.

The term "arthritis" literally means inflammation of a joint, but is generally used to describe any condition in which there is damage to the cartilage. inflammation, if present, is in the synovium. The proportion of cartilage damage and synovial inflammation varies with type and stage of arthritis. Usually the pain early on is due to inflammation. In the later stages, when the cartilage is worn away, most of the pain comes from the mechanical friction of raw bones rubbing on each other.

The arthritic femoral head (i.e., the femoral head) is removed, and replaced with a metal ball. The ball has a metal stem which is anchored into the hollow space inside the femur bone with bone cement. The worn out socket is replaced with a plastic socket. The painful parts of the arthritic hip are thereby completely replaced with metal and plastic surfaces. The plastic socket has a very low frictional resistance, and a very low wear rate against the metal ball. Hip replacement was first performed in the United States around 1969.

Many hundreds of thousands of replacements have been performed in the U.S. since then.

Total hip replacement is successful around 98% of the time.

Newer Developments in Hip Replacement

The major long-term problems with hip replacements are wearing out of the socket, and loosening of the bond between the implant and the bone. In time the cement can crack, directly resulting in loosening. Secondly, the body reacts to minute fragments of cement and plastic, and attempts to remove them, but unfortunately the process also removes bone adjacent to the particles, leaving the bone structurally weakened. If the implant loosens, a second surgery may become necessary to reattach it. There has been much research into the loosening problem. It was widely believed that the solution was to eliminate the cement. This led to the development of the: Cementless Hip Replacement in which the surface of the metal parts is porous, and looks like coral. Bone can grow into the metal pores and bond the implant to the bone without the use of cement. There are many manufacturers and many brands of hip replacement implants. The AML Total Hip Replacement (manufactured by DePuy/Johnson & Johnson) is the most widely used cementless implant in the world, and has the longest track record (since 1978). Dr. Thomas uses the improved AML hip replacement known as the Prodigy. The long-term results with the AML Hip have so far been excellent, especially in people with good bone quality. Initially, the cementless hips were used in patients of all ages, but it was soon found that in people with soft bones (osteoporosis), the femur bone does not always bond to the porous metal.

Cement is still used with very soft bones, regardless of age. Bone quality can usually be determined from the hip x-ray, but, quite frequently a true assessment of bone quality can only be made at surgery.The final decision on the question of cement will be made in your best interest. Cement is rarely used on sockets nowadays.

Other Surgical Considerations During Hip Replacement

Bone grafts are occasionally needed to restore bone defects. If so, the bone may be obtained from the discarded femoral head, or from the pelvis, through a small separate incision. Occasionally it may be necessary to cut tendons in the groin (“Adductor Tenotomy”) if these tendons restrict hip motion. This is done through one or two separate half-inch incisions in the groin, and does not result in loss of function. It is possible to perform two hip replacements under the same anesthetic, and Dr. Thomas does do it in selected cases, but generally does not recommend it, since it greatly increases the risk of complications. If you need two hips replaced, a better course is to have the more painful hip replaced first, and to wait 12 weeks or more before undergoing the second operation.

Surgical Exposure of The Hip Joint

The Posterior Approach 

This approach is the one used by most surgeons. Small, unimportant tendons (short rotators) are detached to get to the hip joint, and re-attached later in the operation. Normal walking returns much sooner than with the antero-lateral approach, sometimes in less than six weeks.

The mini-incision hip replacement is an important recent development. It is used with the posterior approach. In the past the skin incision was ten or more inches long. With special new instruments, this approach is now possible through an incision as small as three inches in thin patients. In obese patients, the incision is less than half what it would otherwise have been.

A smaller incision means less blood-loss. There is also less trauma to the muscles and ligaments around the hip, so much less pain, and an even quicker return to normal walking. Few orthopedic surgeons have learned the posterior mini-incision. Fewer still can do a perfect hip replacement, with accurate leg length, through such a small incision. Dr. Thomas routinely uses the mini-incision posterior approach, or the two incision approach (see below).

The Antero-Lateral Approach

The second most commonly used. The chance of hip dislocation is thought to be less with this approach. However, there is a trade-off. About one third of the most important hip muscle (gluteus medius) is detached from the bone, and Later re-attached. This weakens it, leaving most patients with a limp, sometimes for up to a year.

In The Anterior Approach 

The whole operation is done through a single incision in the groin. The muscles are not cut, but are spread apart. The ligaments that hold the hip together still have to be cut. The procedure is done under x-rays. The operation is risky, even in the best hands. It is very difficult to line up the femur bone through this incision, and see it clearly. There is much room for error in the placement and sizing of the femoral component, as well as in getting the leg length right.

The Two-Incision Approach 

A more recent development. Two 3-inch incisions are made: one in the groin, as in the anterior approach, and one over the back of the hip. Muscles are not cut, but are spread apart, and are disturbed less than with any other approach. Normal walking returns sooner with this approach than with any other, sometimes in as little as two weeks. The operation is difficult, and very few surgeons have been trained to do it. Dr. Thomas has taken special cadaver training in the use of this approach.

Dr. Thomas performs the operation using a posterior approach with a mini-incision, or the two-incision approach. With either approach there is much less pain, less blood loss, and normal walking returns sooner.

Hip Implant Designs and Materials

Metal on Metal Vs Ceramic on Ceramic Hip Replacement

There are many hip implant designs available to the surgeon. There is no universal agreement as to which design is best. Each surgeon selects what he believes is best for the patient, or what he was trained to use, or in some cases, what his hospital forces him to use. Find out what implant your surgeon plans to use.

Each type of implant has unique surgical aspects and considerations which can only be learned by experience with many cases. Preferably, your surgeon should have had experience with hundreds of cases of the implant selected.

The basic design of the implant is similar regardless of brand.

The plastic socket (high-density polyethylene) is the hip implant’s weakest link. The plastic wears away at the rate of about one millimeter per year (about 1/40th of an inch), against a metal ball, giving the implant a life expectancy of 10 to 15 years.

Johnson & Johnson’s “Marathon.” polyethylene is one of several recently developed “cross-linked” polyethylenes that wear at a slower rate in the lab. It will take at least ten years of human usage before we will be sure that cross-linking is a true improvement.

Microscopic plastic particles are produced by daily wear, even with the cross-linked poly. They migrate between the implant and the bone. The body reacts to these “foreign particles” by producing enzymes which slowly dissolve bone. This may eventually result in loosening of the implant.

The metal parts of the implant are manufactured of Cobalt-chrome or Titanium. There is no agreement as to which is better. In some circumstances, each has advantages over the other. Cobalt-chrome has been used in the manufacture of orthopedic implants for 65 years, and is extremely well tolerated by the body. The AML stem is made of Cobalt- chrome. The socket is made of Titanium. In rare cases patients with metal allergies may have skin rashes, or chronic pain and swelling of the replaced joint which may be due to metal allergy. True rejection of the implant has never been reported. If you are allergic to any metals you need to let Dr. Thomas know.

A huge breakthrough came with the 2002 introduction to the USA of metal-on-metal hips. Both ball and socket are made of Cobalt-chrome. It is believed that these will never wear out. Patients are allowed greater freedom of activity than with plastic sockets. There is some concern that the long-term frictional release of cobalt or chrome ions from the joint may be harmful to the human body. So far no deleterious effects have been reported in over sixteen years of use and careful study in Europe.

Note that in about 30% of patients, the metal parts of the hip replacement may trigger airport security devices. Simply tell airport security you have a hip replacement.

An even newer development is ceramic-on-ceramic hips. The ball and socket are made of ceramic, which is not pottery, but the oxide of any metal, in this case, aluminum oxide. Wear is even less than with metal-on- metal surfaces, and there are no metallic ions to worry about. However, there is a 1 in 25,000 risk of the ceramic components fracturing. Re-operation may be required if either the ceramic ball or ceramic socket fractures.

The all-metal and all-ceramic implants are expensive, and many hospitals and insurance plans refuse to pay for them, despite their long-term advantages. Dr. Thomas recommends that patients consult the A.A.O.S. website to obtain more information regarding the options of metal-on- metal or ceramic-on-ceramic implants.

Choosing the Best Implant For You

The hip implant parts are expensive, and there are many competing brands. Many hospitals contract with suppliers for a volume discount, and then restrict the surgeon’s choice to the contracted brand. That product may not be the best vailable, or the best implant for you.

Worse yet, some hospitals carry cheap, bottom-of-the-line, “low demand implants” for older patients, often defined as being over 65. The new metal-on-metal, and ceramic–on-ceramic designs are more expensive, and hospitals often restrict their use, even for younger patients.

You should find out if your surgeon is restricted in his choices by his hospital. If so, find a good surgeon who works out of a hospital that leaves the implant choice to him. Alternately, offer to pay the hospital the difference for the more expensive implant. The difference could be as little as $1000 before the hospital’s mark-up, and worth every penny. Many older people these days are in good health, live very active lives, and expect to live well into their nineties. Clearly these are “high demand” patients who deserve a high quality hip replacement that will serve them well for the rest of their lives.

Choose the best surgeon, have him tell you exactly what implant he plans to install in your hip (brand name, manufacturer, and what the parts are made of), and then do some research to find out if it is what you want and need.

*Dr. Thomas’ hospitals do not restrict his selection of implants. He uses the only the best implants available, manufactured by the most respected company in the world, the Johnson and Johnson Company. His choices and decisions are based solely on what is best for you.

Leg Length After Hip Replacement

A leg that is too short or too long is the most common reason for a lawsuit after hip replacement. A difference in leg length is more than an inconvenience requiring costly and unsightly shoe lifts. It can cause a limp, weakness of the hip muscles and chronic back pain. The final length of the operated leg is determined by the level at which the “neck” of the femur is cut, the depth to which the socket is machined, and the size of the implants used.

Most surgeons try to make all these decisions before surgery, by measurements made on the hip x-rays. However, x-ray pictures are always magnified, by anywhere from 10% to 25%, compared to the real size of the bones. Most surgeons simply assume a magnification of 18% or so, as a “ball-park number”. In one study, the actual size of the
implants was incorrectly predicted before surgery 60% of the time (six out of every ten patients!).

There are other sources of error. The anatomical landmarks for determining the level of the “neck” cut are not exact. The depth to which the socket is machined varies, depending on the shape of the socket, and the hardness of the bone.

Accurate leg length measurement is difficult, even with an uncovered patient, lying face upwards. But during surgery the patient is covered with layers of sterile drapes, and the operation is commonly done with the patient lying on the side. The “bottom leg” is bent at the hip and knee, is completely covered, and can not be measured easily for comparison. Finally, sometimes the surgeon will deliberately lengthen the leg a little for stability.

For all the above reasons, the leg length can be off by a quarter inch or more, and still be within an acceptable standard of care.

* Dr. Thomas uses a very accurate method for measuring leg length during the operation.

The Correct Implant Size

Patients return to normal walking much faster if they can put all their weight on the operated leg, starting the day after surgery. Most surgeons do not permit full weight for six weeks with uncemented implants. This allows time for bone to grow into the implant until it is stable. However, if the fit is perfect, and the implant is totally stable at surgery, many surgeons allow full weight the next day.

If the implant is too large, the femur can fracture as it is driven down inside the bone, so the tendency is to under-size for safety. But, if the implant is very under-sized, the bone may fail to bond to it.

The correct implant size is therefore very important.

Most surgeons decide the size from hip x-rays taken before surgery, which is not very accurate, and make their final decision based on the “feel” of the instruments used to prepare the femur during surgery. In one study the size was incorrectly predicted from the x-rays 60% of the time (see “Leg Length” above).

There are twenty four stem sizes available with the Prodigy system, each slightly larger than the next. Dr. Thomas makes his final selection based on an x-ray taken in surgery, with a metal sizing rod placed inside the femur. Very few surgeons do this. With such accurate sizing the implant fits perfectly every time, and more than 95% of Dr. Thomas’ patients are allowed to bear full weight on the leg the day after surgery. However, if the bone is found to be very soft at surgery, and an uncemented implant is nonetheless selected because of your age, weight bearing may be restricted for six weeks.

Dr. Thomas takes an x-ray of the femur bone during surgery, with a sizing rod in place. This allows very accurate sizing of the non-cemented implant, and allows full weight on the operated leg, starting the day after surgery

Other Surgical Treatment Alternatives

There are other operations that can be useful in treating hip disease:

1. Hip fusion (arthrodesis)

Was frequently performed before the era of hip replacement. The hip ball is fused to the pelvis. This is a single- operation, permanent-cure for the painful hip. Lost hip motion is made up by extra movement of the knees and spine. You must have a normal spine, normal knees, and a normal opposite hip for arthrodesis to be even considered. Few people today will accept the inconvenience of a stiff hip joint. It is usually only offered to very young people whose work involves heavy manual labor.

2. Osteotomy of the thigh bone

May be an alternative for very young patients. The femur is cut and re-aligned to change the direction of forces across the arthritic hip. It takes three months for the cut bone to heal and the results are unpredictable and almost never permanent. The procedure is much more popular in Europe than in America.

3. Femoral Hemiarthroplasty (“half a hip replacement”)

Sometimes offered to younger patients, when the hip ball is damaged, but the socket cartilage is normal, such as in patients who have osteonecrosis . The socket is not replaced. The femur component is similar to that of a total hip replacement, but it has a large ball, sized to fill the socket. The metal ball moves directly against the socket cartilage, which can wear out and become painful, requiring a second operation to install an artificial socket. In general, Dr. Thomas does not recommend hemi-arthroplasty for hip disorders, other than for hip fractures in the elderly. These are usually displaced fractures of the neck of the femur (see figure below). The implant is almost always cemented for hip fractures, except in patients under 65 or so, depending again on bone quality.

* Everything in this booklet concerning total hip replacement (complications, postoperative course, short and long-term care, etc.) applies equally to femoral hemi-arthroplasty.

Because Dr. Thomas is an expert on the treatment of disorders of the hip joint, he also treats numerous hip fractures.

4. Surface Replacement of the Hip

Had a vogue in the early 80’s. An improved version is now making a come-back of sorts. Instead of removing the femoral head, it is shaved (like peeling an apple) and a metal cap is cemented onto it. In some versions the socket is left untouched as in the hemi-arthoplasty (above). In others, the socket is lined with a metal shell (metal-on –metal).

5. Pseudoarthrosis (Girdlestone)

Involves removing the femoral head and leaving the hip without any replacement. The procedure is sometimes used as a last resort treatment for persistently infected hip replacements, or when the bone is totally inadequate for further reconstruction after multiple failed hip replacements. It leaves the patient with a short leg and an unstable hip and the need to use two crutches permanently.

6. Core Biopsy

involves removing a core of bone about one quarter-inch in diameter from the femoral head using a coring device. It is used in the earliest stages of osteonecrosis in the hope that it will allow the blood supply to return to the femoral head. Some doctors report 85% success rate with this procedure, but generally the results are much less optimistic. Because there is a danger of fracturing the weakened bone, patients have to be on crutches for six weeks. If the procedure is unsuccessful, you will almost certainly need a hip replacement.

7. Hip arthroscopy

Has a very limited role in the management of hip arthritis. It is occasionally used to help diagnose perplexing hip symptoms. It has no role for removing bone spurs or smoothing over the arthritic bone, as some surgeons claim.

8. Other “possibilities”

There are several other possibilities that patients frequently ask about. These include:

1. Is it possible to restore the cartilage to the joint? It is now possible to implant new cartilage cells in a young knee with minimal, localized damage. It is not applicable to the hip.

2. Does “robot surgery” improve the outcome? Recently the media has focused on attempts at “robot” surgery: little more than a milling machine, used to do a small part (about 10%) of the operation. It prolongs the procedure and has not been shown to be superior to conventional surgery. “Navigation”, i.e. computer guided surgery, somewhat akin to GPS missile guidance, may revolutionize hip replacement in the future.

3. Are custom implants better than standard implants? Custom implants are extremely expensive because each is manufactured specifically for one patient. They are rarely needed, and add very little to the ultimate success of the operation.

Currently, the biggest problems associated with hip and knee replacement, are with the materials used in the manufacture of the implants, and not with the surgical technique.