Hip Resurfacing – The Pros and Cons

Hip Resurfacing Overview

Three Recent Important
Articles on Hip Resurfacing

Hip resurfacing remains a controversial topic among hip surgeons. While used commonly in some countries in Europe and Australia, other countries such as the United States have been slow to follow suit, largely because of catastrophic results with the earliest designs in the 1970’s and restrictions placed on its use by the FDA. However, two hip resurfacing devices have been approved by the FDA for use in the United States. On May 9, 2006, the Birmingham Hip Resurfacing System (BHR) sold by Smith & Nephew Orthopaedics, and on July 3, 2007 the Cormet sold by Stryker/Corin were approved for use in the U.S. Other companies’ resurfacing devices are sure to follow. But, what is a hip resurfacing anyway, and how does it differ from a total hip replacement?

Hip resurfacing is a type of hip replacement that involves resurfacing the femoral head (hip ball) with a smooth, spherical metal cap (femoral component), similar to the way in which a dentist caps a tooth. If the acetabulum (hip socket) is in good condition, only the ball is resurfaced, and the socket is left alone. More commonly, when the socket is worn, it too must be resurfaced with a smooth, artificial metal socket (acetabular component). This contrasts with the surgical technique used in conventional total hip replacement which removes the worn out ball and replaces it with an artificial ball attached to a stem anchored inside the hollow femur (thigh bone). The artificial socket for both total hip replacement and resurfacing replacement are very similar, although resurfacing sockets are always made of metal while total hip replacement sockets may use metal, ceramic, or plastic (polyethylene).

So what are the advantages and disadvantages of a resurfacing replacement vs. a total hip replacement?

Advantages:

1. Bone Preservation

Bone preservation is often noted as one of the main advantages of hip resurfacing. Because the femoral head is resurfaced rather than removed, some of the ball is left intact, and the femoral canal is left untouched for the most part, thus preserving femoral bone. While conserving bone on the femoral side, some resurfacing hip sockets require that a bit more bone be removed from the acetabulum than in a conventional total hip replacement to allow the resurfaced femoral head to fit within the socket. Recently, manufacturing companies have begun addressing this issue by producing thinner acetabular shells for their resurfacing prostheses. Most important, proponents of hip resurfacing stress that because the femoral head has not been removed and the femoral canal has been left largely untouched, when it comes time to revise the prosthesis, it can be done very easily because of minimal bone loss.

2. Less Stress Shielding

“Stress shielding” refers to bone loss (osteoporosis) caused by lack of use of certain parts of the body’s skeleton. The resurfaced hip replacement more closely approximates a natural hip because the upper femur is largely left intact and therefore sees stresses that more closely resemble those in a normal hip than does the femur after a total hip replacement. Many advocates of resurfacing say that bone density at the head/neck junction (just at the base of the hip ball) is maintained and, in many cases, increased with use of the hip, although not all studies agree. However, although the real answer may not be entirely clear, it stands to reason that the more naturally the hip is loaded, the more likely stress shielding will be minimized.

3.Low Dislocation Rate

Dislocation of a hip prosthesis is in part related to the size of the ball and socket used. The larger the ball and socket, the less likely the hip will dislocate. Because a resurfacing prosthesis mimics the size of the natural ball and socket, it is less likely to dislocate than the ball and socket used with many total hip replacements, which may be only 1/2 – 2/3 the size of the natural hip ball. However, with newer instrumentation for standard total hip replacement allowing more accurate placement of components, use of larger balls and sockets, and better soft tissue repair techniques (see Capsular Noose page), dislocation even after total hip replacements is becoming quite rare.

4. Ability to Resume Athletic Activities

Hip resurfacing is often uplifted as a panacea for athletes wanting to return to high-impact sports due to its metal on metal wear resistance and low dislocation rate. Some also assert that patients with resurfacings vs. total hip replacements claim they are better able to return to competitive sporting activities, although the literature is not clear on this point. However, contrary to what some might say, hip resurfacing is not without risks if subject to high-impact activities.

“Endorphin junkies are fooling themselves if they believe bionic parts–no matter how sturdy they seem–give them license to resume high-impact activities such as running, basketball, and singles tennis.” warns Richard Laskin, MD, chief of the Division of Arthroplasty, Hospital for Special Surgery.

“I don’t like them doing high-impact things,” says Ronan Treacy, an orthopedist who helped develop the Birmingham metal/metal resurfacing hip Replacement, “but the fact is, most people ignore their surgeon’s advice.”

At a time when total hip replacements totally relied on polyethylene sockets (which are much less wear-resistant than metal) and when dislocation was much more common with athletic activities, this theory made some sense. However, total hips now utilize metal-metal and ceramic-ceramic articulations which provide tremendous longevity even in young, active patients, and cementless fixation provides for survival of the standard total hip replacement which may outlive even young, active patients. In contrast, the currently available resurfacing head is cemented into place, so there is technically even more reason to avoid impact loading activities – because these activities tend to crack the cement and lead to earlier loosening. Whether or not the various resurfacing devices will actually hold up to repetitive high-impact exercise is unknown because of a lack of long-term data. What is known is that all cemented implants eventually loosen, so it stands to reason that subjecting any cemented implant to high-impact loading stresses will lead to earlier loosening and need for revision surgery.

Disadvantages:

1. Early Failures

The biggest disadvantage of hip resurfacings is that the early failure rate is much higher than that after total hip replacement. Although in the innovators’ hands, failures are less likely, the risk of a femoral neck fracture (the ball breaking off of the femur bone) in the first year is significant, as high as 3% or more. Fractures are often related to surgeon error, most commonly notching of the femoral neck or placing the artificial ball in a position which is too low (called “varus malpositioning.”). An additional factor may be disruption of the blood flow to the remaining femoral head, a condition known as avascular necrosis. A review of the published literature shows that the short term survival rates with resurfacing replacements vary from 75-100% while most well-designed total hip replacements have short and mid-term survival rates exceeding 95%, even in young, active patients.

2. Late Failures

Just as disconcerting are failures occurring more than 5 years after implantation. The BHR femoral head prosthesis must be cemented in place while the socket is cementless. All cemented components will eventually loosen. So given enough time, all cemented resurfacings, just like cemented total hips, will loosen and fail, guaranteeing a second trip to the operating room for revision surgery at some point in young, active patients. In contrast, many cementless total hip replacements may remain intact indefinitely. Therefore, using the currently available cemented resurfacing prosthesis in a young, active patient, although conserving bone for a future revision, ironically almost guarantees that a revision surgery will be necessary at some point due to loosening of the cement. In contrast, using an entirely cementless total hip replacement could potentially last a lifetime, even in a young, active patient.

3. Surgical Procedure

Resurfacing hip replacement requires a larger surgery with a more invasive surgical approach than conventional hip replacement because the femoral head must be preserved and moved out of the way to visualize the hip socket. Resurfacing incisions have historically been 8-12 inches in length, although recent advances in surgical technique have allowed them to be performed through more conservative approaches. In addition, resurfacing patients occasionally are required to be on restricted weight-bearing using crutches for 6 weeks post-operatively. In contrast, conventional total hip replacement can be done through a minimally invasive, muscle sparing, 3-4 inch incision, and patients are allowed to walk on their new hip fully weight-bearing immediately. Although bone preservation is attractive, minimizing muscle trauma and scarring is also important to allow a rapid recovery and minimize complications.

4. Metal/Metal Wear and Toxicity

Probably the most disconcerting yet still uncertain potential complication of any metal-on-metal replacement is metal ion toxicity. Conventional hip replacements can utilize a variety of bearing surface combinations including metal-metal, ceramic-ceramic, metal-polyethylene, and ceramic-polyethylene. Polyethylene may be either standard or highly crosslinked (a more durable form of polyethylene). Resurfacing prostheses today are only able to utilize a metal femoral cap paired with a metal acetabular socket, although eventually other bearing surfaces will likely become available.

Although the wear rate of metal against metal is extremely low, the metal particles generated often lead to elevated levels of cobalt and chromium ions in the bloodstream and body tissues. Additionally, all metal ions must be filtered and removed by the kidneys. The graph below illustrates the volume of particles generated by various bearing surfaces (from Greenwald & Garino, JBJS, 2001):

Although many experts fear a possible carcinogenic effect from these high metal ion levels, no long-term study has confirmed this potential risk. However, ridding the body of these ions may be a problem in a patient who has kidney problems, so patients with kidney disease or a history of kidney problems in the family should probably avoid a metal-metal hip replacement or resurfacing. Additionally, women of childbearing age should probably avoid metal/metal bearings because of potential toxicity to a fetus from the wear debris.

An additional type of metal toxicity known to occur around metal-metal replacements is a type of allergic reaction, called a “hypersensitivity reaction.” Local and systemic reactions to metal particles have been documented, most commonly an inflammatory reaction adjacent to the prosthesis that leads to bone loss called “osteolysis” (i.e. ”dissolving bone”). Osteolysis around the metal prosthesis will eventually cause weakening of the bone and loosening of the prosthesis or fracture of the bone.

5. Inability to Use with Deformity or AVN

Because resurfacing arthroplasty only caps the existing anatomy, it cannot be used in cases of deformity of the femoral head, such as with Perthes disease, Slipped Capital Epiphysis, or high grade Dysplasia of the hip. Additionally, its role in a condition known as Avascular Necrosis (AVN) is not well defined, and resurfacing often cannot be used when the area of blood loss to the femoral head is large. Many young patients develop arthritis due to one of these conditions. However, resurfacing arthroplasty should not be used in these conditions because it cannot restore the leg’s normal length or the mechanics of a deformed hip joint as can be done with total hip replacement.

Summary

Although metal-metal resurfacing hip replacements may be the right choice for many young, active individuals, it is not the panacea that many advertising campaigns have made it out to be. The resurfacing arthroplasty may be used by some surgeons to promote their practices, in spite of the fact that much is still unknown about the long-term results of this procedure. In fact, favorable studies exceeding 5 years are rare in the published literature. In contrast, favorable studies of total hip replacements up to 25 years and longer are available in the peer-reviewed literature. Additionally, as with total hip replacements, favorable results with one design of resurfacing arthroplasty do not guarantee favorable results with another brand of resurfacing device.

On the other hand, with further study and advances in technology, this type of replacement may hold much more promise in the future. Better metal alloys, manufacturing processes, surgical techniques, development of cementless femoral fixation, and use of alternative bearing-surface materials such as ceramics may ensure more durable, long-lasting results even in young, active patients, without the risk of early failures, loosening, metal ion toxicity, or other complications. So, just as with all other new technologies, Caveat Emptor (“Buyer Beware”).

For more information about resurfacing visit the Link Library.

Tweeter button Facebook button Youtube button