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Thu 20 Mar 2008 04:00 AM

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Exploring advances in hip replacement surgery

A look at recent developments in hip replacement, including minimally invasive surgery, hip resurfacing, and new materials.

A look at recent developments in hip replacement, including minimally invasive surgery, hip resurfacing, and new materials.

Hip replacement, or total hip arthroplasty, is one of the world's most common orthopeadic surgeries thanks to its remarkable success in relieving pain and restoring function to people with disabling osteoarthritis of the hip joint.

Over the past decade, implant manufacturers have taken many measures to make artificial hips last longer.

The procedure is often recommended for patients who suffer from severe degenerative joint disease that does not respond to conservative approaches, such as weight loss, non-steroidal anti-inflammatory drugs, or physical therapy. Other patients who may benefit from hip replacement include those with rheumatoid arthritis, hip fracture, bone tumors, and avascular necrosis of the femoral head.

More than a million joint replacements (mostly hip and knee) are performed each year worldwide and with an aging population, this number is expected to continue to grow. Many consider total hip replacement to be one of the most significant advances in orthopaedic surgery. It is also big business.

Device manufacturers are continually investing in new prosthesis designs and fabrication, bearing surfaces, and custom-made components. Meanwhile, surgeons have been exploring techniques involving smaller incisions in the hope of speeding patient recovery, minimizing pain, sparing muscle, and reducing blood loss. But how much do these new technologies and techniques really improve patient care?

In this "In Practice," Douglas K. Ayres, M.D., M.B.A., Vice Chief, Department of Orthopedic Surgery at Beth Israel Deaconess Medical Center in Boston, Massachusetts, weighs in on some of the recent developments, including minimally invasive surgery, hip resurfacing, and new materials.

The basics

In conventional total hip arthroplasty, a surgeon makes a 15 to 25 cm incision and cuts through the hip capsule and several overlying tendons to access the hip joint. The surgeon removes the head and much of the neck of the femur and replaces them with a femoral head and stem, and reams away the outer bony surface layer of the acetabulum, replacing it with an artificial socket attached to the pelvis.

Cement may be used to bond these porous metal alloy implants to the existing bone, or most often, the artificial parts may be made of a porous material that supports bone growth, which holds the parts in place.

Although many patients who undergo hip replacement surgery are between age 60 and 80, younger patients may benefit from the procedure if they suffer extreme pain, disability, and loss of function due to their injured or diseased joint.

Minimally invasive surgery (one-incision and two-incision)

Almost a decade ago, minimally invasive techniques were introduced that use specially designed instruments to insert hip prostheses through smaller incisions (usually measuring 10 cm or less in total), often using fluoroscopy. The goals of minimally invasive surgery (MIS) are to reduce soft tissue injury and blood loss, and to shorten post-op recovery, with results comparable to the excellent outcomes of conventional hip replacement.

So far, some orthopedic surgeons operating on select patients in high volume medical centers have shown results that suggest MIS can indeed lessen blood loss and provide faster rehabilitation. But whether or not the overall results are better than (or even comparable to) traditional hip replacement surgery remains a subject of much debate.

Ayres takes the middle ground. While MIS has been shown to improve recovery in the short term, these results have occurred primarily in studies involving higher-functioning, motivated, younger patients with relatively straightforward anatomy at high-volume centers, he says. Once you look beyond these selected parameters the number of undesirable outcomes often begins to rise.

Problems include nerve and artery injuries, wound healing problems, infection, fracture, prosthetic joint dislocation, leg length differences, and diminished ingrowth, which can be devastating. In many instances, the downside risks of MIS may outweigh its potential advantages.

As a result, Ayres says pure MIS was a wave that "seems to have crested a few years ago and has subsided somewhat. These days, many hip replacement surgeons have tailored their practice, so they do excellent muscle, capsule and tendon sparing surgeries through smaller incisions than were common just a few years ago," which are in the range of 10-12 cm (vs. 6-10 cm).
Ayres says that even some of the centers that aggressively promoted MIS hip replacements only use the MIS technique for a small portion of their total hip replacement surgeries, because many patients have confounding factors such as severe arthritis or hip dysplasia, are very significantly obese, require cemented hip prostheses, etc., that may prevent them from being good candidates for MIS.

"It is really only a select group of patients that are good candidates for MIS. For many other patients, the potential benefits may not justify the added risks."

Hip resurfacing

Hip resurfacing has emerged as a potential alternative to total hip replacement.

With advancements in technology, hip resurfacing has emerged as a potential alternative to conventional total hip replacement, especially for younger and more active patients. As in most total hip replacements, the surgery involves using a metal alloy femoral head and acetabular cup, but the femoral neck and part of the femoral head are preserved.

This conservation of healthy femoral bone makes it easier and less complicated to perform a revision hip replacement in the future when the resurfacing fails, compared with revising a conventional failed total hip replacement. But questions remain about the relative health benefits and risks.

"Hip resurfacing is not all that novel an idea, as variations on the theme have recurred over the years," says Ayres. Still, hip resurfacing has a track record that is relatively short and involves far fewer patients compared with total hip replacement.

There is also a higher fracture rate of the proximal femur, though Ayres points out that this result could be due to the fact that patients having hip resurfacing may tend to be more active than those with total hip replacements, and thus put more stress on the bone stock.

As newer generations of total hip replacement yield better outcomes with longer wearing components, the advantage of preserving bone for a future replacement starts to diminish, as there is less likelihood of needing a revision. Another advantage of hip resurfacing, joint stability and greater range of motion, is now approximated in total hip replacement.

"We now use larger prosthetic femoral heads with thinner bearing surfaces or metal-metal bearing surfaces than just a few years ago, so dislocations have been greatly minimized for both THR and for hip resurfacing," says Ayres.

In addition, metal-on metal and metal-on-ceramic implants increase stability, further narrowing the gap in both stability and range of motion.

Today's materials

Traditional hip replacement consisted of a hard metal (usually a cobalt-chromium alloy) femoral head that fits into a polyethylene socket liner. The primary drawback of this combination has been osteolysis, which results from wear on the plastic cup.

When polyethylene particles shed they trigger the local production of cytokines, enzymes, and other factors that weaken host bone. As a result, the artificial hip loosens and becomes painful, and the patient needs a revision.

Over the past decade, implant manufacturers have taken many measures to make artificial hips last longer. "Today, with the advent of more durable, highly cross-linked polyethylene, that wear has been markedly reduced," says Ayres.

And now metal-on-metal, ceramic-on-ceramic and ceramic-on-polyethylene alternatives can eliminate plastic altogether.

Ceramic components were initially so brittle as to increase the risk of catastrophic material breakage that resulted in higher-than-normal failure rates. But improvements in design and material have improved the durability and popularity of ceramic implants. Another downside, says Ayres, is that occasionally ceramic-on-ceramic implants can squeak as the patient walks.

Studies suggest that metal-on-metal hips have low wear rates, and concerns that associated elevations in serum levels of chromium might be carcinogenic have fortunately been unfounded.

The main drawback to both the metal-on-metal and ceramic implants is cost - they can be as much as 40-50% more expensive compared with polyethylene and cobalt chrome bearing surfaces, says Ayres. As a result, many hospitals put restrictions on maintaining inventories of these new materials.

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