Special AHIMA Edition September 2013

It’s Hip to Be Repaired
By Carolyn Gutierrez
For The Record
Vol. 25 No. 13 P. 22

Advances in hip replacement surgery have allowed thousands to maintain an active lifestyle.

The Lancet dubbed total hip replacement surgery, which is performed on approximately 285,000 arthritis sufferers in the United States each year, “the operation of the century” in 2007. The procedure, which can be performed through various techniques, has been a pain reliever for many, but it also has stirred some controversy thanks to a recent implant recall.

A Brief History
Also known as total hip arthroplasty (THA), attempts at hip replacement surgery were made as far back as the 1890s, when German professor Themistocles Gluck formulated a replacement for the femoral head (the round top of the femur bone connecting to the hip) out of ivory. In the 1920s, Norwegian-born American surgeon Marius Smith-Petersen created a glass mold to fit over the femoral head. Because glass was prone to shattering, Smith-Peterson also experimented with celluloid, Bakelite, and Pyrex.

Along with English surgeon Philip Wiles, Smith-Petersen formulated early stainless steel components that, in 1938, resulted in the first THA fitted to the bone with bolts and screws. An early metal-on-metal hip prosthesis, which was modified and used in the 1950s by English surgeon George McKee, seemed to show promise until it was discovered that the steel implants shredded metal particles after years of use.

Considered to be the father of modern THA, English orthopedic surgeon Sir John Charnley refined the basic blueprint for future hip replacement surgery in the early 1960s by creating a long-lasting hip prosthesis made of metal, polyethylene, and acrylic bone cement. The revolutionary design streamlined the procedure and earned Charnley a knighthood.

The Joints Are (Not) Jumping
“The most common reason to need hip replacement is osteoarthritis, the wear and tear of the joint,” says Scott Ball, MD, an associate clinical professor and the chief of adult joint reconstruction at the University of California, San Diego School of Medicine. Also known as degenerative joint disease, osteoarthritis is localized in weight-bearing joints such as the hip and knee. As cartilage that cushions the joint wears down over time, severe pain and stiffness results. Generally occurring in people over the age of 50, this type of arthritis usually can be traced across generations.

Other types of arthritis can hasten the need for hip replacement over time. Rheumatoid arthritis, the most common type of inflammatory arthritis, is an autoimmune disease in which the synovial membrane (the connective tissue lining the joint) is chronically inflamed and thickened, leading to cartilage damage and persistent pain. Posttraumatic arthritis resulting from a hip fracture or injury, and avascular necrosis, in which the surface of the bone underlying the cartilage of the femoral head collapses because of a lack of blood, also cause disabling hip pain that may be relieved by THA.

Some patients may experience hip joint difficulties because of minor structural abnormalities they have had since childhood. Ball routinely performs hip replacement surgery for patients in their 40s who present with structural problems on the socket side of the hip joint. In these patients, “The socket is, by birth, just a little bit shallow,” he says. “It could also be that the head of the femur is not perfectly round—it’s a bit of an elliptical shape—so this can cause premature wearing of the joint. I always tell patients it’s kind of like having a car where the alignment is just a little bit off.”

Nonsurgical options such as anti-inflammatory medications, physical therapy, walking supports, and cortisone injections are explored first. Once these treatments have been exhausted, THA can be considered. The decision to have surgery is a highly individual one contingent on quality-of-life concerns. If persistent hip pain and immobility prevents a patient from engaging in work and recreational activities and also impedes his or her sleep, THA may provide a solution.

According to literature from the American Academy of Orthopaedic Surgeons, there are no age or weight restrictions on patients who elect to have total hip replacement. Although the average age of a hip replacement patient is 60, recommendations for the procedure are based on pain and disability, not age.

Ball’s patient population in San Diego is active, with athletically inclined patients ranging from skiers and hockey players to surfers and martial artists. “I’ve got people who do triathlons with artificial hips,” he says. “People can go back to pretty much whatever activity they want to go back to, so from that standpoint, the benefit of it is very clear. I’d say nine out of 10 patients are ecstatic and wish they had done it sooner. It’s gratifying because patients typically do amazingly well. That’s the obvious upside.”

Ball says potential problems following THA include dislocation (an approximate 1% risk), infection (0.5% risk), and blood vessel or nerve injury (0.5% risk), complications that usually require revision surgery. Other rare surgical risks include instability, aseptic loosening of the implant, a femur fracture (either shortly after surgery or years later), and leg length inequality. Generally speaking, these risks have declined dramatically over the past 50 years as THA technology has improved and surgical protocols have been established.

Surgical Approaches
A commonly used surgical technique for THA is the posterior approach, in which the hip is accessed through a large curved incision centered over the buttock. This approach, which is thought to provide better visualization of the hip, involves splitting muscle tissue. Another popular route, the anterolateral approach, involves a straight incision over the side of the hip and requires less cutting of muscle tissue than the posterior technique.

Currently receiving much attention is the minimally invasive direct anterior approach in which the surgeon makes one or two small incisions over the front of the upper thigh. Although there is less muscle disruption through this approach, visualization of the hip joint is limited, making it a more technically challenging procedure.

There is ongoing controversy within the orthopedic medical community as to which surgical method is the most beneficial. To date, studies have not found significant postoperative differences in dislocation rates and other complications or distinct advantages among any of the various approaches to THA. “Ultimately, the difference is where you are going to see your scar three months after surgery: the front or back of the body,” Ball says.

The advantages within each approach mainly lie in which method the surgeon is most adept. For a high level of proficiency, Ball recommends that patients look for a surgeon who performs at least 50 hip replacements per year, preferably in a hospital where about 500 of the surgeries are done annually. “The hospital experience really matters as well,” Ball says. “You want the patient to go to a relatively high-volume surgeon who works in a relatively high-volume hospital to ensure that they have good nursing care, therapy, and follow-up.”

Implants
Essentially, traditional THA involves removing the arthritic ball-and-socket hip joint and replacing it with biocompatible components made from metal-on-polyethylene, metal-on-metal, ceramic-on-ceramic, or various hybrid systems incorporating elements of all three.

 The implant generally is composed of three sections: the stem fitting into the top end of the femur; the ball, which acts as a replacement for the femoral head; and a cuplike hollow socket to replace the acetabulum, the cavity of the pelvis holding the joint in place. The implant components either are cemented into place using an acrylic polymer or “press fit” into the bone, a process in which new bone growth actually latches onto a special surface coating of the implant. A combination of cemented and cementless components also is commonly used.

For patients wanting to avoid a traditional total hip replacement, hip resurfacing may be an option. In this procedure, although the acetabulum is replaced, the top end of the femur is not removed, and the femoral head is encased with component material.

The implant’s artificial ball-and-socket joint is known as the bearing surface. Throughout the 50-year history of THA, formulating a bearing surface that perfectly withstands the pressure and wear of everyday movement without creating wear-and-tear particles over the long term remains a work in progress for orthopedic scientists.

Since the days of Charnley, implants made of metal-on-polyethylene have been the most commonly used and are considered to be safe and durable. More than 90% of metal-on-polyethylene implants last 10 years. The primary disadvantage is the possibility that the polyethylene, a high-performance plastic resin, will shed debris over time, possibly leading to osteolysis, an autoimmune response to the debris in which there is dissolution of the bone and aseptic loosening of the implant.

Ceramic-on-ceramic implants are considered to be beneficial because of having a hard surface, high wear resistance, low friction, and less debris particles. However, ceramic-on-ceramic implants are costly and require extreme proficiency when implanting. Also, early dislocation is possible if the insertion is not exact. In addition to those potential drawbacks, these implants have been known to produce a disconcerting squeaking sound in the hip.

Metal-on-Metal Implant Complications
According to Jeffrey Lozman, MD, an orthopedic surgeon at Capital Region Orthopaedics in Albany, New York, use of the metal-on-metal technique has steadily increased over the past 15 years until recently. Ball concurs, noting that “there are studies that show between 2005 and 2006, more than one-third of hip replacements in America were done with metal against metal.”

Concerned about debris from polyethylene components, surgeons have revisited the metal option with the belief that modern technologies have improved the alloys and the problematic designs that compromised them in the 1970s. Because metal is less brittle than polyethylene, scientists could formulate a larger and stronger femoral ball, creating enhanced stability and, in theory, decreasing the risk of dislocation.

All hip implants, regardless of their construction, shed varying amounts of debris over the short and long term. Although extremely durable, wear and corrosion on the metal-on-metal bearing results in elusive microscopic debris, or nanoparticles, that release metal ions such as cobalt and chromium that can enter the bloodstream. Over time, concentrations of these metal ions may cause inflammation in the joint as well as pain, tissue damage, and implant loosening. Eventually, revision surgery to replace the implant may be necessary.

A 2008 study of patients who had metal-on-metal hip resurfacing uncovered several cases of pseudotumors. Although it was a small finding—1% after five years—it is troubling because these tumors ultimately can compromise muscle and bone.

In 2010, DePuy Orthopaedics recalled Articular Surface Replacement (ASR) hip resurfacings and ASR XL total hip replacements after the UK Medicines and Healthcare Regulatory Products Agency issued safety alerts for ASR and all metal-on-metal implants. Two joint registries from Australia and the United Kingdom also had reported high failure rates for the ASR devices and all large-diameter metal-on-metal total hip replacements.

“What happened was that [studies] found an increased incidence of metal-on-metal wear and metal debris in and around the hip joint in an increasing percentage of patients,” Lozman says. “Now that we are keenly aware that this is occurring, we’re looking for it, and we’re seeing it more often than we intended on seeing it when [the implants] became popular over a decade ago.”

Experts believe that a design flaw in the hip “cup” has made the ASR components prone to what orthopedic surgeons call “runaway wear,” in which the implant’s normal wearing process is accelerated, resulting in even higher concentrations of metal ions.

The ASR systems were used in more than 93,000 patients worldwide, with approximately one-third of them in the United States. It was estimated that 40% would experience implant failure within five years. Perhaps as a result of the findings, the use of metal-on-metal hip components has decreased dramatically in the last two years.

The FDA recommends that patients with metal-on-metal implants who have no discernable symptoms follow up with their orthopedic surgeon every one to two years for monitoring. Patients experiencing pain or any other adverse reactions are advised to have physical examinations, X-rays, and blood tests to measure metal ion concentration. In some cases, MRI and ultrasound may be recommended.

“It’s extremely unfortunate, this whole thing with the ASR, because a lot of patients have been hurt by it,” says Ball, a specialist in metal-related implant research who routinely performs revision surgery. “Fortunately, the ASR represents a very small percentage of hip replacements that have been performed. [Nevertheless], I feel like it’s overshadowed the benefits of a hip replacement a little bit.”

Gender Differences
Analyzing data from a cohort of more than 35,000 patients who have undergone THA, researchers found that women have a 29% higher risk of implant failure than men, regardless of type of implant.

The team, led by Maria C. S. Inacio, MS, an epidemiologist with the Southern California Permanente Medical Group in San Diego, studied patients registered in the largest total joint replacement registry in the United States between 2001 and 2010. The project, which analyzed patient outcomes during a three-year window following THA, focused on failure rates and failure types based on gender.

“In a very comprehensive analysis of our registry data, we found that women—regardless of adjusting for several factors, including comorbidities, implant characteristics, and surgical techniques—were at a higher risk of failure at the follow-up times,” Inacio says.

Because the study is ongoing, the researchers can only speculate that anatomical differences may be the main reason for the disparity between the sexes. Larger femoral heads, which are less likely to cause dislocation, are more commonly used in male hip replacement patients. Researchers also surmise that the difference between men’s and women’s bone quality may somehow affect the body’s interaction with the implant.

“The FDA has advised [that] medical device research be done in both women and men, so I think the study highlights the importance of conducting a study that takes women into consideration, as there are differences in how medical devices perform in this subgroup,” Inacio says. “Obviously gender is a nonmodifiable risk factor, so I think it’s very important for physicians to be aware of how they choose implants for women.”

— Carolyn Gutierrez is a freelance writer in New York City.