Total hip replacement has become one of the most common and most successful operations performed in the United States, and perhaps the world. Over 200,000 hip replacements are performed on Americans each year, a number that may almost triple over the next couple of decades as baby boomers get older and their hips creakier.
With this popularity has come choice in everything from the type of incisions (minimally invasive vs. conventional) to the material used in the new hip (metal, polyethylene, ceramic) to how the parts of the new hip are attached to the leftover bone.
Recently, a modified version of total hip replacement has made a comeback, so there's another item on the hip-repair menu. Hip resurfacing preserves the neck and head of the thighbone, or femur. The operation is being promoted as an option for younger patients who might otherwise be told to hold off on getting the traditional hip replacement operation till they are in their mid-60s or older.
There are questions, though, about whether hip resurfacing is being oversold. And, as is often the case with new surgical procedures, large, well-designed randomized trials are lacking. Patients and doctors have only scraps of evidence to go on.
*Hip replacement: The femoral component replaces the neck and head of the thighbone (the femur) after they've been removed.
*Hip resurfacing: The neck and head of the femur are preserved. The femoral ball component fits over the femoral head.
The hip is a ball-and-socket joint, a design that allows for more range of motion than if it were a simple hinge. The ball -- the knobby top of the femur -- fits into the socket, the cup-shaped area in the pelvis called the acetabulum (a colorful word that comes from the Latin for vinegar cup). Muscles, ligaments and a rim of cartilage around the acetabulum, called the labrum, help keep the femoral head snugly in place.
In a total hip replacement operation, the surgeon saws off the end of the femur, including the neck and knobby head, and smoothes out the acetabulum. The replacement has two parts: the femoral component, which has a ball-shaped top and a stem that is inserted into the shaft of the femur, and the acetabular cup, which fits inside the curved space of the acetabulum.
The operation takes a couple of hours. Most patients get general anesthesia, but regional anesthesia is an option for some.
Hospital stays after this operation have shrunk to two or three days. Patients are encouraged to walk as soon as possible, although full recovery may take several months. Usually the joint's range of motion is diminished somewhat after a replacement, but that's offset by reduced pain and the return to normal movement for most everyday activities.
There's always room for improvement, and the minimally invasive approach has added a new wrinkle, but, by and large, surgeons have mastered the surgical aspects of hip replacement. The in-hospital mortality rates are now low -- less than 1 percent. And the postoperative period is ordinarily uneventful, if not without pain. Patients do have to be careful about avoiding movements that increase the chance of the new joint dislocating.
People sometimes have unrealistic expectations about what a new hip will mean. The goal is to regain functioning, not to start doing things that you were never able to before the operation.
One of the major challenges of hip replacement has been coming up with implanted components that don't loosen or degrade, so a second operation -- a "revision" -- isn't necessary. Making the replacement parts last as long as possible has become even more of a priority because of lengthening life spans and a shift toward people becoming candidates for joint replacement at a younger age. To that end, there's been a lot of experimentation (and a few fads) over the years with making implants out of different materials and affixing them in different ways.
Here are some of the current choices:
*Metal-on-metal. Both the femoral component and the acetabular cup are made out of a titanium- or cobalt-chromium-based alloy. As you might expect, metal-on-metal hip implants are durable.
One concern has been that normal wear and tear results in the release of microscopic metal particles that can, in some instances, cause inflammation and cysts that affect bone tissue. As a result, the implant loosens.
Tests have also shown that some people with metal-on-metal hips have elevated levels of chromium in their blood, but so far it hasn't been shown that those levels result in harm.
*Metal-on-plastic. The femoral component is made of metal, but the acetabular cup is made of a tough plastic like polyethylene. The polyethylene may have a metal backing that's affixed to the bone, but the load-bearing surface is the layer of polyethylene.
As with the metal-on-metal implants, wear and tear has been an issue. Friction releases tiny bits of plastic that can result in inflammation. The good news is, the polyethylene used in today's implants is much tougher than the material used in the past.
*Ceramic-on-ceramic. The head of the femoral component and the lining of the acetabular cup are made of a high-tech ceramic. The smoothness and hardness of the material reduces the amount of wear so the implants last longer. Newer ceramic materials have also resulted in fewer component fractures, a problem that plagued earlier generations of ceramic implants. There have been reports that a small percentage (7 percent in one study) of ceramic-on-ceramic hips produce an audible squeak. The squeaking may be caused by uneven wear.
*Cementless attachment. Traditionally, the components of artificial hips have been attached with an acrylic cement. The cement sets quickly, which hastens recovery, but over time, it may weaken or crack, so the implant loosens. Now an increasing number of replacements are done with cementless components. They have semi-porous surfaces that allow bone tissue to grow into the component. In theory, cementless components stay more firmly attached than cemented ones, but whether that's true over long periods is still unclear.
*Resurfacing resurfaces. Hip resurfacing is billed as a more conservative operation than a traditional hip replacement. Instead of removing the neck and head of the femur, the surgeon reshapes the head of the bone and puts a rounded metal cap on it. Hip resurfacing is in the midst of a revival after falling out of favor in the '70s. The big difference is that the acetabular component is now made of metal instead of polyethylene.
The components implanted in a total hip replacement tend to last between 15 and 20 years before they wear out or loosen. If the pain and disability aren't too bad, people are encouraged to wait until their 60s or early 70s to get a hip replacement, so another operation won't need to be done before they die.
Hip resurfacing might eliminate the waiting for some people: Surgeons who perform the operation say the ideal candidates for it are relatively young -- in their 50s or early 60s. One advantage of hip resurfacing is that if patients need a hip operation later on, the femur will be more intact, so a second surgery is likely to be more successful.