Osteonecrosis, Part 2

What Your Doctor May Not Tell You About Hip and Knee Replacement Surgery
by Ronald P. Grelsamer, M.D.

(Page 3 of 6)

The “staging” of AVN. AVN exists in varying degrees of severity. A number of staging classifications have developed over time to assist in communication and treatment. For example, your doctor might indicate in his report that you have Stage III AVN, thus communicating to any orthopedist the extent of your condition. There are four commonly accepted stages for AVN of the hip:

In Stage I, the AVN is not visible on the X-ray but is already visible on an MRI.

In Stage II, the dead bone is now apparent on the X-ray, being whiter than the surrounding bone.

In Stage III, the bone no longer has a smooth, round, convex shape. It has collapsed at the edge-the so-called crescent sign on a hip X-ray.

In Stage IV, the bones on either side of the joint have come together as the cartilage has deteriorated. To say that someone has Stage IV AVN is tantamount to saying that they now have arthritis.

Other classifications featuring groups and subgroups have been devised, but the stages I've outlined above are considered classic.

The treatment of AVN. Since we don't always know where AVN comes from, it isn't always easy to come up with a rational plan, and because there are multiple causes of AVN, it would make sense for the medical community to come up with a treatment specific to each cause. It hasn't happened yet.

The normal femoral head is round. Note here how the bone has collapsed and the femoral head is no longer a smooth, round structure. Choices include using crutches to alleviate pain and to allow the bone to heal itself without collapsing, making holes in the bone (more on this shortly), bone grafting (bringing bone from elsewhere), applying electrical stimulation to the affected area, and replacing the joint.

The drilling of holes in the hip is called a core decompression since you take a core of bone out to decompress it. The concept of making holes in the bone comes from the observation that abnormal pressures build up in bones with AVN. This pressure causes pain and can contribute to cell death.

Drilling holes in the affected area lowers the pressure. It may also stimulate the bone to remove the dead bone and replace it with healthy bone. At least so goes the theory. Since new bone is soft bone and soft bone is more prone to collapse, do you really want a large area of fresh, soft bone? Orthopedists have been debating this for years. There is no risk of aggravating the AVN by performing a core decompression, and so it is a commonly accepted procedure.

But by making a hole in the shaft of the femur to gain access to the femoral head, the surgeon creates an area of weakness in the bone. This weakness predisposes you to a serious hip fracture should you take a misstep. Your surgeon will, therefore, have you walk with crutches for six to twelve weeks. Yes, back to the crutches. Cynics say that the core decompression is just a way of enforcing crutch walking that would otherwise be unacceptable to the patient.

Having made a hole in the bone, most surgeons are content to leave that hole unfilled. Another approach, though, is to fill the hole. Some surgeons have used a piece of fibula, the little bone that runs along the lower leg. But that piece of bone dies as soon as it is separated from its blood supply-that's what happens anytime you take a piece of bone out of someone.

Other investigators have, therefore, harvested the fibula along with the main blood vessels feeding it. The vessels are then connected to arteries about the hip joint, thus maintaining the fibular graft alive. This is a relatively complex and time-consuming operation, but at least there is now a live piece of bone inside the femoral head. The extent to which this technique represents an improvement over the straightforward core decompression remains to be determined.

Electromagnetic stimulation was tried in the 1980s. The theory was that coursing a specific electromagnetic field across the affected hip would stimulate the healthy bone to replace the dead tissue. This was not as far-fetched as it seems, since electromagnetic stimulation has been shown to assist in the treatment of hard-to-heal fractures. Some investigators reported success early on, but this success has not been duplicated.

When all else fails and the AVN has reached Stage III or IV, a hip or knee replacement can be contemplated. The question that comes up in orthopedic circles is whether both sides of the hip joint need to be replaced. In other words, in addition to removing the afflicted femoral head, should the surgeon remove the articular cartilage covering the acetabulum and replace it with a cup? Here are the two sides of the argument: Only the femoral head is diseased, so why mess with the healthy cartilage on the acetabular side? Why not simply implant a partial hip replacement, a so-called hemiarthroplasty, the literal translation of which is “half ” a replacement?

In a hemiarthroplasty, the femoral head is replaced with a metallic ball that matches the size of the bone that was just removed. This differs from a total hip replacement in two significant ways:

1. The acetabulum is left intact. No cup is implanted. The operation, therefore, takes less time than a total hip replacement.

2. The femoral head will usually measure somewhere between 44 and 54 millimeters as opposed to the 22- to-32-millimeter range found in a total hip replacement. The hip is, therefore, less likely to dislocate (pop out) after surgery.

The large, round metallic ball is the size of the femoral head that has been removed. This metallic ball is not fixed to the acetabulum (pelvis). Only the stem portion is fixed to bone.

On the other hand, if a partial replacement is implanted, the large metallic ball will rub against the cartilage of the acetabulum.

In a relatively young person, this type of repeated rubbing will eventually lead to the wearing out of the acetabular cartilage and to pain, pain that will require the surgeon to return into the hip to insert a cup. If this conversion from a partial to a total hip replacement takes place many, many years after the initial operation, the surgeon will have won his gamble, for the patient will have benefited from a pain-relieving procedure associated with a very low risk of dislocation. If the conversion needs to be done soon, however, it will be argued that the surgeon might as well have performed a total hip replacement right from the start! The problem, as you can see, is that no one knows in any given patient how long a partial replacement will last.

So there you have it. You can see how the arguments balance out. I've tended to favor the total hip replacement approach, but have on occasion opted for the partial replacement in patients at particular risk for dislocations.

Copyright © 2004 by Ronald Grelsamer, M.D.

About the Author

RONALD P. GRELSAMER, M.D., is the chief of hip and knee reconstruction at Maimonides Medical Center, and a staff orthopaedic surgeon at the NYU Hospital for Joint Diseases/Orthopaedic Institute.