Spokane orthopedic surgeon Dr. Craig Bone has begun using a computer-assisted navigation system that helps surgeons position prosthetic knees more accurately during knee-replacement surgeries. Use of the system, which includes significant changes in how the surgery is done, can increase the life span of prosthetic knees and speed the recovery time for patients who undergo knee replacements, says Bone.

Bone began using the system at Holy Family Hospital in June and so far has employed it in 15 knee-replacement procedures there. The system, called eNact, has been around for about 10 years, but has been slow to catch on, partly because Medicare doesn't reimburse for the full price of the procedure, he says. The system is made by Kalamazoo, Mich.-based Stryker Corp.

Bone is leasing the system he uses from a Seattle-area physician, and Stryker's representative here, Tom Tabbert, says Bone is the first surgeon in the Spokane market to use it. Tabbert says several of the systems are in use by surgeons in Seattle, and one is in use in the Tri-Cities and two in Western Montana.

eNact includes computer software and wireless instruments that enable a surgeon to create a three-dimensional computer model of a patient's knee and surrounding bone structure during a surgery.

Tabbert says the computer model is accurate within one-quarter of a millimeter. The surgeon can view the real-time graphic display on a screen throughout the surgery to determine the depth and angle of cuts that need to be made into the patient's bones, and the alignment of the prosthetic pieces emplaced in a total knee replacement.

In a total knee replacement, a surgeon shaves off the bottom end of the femur and the top end of the tibia where they meet at the knee, inserts metal prosthetic covers over the bottom of the femur and the top of the tibia, then fits a plastic joint into the space between the two metal pieces. The surgeon also replaces the undersurface of the kneecap, which glides over the front of the knee.

While that part of the surgery doesn't change with the use of eNact, what does change is the process of aligning the prosthetic, which eNact does in a less invasive way, says Bone.

Bone says that although total knee-replacement prosthetics have become more sophisticated over the years, surgeons still use a method of aligning the prosthetics that dates back to the early days of the procedure, in the 1960s. That method consists of inserting a metal rod several inches into the center of the patient's femur to gauge the angle for placement of the knee. He says that most surgeons then use the position of the rod to "eyeball" the alignment of the bottom prosthetic piece that will be attached to the top of the tibia. Sometimes a rod also is inserted into the tibia to help in the visual alignment.

Using eNact, it's no longer necessary to drill a hole into the center of the femur and drive a metal rod into it. Instead, a surgeon drills a tiny hole into the outside surface of both the femur and tibia and threads into each hole the slender post of a wireless device that resembles a high-tech compass, called a tracker.

The trackers transmit signals wirelessly to a nearby computer, which uses the data sent by those signals to create a three-dimensional electronic model of the patient's leg and joint system. With the trackers in place, the surgeon rotates the patient's entire leg and bends the leg at the knee to establish the range of motion and location of the hip socket, Bone says.

With the eNact system, the surgeon supplements that data by using a handheld wireless wand to identify the location and surface anatomy of other key points along the patient's leg, touching the wand at various points and pulling a trigger on the wand to save the data. Among those points are the bottom of the femur, the top of the tibia, the hip, and the ankle.

Once the three-dimensional model is generated from that data, the computer screen displays, in real time, the angles of the bones as the doctor manipulates or adjusts the position of the leg, helping the surgeon to make decisions about the angle and depth of the cuts he will make in the femur and tibia by calculating the depth and angle of the cuts needed on the ends of each of the two bones to align the patient's prosthetic knee components optimally.

The goal of any knee-replacement surgery, Bone says, is to get the prosthetic lined up within 3 degrees to 5 degrees of the patient's natural leg alignment. He asserts that surgeons get their patients' prosthetics aligned within that ideal range only about 70 percent to 75 percent of the time using conventional surgical methods. Bone claims that the accuracy of alignment rises to 98 percent with eNact.

A less-than-ideal alignment shortens the lifespan of the artificial knee because the pieces rub against one another more and wear out faster, he says.

"If you're out of the patient's alignment by more than 5 percent, it's 11 times more likely to fail within 15 years," Bone says. If the prosthetic knee is aligned well, it can last between 18 and 25 years, depending on the person's level of activity, he says.

Bone says problems with the alignment of a knee often don't become apparent right away.

"It's like the tires on a car," he says. "If your tires are out of alignment, the first 5,000 or 6,000 miles you don't notice," but eventually the tread wears out too quickly in one spot.

Another drawback of the conventional surgical methods is that the insertion of the metal rod into the femur disrupts the marrow in the center of the bone, which can lead to added blood loss and longer recovery time for the patient, Bone says.

To install the prosthetic, the surgeon attaches cutting jigs to the ends of the femur and tibia. The devices help the surgeon to make accurate cuts by using information provided by the computer model.

Also, because a surgeon doesn't have to see as much of a patient's bone when making cuts with the eNact system, a smaller incision can be used during the surgery, shortening recovery time, Bone says.

The eNact technology offers so many benefits that Bone says he'd like to use it ultimately for all of the total knee-replacement surgeries he performs. The biggest limitation to achieving that, he says, is cost, because Medicare doesn't cover use of the computer navigation system, and thus many patients must pay between $500 and $700 extra out of pocket for surgery, on top of what they normally would pay. Bone says some private insurers will reimburse patients for the cost of using the navigation equipment. Hospital services for a typical knee replacement surgery can cost more than $23,000.

That cost factor, Bone says, is a main reason the technique has been slow to catch on here. The system costs $300,000, and hospitals and surgery centers here so far have been reluctant to invest in the expensive equipment, he says.

Bone, who has been an orthopedic surgeon since 2000, says he's been familiarizing himself with eNact for about four years, including training on the equipment over a period of 14 months at the Evergreen Orthopedic Research Lab, in Kirkland, Wash.