Painful arthritis of the knee can make it difficult to take even a short walk. Knee replacements can give osteoarthritis patients a new lease on life. But some patients, particularly younger ones, don't need the entire joint replaced, and they may benefit from a minimally invasive, partial knee replacement. Patients have fewer complications and faster recovery times following such surgeries, but they're tricky to perform and not routinely done. The robotic guidance system helps orthopedic surgeons create and execute detailed plans for this complex surgery.

The surgical system was developed by MAKO Surgical Corp., of Ft. Lauderdale, FL, and approved by the Food and Drug Administration in 2005. Based on preoperative CT scans, the system generates a 3-D model of a patient's knee, which surgeons use to determine how much bone to remove from the tibia and femur, and where to place the implants that replace inflammed parts of these bones. Instead of removing the bone with a burr saw and working by eye, the MAKO system lets the surgeon manipulate a robotic arm that is equipped with a burr. The surgeon is free to remove bone until she reaches the boundaries prescribed during the planning stages. Haptic feedback creates a wall of resistance if the surgeon ventures outside the planned area, and if she persists, an audio alarm will sound, and the saw will turn off. This helps the surgeon minimize the trauma to the knee and preserve the maximum amount of tissue.

Andrew Pearle, an orthopedic surgeon and founder of the computer-assisted surgery center at the Hospital for Special Surgery in New York City, has been performing partial knee replacements with the MAKO system since June. He says that it's possible to perform these surgeries manually, by eye, but it's very difficult. The MAKO system makes this surgery "more precise and reproducible," says Pearle, who has no financial interest in the company.

During surgery, the system carefully monitors the doctor's progress as she works with the robot. Instead of looking at the bone itself to determine whether enough has been removed, the doctor follows her progress on an LCD depicting the bone and the saw position. Bone that needs to be shaved off is shown in green; when too much has been removed, the area appears in red. This allows for accuracy within about two millimeters.

Crucially, the system also tracks the position of the leg throughout surgery. The leg cannot be immobilized during partial knee replacements because the surgeon needs to adjust and test it to make sure that she leaves the musculoskeletal system in proper alignment. Other robotics systems for orthopedic surgery, including one in use in Europe for hip replacements, are clamped directly onto the bone to make the cuts, so they don't face the problem of movement. Because the MAKO system is not fixed relative to the bone, it must remain oriented even when the patient's leg moves. An infrared camera tracks the position of the robot base and the position of reflective bone screws placed in the femur and tibia in preparation for surgery.