Osteoarthritis is a form of arthritis that's caused by degeneration of the joint. In a healthy joint, the ends of the bones are covered with a smooth layer of tissue, called cartilage. The cartilage protects the ends of the bones and keeps them from rubbing together during movement.

As the cartilage breaks down or thins, the bones start to rub, causing pain, swelling and stiffness. The bones thicken and tiny deposits can form on the edges of the joint. Tiny pieces of bone may break off and remain inside the joint, causing even more pain.

Osteoarthritis is the most common form of arthritis. According to the Arthritis Foundation, it affects 27 million Americans. Risk increases with age. Excess weight (which can put stress on a joint), trauma, repetitive stress injury and participation in certain high-impact sports (like soccer, tennis and long-distance running) are also associated with an increased risk for the condition.

Knee Replacement for Osteoarthritis

In severe cases of knee osteoarthritis, doctors may recommend joint replacement. Traditionally, that's done by surgically removing the diseased joint and replacing it with a man-made joint, or prosthesis.

According to the American Academy of Orthopaedic Surgeons, about 300,000 knee replacements are performed in the U.S. each year. More than 90 percent of patients have a successful outcome (i.e., decreased pain and improved quality of life). However, there are some limitations in activity after a total knee replacement. Excessive activity can cause wear and tear that eventually leads to loosening of the joint and joint failure.

A Robot Guide

Another option for knee replacement for osteoarthritis is unicompartmental knee replacement, or partial knee replacement. In this procedure, only the damaged portion of the knee is removed. The section is then replaced with an implant that is matched in size and shape to the area that was removed.

Andrew Pearle, M.D., an Orthopaedic Surgeon with the Hospital for Special Surgery in New York City, says unicompartmental knee replacement is technically very difficult and takes a lot of skill to precisely place the implant. He now uses the MAKO Tactile Guidance System™ to aid him in planning, preparation and implantation of a partial knee implant. The technique is called, MAKOplasty®.

Prior to surgery, CT scans are taken of the knee. These scans are used to create a 3-dimensional model of the patient's knee joint. The model enables the surgeon to plan and practice the surgery on the computer and determine the location and depth of the cuts and precise placement of the implant for the best outcome.

At the time of the surgery, metal rods with reflective markers are inserted into the bones above and below the knee joint. Surgical instruments are attached to the MAKO Tactile Guidance system. During surgery, the system uses the markers to make an overlay of the CT scan. This enables a computer to make real-time comparisons of the scanned images, the patient's natural anatomy and the movement and cuts made by the surgeon.

Unlike other types of robotic surgery, in which the surgeon manipulates the robotic arms, in MAKOplasty the surgeon maintains full control over the procedure. The robot only serves as a "safety" guide. The target area shows up on the CT image colored in green. If excess bone is shaved or cut, the color turns red, indicating the surgeon is moving beyond the boundaries of the planned cuts. In addition, the system sounds a warning. If the warning is ignored, the robot sends a signal that makes the cutting instrument feel like it's hitting a brick wall. As a final safety check, the robotic arm will shut off the cutting device to prevent the surgeon from moving outside the cutting zone. The fail-safe mechanisms built into the system reduce the margin of error to about one-half a millimeter.

In addition to tracking the surgical progress, the system uses an infrared camera to track the position of the robot against the leg. This enables the physician to manipulate the leg during surgery and ensure proper alignment as the knee is bent.

Pearle says MAKOplasty enables him to make complex, yet precise cuts when planning and placing a partial knee implant. The extra precision also allows the surgery to be performed through a two- to three-inch incision, rather than an eight-inch cut normally made for total knee replacement. Patients are able to maintain more of their natural knee joint and don't face the same activity restrictions as those who have total knee replacement. In addition, the bones in the joint will still be healthy enough to undergo a total knee replacement if it is needed at some point in the future.

For information on MAKOplasty® or the MAKO Tactile Guidance System™, log onto the company's website at http://www.makosurgicalcorp.com.

For information on knee replacement:

American Academy of Orthopaedic Surgeons

The Knee Society

For information on knee arthritis:

Arthritis Foundation

National Institute of Arthritis and Musculoskeletal and Skin Diseases