Computer-guided surgery

Computer-guided surgery is the use of a computer to aid in the planning and execution of an operation.

In the knee, this technology can be used in:

Total knee replacement

Unicompartmental knee replacement

Knee osteotomy

ACL reconstruction

Traditionally, these operations have used invasive guides, approximations, or surgeon judgment to determine how implants are inserted. Using computer-guided surgery, a surgeon can plan an operation more accurately, and execute this more precisely. This may result in better outcomes. Success in total knee arthroplasty is a function of very accurate positioning of the implants, and precise balancing of the soft tissues. Studies have suggested that when knee replacements are inserted as close as possible to an “ideal” alignment, these replacements are likely to last longer. Using a computer has been shown to reduce the number of cases which deviate greatly from this ideal alignment.

There are two types of computer-guided knee surgery – navigation and patient-specific guides (PSI).


Navigation is used during an operation to map the anatomy of the knee, and to then use landmarks within the knee as a reference point for the operation. It is highly accurate to around 1mm and 1 degree.

Typically, pins are inserted into the femur (thighbone) and tibia (shinbone). To these pins, arrays are attached which have spheres that are visible to a 3D infrared camera that is in the operating room. These provide a fixed reference point on each bone. The surgeon then uses a device to map landmarks around the knee. This data is then used to build a model of the patient’s knee within the computer. The surgical tools are visible to the computer, and their position is seen on the screen along with the ideal planned position. The surgeon then matches these positions, and the surgery is performed.

Navigation allows instantaneous feedback to the surgeon, as well as multiple measurements, allowing excellent control of the surgery. It is very accurate. However it takes a little longer than conventional surgery, and requires fixation pins outside the field of the main surgery. These have a small risk of infection, and a very small risk of fracture of the bone.


Patient-specific guides involve acquiring imaging data several weeks prior to the surgery, and then building a model of the knee within a computer. This information is acquired using an MRI scan, and a long-leg standing x-ray. The surgeon then uses this anatomical model of the actual patient to plan the position of the implants prior to surgery. This allows assessment of sizing, precise positioning of the implants, and also gives indication of some of the specific problems that the surgeon may face. This might include how to deal with severe deformity, sizing of smaller or larger patients, and alignment issues.

Once the surgeon has finalised the plan for surgery, nylon guides are 3D printed. The guides are then sterilised, and shipped in time for the operation. The guides are made to fit only that specific knee. These are designed to be fitted to the knee during surgery and pinned in place. These guides then have pre-made slots for saw cuts or for guide pins, which the surgeon uses to perform the knee replacement. During the surgery the surgeon verifies the cutting positions of the guide with landmarks within the knee, to ensure that correct cuts are made. This is a critical step in insuring precise surgery.

The process from imaging to surgery takes around 5-6 weeks.


Both technologies should be considered a tool to deliver the implants to their ideal positions. Each has their advantages and disadvantages. However, both technologies have been shown to be more accurate than standard instrumented knee replacement. They also have the advantage of avoiding instrumenting the femoral and tibial canals. Instrumentation of the canals have been shown to increase bleeding. It also offers the surgeon the ability to perform a slightly smaller exposure, leading to decreased pain.

The Australian National Joint Registry, which tracks all of the knee replacements performed in Australia, has recently shown that using computer navigation leads to improved survival of total knee replacements.


In my practice I routinely use PSI for total knee replacement, and unicompartmental knee replacement. I use navigation for osteotomy cases, and have previously used it for ACL reconstructions.

PSI offers the option to plan surgery well in advance, and to identify problems and solve them pre-operatively. It also allows smaller exposures, leading to less tissue disruption and pain. It avoids the problem of instrumenting the bony canals, decreasing bleeding, pain, and inflammation. In my hands, it has been highly accurate in delivering precise cuts which match the planning. However, there is the disadvantage of the 5-6 week lag time to surgery while the guides are planned and produced. PSI cannot be used when there is metallic hardware from previous surgery near the knee joint, or if the patient is unable to have an MRI scan for any reason.

Navigations offers the benefit of real time feedback, and the ability to accurately verify cuts at various stages of the operation. It feeds back a wealth of information including on alignment, tracking, and soft tissue balance. However it requires pins to be inserted outside the field of the main surgery, and is associated with increased operative time. I use navigation in patients who are unable to undergo MRI scanning, or are unable to wait for surgery.


No. Both systems are absorbed into the cost of the prosthesis. This cost is paid by the private health fund or public hospital. In addition, the scans used in MRI planning are also bulk billed at the radiology providers who are accredited to perform them.


Very much the opposite. The surgeon is critical in optimising the position of the implant, and balancing soft tissue tension. The surgeon is actively involved in planning the surgery pre-operatively. The exposure of the knee requires careful soft tissue management to reduce pain and swelling. While the guides and navigation are accurate, they should always be verified against multiple anatomical landmarks during various stages of the operation. Finally, the soft tissues must be accurately balanced with ligament releases.

How the guides are made

In this video Andrew Porteous, my fellowship mentor, demonstrates a total knee replacement using PSI technology

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