How Tracking Arrays Work During Robotic Knee Replacement

Written by Dr. Cory Calendine, Orthopedic Surgeon | Bone and Joint Institute of Tennessee

Last updated: February 2026

If you're set for a robotic knee replacement, you may have heard your surgeon talk about "arrays." These small reflective devices are pinned to your bones during surgery. They give the robotic system a live, 3D view of your knee. Think of them as a GPS for your bones. Without arrays, the robotic arm can't know where your femur and tibia are in space -- and precision is the whole point of robotic knee surgery.

What Are Tracking Arrays in Robotic Knee Replacement?

Tracking arrays are small tools with reflective markers on them. During robotic knee replacement, your surgeon places two sets of bone pins -- one in the femur (thighbone) and one in the tibia (shinbone). The arrays clip onto these pins.

An infrared camera in the operating room reads the markers on each array nonstop. This creates a live 3D map of your leg's position and movement throughout the whole procedure.

The Three Types of Arrays Used

There are three arrays at work during a robotic knee replacement:

• Femoral array: This one attaches to the thighbone. It reports the exact spot of the femur to the computer in real time.
• Tibial array: This one sits on the shinbone. It does the same job for the tibia, tracking its spot and rotation during surgery.
• Instrument array: This one mounts on the robotic saw. It tells the system where the cutting tool is at every moment.

Together, these three arrays give the computer a complete, always-updating picture of the surgical field.

Why Tracking Arrays Matter for Your Knee Replacement

Here's what I tell my patients: the arrays are what connect the plan to the actual surgery. Before your procedure, we get a CT scan of your knee. That scan builds a detailed 3D model of your bone shape. We use that model to plan where to put your implant -- down to the millimeter -- in three planes: front to back, side to side, and rotational.

But a plan on a screen doesn't help unless we can match it to the real bone on the table. That's where the arrays come in.

Once the arrays are in place, the computer lays the 3D plan right over the live position of your bones. Every time we move your leg -- bending, straightening, rotating -- the arrays report that change right away. The system knows where every bone surface is at all times.

Real-Time Adjustments During Surgery

One of the biggest perks of array-based tracking is the ability to make changes on the fly during surgery. After placing the arrays, we check your ligament tension using a digital tool. This gives us hard numbers on how tight or loose the inner and outer sides of your knee are.

Studies show this tool gives very consistent readings between different users, with reliability scores above 0.90. That means the data is solid -- not a guess.

Based on those numbers, we can shift the implant position in real time to balance the soft tissues around your knee. The arrays track every change, making sure each one stays within a safe and accurate plan.

How Haptic Boundaries Protect Your Knee

The tracking arrays also power one of the most useful safety tools in robotic knee surgery: haptic boundary technology.

Once the plan is set and the arrays are tracking, the system builds a virtual cutting zone. This is a boundary shaped to your anatomy around the bone that needs to be cut. As the surgeon guides the robotic saw, the arrays keep checking the tool's position against the bone.

If the saw blade gets close to the edge of the planned zone, the robotic arm pushes back. It gives the surgeon physical resistance in real time. This stops over-cutting and shields the soft tissues around the knee, including the ligaments and tendons your new joint needs to work well.

Research shows this haptic approach causes less soft tissue damage than manual cutting with static metal blocks.

What Happens to the Arrays After Surgery?

This is a question I get a lot. The arrays and their bone pins are fully temporary. Once the implant is locked in and the final position is checked, everything comes out. The small pin holes in the bone heal on their own. They don't need any extra hardware or stitches.

The arrays leave nothing behind. They do one job -- guide the most precise robotic knee replacement possible -- and then they're done.

Do Tracking Arrays Actually Improve Outcomes?

The data backs up what we see in the operating room. Studies show that 3D CT-based planning with haptic boundaries helps surgeons place implants closer to the target compared to manual methods.

Patients who have robotic-assisted total knee replacement report less pain after surgery. They also need fewer pain pills compared to those who had a manual procedure. Research shows these patients hit recovery goals faster and go home sooner.

Long-term data tells the same story. The 2024 Australian Joint Replacement Registry found that six-year implant survival improved by 19% with robotic placement versus manual. When a kneecap component was added, that number jumped to 30%.

Those numbers matter. A knee that lasts longer means fewer repeat surgeries down the road.

What You Should Know Before Your Robotic Knee Replacement

If your surgeon uses a robotic-assisted platform for knee replacement, tracking arrays will be part of your procedure. Here are a few practical points to keep in mind:

• The arrays don't add significant time to the surgery. Pin placement and array registration take only a few minutes.
• You won't feel the pin sites after surgery. The small holes in the bone heal quickly and are not a source of postoperative pain.
• The CT scan is essential. Your preoperative CT creates the 3D plan that the arrays help execute. Don't skip this step.
• Not every patient needs robotic surgery. Your surgeon will help you decide if robotic-assisted knee replacement is the right fit based on your anatomy, condition, and goals.

Key Takeaways

• Tracking arrays are small, temporary tools pinned to the thighbone and shinbone during robotic knee replacement.
• They give the system live 3D bone tracking that links the CT plan to the real surgical field.
• Arrays power haptic boundary technology. This shields soft tissues and stops over-cutting.
• Studies show robotic knee replacement with array tracking gives better accuracy, less pain, faster recovery, and longer-lasting implants.
• All arrays and pins come out at the end of surgery. No hardware stays behind.

Medical Disclaimer: Surgical procedures carry risks and benefits that vary for each individual. This information is general in nature and should not be used as a substitute for consultation with a qualified surgeon. Always discuss your specific situation, risks, and expected outcomes with your healthcare team. Individual results may vary based on personal health circumstances.

Ready to Learn More About Robotic Knee Replacement?

Dr. Cory Calendine is a board-certified orthopedic surgeon specializing in hip and knee replacement at the Bone and Joint Institute of Tennessee in Franklin, near Nashville. If you're considering knee replacement and want to know whether robotic-assisted surgery is right for you, schedule a consultation to discuss your options.

About the Author

Dr. Cory Calendine is an orthopedic surgeon at the Bone and Joint Institute of Tennessee, part of Williamson Health. He focuses on total and partial knee replacement, hip replacement, and robotic joint surgery. He is based in Franklin, near Nashville, Tennessee.

Frequently Asked Questions

What are tracking arrays in robotic knee replacement?

Tracking arrays are small tools with reflective markers. They are pinned to the thighbone and shinbone during robotic knee surgery. An infrared camera reads the markers and tracks each bone's exact spot in 3D space. This helps the surgeon place the implant with great precision.

Do tracking arrays stay in the knee after surgery?

No. The arrays and their bone pins come out at the end of surgery. They are only used during the operation. No hardware is left in the body.

How do arrays improve the accuracy of robotic knee replacement?

Arrays give the system live 3D data about bone position. The computer uses this to lay the CT-based plan over the real anatomy. This lets the surgeon make bone cuts and place the implant with sub-millimeter accuracy.

Does robotic knee replacement with tracking arrays lead to better outcomes?

Studies show that robotic knee replacement with 3D tracking gives better accuracy, less pain after surgery, less need for pain pills, faster recovery, and better implant survival at six years compared to manual methods.

Is a CT scan required before robotic knee replacement?

Yes. A CT scan before surgery creates the 3D model of your knee. This model is the base of the surgical plan. The tracking arrays help carry out that plan during the operation.

References: Stryker Mako SmartRobotics clinical data; 2024 Australian Orthopaedic Association National Joint Replacement Registry; peer-reviewed studies on robotic-assisted total knee arthroplasty outcomes.