Chapter 122 Robotics in Total Knee Arthroplasty

Sabine Mai, Werner E. Siebert, Peter F. Heeckt

Degenerative joint disease of the knee is treated with total knee arthroplasty (TKA) after conservative therapy options have been exhausted. However, despite conscientious planning and carefully performed procedures, surgeons are often unsatisfied with implant alignment. Various studies have described significant axial or rotational malalignment, and mediolateral and ventrodorsal tilt.^2,^3,^15,²⁶ Seemingly small displacements of 2.5 mm potentially alter the range of motion by as much as 20 degrees.⁸ None of the contemporary improvements in implant design and instrumentation have alleviated these problems.

To improve precision in surgery, robotic systems were developed. Robots are able to position and move tools accurately, thereby reducing human error. These systems rely on preoperative imaging, registration, and planning. The first clinical use was reported in 1985 in the field of neurosurgery.¹⁶ In 1992, orthopedic surgeons started using robot systems for total hip arthroplasty.⁴ The active surgical robot CASPAR (computer-assisted surgical planning and robotics) was initially adapted for total hip arthroplasty and for anterior cruciate ligament repair.²⁰ Later it was developed for total knee arthroplasty. The first robot-assisted knee replacement with this system was performed in March 2000 at the Kassel Orthopedic Hospital (Kassel, Germany). A total of 108 consecutive procedures were performed at this institution and followed up for at least 5 years in a prospective study.²⁵

Surgical Technique

Robot-assisted TKA consists of the placement of fiducial markers, computed tomography (CT) scanning, preoperative planning, and surgery.

Placement of Fiducial Markers

To facilitate orientation, the robot requires placement of femoral and tibial pins that serve as fiducial markers for each bone (Fig. 122-1). The robot uses these pins for spatial orientation and performs geometric calculations based on their location.

Figure 122-1 Fastening of the special CT cross on the tibial pin.

Computed Tomography Scanning and Preoperative Planning

After the pins have been placed, a helical CT scan is obtained. Particular attention is paid to the areas of the femoral head, pins, knee, and ankle. The CT data are then transferred into the computer-based planning station. The technical quality of the scan is automatically checked and the pin position is verified. The surgeon identifies specific anatomic landmarks. The anatomic and mechanical axes of the femur and tibia are then calculated in the frontal and sagittal planes. The joint line, epicondylar twist, torsion of the tibia, and relationship of the dorsal part of the tibia and condylar line serve as additional important parameters.

The system allows the user to select and position a specific implant in different sizes. With computer-assisted planning, the strong interdependence of all parameters, including the mechanical axes, becomes evident. Implant fit can be accurately assessed by scrolling through the scan. Unintentional notching can easily be avoided. All angles and possible geometric translations are displayed on the video screen during the planning procedure (Fig. 122-2). The system informs the user about the expected change in extension and flexion gaps and the resulting ligament tension. After positioning the implants, the milling areas are specified in order to protect the surrounding soft tissue. As a last step, the system prints out an overview of the final plan. All data are stored on a PC card and transferred to the robot control unit before surgery.

Figure 122-2 Original screen shot from the planning station showing the PC-based planning of the femoral component and the resulting mechanical leg axis.

Robot-Assisted Surgery

A conventional median incision with parapatellar approach to the knee joint is used. The knee joint is secured by a transfemoral and transtibial self-cutting screw to a specially designed frame. This rigid frame is also used for fixation of self-holding soft tissue retractors. To control for unwanted micromovements of the leg during robotic surgery, rigid bodies with light-emitting diodes (LEDs) are firmly attached to the frame. The LED signal is constantly monitored by an infrared camera system, which will automatically shut off the robot in the event of excessive motion (Fig. 122-3). After registration of the fiducial markers, robotic milling is started by the surgeon. The cutting tool is equipped with internal water cooling and irrigation. A splash guard helps keep the operative field and LEDs dry and clean (Fig. 122-4). Milling heads are changed during the procedure, depending on the type of cut to be made. Varying with the size of the implant and bone density, the entire milling procedure takes approximately 18 minutes. If required, it is possible to revert to conventional manual technique at any point during surgery.

Figure 122-3 View of the working robot. Unwanted motion is detected by an infrared camera system, as seen in the background, and corresponding rigid bodies fixed to the frame, as seen in the foreground.

Figure 122-4 Knee securely fixed with cutting tool and splash guard in place just before femoral milling action commences. The tibial registration cross is still in place at the distal end of the incision.

The resulting bone surfaces are accurately shaped and smooth (Fig. 122-5). After the fixation frame and pins are removed, soft tissues are balanced and the components of the implant are inserted. In this study, we started with the cemented LC Search Evolution knee system (Aesculap, Tuttlingen, Germany) in the robotic group because this was the first knee implant system geometry that was loaded into the planning software.