Kinematic Alignment With Image-Based Robotic Instrumentation

Overview

Robotic assistance was recently reintroduced in total knee arthroplasty (TKA) to improve surgical precision and to minimize errors. However, the technique stills focuses on mechanical alignment (MA) philosophies, with minor adaptations. This chapter is meant to highlight the surgical technique of robotic assistance and its suitability for achieving true kinematic alignment (KA) in TKA and discusses its potential benefits over conventional or other computer-assisted techniques.

The first section gives a brief overview on current devices for robotic-assisted TKA. Differences between the systems and their underlying principles are discussed. Special focus is on the potential advantage of image-based concepts with three-dimensional KA planning.

The second section outlines a step-by-step algorithm for a KA preplan and discusses the major parameters of interest. How this plan can be adapted to individual anatomic variations is outlined.

In the third section, we describe a distinct intraoperative workflow for KA using image-based robotics. It is based on a hybrid approach, with a true measured resection philosophy for the femur to restore the native joint surfaces, and a gap-balanced tibial resection to restore a rectangular extension space. For illustration, three clinical cases are discussed in terms of implant alignment and balancing.

Introduction to Robotic-Assisted Technologies for Kinematically Aligned Total Knee Arthroplasty

Several technologies have been introduced over recent years that can be called robotic surgical assistance. However, there are marked differences in the methods for technically implementing the use of robotic platforms and in the principles relied on to execute the surgery. This results in varying workflows for KA TKA and different potential to overcome the known limitations of current technologies or manual KA instrumentation.

Motivation to use computer and robotic assistance for kinematically aligned total knee arthroplasty

There is growing evidence that computer assistance in general results in higher precision in TKA component position and reduces outliers, compared with manual instrumentation. ^, However, it has not been shown that this improved precision results in better patient outcomes in the context of MA. KA has the potential to improve patient outcomes compared with MA. Several studies have shown that complications and outcome are related to the precision of component position. ^, Higher failure rates have been reported when the sagittal profile (femoral flexion or tibia slope) is changed ^, and inferior outcomes have been seen when desired KA alignment was not achieved. Additionally, changing joint line obliquity from the native results in increased joint loading and possible higher wear.

This apparent need for accurate three-dimensional implant positioning of both components in KA TKA raises interest in computer-assisted technologies. However, current available technologies have their limitations. Some important parameters, such the reconstruction of the anterior aspect of the knee or the sagittal profile, are not solved satisfactorily in manual or navigation-based techniques. Purely image-based technologies, such as patient-specific instruments (PSI), are limited in that the soft tissues are not included in the planning and wear is often difficult to predict precisely before surgery. This makes intraoperative adaptations still necessary. These limitations make further evolution in technology for KA TKA reasonable.

Introduction to robotic-assisted technologies

Currently, five robotic systems for TKA are available, which can be categorized according to four basic principles. First, there are systems that actively position the cutting jig, which is assembled on a robotic arm to the desired cutting plane in the knee (Rosa®, Zimmer Biomet, Warsaw, IN, USA; OMNIBotics®, Corin Group, Cirencester, United Kingdom [UK]). The cut itself is conducted traditionally by the surgeon through the jig. Second, one system is based on a navigated burr that is used to mill the implant bed or to accurately position traditional cutting jigs (NAVIO, Smith & Nephew, Watford, UK). The burr has a retraction mechanism that prevents removal of any bone outside the desired resection plane or drill hole position. A third system employs a saw directly assembled onto a robotic arm with haptic control (MAKO, Stryker, Warsaw, IN, USA). ^, The robotic arm controls the desired resection plane and sets boundaries in which the saw can be activated by the surgeon. Fourth is an automatic robot conducting the bone resections on its own, based on a surgeon’s planning (TSolution One® Surgical System, Think Surgical, Fremont, CA, USA). So, considerable differences can be recognized already between these robotic systems in the planning programs and methods of executing the bone cuts.

In terms of the underlying navigation and planning, most systems rely on bone models selected from an atlas of knees. During surgery, several landmarks are captured on the surface of the femur and tibia by the surgeon, to which a best-fit bone model is selected to represent the patient’s knee. One system has the option to use preop imaging (X-ray based) to preselect a bone model from the database, best matching the dimension of the patient’s knee (Rosa®, Zimmer Biomet, Warsaw, IN, USA). This also enables preplanning of the prosthesis position following KA principles before surgery. Another system has the option to create a surface map of the bone during surgery, in addition to the atlas model (NAVIO, Smith & Nephews, Watford, UK). From that surface map, fake computed tomography (CT) is created on the planning screen that can be used to kinematically align the prosthesis with the femur. The two other systems are image-based (MAKO, Stryker, Warsaw, IN, USA; TSolution One® Surgical System, Think Surgical, Fremont, CA, USA). They rely on a CT scan of the hip, knee, and ankle, conducted before surgery. The bone morphology is segmented to an individual knee model. Based on this, a KA preplan can be created before surgery and adapted during surgery with respect to soft tissue, as described later.

All the systems presented are equipped with virtual planning software. This means that the prosthesis planning is initially based on the principles of measured resection. Intraoperatively, the flexion and extension spaces are recorded, and the position of the components can be adapted to achieve a balanced joint without releases. These adaptations are virtual and made before the final bone cuts are conducted. The effect on the gaps is predicted by the software. Differences between the systems are in terms of the knee positions that are included in the planning; some analyze the whole range of motion, others just the extension and flexion space at 90 degrees.

Benefits and limitations of current robotic systems for kinematic alignment

Generally speaking, all said technologies can be used to achieve KA TKA. In particular, the hybrid approach of measured resection of the femur and gap-balanced planning of the tibia cut in extension to establish a tight symmetrical extension space fits perfectly with the KA concept. With virtual planning and the precision of the technology, errors and recuts can potentially be minimized. Also, the final alignment can be controlled, helping the decision-making process during surgery. Besides this, each system has some advantages and disadvantages.

The advantages of imageless robotic-assisted technologies over manual instrumentation are pretty much the same as for conventional navigation (e.g., higher precision, visualization of alignment, objective balancing, virtual planning of component position before cuts). So is the intraoperative workflow analog to navigated KA, as described in Chapter 7. However, with the use of any standard knee model there is still a lack of information on the anterior aspect of the knee, the aspect that usually shows the greatest variability in knees. Although KA in general leads to better restoration of patella kinematics compared with MA TKA, the individual relationship between trochlea orientation and posterior condylar line remains unknown and thus the individual effect of the replacement surgery on patella tracking. Also, the individual anterior offset representing the lever arm of the quadriceps is unknown. With the addition of individual surface mapping, as described for the second technology, this limitation is addressed. With a better visualization of the patients’ individual knee geometry, more parameters are available for correct component sizing, flexion, and mediolateral (M-L) position. However, the model creation and planning are done during surgery, which takes time. Also, the more posterior aspect of the knee is difficult to reach with the probe. Thus, surface-based technology has its limitations, especially in visualizing the tibial slope.

Compared with surface-based technology, there are several advantages to image-based technologies, especially in the context of KA TKA. Analog to the mentioned surface model, the individual anterior knee aspect is visible and accessible for appropriate planning. In addition, the tibia slope can be visualized in CT-based planning and precisely adjusted to the native situation (see Basic Bernese kinematic alignment planning workflow). Second, new algorithms can be created to set the tibia rotation with reference to the transverse kinematic femoral axes, as described below. This is difficult to control in imageless navigation, where only the surgeon-defined landmarks are available. Third, the osteophytes affecting soft tissue balance are clearly visible. With image-based navigation, osteophytes can be identified during surgery and removed with navigation control (see section Bernese surgical workflow robotic-assisted kinematically aligned total knee replacement). This helps to ensure a reliable soft tissue analysis with the system, especially in difficult cases. Last, the image-based model enables the measurement of individual cartilage thickness during surgery, potentially resulting in a better reconstruction of the individual joint line. In model or surface-based methods, the cartilage is routinely assumed to be 2 mm on average. ^,

From the surgical point of view, the robotic devices themselves are of less interest in KA than the underlying navigation component. One difference between robotic systems could theoretically be the precision of each system. However, no comparative data are available. Another aspect would be possible active soft tissue protection by robotic systems. This would be applicable for the retracting burr or the saw with haptic boundaries, but not for jig-based systems used with traditional saws. However, there is little information yet on positive effects on patients’ outcomes.

In summary, the authors see several advantages to image-based planning and surgery over imageless robotics or manual techniques, making these technologies an interesting option for achieving KA. Potential pitfalls owing to individual knee morphotypes, massive osteophytes, or bone defects are more easily visible, helping to make the surgery reproducible and safe. Key is to correctly coalign the implant to the prearthritic surface of the knee. The more information available, the better this works. Additional to that, the balance of the knee is objectified. The robotic device assists the transfer of the plan to the knee, with the potential benefit for systems with resection boundaries to protect the soft tissue envelope. In the following, our KA planning and surgical workflow for one specific image-based robotic arm–assisted surgical system is described in detail.

Preoperative Image-Based Kinematic Alignment Planning

The basic principle of KA is to reconstruct the prearthritic surface of the knee joint with the prosthesis and thus coalign the kinematic axes of the implant to those of the native knee joint. In image-based robotic-assisted surgery, the correct anatomic position of the components is primarily determined preoperatively using a computer model of the individual knee, segmented from a preoperative CT scan. Being a key factor for success, this planning should follow a standardized workflow, as described here.

Basic kinematic alignment planning workflow

Femur

Following the principles of KA, first, the distal and posterior femoral resections are set symmetrically and are based to match the thickness of the distal and posterior femoral condyles of the implant. As the model is CT based, the cartilage thickness needs to be considered to restore the original joint line. Calculating with an average cartilage thickness of 2 mm on the femoral condyles, the magic numbers for bone resection thickness on the segmented bone model are thus set to 6 mm for each distal and posterior resection, when aiming for a symmetric 8-mm thick prosthesis ( Fig. 8.1 ). This results in an individual varus-valgus (V-V) orientation of the femur and an internal-external (I-E) rotation of the component to 0 degrees of the posterior condylar axis (PCA). The rotation to the transepicondylar axis (TEA) is of no interest.

After setting the distal and posterior femoral resection, the third parameter concerns how to size and reconstruct the geometry of the anterior aspect of the femur, including the trochlea. This is best planned on axial cross-sections of the CT scan. The idea is to choose the implant size best reconstructing the anterior femoral offset without producing overhang. From the authors’ experience, the largest component still fitting in the M-L dimensions of the knee is most likely to be selected. The M-L position can be adapted to best reconstruct the position of the trochlea groove.

The third and final planning step for the femur is to adjust the femoral component flexion. Because the orientation, position, and size are already defined, this is used to create a smooth transition to the anterior femoral cortex with a low risk of notching. With the use of the largest possible component fitting to the knee, the average flexion is usually conservative at between 0 and 5 degrees with reference to the hip center. Care needs to be taken not to influence the other position parameters when adjusting the flexion. Therefore, the center of rotation about which the component is flexed is positioned in the kinematic center of the prosthesis ( Fig. 8.1 ).

Therewith, the femur is preplanned based on the kinematic principle. During surgery, only minor adaptations are made to this plan, based on the cartilage thickness, potential defects, or the cartilaginous trochlea groove, as described in the section Pitfalls, tips and tricks for image-based kinematic alignment planning.

Tibia

The planning of the tibia position is preliminary and underresected based on best judgment, which leaves room for fine tuning the V-V and slope orientations of the tibial resection that are determined intraoperatively. Because bone defects are much more likely on the tibia than on the femur, the correct estimation of the original joint line orientation and height is much more difficult. Thus, the final tibia alignment is determined by the soft tissue balance in extension during surgery, to create negligible V-V laxity in full extension, like the native knee (see Bernese surgical workflow robotic-assisted kinematically aligned total knee arthroplasty).

Depending on the bony situation, we usually preset the tibia orientation close to what we think the original joint line obliquity is. This is best visualized on coronal CT cross-sections, positioning the tibia parallel to the unworn plateau ( Fig. 8.2 ). In the authors’ experience, it is better to stay on the conservative side (usually 0- to 3-degrees tibia vara) in the beginning, so not to overresect the medial compartment in excess tibia varus. Also, the resection thickness should be planned conservatively so that adaptations and recuts can be easily performed. Usually, we set the initial bone resection at around 4- to 5-mm maximum with reference to the bony topography of the CT-based model. This outputted resection level is not very helpful in setting the correct angulation or resection depth. Usually, the resection landmarks are positioned directly onto the worn plateau. Thus, the resection numbers might refer to bone defects. Also, the cartilage surface is initially not visible on the CT-based model. Again, an intraoperative adaption based on the unworn cartilage level can be used to correctly set the resection level and tibia orientation, as described later (section Pitfalls, tips, and tricks for image-based kinematic alignment planning).