Robotic Applications for Total Knee Arthroplasty

Kenneth Gustke, MD

INTRODUCTION

Orthopedic surgeons continue to seek a better technique to perform total knee arthroplasty to match the greater perceived outcomes of total hip replacement surgery. There may be an element of inappropriate expectations for total knee arthroplasty.¹,² Several series of revision knee arthroplasties report malalignment in 25% and instability in 20% to 25% of revisions.³,⁴ Total knee imbalance with either too tight or loose soft tissues accounts for up to 54% of revisions in one series.⁵ Gross component malalignment will lead to increased stress on the polyethylene, locking mechanism and supporting bone, thereby resulting in polyethylene wear and early loosening. Gross instability is easily perceived by patients necessitating early revision. Subtle instability and poor kinematics are more difficult to diagnose by both patients and surgeons. It can result in stiffness and perhaps may be the reason for the approximate 20% of unsatisfied patients who live with their arthroplasty.²,⁶ Being able to consistently obtain an accurately aligned knee arthroplasty within acceptable alignment parameters that would not be expected to overstress the polyethylene and implant interfaces and obtain a perfectly balanced knee should result in the best short- and long-term outcomes.

Total knee arthroplasty performed with standard instruments has an inherent margin of error with implant and axial alignment. Even though many native knees are in constitutional varus or valgus,⁷ most surgeons will aim for neutral mechanical alignment to minimize final alignment outliers outside of 3° of mechanical varus to 3° of mechanical valgus. Despite usual oblique joint lines in native knees, 90° angle proximal tibial cuts are usually performed because they are easier to make with conventional instruments. Unfortunately, following these techniques, axial alignment within 3° is obtained in only 70% to 80% of cases using standard instruments.⁸,⁹ This lack of accuracy certainly prohibits consideration of following a constitutional, true kinematic, or hybrid kinematic alignment philosophy.¹⁰,¹¹,¹²,¹³ Standard total knee kits lack instruments to assist with soft tissue balancing. Soft tissue balancing is based on feel and visualization of gaps under applied varus and valgus stress. Subjective balance is less accurate and highly influenced by surgeon experience. Instrumented sensor trials can be helpful by providing quantitative information while balancing the soft tissues mainly through tissue releases may provide better short-term outcomes.¹⁴,¹⁵,¹⁶,¹⁷,¹⁸ However, excellent long-term outcomes are predicated on both a perfectly balanced and aligned knee.

Computer navigation was developed to improve accuracy of implant alignment. Studies have shown a decrease in alignment outliers.¹⁹,²⁰,²¹ This technology may allow surgeons to consider constitutional or kinematic alignment of implants.²² Computer navigation may aid with soft tissue balancing by showing gap equality and symmetry. However, most outcome studies of computer-navigated knee arthroplasty have not shown a difference in patient satisfaction, clinical function, and positioning of implants.²³,²⁴ The Australian National Joint Registry has shown better survivorship of computer-navigated total knees in patients less than 65 years of age.²⁵ Younger patients would be expected to apply more stress on the polyethylene and component interfaces, thereby revealing a difference in survivorship which may not be noted in older, less demanding patients. Perhaps the use of sensor trials with computer-navigated knees, providing quantitative information on balance, may demonstrate improved short-term outcomes.

Patient-specific instrumentation (PSI) was also developed to assist with bone cuts to more accurately obtain implant alignment. Chosen alignment is based on the surgeon’s preoperatively determined alignment parameters. Some alignment inaccuracy has been reported, particularly with the tibial cut.²⁶,²⁷,²⁸ Unfortunately, back-up instruments may still be needed if intraoperative changes are necessary.²⁹ One study demonstrated worse alignment with PSI than with computer navigation.³⁰ Typically, these systems do not include instruments to assist with soft tissue balancing. Use of custom-manufactured implants to match patient’s anatomy inserted with PSI has been developed to theoretically reproduce better kinematics than with use of off-the-shelf implants. This may not be achievable without having anterior and posterior cruciate ligaments and in severe deformities. Callies et al have shown comparable results between kinematic aligned total knee arthroplasties and patient-specific instruments to mechanically aligned total knee arthroplasties with standard instruments.³¹ However, they noted more outliers with poor outcomes in the PSI kinematic aligned group.

Robotic assistance for knee replacement can also accurately create bone cuts to align components in exactly the surgeon’s desired position.³²,³³,³⁴,³⁵,³⁶,³⁷,³⁸ It is a unique technology because it can also allow individualized patient component alignment that can be adjusted preoperatively as well as intraoperatively. It also gives the surgeon the ability to modify alignment confidently within their desired parameters off the neutral mechanical axis in order to balance the knee and minimize soft tissue releases.

ROBOTIC ARM-ASSISTED TOTAL KNEE ARTHROPLASTY (MAKO—STRYKER ORTHOPEDICS)

General Overview

This system utilizes a saw that is attached to a robotic arm which places the saw cutting blade in the correct cutting plan (Fig. 35-1). This is a semi-active system that requires the surgeon to press the trigger to active the saw. The surgeon manipulates the robotic arm placing the saw tip close to the desired bone cut. Initial depression of the trigger will orient the saw tip in the exact plane and position to perform the bone cut. The tip of the saw operates only within haptic boundaries that are determined by the edges of the component positions (Fig. 35-2). Haptic boundaries significantly reduce the risk for soft tissue injury during bone resections.³⁹,⁴⁰,⁴¹ Component position, size, and axial and rotational alignment are determined from preoperative planning of a 3D virtual model obtained from a computed tomography (CT) scan. The CT scan is performed under a protocol with scanning of just the hip, knee, and ankle. The radiation exposure is approximately 4.0 millisieverts (mSv), which is equivalent to 1/2 the dose of an abdominal CT, 1/4 the dose of a coronary angiogram,⁴² or 1/13 of the occupational dose limit of 50 mSv.⁴³ Femoral and tibial infrared tracker arrays are inserted either inside or outside the incision depending on length of incision and surgeon’s preference. The knee is surgically exposed. Various anatomic points and hip center are registered with probes to synchronize the patient’s anatomy to the preoperative CT scan virtual model.

After removal of visualized osteophytes, varus and valgus stresses are applied to the knee in extension and flexion to demonstrate gaps and the soft tissue balance with the component aligned within the preoperative plan. A hybrid gap balancing/measured resection technique is used. The preoperative plan can be modified to change component position and alignment within the surgeon’s acceptable parameters to facilitate better soft tissue balance and minimize soft tissue releases (Fig. 35-3A and B). Component realignment may be preferred to performing soft tissue releases due to the concern that soft tissue releases can be unpredictable and may stretch out postoperatively.⁴⁴,⁴⁵ After all initial bone cuts are made and with trial components in place, varus and valgus stresses are again applied in extension and flexion to visualize gaps and soft tissue balance. Alternatively, some of the bone cuts can be made and tensioner instruments used to show gap balance prior to trialing components. Further modification of component position and alignment can be considered and additional bone cuts are performed under haptic guidance to facilitate soft tissue balance. Generally tibial component modifications are performed if a change of balance is needed in both flexion and extension, and the femoral component alignment is adjusted if only extension rebalance is needed. Component alignment and position modifications are considered as long as they are still within the ranges acceptable to the surgeon. My final desired coronal alignment is between 2° of valgus and 3° of varus. Otherwise, soft tissue releases are performed to
further balance the knee. Use of sensor trials that show compartment load balance throughout range of motion, maximum load contact points, and tracking pattern can be used to also titrate the release of soft tissue and assist with determining the amount of component realignment needed⁴⁶ (Fig. 35-4A and B).

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