CHAPTER 14 Computer-Guided Partial Knee Replacement
Introduction
Over the past 10 years, partial knee replacement has risen in popularity. In large part this is due to the development of minimally invasive surgical techniques, which have become increasingly popular in all areas of surgical intervention. Minimally invasive techniques have promoted rapid recovery, lessened blood loss, and reduced postoperative pain.1 In surgery of the knee, however, they have also increased the difficulty of arthroplasty surgery, particularly in the partial replacement environment, which utilizes even smaller incisions with the resultant constrictive soft tissue envelope. Such surgical constraints may lead to malalignment or malposition of implants by all but the most experienced of surgeons.2,3 The same time period has seen rapid advancements in computer and electronic technology, and industry has utilized this technology to develop robotic machines that enhance the speed, efficiency, and accuracy of manufacturing far beyond the capabilities of human workers. Engineers and visionaries have long dreamed of the day such technologies could be applied to human surgical endeavors, and such a machine has been developed for the orthopaedic community by MAKO Surgical Corporation (Ft. Lauderdale, FL). The author first began use of this machine in June 2007, and has since performed approximately 300 robotic-assisted partial knee replacements with excellent results. This chapter outlines the theory, setup, performance, and early results of robotic-assisted partial knee arthroplasty.
The RIO (Robotic Arm Interactive Orthopaedic System) surgical system utilizes a mobile robotic arm that gives the surgeon tactile feedback of the cutting zone, so that bone cutting occurs only within the desired footprint area of the proposed implant, thus eliminating the risk of surgical bone-cutting error. The motorized burr is navigated by the computer, and if maneuvered out of the desired cutting area, simply shuts off. This is the first fully Food and Drug Administration–approved instrument widely available in the United States that enables surgical navigation of the cutting tool, thus taking computer navigation to the next level of sophistication.4 To better exploit the capabilities of this novel cutting system, the engineers and surgeon consultants with MAKO Surgical Corporation have designed a complimentary robotically optimized implant system that uses the robot’s unique ability to accurately sculpt bone into complex curves and shapes, thus creating a unique implant system of thin-section shape-matching implants that are ideally suited to partial knee replacement.5 Because the implants measure approximately 3 mm in cross section, they are ideally suited to use in younger, more active patients who may in the future require revision surgery. The use of these thinner implants should ease revision issues of bone loss for the surgeons of tomorrow. This implant system (Restoris MCK; MAKO Surgical Corporation) is modular and multicompartmental in nature, consisting of tibial onlay or inlay components, femoral condylar components, trochlear components, and patellar components. The components can be mixed and matched based on computed tomography (CT) scan planning like jigsaw puzzle pieces, giving the patient a knee construct that is individualized for his or her unique anatomy, while still maintaining the efficiency of off-the-shelf manufactured components.6 Like any partial knee replacement system, the ligamentous structures are retained, and the navigation component of the robotics allows nearly perfect dynamic gap balancing to allow excellent anterior cruciate ligament (ACL) and posterior cruciate ligament function of the partial knee construct.
Patient Selection
Since the partial knee replacement relies on intact ligamentous structures for stability, it is important that all ligaments remain intact and generally normal range of motion (ROM) is desired. For medial compartment arthritis, it is generally considered optimal to have an intact ACL, although this is not an absolute requirement.7 If the patient does not have symptomatic instability, a medial unicompartmental knee arthroplasty (UKA) can be performed with good success. ACL reconstruction can be performed at a later date if instability becomes a problem. For isolated patellofemoral (PF) arthroplasty, it is important that the remaining compartments of the knee have minimal pathology, as failure of these implants is usually caused by progression of disease in the other compartments. It is probably also ideal to avoid cases with severe patellar malalignment issues or excessive bony deficiency. Bicompartmental arthroplasty combines medial UKA with PF arthroplasty. The unique shape-matching ability of the Restoris MCK system allows computer matching of various sizes of implants to yield a perfect implant fit. It is still important to assure intact cartilage of the contralateral compartment and intact ligamentous structures to allow anatomic tracking of the construct.
Planning
The robotic technology is based on three-dimensional CT images of the patient’s knee. Once a compatible CT scan is obtained, the scan segments are inputted into the robotic computer, and a process called segmentation is performed. In this process each CT image is outlined with a computer probe and the cortical bone shape thus inputted into the computer. The robot then utilizes this information to construct an accurate three-dimensional image of the knee. The Makoplasty technician then uses shape matching to align computer models of the implants to the cortical bone surface, planning for the implants to be approximately 1 mm proud of the bone to mimic the thickness of hyaline cartilage. The computer allows accurate positioning of the implants on the bone, and measures the alignment in relation to the mechanical axis of the extremity as determined from the CT scout views of hip, knee, and ankle.8
Implant placement of UKA components is based on fairly traditional alignment parameters that can be found in any tome on unicompartmental arthroplasty. Once alignment of the trochlear component is begun, however, positioning parameters are less clear since this is a shape-matching component. The author has had good success with generally matching the cortical shape of the trochlear groove, taking care to deepen the groove slightly to avoid overstuffing the patellofemoral joint. It is very important to pay close attention to the medial and lateral transition zones to assure smooth transition of the patella from the trochlea onto the femoral condylar surface, and thus avoid a “speed bump” effect when the patella tracks through the range of motion (Fig. 14–1).
