Robotic assistance is a relatively new technology for unicompartmental (UKA) and total knee arthroplasty (TKA), as well as for certain aspects of total hip arthroplasty (THA). Currently, two systems have been approved by the U.S. Food and Drug Administration (FDA) and are commercially available in the United States—RIO® (MAKO Surgical, now owned by Stryker Corp.) and ROBODOC® (Curexo Technology).
The hypothesis and rationale for such systems is that robotic assistance may result in improved component positioning and alignment that influences long-term clinical outcomes. According to reports for TKA, for example, conventional surgery achieves neutral alignment (within 3° of the mechanical axis) only 75 percent of the time, and coronal suboptimal alignment greater than 3° correlates with worse outcomes. In the case of UKA, robotics may aid component alignment and fixation within the limited surgical exposure.
Although the relationship between alignment and outcomes is complex, research on robotic arthroplasty has focused on its role in improving component positioning and alignment as surrogate markers for long-term clinical outcomes.
History and function of systems
The Robodoc system was one of the first robots to be used for joint arthroplasty. In the early 1990s, Howard A. Paul, DVM, and William L. Bargar, MD, teamed up to develop a system to prepare the femoral side of a THA to facilitate the use of cementless stems and improve bony ingrowth.
Once the system was positioned, fixed to the patient, and registered with fiducials (markers in the surgical field used as a reference for image guidance) anchored in bone, the robot would automatically mill a cavity in the femur for the stem. Initial pilot studies were performed in dogs, and human trials began in 1992.
Early studies claimed that the robot could accurately prepare the femur to within 0.4 mm, with 96 percent precision (compared to 75 percent for manual broaches). Worldwide, the system has been used for more than 24,000 joint replacements. The U.S. experience has been more limited, since the system did not receive FDA approval until 2008. More recently, the system has been expanded to focus on TKA using a similar technical approach.
Another one of the early robots—the Acrobot® (Acrobot Company Ltd)—took a different approach. In this system, the surgeon controls the entire milling process, while the robot uses force-feedback (haptic feedback) to constrain the surgeon to the safe zones for the procedure. The femur and tibia are immobilized, and the robot is placed in continuity with the leg and registered with landmarks. Then, the surgeon prepares the femur and tibia robotically, while the patella is prepared conventionally.
One small randomized study of conventional (n = 15) and robotic (n = 13) UKAs showed that American Knee Society scores at 6 months were better for robotic techniques but the operative time averaged 10 minutes longer than conventional techniques. Acrobot was recently acquired by MAKO as part of a settlement in an intellectual property litigation.
The current MAKO system is a semi-active system controlled by the surgeon. It provides both auditory and haptic feedback, limiting milling of the tibia and femur to certain regions. Prior versions required rigid fixation to a frame, but newer systems have a dynamic tracking feature that results in better implant positioning relative to conventional UKA, according to one study. The MAKO system has been focused on UKA; an expansion into acetabular preparation for hip arthroplasty is relatively recent.
Current studies of Robodoc
Cadaver studies have demonstrated that Robodoc can deliver more accurate component positioning for TKAs than conventional approaches. One study involved performing bilateral TKAs on a cadaver; one knee was done using the Robodoc system and the other using a conventional approach. The results showed that using the Robodoc system resulted in significant improvements in some radiologic parameters, as evaluated by postoperative computed tomography (CT) scans.
These improvements were translated into clinical practice in another similarly designed study involving 30 patients who received bilateral TKAs, using the Robodoc system on one knee and conventional techniques on the opposite knee. Component positioning and alignment were within 3° of neutral for all robotic knees, but only 76.7 percent of conventional knees met that standard. However, surgery took an average of 25 minutes longer to perform using the Robodoc system.
Another study involved 100 patients who underwent unilateral TKA. Patients were randomized to receive either robotic or conventional surgery. In this study as well, 77 percent of patients who received conventional surgery and 100 percent of patients who received robotic surgery using the Robodoc system had less than 3° of deviation from a neutral mechanical axis. In both studies, clinical outcomes, as assessed by WOMAC and Hospital for Special Surgery functional scores, were no different between the groups at follow-up (1 year for the bilateral knee study and approximately 3.5 years for the randomized study).
MAKO Surgical is a more recent player in the field of robotic knee arthroplasties, but the company has expanded quite rapidly in the United States. As of September 2011, the system has been used in more than 10,000 knee arthroplasty procedures.
One clinical study compared 31 MAKO robotic system procedures to 27 conventional UKA procedures. The UKAs performed with the robotic system had improved positioning in terms of decreased variance, more accurate posterior tibial slope, and better coronal plane alignment than those performed with conventional techniques.
In addition to better bony alignment, computer assistance may also improve soft-tissue balancing. One study of UKAs performed using the MAKO system found that 83 percent of patients had ligament balancing within 1 mm of the pre-incision balance plan determined under valgus stress.
Only short-term follow-up studies have been reported thus far. One such study that compared 45 conventional to 36 robotic UKAs found no significant difference in Knee Society Scores at 12 weeks after surgery.
An early trial reported a 41-minute setup time for the robot, 7.5 minutes for registration, and 34.8 minutes for robot-assisted burring. By the tenth case, however, surgeons were able to shave 20 minutes from the entire procedure to average 120 minutes for the entire surgery. Additional research is needed to determine the time required by a surgeon experienced in using robotic systems and the number of surgeries necessary to achieve that proficiency.
Pros and cons
Overall, the use of robotics for TKAs and UKAs has demonstrated the ability to improve component positioning in some cases; however, no study has demonstrated improved functional outcome in near-term follow-up. This may be due to limited sensitivity of clinical outcome measures or to the limited follow-up period. Longer term follow-up will be needed to demonstrate whether the improved positioning will result in clinically significant improvements in patient outcomes.
Custom cutting blocks are an alternative approach to improve alignment over conventional techniques, but one early study reported that their use does not lead to improved component alignment. However, time in the operating room was less with the blocks than with conventional approaches.
Robotic systems have several drawbacks. They often require a preoperative CT to perform the necessary image registration, thus exposing patients to additional radiation over a typical preoperative TKA evaluation. All robotic procedures have been found to require additional surgical time in most circumstances. This raises concern about the correlation of surgical time with infection risk. In addition, the learning curve can be substantial, with a decrease in surgical time within about 20 cases. Finally, depending on the type of registration used, there can be a risk of pain or fracture from the fixation system.
Perhaps the largest question underlying robotic surgery is the cost-benefit trade-off. Incorporating robotics into a practice requires the upfront capital expenses for acquiring the robot, the additional costs for servicing the robot, and the generally increased cost of disposable equipment used for each surgery. The initial capital requirement can approach the $1 million mark for some systems. In an era of cost-benefit awareness, substantial evidence supporting improved clinical outcomes must distinguish systems that are truly beneficial from systems that support the marketing of a robotic service line.
Overall, the use of robotics in knee arthroplasty may have the potential to deliver better implant positioning than conventional approaches, although some drawbacks—such as increased surgical time—also exist. Although no near-term clinical impact has been detected, the relationship between component positioning and long-term outcomes indicates that additional clinical studies are required to assess the true impact of the technology. Surgeons should evaluate whether they believe such a system will aid in their technique to optimize their patient outcomes.
Despite these caveats, robotic assistance has the potential to be an exciting new addition to the long list of technologies that have incrementally improved the practice of arthroplasty. It remains to be seen whether Stryker will be able to integrate its arthroplasty line with the MAKO system to lead to improved outcomes. If robotic arthroplasty can be shown to result in improved component positioning that leads to better long-term outcomes, the cost-benefit ratio may be favorable. This level of evidence will be particularly important in an era of increasing emphasis on outcomes-based reimbursement.
Disclosures: Dr. Ponnusamy—Port City Group, Globus Medical. Dr. Golish—U.S. Food and Drug Administration, Cytonics, Inc., OKO Section Editor, AAOS Biomedical Engineering Committee.
For a point/counterpoint on using robotics in arthroplasty, see “Face Off: Robotic Surgery” on page 18.
Karthikeyan E. Ponnusamy, MD, and S. Raymond Golish, MD, PhD, are members of the AAOS Biomedical Engineering Committee.
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