What Is the Role of Robotics and Navigation in THA?
Navigation and robotics systems are intended to help improve accuracy of component positioning and reduce outliers in total hip arthroplasty. The authors review the evidence for and against using these systems.
Amy S. Wasterlain, MD, and Jonathan M. Vigdorchik, MD
The authors have no disclosures relevant to this article.
Standard total hip arthroplasty (THA) techniques have traditionally produced excellent clinical results. Nonetheless, hip instability and limb length inequality may lead to patient dissatisfaction, ultimately requiring revision THA. 
Navigation and robotics have recently been introduced in an attempt to improve the reproducibility and accuracy of THA, as well as other orthopaedic surgeries.
Navigation systems use patient-specific anatomic data and information about the relative positioning of surgical instruments and implants to convert qualitative human judgments into calculated surgical decisions.
Robotic navigation refers to the use of computerized instruments that assist the surgeon in executing specific parts of the procedure. Despite evidence supporting improved accuracy and fewer outliers with robotic THA, it is not yet clear whether these radiographic benefits translate into better clinical outcomes.
Types of Navigation
Navigation systems are classified based on whether they require preoperative imaging and whether they use a robotic assistant.
Currently, all robotic systems require preoperative CT imaging. CT-based non-robotic and robotic navigation systems generally involve 4 main steps: 
- Digital modeling based on preoperative imaging
- Preoperative implant templating
- Intraoperative bone registration
- Component positioning and implantation
Imageless and fluoroscopy-based navigation systems do not use preoperative imaging and therefore omit steps 1 and 2.
Common navigation systems available in the US are listed in Table 1.
Table 1. Commercially Available THA Navigation Systems (listed alphabetically)
Potential Benefits of Navigation
The primary benefit of navigation appears to be a reduction in the number of cups placed far outside the acceptable safe zone. [3-9]
A systematic review including 400 patients revealed no significant difference in mean cup inclination or anteversion between conventional and navigated THAs, but navigated THAs were associated with significantly less variability in cup position and lower risk of placing the cup outside the safe zone. 
A retrospective review of 1980 THAs showed that navigated and robotically placed cups were significantly more likely to be within the safe zone than manually placed cups. [5
]Domb et al  found that 100% of cups placed using the Mako THA robotic system were within the Lewinnek safe zone, compared with 80% of conventionally placed cups.
Similarly, Elson et al  reported 95% of cups were placed within 3.5 degrees of the intended position using the Mako system.Navigation can also assist in restoring limb length equality. One study showed that navigated THA was associated with fewer outliers more than 5 mm outside the targets for limb length and femoral offset. 
Others have reported limb length restoration to within 6 mm of the contralateral limb in more than 95% of cases,  although there is currently no clear evidence that imageless navigation restores limb length better than conventional THA.
Robotic THA has also been shown to achieve limb length equality. 
Potential Drawbacks of Navigation
Perhaps most importantly, evidence demonstrating improved clinical outcomes with navigation is lacking. Some surgeons argue that in the absence of a clinical benefit, navigation is not worth the extra expense and hassle.
In a randomized controlled trial (RCT) comparing conventional THA to THA with imageless navigation, patient satisfaction, clinical outcomes, and manual range of motion testing were no different at 1 year. 
Another RCT demonstrated no difference in clinical scores or polyethylene wear at 10 years when comparing conventional THA and navigated THA outcomes. 
Other disadvantages include:
Increased cost.Robotic systems require substantial upfront financial investment for the robot and software (up to or exceeding $1 million), as well as annual maintenance and disposables for each case. CT-based navigation is associated with increased cost and increased radiation exposure from the preoperative CT, as well as increased time for preoperative computer modeling.
Increased surgical time. With imageless navigation, surgical time is lengthened by 12 to 18 minutes due to additional registration steps, although this decreases with experience using the system. [10,13,15]
Surgical time is also increased with robotic THA, although it decreases throughout the surgeon’s learning curve. Total operative time decreased from 80 to 70 minutes during the surgeon’s first 35 and 100 MAKO robotic cases, respectively. 
Increased risk of certain types of complications. The first robotic assistant, called ROBODOC, was associated with several major complications when it first launched. In some studies, patients had higher dislocation (18%), revision (15%),  and heterotopic ossification (27% vs 16%) rates  when ROBODOC was used in their surgery, possibly due to abductor damage during robotic milling.
Other technical complications included femoral shaft fractures requiring cerclage wiring, and milling defects in the acetabulum and greater trochanter. [18, 19]
Newer robotic systems have not been associated with the problems reported with use of the older systems like ROBODOC.
Which Type of Navigation Is Best?
Currently available evidence does not definitively support one type of navigation over the others. Each navigation type has advantages and disadvantages, the relative importance of which may depend on the specific surgical case.
Benefits of imageless navigation: Compared with robotic navigation, imageless navigation requires less capital investment, spares the patient radiation exposure and the expense of preoperative CT, and requires only minimal set up for each surgical case. Robotic assistants must be designed to accommodate each implant system, and they are typically compatible with only a few implant systems. On the other hand, imageless navigation can be used with most systems.
Benefits of image-based and robotic navigation: Image-based navigation (including robotic THA) can improve accuracy through minimally invasive approaches and in obese patients, both of which have been associated with an increased risk of component malpositioning. [20, 21] By contrast, the accuracy of imageless navigation depends on the surgeon’s registration technique. Poorly identified bony landmarks result in poorly placed components, an issue termed “garbage in, garbage out.” Robotic navigation can be especially useful to ensure accurate component positioning in dysplastic hips or in other cases in which normal anatomic landmarks are obscured.
Computer navigation and robotic assistance represent new surgical tools with the potential to minimize outliers in component positioning and to assist in challenging cases in which the patient has irregular anatomy.
Semi-active robotic systems offer the most accurate patient-specific preoperative planning tools, and they ensure that the plan is executed accurately during surgery. However, robotic systems add considerable cost and radiation, and have a steep learning curve.
Imageless navigation systems aid the surgeon in ensuring accurate component positioning and limb alignment intraoperatively, with comparatively less expense and radiation, as well as simpler intraoperative technique.
One challenge of evaluating outcomes and efficacy of navigation systems is that computer technology changes so rapidly that it is obsolete by the time long-term clinical data are available. Therefore, studies reporting the outcomes of patients who underwent navigated THA 10 years ago may not accurately reflect today’s technology.
Meaningful clinical benefits of navigation may emerge as existing devices develop a track record and as our technology evolves.
Amy Wasterlain, MD, is a resident in orthopaedic surgery at the NYU Langone Medical Center’s Hospital for Joint Diseases, New York, New York. Jonathan M. Vigdorchik, MD, is an Assistant Professor of Orthopaedic Surgery, Associate Fellowship Director in Adult Reconstruction, and Co-Director of Robotics in Orthopaedic Surgery at NYU Langone Medical Center’s Hospital for Joint Diseases, New York, New York.
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