It’s expensive and it takes longer. But these surgeons say the results are worth it.

For many orthopaedic surgeons, making the transition from standard techniques to computer-assisted surgery (CAS) is “a leap of faith.” At the 2006 annual meeting of the American Association of Hip and Knee Surgeons (AAHKS), five orthopaedic surgeons discussed their experiences with computer-assisted navigation in total knee arthroplasties (TKA) and the improvement they believe this technology has brought to their patients.

Mary I. O’Connor, MD; William J. Robb III, MD; James B. Stiehl, MD; Michael L. Swank, MD, and Richard L. Wixson, MD, shared their experiences.

Making the good better

“We all think we’re fabulous surgeons, doing a great job and getting our implants perfectly aligned every time. But the data do not support that belief,” said Dr. O’Connor. The panel explored three areas in which CAS-navigation appeared to improve primary TKA.

Mechanical axis alignment—Because malpositioned knees have a higher risk of failure, most surgeons attempt to achieve a mechanical axis alignment within 3° of neutral. In a review of 12 recent studies, 73 percent of manual TKAs and 94 percent of CAS TKAs were within that range, “compelling data that CAS is a big plus for mechanical alignment,” said Dr. Stiehl.

“With navigation, you can preserve the ability to get correct angles even in minimally invasive situations,” said Dr. Wixson, who shared his personal data comparing mechanical axis alignment with three different approaches (Table 1).

“I’ve reduced the range of variance from 8.7° using the standard incision intramedullary approach to just 4.1° using a mid-vastus minimal incision with navigation,” he explained. “Essentially, I’m getting better as I get older and doing a better job, even with the smaller incision, using navigation.”

Component rotational positioning—Dr. O’Connor acknowledged the debate regarding exactly what the correct reference should be for setting femoral component rotation. “Some data suggest the transepicondylar axis (TEA) most consistently recreates a balanced flexion gap and is the most reliable landmark,” she noted, “but intraoperatively it can be difficult to correctly identify the TEA, creating the potential for error.”

Dr. Robb’s intraoperative data (Table 2) reinforces the value of CAS. “The surgical goal of the total knee that I use is to place the femoral component in 3° of internal rotation. CAS complements visual and mechanical references, increasing the reliability of proper rotation,” he said. “In knees with significant trochlear or condylar deformity still have more variability than optimal. Methods to reduce this variability will require further study.”

Ligament balancing—“I think ligament balancing is one of the strong point of CAS,” said Dr. Wixson. “It gives me a sense of the initial alignment and range of motion and at the end of the procedure I can see what I’ve achieved. I shoot for a couple of degrees of laxity in varus and valgus, and I take measurements throughout the procedure so I can make decisions based on that information.”

Drs. Stiehl and Swank agreed. “I think this is a highly important application,” said Dr. Stiehl. “I routinely use it and it really has helped my balancing problems.”

“It’s this quantitative feedback—giving you your gap balance in both flexion and extension, medially and laterally—that I find extremely helpful,” said Dr. Swank. “I’m a routine user of mobile-bearing knees and I haven’t had a bearing spin-out problem ever since we’ve been using the computer. In 112 cases, we achieved ligament balance within 3 mm side-to-side in 95° flexion and full extension”

Understanding the deformity

“My eyes were opened as I began to better understand knee deformity,” said Dr. Robb. “Even experienced knee surgeons have a hard time separating how much of the deformity is flexion, or how much is varus or valgus. Navigation enables you to break down the components so that you understand from the beginning how the surgery has to address the components of the deformity. This allows us to address all components much more predictably and accurately during the surgery itself.”

In making the transition from standard to navigated TKA, Dr. Robb recommended an “integrated” approach that relies on two independent data sets (one virtual and one conventional) for each surgical decision. “With two sets of values for every step, you can choose that value that you think is more accurate. In some cases, it may be the standard reference; in other cases, the virtual reference,” he explained.

Dr. O’Connor reinforced that the computer is not a substitute for surgical judgment. “If the computer gives you a value that seems inappropriate, you should question it. Check for accuracy of the model to ensure that your reference arrays have not moved and that your initial registration produced a model of the patient’s knee that makes sense,” she advised.

Sizing the implant

A closed platform, noted Dr. O’Conner, can clearly help with sizing of the femoral component because all sizing data is preloaded. The relationship of the anterior cortex and position of the component can be seen and adjustments made to avoid femoral notching and overresection of the femoral condyles for patients who are “between sizes.”

Although Dr. Swank uses a closed system, he admitted to not using the sizing feature initially “because I was just getting familiar with the alignment and measurement techniques. But the better you get, the more features you use. Now using that sizing feature is, for me, the single most important aspect of the system, the place I where spend the most time. It’s one of the things that has improved my clinical results.”

Although Dr. Wixson does all his sizing off the computer, Dr. Stiehl admitted to not using the computer to size at all. “In my cases, I want to know the mechanical alignment for balancing; the proximal tibial cut, because that’s the basis of my particular technique; where the anterior cortical reference is for femoral component placement, and help in balancing at the end of the procedure. That’s what the computer is doing for me. It’s not sizing my implant,” he said.

“Traditionally, I think orthopaedic surgeons are sensitive to not undersize the femur,” said Dr. Robb. “We don’t want to put the anterior cortex as risk. With virtual images, the surgeon can overlay a femoral component placed in 3° of flexion on the femur to see where the cut will exit anteriorally. CAS allows us to make more accurate sizing decisions for the femoral component.”

Complications?

According to Dr. O’Connor, CAS TKAs may have more complications with the technology than with the patient. In her nearly completed prospective randomized study of 200 patients, there was one complication related to navigation: an injury to the medial collateral ligament from pin placement. In several cases, however, line of sight issues limited the use of the computer during the surgery. “We found that the spectrum of light emitted by new lights placed in our operating rooms midway through the study were interfering with the computer beams,” she reported.

Dr. Robb reported a 1 percent complication incidence among 671 cases, but none of them was directly attributable to navigation.

Electromagnetic engineering is “compelling technology,” pointed out Dr. Stiehl, but “the signals can be bent or disturbed by just about anything in the operating room.”

Happy patients

Dr. Robb also shared perioperative data on 162 TKA patients; half were standard procedure and half were navigated surgeries. “There was a trend toward lower white counts, fewer fevers, almost statistically significance in fewer transfusions and a definite statistical significance in fewer cardiac arrhythmias in the patients who had navigated surgeries,” he reported. “They were also walking farther prior to discharge and they got rid of their walking devices sooner.”

What’s behind the faster recoveries? “Medically, I think it’s because CAS enables us to avoid the medullary canal,” said Dr. Robb.

Dr. Swank agreed with the postoperative assessment. “Looking at groups done at the same time, we can prove that the CAS group does better than the non-CAS group,” he says. “I found that I got busier, because my patients were happier and they were coming back and sending their friends.”

His studies showed that patients who received CAS had a statistically significant improvement in Knee Society scores after the first two to three months. Although that improvement diminished after two to three years, as anticipated, it was an unexpected result. “In every instance, including pain and function scores, the CAS group made greater improvements sooner,” he reported.

In closing, Dr. O’Connor emphasized the need for prospective, randomized clinical trials to advance surgical knowledge of computer-assisted navigation in TKA.