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Fig. 1 Alignment after registration displaying initial varus-valgus angle, flexion-extension angle, and femoral-tibial rotation measurements for intra- and inter-observer analysis.
Courtesy of Andrew D. Pearle, MD

AAOS Now

Published 8/1/2008
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Annie Hayashi

Putting computer navigation to the test

Study examines accuracy, reliability of navigation in HTOs

Integrating quantitative feedback with the art of surgery is just one of the challenges facing orthopaedic surgeons who use computer-assisted surgery, according to Andrew D. Pearle, MD, co-chair of the AAOS/Orthopaedic Research Society Advanced Imaging and Computer-Assisted Surgery of the Knee and Hip research symposium.

“Computer navigation has been the only available dynamic, intraoperative tool for monitoring simultaneous coronal, sagittal, and axial alignment in high tibial osteotomies (HTO),” said Dr. Pearle, who also presented the results of his study on the reliability of navigated lower limb alignment in HTOs.

“It improves the overall accuracy, reduces radiation time and allows precise measurement of intraoperative leg axes,” he said.

The gold standard?
Dr. Pearle and his colleagues investigated an aspect of image-free navigation that had not been tested—the reliability and accuracy of monitoring changes in long-leg alignment.

“To assess the navigation system’s accuracy to monitor changes in three dimensions during HTO,” he explained, “we compared it with measurements taken from a novel three-dimensional computed tomography (3D CT) method used at our institution.”

Reliability was measured by intra- and interobserver of noninvasive registration for HTO in 3D with navigation.

“Image-free navigation now offers 3D visualization of long-leg alignment during an osteotomy. The surgeon can see where the weight bearing line is going in the coronal plane and in the sagittal plane (Fig.1). The surgeon also wants to be able to see whether the distal fragment is rotating.

“My assumption was that the navigation was accurate and reliable in all three planes,” he said.

Fig. 1 Alignment after registration displaying initial varus-valgus angle, flexion-extension angle, and femoral-tibial rotation measurements for intra- and inter-observer analysis.
Courtesy of Andrew D. Pearle, MD
Fig. 2 Postoperative alignment following navigated HTO displaying varus-valgus angle, weight bearing line location, change in tibial rotation, and change in tibial slope for navigation comparison to 3D CT.
Courtesy of Andrew D. Pearle, MD
Fig. 3 Software tools were used to identify various landmarks on the 3D models to allow for alignment measurements. Here the center of the femoral head was defined with a sphere of best fit generated from a cloud of points picked by the user from the head surface with GeoMagic 9.
Courtesy of Andrew D. Pearle, MD

Measuring image-free navigation
The study used six cadaver legs to test the intra- and interobserver registration reliability of three observers. A navigation system with an image-free module for navigated HTO measurements was used for all of the testing.

“During each of the three observation periods of the intra- and interobserver analyses, each observer registered navigation data for six cadaveric limbs at a minimum of 36 hour intervals,” he explained.

After the intra- and interobserver analyses were completed, various reference arrays and registrations were strategically placed prior to the surgical HTO for an assessment of the accuracy of the navigation system. These were needed to track the mechanical axis in the coronal, sagittal, and axial planes of the affected lower extremity.

“Upon completion of the reliability analyses, navigated HTOs were performed on each of the 13 cadaveric specimens and the initial, postoperative and relative alignment data were recorded (Fig. 2),” said Dr. Pearle. These were compared to equivalent measurements from the 3D CT to assess the accuracy of the navigation system.

Gathering CT 3D data
CT scans were taken of the cadavers at the hip, knee, and ankle. The data from the scans were imported into the software to create 3D models (Fig. 3). “With these models, varus-valgus angle, tibial slope, and tibial torsion could be measured both pre- and postoperatively,” Dr. Pearle explained. “The medial-lateral position of the weight-bearing line was also determined both pre- and postoperatively.”

After the CT data was imported into the software, 3D “solids”—reconstructions—were produced for the proximal femur, distal femur, proximal tibia, and ankle. On the 3D solids, the user could define various anatomic points to generate alignment measurements in all planes.

How did navigation do?
“We found that surgical navigation was reliable and precise in determining real-time coronal plane alignment. Caution should be used in using image-free navigation systems for axial and sagittal plane alignment,” advised Dr. Pearle.

“In 10 of 13 trials, the navigation system measured a change in tibial torsion in the direction opposite to that reported by the CT. It also had poor intra- and interobserver reliability in rotational alignment.”

“Although navigation systems offer great promise for augmented interoperative visualization, they need to be rigorously studied before we can be sure that the data generated are reliable,” he concluded.

Annie Hayashi is the senior science writer for AAOS Now. She can be reached at hayashi@aaos.org