Accurate knee alignment measurements seen with computer-assisted navigation

Updated 2/10/2010

SAN FRANCISCO - Knee and limb mechanical axis alignment measurements made using radiographs were found to be inaccurate when compared to measurements obtained intraoperatively using computer-assisted navigation, according to results of a study presented here.

"From my point of view, navigation seems to be a valuable tool for assessing component alignment but, more importantly, it is a more-validated tool than radiographs," S. David Stulberg, MD, said during the Knee Society/American Association of Hip and Knee Surgeons Specialty Day Meeting.

Ensuring measurements are accurate is critical to successful total knee arthroplasty (TKA) outcomes and minimizing component wear, he noted.

Stulberg and colleagues analyzed 58 TKAs performed using an image-free computer-assisted navigation system (B. Braun Aesculap).

The investigators measured limb alignment according to AP mechanical limb axis, and lateral femoral and lateral tibial implant axes according to the Knee Society standards on pre- and postoperative radiographs. They then obtained the same measurements intraoperatively using the navigation system.

"Radiographs consistently overestimate varus in the AP mechanical axis," Stulberg said. "They overestimate the posterior slope of the sagittal tibial axis and they overestimate flexion in the sagittal axis of the femoral component," he said.

The difference between the radiographic and navigation measurements often varied by as much as 12°. In cases where such a discrepancy existed, intraoperative measurements from the navigation system were frequently within about 1° of the surgeon's desired limb alignment, according to the study abstract.

"Radiographs are not sensitive enough for accurate postoperative measurement," Stulberg said.

Stulberg noted that many factors affect the accuracy of the measurements used in the study. But, in general, the radiographs tended to have substantial variation compared to the narrow margin of variation seen with the navigation system's measurements, he said.

For more information:

Yaffe MA, Koo SS, Stulberg SD. Correlation of intra-operative navigation with post-op radiographs. Symposium IV: Computer navigation meets MIS TKA. Presented at the Knee Society/American Association of Hip and Knee Surgeons Specialty Day Meeting. March 8, 2008. San Francisco.