Navigated femoral anteversion measurements: general precision and registration options
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Intraoperative reduction of femoral fractures can result in rotational malalignment. Navigation modules allow fluoroscopy-based femoral anteversion (AV) measurements; however, their clinical feasibility has not been fully evaluated. An important technical consideration when obtaining navigated femoral rotational alignment is the necessity for orthogonal navigated fluoroscopic images.
The current investigators hypothesized that there would be a critical imaging angle between the fluoroscopic images which optimized the acquisition of accurate rotational measurements. Following initial testing in plastic femora, 14 intact human cadaveric femora were subjected to CT and navigated fluoroscopic assessment of AV. The navigated registration process included the following test series: Standard perpendicular AP and lateral imaging at a 90° angle; planar rotation of the lateral fluoroscopy position perpendicular to the axis of femoral neck and angled to the AP position at 75°, 60°, and 50°; inlet and outlet imaging of the axis of the femoral neck at angles of 10°, 20°, and 30°. For all setups, the difference between the AV angle of the navigation system and the CT-measured angle was calculated.
Results revealed no relevant differences between CT and navigated determination of plastic femora (1.5°). No significant deviations of cadaver femora with 90° angle imaging (mean 2.00°, range 0.00°–4.33°), 75° angle imaging (mean 1.95°, range 0.00°–3.33°) and 60° imaging (mean 2.00°, range 0.00°–3.33°) were found. However, significant deviations were found for the navigated 50° angled imaging technique (mean 5.02°, range 2.33°–7.67°).
These data demonstrated that the femoral AV angle can be calculated accurately by using two navigated fluoroscopic images when a minimum angle of 60° is utilized between the angled fluoroscopic images for the registration process. A difference of <60° does not allow for accurate navigated measurements.
KeywordsFemoral anteversion Measurement CT Navigation system Validation Fluoroscopy Accuracy
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