• S Blendea, J Troccaz, and P Merloz.
• Service Orthopédie-Traumatologie, CHU de Grenoble, Hôpital Nord, BP 217, 38043 Grenoble Cedex 9. soblend@yahoo.fr
• Rev Chir Orthop Reparatrice Appar Mot. 2007 Apr 1; 93 (2): 157-64.

Purpose Of The StudyStandard methodology is lacking for evaluating the accuracy of surgical navigation systems. The purpose of the present study was to propose a new approach to error measurements of an image-free navigation system used for total hip arthroplasty.Material And MethodsThis new approach evaluates the overall accuracy of the system and quantifies the influence of clinical application on this global error. The majority of hip navigation systems use the anterior pelvic plane as part of the reference system. With image-free systems, anatomic pelvic landmarks must be acquired intraoperatively in order to define the anterior pelvic plane. This step could potentially introduce a significant error for navigation. Two studies were performed to measure this error, one on patients and the other on pelvic phantoms. Both used the difference between the intraoperative cup orientation, as displayed by the navigation system and the postoperative cup position, measured on computer tomography (CT) data. The CT measurements used the same reference system as the navigation system.ResultsThe intraobserver measurement variability ranged from 48.4 degrees to 49.5 degrees for cup abduction and from 12 degrees to 13.5 degrees for anteversion. The interobserver variability ranged from 47.5 degrees to 19 degrees for cup abduction and from 11.8 degrees to 13.8 degrees for anteversion. Overall errors were calculated for cup abduction and anteversion. Cup navigation was accurate on pelvic bone phantoms. The anteversion error ranged from 0 degrees to 2.5 degrees (mean 0.9 degrees, standard deviation 0.7 degrees). For the clinical study, abduction errors ranged from 2.1 degrees to 16.7 degrees. The mean abduction error introduced by the acquisition of anatomic landmarks was 7.2 degrees.DiscussionThe proposed simple clinical end-to-end accuracy evaluation model provides the surgeon with sufficiently accurate information. The evaluation model was able to identify and more importantly to quantify the clinically induced error. This study proves that ameliorating the reference system acquisition would improve the system's overall accuracy.

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