Small differences between anatomical and mechanical sagittal femur axes: a radiological and navigated study of 50 patients.

Background And Purpose: The purposes of the study were to define the orientation of the anatomical and mechanical axes of the femur on the lateral plane on long leg X-rays including the femoral head, and to compare these results to the intra-operative measurement of the mechanical axis by a nonimage-based navigation system.

Methods: Fifty cases of patients operated on for total knee replacement were included. All patients had digital pre- and post-operative long leg lateral X-rays including the femoral head. All prostheses were implanted with a nonimage-based navigation system. Five sagittal femur axes were measured on pre- and post-operative X-rays. The sagittal orientation of the distal femoral resection was measured by the navigation system and on the post-operative X-rays.

Results: Significant, but small differences were observed in the orientation of the different axes on the pre- and post-operative X-rays. The correlation and the agreement between all axes were good. There was no significant modification on the post-operative X-rays. There were significant, but small differences between the navigated and radiological orientation of the distal femur resection, but the correlation and the agreement were good.

Interpretation: The differences observed are small and have probably little clinical relevance. The distal cortical axis may be the best compromise. The navigation system allows measuring accurately the orientation of the distal femoral resection in the sagittal plane.