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A hybrid CT-free navigation system for total hip arthroplasty.

OBJECTIVE
To design and evaluate a novel CT-free image-guided surgical navigation system for assisting placement of both acetabular and femoral components in total hip arthroplasty (THA).


MATERIALS AND METHODS
The methodology in this paper is conceptually based on our previous work on CT-free cup placement. For femoral component placement, two patient-specific reference coordinate systems are first defined: One for the pelvis, based on the so-called anterior pelvic plane (APP) concept, and one for the femur, using the center of the femoral head, the posterior condylar tangential line, and the medullary canal axis of the proximal femur. A hybrid method is used for the associated landmark acquisition, which involves percutaneous point-based digitization and bi-planar landmark reconstruction using multiple registered fluoroscopy images. The following clinical parameters are computed in real time: cup inclination and anteversion, antetorsion and varus/valgus of the stem, lateralization, and change in leg length for complete THA. In addition, instrument actions such as reaming, impaction, and rasping are visualized for the surgeon by superimposing virtual instrument representations onto the fluoroscopic images.


RESULTS
A laboratory study of computer-assisted measurement of antetorsion and varus/valgus, change in leg length, and lateralization for femoral stem placement demonstrated the high precision of the proposed navigation system. Compared with CT-based measurement, mean deviations of 1.0 degrees, 0.6 degrees, 0.7 mm, and 1.7 mm were found for antetorsion, varus/valgus, change in leg length, and lateralization, respectively, with standard deviations of 0.5 degrees, 0.5 degrees, 0.6 mm, and 0.7 mm, respectively. A pilot clinical evaluation showed that THA could benefit from this newly developed CT-free hybrid system.


CONCLUSIONS
The proposed CT-free hybrid system promises to increase the accuracy and reliability of THA surgery.