[Discrepancy between radiographic and true cup position after total hip arthroplasty : Are we interpreting our radiographic quality indicators correctly? Video article].

BACKGROUND: After total hip arthroplasty (THA), objective postoperative quality control is done via X‑rays by as component position assessment. The cup position is defined by its version and inclination. However, there is a discrepancy between radiographically measured and true (anatomic) cup position, which may lead to misinterpretation.
METHODS: To visualize the discrepancy between true and radiographically measured cup position, in this video, a cup holder was used to set the angular cup version and inclination. Hereby, the cup position (anteversion and inclination) can be characterized in its radiographic and anatomic definition in greater detail. The viewer of this video should receive an impression as to when radiographically measured cup angles must be considered with caution.
RESULTS: In a simultaneous X‑ray and image sequence, this video shows decreased radiographic inclination measurement with increasing anterior rotation of the cup exceeding 20° of anteversion, yet with unchanged true inclination on the cup holder. Isolated consideration of the radiographic angles of anteversion and inclination may cause misinterpretation of true cup position. In pectoral illustration we show that variations in cup version and inclination may remain undetected when considering isolated the radiographic cup parameters.
CONCLUSION: True cup position in its anatomical definition can be calculated from the radiographically measured position. For this purpose, both cup parameters (radiographic anteversion and radiographic inclination) have to be taken into account.