Intraoperative measurement of cup inclination using fluoroscopy: do we need a different correction factor when using...

The direct anterior approach (DAA) in THA allows for intraoperative fluoroscopy imaging (IF). A previous study has shown that the intraoperative determination of radiographic cup inclination (RI) using IF during DAA THA is reliable and reproducible. However, a 5° correction factor should be applied to intraoperative measurements to adjust for parallax when using a standard 12-inch GE 9900 elite C-arm (GE Healthcare, Chicago, IL). The current study evaluates if the use of a flat panel C-arm requires a different correction factor.

The current study included 112 patients who underwent primary THA utilizing DAA with IF for cup placement. RI was measured using intraoperative fluoroscopy images of two different C-arm models (12-inch GE 9900 elite C-arm and OEC Elite flat panel and compared to postoperative AP pelvis radiographs.

Using the 12-inch GE 9900 elite C-arm (n = 76) mean intraoperative RI measurements (35.5°, range 28-42°) were on average 6.4° lower compared to mean postoperative measurements (41.9°, range 36-49°). Using the OEC Elite flat panel C-arm (n = 36) mean intraoperative RI measurements (36.1°, range 29-40°) were on average 5.6° lower compared to mean postoperative measurements (41.7°, range 37-48°).

The current study confirms that a correction factor needs to be applied to intraoperative measurements of cup inclination to adjust for parallax regardless of the type of C-arm.