A correction factor is needed for the intraoperative measurement of cup inclination in total hip arthroplasty

The cup inclination in total hip arthroplasty is key to minimising complications. Stereometric effects (parallax) in two-dimensional projections can alter angle measurements. Even for different approaches, fluoroscopy causes different results in the measurement of inclination. A previous study has introduced a corrective factor for intraoperative radiographic cup inclination measurements compared to the postoperative standing radiographs.

The aim of this study was to find out whether, first, the correction factor is reproducible and second, whether the correction factor is independent of the surgical approach and C-arm model used.

A series of 377 cases of primary total hip arthroplasty was reviewed. We compared the cup inclination angle in the intraoperative and postoperative radiographic images. Based on this, it was possible to specify a standard of correction factor in defined ranges.

The mean cup inclination in intraoperative images was 37.47∘ and the mean angle in postoperative images was 41.42, resulting in a mean difference of 3.95∘ with a strong correlation (r= 0.706).

An added correction factor of 4∘ should be respected in intraoperative inclination measurements to adapt for parallax. An increased correction factor of 6∘ in particularly low-positioned cups (< 31∘) and a decreased correction factor of 2∘ in particularly steeply positioned cups (> 43∘) is recommended.