The Safe Zone Range for Cup Anteversion Is Narrower Than for Inclination in THA

BackgroundCup malposition is a common cause of impingement, limitation of ROM, acceleration of bearing wear, liner fracture, and instability in THA. Previous studies of the safe zone based on plain radiographs have limitations inherent to measuring angles from two-dimensional projections. The current study uses CT to measure component position in stable and unstable hips to assess the presence of a safe zone for cup position in THA.Questions/purposes(1) Does acetabular component orientation, when measured on CT, differ in stable components and those revised for recurrent instability? (2) Do CT data support historic safe zone definitions for component orientation in THA?MethodsWe identified 34 hips that had undergone revision of the acetabulum for recurrent instability that also had a CT scan of the pelvis between August 2003 and February 2017. We also identified 175 patients with stable hip replacements who also had a CT study for preoperative planning and intraoperative navigation of the contralateral side. For each CT study, one observer analyzed major factors including acetabular orientation, femoral anteversion, combined anteversion (the sum of femoral and anatomic anteversion), pelvic tilt, total offset difference, head diameter, age, sex, and body mass index. These measures were then compared among stable hips, hips with cup revision for anterior instability, and hips with cup revision for posterior instability. We used a clinically relevant measurement of operative anteversion and inclination as opposed to the historic use of radiographic anteversion and inclination. The percentage of unstable hips in the historic Lewinnek safe zone was calculated, and a new safe zone was proposed based on an area with no unstable hips.ResultsAnteriorly unstable hips compared with stable hips had higher operative anteversion of the cup (44 degrees 12 degrees versus 31 degrees 11 degrees, respectively; mean difference, 13 degrees; 95% confidence interval [CI], 5 degrees-21 degrees; p = 0.003), tilt-adjusted operative anteversion of the cup (40 degrees +/- 6 degrees versus 26 degrees +/- 10 degrees, respectively; mean difference, 14 degrees; 95% CI, 10 degrees-18 degrees; p < 0.001), and combined tilt-adjusted anteversion of the cup (64 degrees +/- 10 degrees versus 54 degrees +/- 19 degrees, respectively; mean difference, 10 degrees; 95% CI, 1 degrees-19 degrees; p = 0.028). Posteriorly unstable hips compared with stable hips had lower operative anteversion of the cup (19 degrees +/- 15 degrees versus 31 degrees +/- 11 degrees, respectively; mean difference, -12 degrees; 95% CI, -5 degrees to -18 degrees; p = 0.001), tilt-adjusted operative anteversion of the cup (19 degrees +/- 13 degrees versus 26 degrees +/- 10 degrees, respectively; mean difference, -8 degrees; 95% CI, -14 degrees to -2 degrees; p = 0.014), pelvic tilt (0 degrees +/- 6 degrees versus 4 degrees +/- 6 degrees, respectively; mean difference, -4 degrees; 95% CI, -7 degrees to -1 degrees; p = 0.007), and anatomic cup anteversion (25 degrees +/- 18 degrees versus 34 degrees +/- 12 degrees, respectively; mean difference, -9 degrees; 95% CI, -1 degrees to -17 degrees; p = 0.033). Thirty-two percent of the unstable hips were located in the Lewinnek safe zone (11 of 34; 10 posterior dislocations, one anterior dislocation). In addition, a safe zone with no unstable hips was identified within 43 degrees +/- 12 degrees of operative inclination and 31 degrees +/- 8 degrees of tilt-adjusted operative anteversion. However, the results demonstrate that the historic Lewinnek safe zone is not a reliable predictor of future stability. Analysis of tilt-adjusted operative anteversion and operative inclination demonstrates a new safe zone where no hips were revised for recurrent instability that is narrower for tilt-adjusted operative anteversion than for operative inclination. Tilt-adjusted operative anteversion is significantly different between stable and unstable hips, and surgeons should therefore prioritize assessment of preoperative pelvic tilt and accurate placement in operative anteversion. With improvements in patient-specific cup orientation goals and acetabular component placement, further refinement of a safe zone with CT data may reduce the incidence of cup malposition and its associated complications.Level of Evidence:Level III, diagnostic study.