What Changes in Pelvic Sagittal Tilt Occur 20 Years After THA?

Background Cup orientation in THA in the supine, standing, and sitting positions is affected by pelvic sagittal tilt (PT). Patterns of PT shift between these positions may increase the risk of dislocation and edge loading. The PT has also been reported to change during the aging process; however, there is limited research regarding long-term changes in PT and PT shifts after THA. Questions/purposes (1) What changes occur in PT in the supine, standing, and sitting positions during 20 years of follow-up after THA in patients who have not had revision or dislocation? (2) What factors are associated with the differences between preoperative supine PT and postoperative sitting or standing PT (Delta sitting and Delta standing, respectively) 20 years postoperatively? Methods Between January 1998 and December 1999, 101 consecutive patients underwent THA for appropriate indications. AP radiographs of the pelvis in the supine, standing, and sitting positions preoperatively and at 1, 10, and 20 years after THA were longitudinally performed to evaluate changes in PT. Fifty-nine percent (60 of 101) of patients were lost before 20 years of follow-up or had incomplete sets of imaging tests, leaving 41% (41 of 101) eligible for analysis here. There were no patients who had recurrent dislocation or underwent revision arthroplasty in the cohort; therefore, this analysis regarding postoperative changes in PT indicates the natural course of the change in PT during follow-up of THA. PT was measured based on the anterior pelvic plane. PT shifts with positional changes, Delta standing, and Delta sitting during the follow-up period were calculated. Posterior changes and shifts are represented by negative values. To analyze the factors associated with Delta standing and Delta sitting after 20 years, the correlations between these parameters and preoperative factors (including sex, age, pelvic incidence [PI], lumbar lordosis [LL], preoperative PT, and preoperative PT shift) and post-operative factors (including the occurrence of new lumbar vertebral fractures, lum bar spondylolisthesis, contralateral THA performed during follow-up, and PI-LL 20 years after THA) were determined. Results Median (IQR) supine and standing PTs changed (moved posteriorly) by -5 degrees (-11 degrees to -2 degrees; p < 0.01) and -10 degrees (-15 degrees to -7 degrees; p < 0.01), respectively. Sitting PT did not change during the 20-year follow-up period. Median (IQR) PT shift from standing to sitting changed from -34 degrees preoperatively (-40 degrees to -28 degrees) to -23 degrees after 20 years (-28 degrees to -20 degrees). There were posterior changes in median (range) Delta standing (median -12 degrees at 20 years [-19 degrees to -7 degrees]); Delta sitting did not change during the follow-up period (median -36 degrees at 20 years [-40 degrees to -29 degrees]). Patients with a large preoperative posterior PT shift from supine to standing demonstrated larger posterior tilt of Delta standing at 20 years. Patients with lumbar vertebral fractures during follow-up demonstrated larger posterior tilt of Delta standing at 20 years. Conclusion Patients who demonstrate a large preoperative posterior shift from supine to standing deserve special consideration when undergoing THA. In such circumstances, we recommend that the anteversion of the cup not be excessive, given that there is a relatively high risk of further posterior tilt in PT, which may lead to anterior dislocation and edge loading. Further longitudinal study in a larger cohort of patients with complications including postoperative dislocation and revision, as well as older patients, is needed to verify these assumptions on the potential risk for dislocation and edge loading after THA.