Intraoperative patient movement is a common but underappreciated phenomenon in total hip arthroplasty (THA). Such movement can significantly affect the accuracy with which the acetabular cup component is implanted. To evaluate the effect of intraoperative movement on cup position, we performed a study using mathematical modeling to simulate intraoperative movement. Mathematical simulations were used to simulate pelvic movements during THA. Pelvic axial rotation, tilt, and obliquity were simulated, and the resulting changes in intended cup position were calculated. The rate of change of inclination and anteversion per degree of pelvic movement was calculated, establishing a ratio relating cup angle sensitivity to pelvic movement. These sensitivities were used to construct nomograms showing the per-degree effect of pelvic movement on cup position. The effect of pelvic movement on cup position was multifactorial and dependent on the intended cup orientation. For a cup intended to be inserted at 15° anteversion and 40° inclination, each degree of pelvic rotation induced changes of 0.64° and –0.20°, respectively. For this same cup orientation, each degree of pelvic tilt induced changes of 0.77° for anteversion and 0.17° for inclination. Pelvic obliquity was associated with a 1:1 ratio with inclination, with each degree of obliquity inducing 1° of change in inclination. Anteversion was unaffected by changes in pelvic obliquity. This study demonstrates the consequences of undetected pelvic movement on cup position, including the increased risk of the acetabular component being placed in an orientation that could increase wear or the likelihood of impingement and dislocation.
Despite inaccuracies due to artifact and variations in patient positioning, anteroposterior (AP) radiographs remain the clinical standard for post-operative evaluation of component placement following total hip arthroplasty (THA). However, cup position, specifically anteversion, can be significantly affected by variations in patient positioning on an X-ray. A major cause of such artifact is unaccounted for pelvic tilt. Several methods for correcting the effects of pelvic tilt on radiographic anteversion have been proposed, with varying degrees of accuracy. The purpose of this study was to evaluate the accuracy and reliability of a commonly referenced method for correcting acetabular cup anteversion in a cohort undergoing total hip arthroplasty and determine its appropriateness for use in this population of patients. Radiographs from patients who underwent primary or revision hip arthroplasty between February 2016 and February 2017 were retrospectively reviewed. Corrected anteversion was calculated by measuring the vertical distance between the symphysis pubis and the sacrococcygeal joint, per the method outlined by Tannast et al. This symphococcygeal distance was then applied to Tannast’s nomograms to calculate the magnitude of pelvic tilt. Corrected and uncorrected anteversion values were compared to anteversion values collected intraoperatively using an imageless computer-assisted navigation device. A total of 71 cases were initially eligible for inclusion in the study. The correction method could not be applied in 44% (31/71) of the cases, chiefly due to difficulties in visualizing the required landmarks. In cases where it could be applied, corrected values correlated very poorly with navigation measurements (r = -0.07). Mean corrected anteversion (36.9°, SD: 7.4°) differed from uncorrected anteversion (25.2°, SD: 7.6°) by an average of 13.5° (p<0.001). Mean navigated anteversion (27.4°, SD: 5.7°) differed from corrected values by an average of 10.8° (p=0.16). The evaluated correction method could not be consistently applied to radiographs and did not reliably correct anteversion due to pelvic tilt in this population of patients undergoing hip arthroplasty. This correction method does not appear to be appropriate for use in this patient population.
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