Spinopelvic mobility affects outcome after THA. Whether the sacro-femoral-pubic (SFP) angle, measured on AP radiographs, can be reliably used to estimate pelvic tilt (PT) in hip osteoarthritis patients is unknown. This study aimed to (1) validate the use of the SFP angle in the calculation of PT from AP radiographs, and (2) identify individual patient factors affecting the estimation of PT. A cohort of 100 patients awaiting THA for end-stage hip osteoarthritis was prospectively studied. AP and lateral radiographs, taken in the standing and relaxed-seated positions were evaluated for spinopelvic measurements (SFP, PT, and pelvic incidence [PI]). To validate the SFP angle, estimated PT values using the formula [PT = 75°-SFP] were compared to the true, measured values from the lateral radiographs. Despite good agreement for the estimated and true PT (16.2 ± 5.9° vs. 15.5 ± 8.6°; p = .315), a significantly poorer agreement could be found between the two methods at high or low values of PT. Patient-specific PI correlated with the difference between the two measurement methods (Pearson's r = -0.644; p < .001). However, the change in SFP angle equaled approximately the change in pelvic tilt (∆PT = 2°-∆SFP; Pearson's r = -0.934; p < .001). Absolute values for the sagittal PT should not be estimated from AP pelvic radiographs in patients awaiting total hip arthroplasty. However, the relative change in PT between different positions equals approximately the change in SFP angle. This may allow functional cup orientation after THA to be determined between different postures from an AP radiograph of the pelvis. The SFP angle has moderate accuracy in determining a patient's pelvic tilt; however, it can accurately determine a patient's change in pelvic tilt in different positions.
Keywords: arthroplasty; hip; pelvis; spine; spinopelvic mobility.
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