Sagittal Spinopelvic Translation Is Combined With Pelvic Tilt During the Standing to Sitting Position: Pelvic Incidence Is a Key Factor in Patients Who Underwent THA

BACKGROUND: Sagittal spinopelvic translation (SSPT) is the horizontal distance from the hip center to the C7 plumb line (C7PL). SSPT is an important variable showing the overall patient balance in different functional positions which could affect the rate of hip instability. This study investigates the SSPT modification in patients who underwent total hip arthroplasty (THA). METHODS: A total of 120 patients were assessed preoperatively and postoperatively on standing and sitting acquisitions (primary unilateral THA without complication). SSPT is zero when the C7PL goes through the center of the femoral heads and positive when the C7PL is posterior to the hips’ center (negative if anterior). Three subgroups were defined based on the pelvic incidence (PI): low PI <45°, 45°< normal PI <65°, or high PI >65°. RESULTS: The overall mean preoperative SSPT change from standing to sitting was 2.2 cm ([-7.2 to 17.4]) (P < .05). The overall mean postoperative SSPT change from standing to sitting was 1.2 cm ([-14.2 to 22.4]) (P < .05). In low- and normal-PI groups, standing to sitting SSPT and preoperative to postoperative changes in standing SSPT were increased significantly after surgery with the C7PL behind the hips’ center (P < .05). In the high-PI group, standing to sitting SSPT was increased postoperatively (P = .034) (no significant changes from preoperative to postoperative status in standing and sitting). CONCLUSIONS: Adaptation from standing to sitting positions combines pelvic tilt and anteroposterior pelvic translation. THA implantation induces significant changes in SSPT mainly for low- and standard-PI patients. This is an important variable to consider when investigating the causes of THA subluxation or dislocation.