Impact of Flatback Deformity and Stiff Spinopelvic Mobility on 3-Dimensional Pelvic and Hip Kinematics after Total Hip Arthroplasty

Background:Spinopelvic abnormalities have been reported to be a risk factor for dislocation after total hip arthroplasty (THA). This study aimed to compare the kinematics of the pelvis and hip joints in patients with and without spinopelvic abnormalities after THA and to elucidate dynamic forward-leaning movement during chair-rising, which are not detectable through static radiographs.Methods:This case series included 108 hips that underwent dynamic anteroposterior radiographic imaging of the sit-to-stand motion after THA. The average age at surgery was 68 ± 10 years, with 95 hips (88%) in women (average body mass index, 23.5 ± 3.2 kg/m2). Kinematic analysis was performed to measure the anterior pelvic plane angle (APPa) and hip flexion/extension angles from seated to standing positions using model-image registration techniques. Pelvic incidence (PI) and lumbar lordosis (LL) were measured to calculate PI-LL.Results:Flatback deformity was present in 45 hips (42%) and stiff spinopelvic mobility (SPM) in 35 hips (32%), with both deformities present in 21 hips (19%). The pelvis was consistently significantly posteriorly tilted in the flatback deformity group throughout the movement compared with the normal group, with the greatest difference observed in the standing position. The hip flexion angles in the flatback deformity group showed significant extension in the standing position (7° greater than that in the normal group). For stiff SPM, a significant posterior tilt in the standing position was observed. Accordingly, the range between the maximum hip flexion and extension was 13° greater. There was no significant difference between the maximal flexion and extension centers.Conclusions:Patients with flatback deformities consistently exhibited posterior APPa, especially when standing. In stiff SPM, a large range of hip flexion and extension while chair-rising increased the risk of impingement, indicating the necessity for a wider range of motion without changing the target orientation. These findings highlight the importance of considering spinopelvic alignment when planning cup positioning in THA to minimize the risk of dislocation.Level of Evidence:Level III. See Instructions for Authors for a complete description of levels of evidence.