Does hip structural damage affect the lower extremity compensations in ankylosing spondylitis patients with thoracolumbar kyphosis before and after pedicle subtraction osteotomy?

To investigate impact of hip involvement on lower extremity compensations in ankylosing spondylitis (AS) patients with thoracolumbar kyphosis before and after pedicle subtraction osteotomy (PSO) and to explore a spinopelvic parameter for evaluating the extent of lower extremity compensations in AS patients. A retrospective study was conducted among AS patients with thoracolumbar kyphosis underwent PSO from April 2017 to November 2019. Radiographic parameters including global kyphosis (GK), sagittal vertical axis (SVA), pelvic incidence (PI), pelvic tilt (PT), sacral slope (SS), sacrofemoral angle (SFA), knee angle (KA), ankle angle (AA), and femoral obliquity angle (FOA) were measured on EOS images. Based on Bath Ankylosing Spondylitis Radiology Hip Index (BASRI-h), all patients were divided into mild hip involvement (MHI) group (BASRI-h:0–2) and severe hip involvement (SHI) group (BASRI-h:3–4). Moreover, patients were divided into 5 subgroups depended on the BASRI-h degree. Fifty-seven patients were recruited in current study. Except for SFA, larger KA, AA, and FOA were observed in group SHI pre- and postoperatively (all P < 0.05). FOA in both groups were associated with KA before and after PSO (all P < 0.001). In subgroup analysis, a linear trend between KA, AA, FOA and the severity of hip involvement could be observed pre- or postoperatively. Lower extremity compensations play a critical role in maintaining sagittal balance in AS patients with severe hip involvement. Residual lower extremity compensations were observed in patients with higher degree hip involvement postoperative. FOA could be served as a reliable value for evaluating lower extremity compensations in AS patients with hip involvement.