P6. Spinopelvic alignment changes between seated and standing positions in pre and post total hip replacement patients

BACKGROUND CONTEXT The inter-relationship between the hip and spine has been increasingly studied in recent years, particularly as it pertains to the effect of spinal deformity and hip osteoarthritis (OA). Changing from standing (ST) to seated (SE) requires rotation of the femur from an almost vertical plane to the horizontal. OA of the hip significantly limits hip extension, resulting in less ability to recruit pelvic tilt (PT) in ST, and requiring increased PT in SE to compensate for loss of hip flexion. To date, the effect of total hip arthroplasty (THA) in altering spinopelvic SE and ST mechanics has not been reported. PURPOSE To investigate the change in spinopelvic alignment parameters between seated and standing positions in pre and post THA patients. STUDY DESIGN/SETTING Retrospective review at a single academic institution. PATIENT SAMPLE Adult patients undergoing THA with full body sitting and standing radiographs pre- and post-THA. OUTCOME MEASURES Spinopelvic alignment measures including pelvic incidence (PI), pelvic tilt (PT), T1 pelvic angle (TPA), sacral slope (SS), sagittal vertical axis (SVA), pelvic incidence and lumbar lordosis mismatch (PI-LL), and lumbar lordosis (LL). METHODS Patients ≥18yo undergoing THA for hip OA with full spine SE and ST radiographs pre and post THA were included. Spinopelvic alignment was analyzed pre-THA and post-THA in both ST and SE positions in a relaxed posture with the fingers on the clavicles. Paired t-test analysis was performed to compare Pre-and Post-THA groups. The effect of TL deformity (SVA>50, TPA>20, PI-LL>10) on these changes was also analyzed. Statistical significance set at p RESULTS There were 192 patients assessed. 179 patients had thoracolumbar (TL) deformity; TPA>20 (N=46), PI-LL>10 (N=55), and SVA>50 (N=78). In standing position, patients have a significant reduction in SVA post THA vs pre THA (34.09±42.69 vs 45.03±46.87, p=0.001) as a result of an increase in PT (15.7±9.74 ᵒ vs 14.6±9.88ᵒ,p=0.028), without significant changes in spinal alignment parameters including lumbar lordosis (-51.26±14.59 vs -50.26±14.87, p=0.092), thoracic kyphosis (35.98±12.72 vs 35.40±13.16, p=0.180), sacral slope (38.15±10.77 vs 38.83±11.31, p=0.205), T1 pelvic angle (14.22±9.94 vs 14.51±10.13, p=0.053) and PI-LL mismatch (2.59±14.61 vs 3.35±14.92, p=0.183). This change in ST_SVA was larger in patients with TL deformity, specifically in those with SVA>50 (61.29±45.69 vs 89.48±35.91, p=0.001), in PI-LL > 10 (59.08±45.49 vs 73.36±48.50, p=0.001) and in TPA>20 subsets (62.14±49.94 vs 82.28±49.55, p=0.001). When moving from ST to SE, the ΔPT was reduced post THA (16.70±15.27ᵒ vs 20.85±17.27ᵒ, p=0.001) in addition to a smaller SE_PT vs pre-THA (32.41±14.47 vs 35.46±14.20, p=0.006). CONCLUSIONS Post Total Hip Arthroplasty (THA), patients demonstrated an increased recruitment of pelvic retroversion to achieve a better global balance by reduction in standing SVA. This compensation was achieved solely by greater mobility of their hip and pelvis, and without a significant change in spinal alignment. ST_SVA reduction was more pronounced in patients with thoracolumbar (TL) spinal deformity (SVA>50, TPA>20, PI-LL>10). On the converse, PT was reduced in sitting (SE) post-THA compared to pre-THA, and the compensatory change in PT was also reduced between ST and SE as a result of restoration of hip flexion. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.