The Diagnosis of and Preoperative Planning for Rapidly Progressive Osteoarthritis of the Hip: The Role of Sagittal Spinopelvic Geometry and Anterior Acetabular Wall Deficiency—A Prospective Observational Study

<b>Background/Objectives</b>: Rapidly progressive osteoarthritis of the hip (RPOH) has unique diagnostic and surgical challenges due to rapid joint degeneration and acetabular structural alterations. This study aimed to investigate correlations between preoperative spinopelvic geometry and anterior acetabular wall bone stock deficiency in RPOH patients and introduce an advanced imaging measurement techniques for cases with amputated femoral heads. <b>Methods</b>: A prospective observational study was conducted that enrolled 85 patients, comprising 40 with unilateral RPOH (Zazgyva Grade II or III) and 45 controls with primary osteoarthritis (OA). Preoperative spino-pelvic parameters (pelvic tilt-PT, sacral slope-SS, lumbar lordosis-LL, and T1 pelvic angle) and acetabular anterior wall characteristics (anterior center edge angle-ACEA, anterior wall index-AWI, and anterior acetabular surface area-AASA) were measured using standardized radiographic and CT imaging protocols, including a new methodology for acetabular center estimation in femoral head-amputated cases. <b>Results</b>: Significant differences were identified between RPOH and primary OA patients in the PT (22.5° vs. 18.9°, <i>p</i> = 0.032), SS (37.8° vs. 41.1°, <i>p</i> = 0.041), T1 pelvic angle (14.3° vs. 11.8°, <i>p</i> = 0.018), and anterior center edge angle (25.3° vs. 29.7°, <i>p</i> = 0.035). RPOH patients exhibited pronounced spinopelvic misalignment and anterior acetabular deficiencies. <b>Conclusions</b>: RPOH is associated with spinopelvic misalignment and anterior acetabular wall deficiency. Accurate preoperative diagnosis imaging and personalized surgical approaches specifically addressing acetabular bone stock deficiencies are mandatory in these cases.