From Our Perspective…Why Total Hip Arthroplasties Dislocate After Spinal Fusion: Functional Mechanics Uniting Hip and Spine Surgery

Background Spinopelvic alignment influences spinal fusion and hip replacement planning yet integrative biomechanical principles remain quantitatively elusive. This study models an acetabular component functionally within the postural system, determining individualized cup orientations in a novel fashion utilizing spinal pelvic tilt (PT) parameters, and quantitatively defines acetabular positional changes following spinal surgery. Methods Intrinsic spinopelvic mechanics of an implanted hemispherical component was studied using applications of a triangular construct consisting of pelvic and acetabular parameters. Utilizing Pelvic Incidence (PI), spinal Pelvic Tilt (PT), Pelvic Acetabular Angle (PAA), Pelvic Femoral Angles (PFA), in standing and sitting, normative sagittal cup Anteinclination (AI) angles were geometrically calculated. Corrections keeping cup AI and PAA optimally oriented for changes in PT and pelvic mobility were determined. Acetabular component angle changes resulting from corrective spinal PT and mobility alterations were derived. Results Cup position tilt adjustment of PT:AI occurs by ratio of 4°:1°, a 4° PI increase for any given PT angle also decreases AI 1°, termed “The Spinopelvic Rule of 4s”. The quotient (AI-PT) bridges the tilt and mobility sensitive cup relationships to the moving pelvis, through the equation, PAA=90-(AI-PT). These relationships revert to a new fixed anatomic PT:AI ratio of 1°:1° following component implantation, subsequent PT change by spinal procedures retrovert the cup AI 0.75° per 1° PT decrease from the new postsurgical optimal orientation. Additionally, in sitting 0.5° retroversion also occurs for each 1° of pelvic mobility lost. Conclusion Hip and spine surgery are biomechanically interwoven and quantifiable through sagittal plane angular functional relationships. Through understanding these conditional ratios of angular change between the hip angles PT, AI, and PAA, changes above and below the pelvis can be fully integrated. This supports common risk planning algorithms to proactively address high-risk patients with dual disease from post-spinal fusion dislocations. Clinical Relevance Longer PT altering lumbar fusions, particularly crossing the sacrum, should be performed prior to THA due to the substantial alterations in pelvic tilt and mobility. When an acetabular component is present prior to multilevel spine fusions, its orientation should be analyzed during planning to assess risk and minimize postoperative posterior hip dislocations.