Pelvic motion the key to understanding spine-hip interaction

Area of Science:

• Orthopedic Surgery
• Biomechanics
• Spinal and Pelvic Anatomy

Background:

• Pelvic motion is vital for balance during bipedalism and compensates for spinal malalignment.
• Pelvic rotation affects acetabular orientation, impacting total hip arthroplasty (THA) cup positioning.
• Degenerative spinal changes often lead to pelvic retroversion, increasing acetabular version.

Purpose of the Study:

• To analyze the relationship between pelvic orientation, spinal mobility, and functional acetabular version in the context of THA.
• To identify patient subgroups (spine users vs. hip users) based on spinal flexibility and their implications for THA.
• To provide guidance on optimizing acetabular cup positioning in THA based on individual pelvic incidence and spinal characteristics.

Main Methods:

• Review of biomechanical principles of pelvic motion and its effect on acetabular orientation.
• Analysis of how spinal malalignment and compensatory pelvic rotation influence functional acetabular version.
• Discussion of the implications of pelvic incidence (PI) and pelvic tilt (anteversion/retroversion) on THA dislocation risk.
• Consideration of patient phenotypes: flexible spine ('spine users') versus stiff/fused spine ('hip users').

Main Results:

• Pelvic retroversion, common in spinal degeneration, increases acetabular version, potentially risking anterior dislocation in THA.
• Patients with stiff spines ('hip users') place higher demands on the hip joint, increasing THA risk.
• Pelvic anteversion or low PI may increase the risk of posterior dislocation during sitting.
• Optimal acetabular ante-inclination varies with PI; low PI may require more ante-inclination, high PI more retroversion.

Conclusions:

• Acetabular cup positioning in THA must account for individual pelvic incidence and spinal mobility.
• Pelvic orientation significantly influences THA stability, affecting dislocation risk in both standing and sitting positions.
• Surgical approach for THA may need to prioritize spinal correction in cases of significant compensatory pelvic retroversion before addressing the hip.