Decreased "Polar Axis Angle" is Associated With Instability After Total Hip Arthroplasty: A New Method to Assess...

Area of Science:

• Orthopedic surgery
• Biomechanical analysis
• Arthroplasty research

Background:

• Hip instability research traditionally focuses on supine component positioning and hip-spine relationships.
• The polar axis angle (PAA) offers a novel measure of component concentricity, evaluable in functional, high-risk positions like sitting.
• Understanding PAA in functional positions may reveal new insights into total hip arthroplasty (THA) instability.

Purpose of the Study:

• To compare the polar axis angle (PAA) in functional positions between patients experiencing postoperative dislocation and a matched control group.
• To investigate the relationship between PAA, component positioning, and hip instability after total hip arthroplasty (THA).

Main Methods:

• Retrospective review of primary THA patients experiencing dislocation.
• Matching of dislocator patients with a control group (2:1) based on age, BMI, sex, and hip-spine classification.
• Radiographic analysis of neck angle, acetabular anteversion, and PAA in both standing and lateral-seated positions.

Main Results:

• Dislocator group showed significantly lower lateral-seated neck angle and PAA compared to controls.
• Significant differences observed in the change of polar axis and neck angles between standing and seated positions.
• No differences in component angles were found between mobile and stiff spine subgroups within the dislocator group.

Conclusions:

• A significantly lower PAA in lateral-seated radiographs is associated with postoperative hip instability after THA.
• Decreased functional femoral anteversion is a primary driver of the lower seated PAA, underscoring its role in THA stability.