Spine Stiffness Leads to High Pelvic Mobility: Uncoupling Native Mechanics and Explaining Why Patients With Stiff...

Area of Science:

• Orthopedics and Biomechanics
• Spine and Hip Biomechanics
• Radiographic Analysis

Background:

• Stiff spines increase instability risk after total hip arthroplasty (THA) due to pelvic stiffness.
• Understanding compensatory mechanisms in stiff-spine patients without hip arthritis is crucial for surgical guidance.

Purpose of the Study:

• To characterize static and dynamic compensatory mechanics in stiff hip or stiff spine patients.
• To determine optimal spinopelvic imaging modalities for uncoupling compensation.
• To assess the impact of spinal fusion length on compensatory mechanics.

Main Methods:

• Prospective, case-control study with three groups: control (n=52), hip osteoarthritis treated with THA (n=52), and post-spinal fusion (n=52).
• Static and dynamic spinopelvic parameters (lumbar lordosis, pelvic tilt, pelvic-femoral angle) were measured using standing, relaxed-, and deep-seated radiographs.
• Correlation between fused spinal segments and spinopelvic parameters was analyzed.

Main Results:

• Spine group exhibited higher standing pelvic-femoral angles and pelvic tilt compared to control and hip groups.
• The spine group showed significantly less lumbar flexion and greater pelvic tilt change between standing and deep-seated positions.
• Deep-seated hip flexion was greater in the spine group; standing to deep-seated assessments effectively uncoupled compensatory mechanisms.

Conclusions:

• Patients with stiff spines demonstrate significant alterations in hip and pelvic mechanics, particularly a large pelvic tilt change.
• This pelvic tilt change may lead to suboptimal acetabular cup orientation, increasing risks of impingement or dislocation post-THA.
• Deep-seated radiographs are recommended for better identification of spinal stiffness and associated compensatory mechanisms.