Fusion angle affects intervertebral adjacent spinal segment joint forces-Model-based analysis of patient specific...

Area of Science:

• Biomechanics
• Spinal Fusion
• Orthopaedic Research

Background:

• Adjacent segment degeneration is a common complication after lumbar fusion.
• The precise mechanisms driving this degeneration, particularly the role of altered biomechanical loading, remain unclear.
• Understanding how surgical alignment affects spinal joint loads is crucial for mitigating post-fusion complications.

Purpose of the Study:

• To investigate the hypothesis that surgically imposed changes in lumbar lordosis influence adjacent intervertebral joint loads.
• To quantify the effects of different fusion lordosis angles on adjacent segment loading and muscle moment arms.
• To determine the optimal fusion alignment for minimizing adverse biomechanical consequences.

Main Methods:

• Development of generic and patient-specific (n=8) kinematics-driven musculoskeletal models.
• Simulation of L4/L5 fusion with in situ (22°), hyperlordotic (32°), and hypolordotic (8°) lordosis.
• Analysis of joint loads and muscle moment arms during simulated bending motion (standing to 45° flexion).

Main Results:

• All fusion types altered adjacent segment (L3/4) joint loads and muscle moment arms.
• Hypolordotic fusion increased L3/4 shear forces by 29% on average.
• Hyperlordotic fusion reduced L3/4 shear forces by 39%, with in situ fusion yielding loads closest to intact conditions.
• Reduced lordosis significantly increased adverse loading at cranial adjacent levels.

Conclusions:

• Surgically imposed changes in lumbar lordosis during fusion significantly alter adjacent segment biomechanics.
• Hypolordotic fusion configurations appear detrimental, increasing shear forces and potentially accelerating degeneration.
• Maintaining or restoring appropriate lumbar lordosis during fusion is critical for minimizing adverse loading and preserving adjacent segment health.