How annulus defects can act as initiation sites for herniation.

Area of Science:

• Biomechanics
• Spinal Research
• Medical Imaging

Background:

• Intervertebral disc (IVD) herniation is a common cause of low back pain.
• Understanding the mechanisms of disc failure, including the role of posture and loading rate, is crucial.
• High-resolution magnetic resonance imaging (HR-MRI) shows promise in visualizing disc microstructural changes.

Purpose of the Study:

• To investigate the correlation between HR-MRI visible disruptions and microstructural features in ovine lumbar spinal segments.
• To elucidate the mechanism by which disc disruption weakens the annulus and contributes to herniation.
• To gain new insights into the process of intervertebral disc herniation.

Main Methods:

• Thirty ovine lumbar spinal segments were subjected to dynamic loading simulating flexion, lateral bending, axial rotation, and compression.
• Discs were analyzed using ultra-high field (11.7 T) HR-MRI and brightfield microscopy.
• Loading conditions were applied for 1000 cycles at 2 Hz.

Main Results:

• Four discs herniated, and seven showed nucleus displacement, all associated with pre-existing defects in the posterior annulus.
• HR-MRI findings corresponded well with microstructural observations of annular disruption.
• The mid-outer annular-endplate junction failure was observed in all examined discs.

Conclusions:

• Pre-existing annular defects are critical for disc herniation, weakening the inner and mid annulus.
• Mechanical compromise of the mid and outer annulus in the presence of defects leads to herniation.
• These findings suggest a potential mechanism for disc herniation within the degenerative cascade.