What causes the motion of the superior, middle, and inferior sacral axis?

Biomechanics of the Sacroiliac Joint: Motion Axes and Their Mechanisms

The motion of the superior, middle, and inferior sacral axes is primarily caused by the coordinated action of surrounding muscles, particularly the gluteus maximus and biceps femoris, which provide dynamic stabilization during weight-bearing activities and gait. 1, 2

Anatomical Basis of Sacroiliac Joint Motion

The sacroiliac joint (SIJ) is a true diarthrodial joint with unique characteristics:

• Contains both fibrocartilage and hyaline cartilage
• Has discontinuity of the posterior capsule
• Features articular surfaces with numerous ridges and depressions
• Well innervated by branches from ventral rami of L4-L5, superior gluteal nerve, and dorsal rami of L5, S1, and S2 3

Motion Characteristics by Axis

1. Superior Sacral Axis:

   • Primarily rotational movement
   • Range of motion during trunk flexion: approximately 0.07° around x-axis in healthy individuals, increasing to 0.57° in those with degenerative lumbar spine disorders 4
   • Motion facilitated by gluteus maximus fibers oriented perpendicular to joint surfaces

2. Middle Sacral Axis:

   • Exhibits minimal motion (approximately -0.02° around y-axis during flexion)
   • Acts as a pivot point for weight transfer between spine and lower extremities
   • Stabilized by surrounding ligamentous structures 4, 5

3. Inferior Sacral Axis:

   • Demonstrates approximately 0.16° rotation around z-axis during trunk flexion
   • Motion increases to 0.19° in patients with degenerative conditions
   • Influenced by biceps femoris activation through its attachment to sacrotuberous ligament 1, 4

Muscular Control of Sacral Motion

The motion of all three sacral axes is primarily controlled by:

• Gluteus maximus: Ideally oriented fibers provide primary stabilization during weight-bearing
• Biceps femoris: Influences joint stability through its proximal attachment to sacrotuberous ligament
• Trunk muscles: Negative correlation between trunk muscle cross-sectional area and SIJ motion 4

Gender Differences in Sacral Motion

Female SIJs demonstrate important differences:

• Wider, more uneven, less curved, and more backward-tilted sacrum
• Higher mobility (particularly during trunk extension)
• Greater stresses/loads and pelvic ligament strains
• More pronounced motion during pregnancy 4, 5

Clinical Implications

Understanding sacral axis motion is crucial for:

1. Diagnostic Imaging:

   • MRI is the preferred modality for evaluating SIJ inflammation and structural changes 6, 7
   • Radiographs have limited sensitivity (19-72%) for early disease detection 7

2. Therapeutic Approaches:

   • Conservative management should focus on pelvic stabilization exercises
   • Muscle balancing of trunk and lower extremities is essential 3
   • Interventional treatments may include intra-articular injections when appropriate 6

3. Pathological Considerations:

   • Hypermobility can result from ligamentous instability or adaptive biomechanical changes 2
   • Malrecruitment of gluteus maximus and compensatory biceps femoris overactivation may lead to joint instability 1

Biomechanical Principles

The SIJ transfers large bending moments and compression loads to lower extremities with limited inherent stability against shear loads. Stability is maintained through:

• Tight wedging of sacrum between hip bones
• Strong ligamentous support spanning sacrum and hip bones
• Dynamic muscular control during movement 5

Normal SIJ range of motion is minimal but essential for proper function:

• Flexion-extension: approximately 3°
• Axial rotation: approximately 1.5°
• Lateral bending: approximately 0.8° 5

Understanding these biomechanical principles is essential for proper diagnosis and management of SIJ-related pain and dysfunction.