How well do you know the biomechanics of the lumbar spine and sacroiliac joint?

Biomechanics and Arthrokinematics of the Lumbar Spine and Sacroiliac Joint

• The lumbar spine has degrees of motion in flexion (60 deg lumbar, 25 deg hips) and extension (35-65 deg), with muscle activity from erector spinae and abdominals.

• Sidebending in the lumbar spine is limited by contralateral flavum, supraspinous ligaments, and interspinous ligaments, with bilateral muscle activation.

• Lumbar rotation is limited by facets, bony impact, capsular stretch, and disc, with contralateral rotators and ipsilateral erector spinae muscle activity.

• The instantaneous axis of rotation (IAR) varies with pathology and changes location in each motion of the lumbar spine.

• The disc allows motion and transmits load between vertebrae, with the nucleus herniating through endplates.

• The sacroiliac joint (SIJ) is a diarthrosis joint, functionally representing an amphiarthrosis due to low mobility.

• Nutation and counter-nutation cause additional motion referred to as winging or flaring of the ilium, attributed to the SIJ alignment and sacrum shape.

• Form closure of the SIJ is created by friction of the articular surfaces and ligamentous structures, adding to horizontal stability.

• Force closure is aided by the strong interosseous ligaments, iliolumber ligaments, sacrotuberous ligaments, sacrospinous ligaments, and posterior SIJ ligaments.

• Muscles aid in stabilizing the SIJ by producing a compressional force that pulls the iliums toward each other and constrains nutation.

• The long dorsal ligament (LDL) aids in stability of the SIJ, with greater muscular support for preventing anterior rotation of the ilium than for posteriorly rotating it.

• Biomechanical regions of the spine have different motions and coupling patterns, with the transitional regions (T9-L2) being the most problematic for some people.Anatomy and Movement of the Innominate Axes and their Effects on Leg Length

• The respiratory axis of the ilia is at PSIS and occurs on a transverse axis in the coronal plane.

• The locomotion axis of the ilia is at the sacrum and occurs in the SI joint corresponding to inferior transverse axis of the sacrum.

• The locomotion axis of the pubis concurs with rotation of the pubic bones around a transverse axis through the symphysis in walking.

• The superior, middle, and inferior transverse sacral axes are important in different types of motion.

• The superior transverse axis is important in SI flexion and extension during respiration.

• The middle transverse axis is the principal axis of regular SI flexion and extension, and the inferior transverse axis is the principal axis of regular iliosacral movements in locomotion.

• Muscle and force couple with sacral motion, and the lumbar moves in the opposite direction of the pelvis.

• Malalignment/positioning of the SIJ can result in a functional leg length discrepancy, with a backward rotation of the ilium resulting in a functionally shorter leg when supine.

• A posterior rotation of the ilium can cause a lower PSIS, higher ASIS, high acetabulum, functionally shorter leg, L SB, R rot, R iliolumbar lig stretch, and lateral deviation of the acetabulum/ER of hip.

• An anterior rotation of the ilium can cause a functionally longer R leg, R sacral base tilts upward, narrowing of R IV foramina and compression of disc, medial deviation of the acetabulum/IR of hip, and R pubic symphysis displaced below.

• Posterior torsion is more prevalent than anterior torsion, and landing on outstretched leg/ischial tuberosity can force innominate into a posterior torsion.

• The inferior transverse axis is the axis around which innominate motion occurs, and the oblique axes run from one side of sacral base to opposite inferior sacrum, near ILA.

Test your knowledge of biomechanics and arthrokinematics of the lumbar spine and sacroiliac joint, as well as the anatomy and movement of the innominate axes and their effects on leg length. This quiz will cover the degrees of motion and muscle activity in the lumbar spine, the limited sidebending and rotation, the variations in the instantaneous axis of rotation, and the function and stability of the sacroiliac joint. Additionally, it will delve into the respiratory and