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.