Functional Anatomy of the Pelvis

Weight transfer through pelvis

Sacrum suspended between innominate bones, weight pushes sacrum inferiorly and is wedged between bones, irregular joint surfaces interlock, ligaments come under tension and holds ilia tight against sacrum, sacroiliac joint self locks

Sacroiliac joint movements due to forces

Small magnitude gliding and rotation movement, anterior rotation of superior sacrum - force applied, anterior rotation of inferior sacrum - force removed

Nutation

Occurs when sacrum absorbs shock - causes superior sacrum to rotate anteriorly, coccyx moves posteriorly relative to ilium - motion opposed by wedge shape of sacrum and ridges/depressions of articular surfaces and posterior sacroiliac, interosseous sacroiliac, sacrospinous, sacrotuberous ligaments

Counternutation

Body’s response to lifting the joint against gravity (taking the weight off), limited movement due to bony architecture and ligaments of the joint - motion opposed by posterior sacroiliac ligament and multifidus muscle

Where does stability of pelvis come from

Stability for high loads transferred through pelvis joints from form and force closure

Form closure

Stability of joint due to pelvic anatomy design - passive structures such as bones

Force closure

Other forces acting across the pelvis joint to create stability - active structures such as fascia, muscles and ligaments

Form closure in the pelvis

Joint structure configuration - ridged, interlocking surfaces, joint surface alignment relative to gravity and bodyweight, tension in the restraining ligaments associated with normal alignment of the segments

Force closure in the pelvis

Interaction of multiple muscle groups that act across the joint to enhance compression on joint surfaces - assists in joint stability, referred to as the active myofascial oblique sling system

Pelvic lift

Coronal plane around ap axis

Muscles producing = concentric ipsilateral hip adductors and contralateral glute med

Muscles controlling = eccentric contralateral hip adductions and ipsilateral glute med

Pelvic drop

Coronal plane around ap axis

Muscles producing = concentric ipsilateral glute med and contralateral hip adductors

Muscles controlling = eccentric contralateral glute med and ipsilateral hip adductors

Anterior pelvic tilt

Sagittal plane around horizontal axis

Muscles producing = concentric hip flexors

Muscles controlling = eccentric hip flexors

Posterior pelvic tilt

Sagittal plane around horizontal axis

Muscles producing = concentric hip extenders

Muscles controlling = eccentric hip flexors

Pelvis rotation to the right

Transverse plane around longitudinal axis

Muscles producing = concentric right hip internal rotators and left hip external rotators

Muscles controlling = eccentric right hip external rotators and left hip internal rotators

Pelvic rotation to the left

Transverse plane around longitudinal axis

Muscles producing = concentric left hip internal rotators and right hip external rotators

Muscles controlling = eccentric left hip external rotators and left hip internal rotators