Pelvic Biomechanics and Anatomy

Pelvic Biomechanics

• The pelvis serves as an attachment point for many muscles of the lower extremity and trunk, transmits the weight of the upper body and trunk to the ischial tuberosities, and supports organs involved with bowel and bladder functions with the help of pelvic floor muscles and connective tissues.

Intrinsic Joints of the Pelvis

• Sacrococcygeal joint: a symphysis between the apex of the sacrum and the base of the coccyx, lined by hyaline cartilage and connected by a fibrous disc, allowing for passive flexion and extension movements to increase the anteroposterior diameter of the pelvis during labor and defecation.
• Pubic symphysis: a joint with hyaline cartilage-lined surfaces, firmly bound by a fibrocartilaginous interpubic disc and ligaments, allowing for up to 2 mm of translation and slight rotation.
• Sacroiliac joint (SIJ): a modified synarthrodial joint that primarily ensures stability and effective transfer of large loads, with functions including transitioning forces from the pelvic to caudal region.

SIJ Joint Stability

• Primary stabilizers: anterior sacroiliac, iliolumbar, posterior sacroiliac, and interosseous ligaments.
• Secondary stabilizers: sacrotuberous and sacrospinous ligaments.

Kinematics of the Pelvis

• SIJ movements: small rotational and translational movements based on articular cartilage compression, with anthropometric measurements of 1-4 dg of rotation and 1-2 mm of translation.
• Nutation: relative anterior tilt of the top of the sacrum relative to the ilium, providing joint stress relief and stability during load changes.
• Counternutation: relative posterior tilt of the top of the sacrum relative to the ilium, also providing joint stress relief and stability during load changes.

Functional Considerations

• Nutation and counternutation help relieve joint stress during high-mechanical-stress activities like walking, running, and giving birth.
• These movements also provide stability during load changes by increasing congruency between the sacrum and ilium.

Muscles that Reinforce and Stabilize the SIJ

• Erector spinal, lumbar multifidi, abdominal muscles, hip extensors, latissimus dorsi, iliacus, and piriformis.

Arthrokinematics of the Pelvis

• Anteversion: anterior tilt with descent of ASIS and ascent of PSIS, corresponding to lumbar extension movement.
• Retroversion: posterior tilt with ascent of ASIS and descent of PSIS, corresponding to lumbar flexion movement.
• Lateral tilt: lateral movement with descent of ASIS on one side and ascent of contralateral ASIS, associated with abduction and adduction movements.
• Rotation: anterior movement of ASIS on one side and posterior movement of contralateral ASIS, associated with gait movements.