Pelvic Biomechanics

Pelvic Biomechanics

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

Intrinsic Joints of the Pelvis

• Sacrococcygeal joint:
  • Located between the apex of the sacrum and the base of the coccyx.
  • A symphysis joint lined by hyaline cartilage and connected by an interposed fibrous disc.
  • Has limited movements of flexion and extension, with one degree of freedom.
  • Function: increases the anteroposterior diameter of the pelvis during labor and defecation.
• Pubic symphysis:
  • Hyaline cartilage lines the surface of the articulation.
  • Joint is firmly bound by a fibrocartilaginous interpubic disc and ligaments.
  • Allows up to 2 mm of translation and very slight rotation.
• Sacroiliac joint (SIJ):
  • A modified synarthrodial joint.
  • Functions: transitions forces from the pelvic to caudal region, and provides stability for effective transfer of loads.
  • Primary stabilizers: anterior sacroiliac, iliolumbar, posterior sacroiliac, and interosseous ligaments.
  • Secondary stabilizers: sacrotuberous and sacrospinous ligaments.

Kinematics of the Pelvis: SIJ

• Mainly small rotational and translational movements of the SI joints.
• Movements based on compression of articular cartilage.
• Anthropometric measurements: 1-4 degrees of rotation and 1-2 mm of translation.
• Nutation and counternutation:
  • Nutation: relative anterior tilt of the top of the sacrum, relative to the ilium.
  • Counternutation: relative posterior tilt of the top of the sacrum, relative to the ilium.
• Functional considerations of nutation and counternutation:
  • Joint stress relief: releases the pelvis from joint stress during support phases.
  • Stability during load changes: increases congruency and stability.

Muscles that Stabilize the SIJ

• Erector spinal
• Lumbar multifidi
• Abdominal muscles
• Hip extensors
• Latissimus dorsi
• Iliacus and piriformis

Arthrokinematics of the Pelvis

• Anteversion:
  • Anterior tilt
  • Descent of ASIS
  • Ascent of PSIS
  • Sacrum: counternutation
  • Lumbar extension movement
• Retroversion:
  • Posterior tilt
  • Ascent of ASIS and descent of PSIS
  • Sacrum: nutation
  • Lumbar flexion movement
• Lateral tilt:
  • Lateral movement
  • Descent of ASIS from one side
  • Ascent of contralateral ASIS
  • Associated with movements of abduction and adduction
• Rotation:
  • Anterior movement of ASIS of one side
  • Posterior movement of the contralateral ASIS
  • Associated with movements during gait