Biomechanics of the Pelvis Flashcards
Functional Anatomy
Two innominate bones
pubic symphisis
sacrum
The Innominate is a fusion of what 3 bones?
Where do they fuse?
• ilium, ischium and pubis
They fuse in the center of the acetabulum
The pubic symphisis is what type of joint?
Amphiarthrosis
• slightly movable joint
• fibrocartilagenous disc
Describe the three joint complex of the SI joint.
• two sacroiliac joints and pubic symphysis
- atypical joint surfaces
- diarthrosis (synovial) joint … joint cavity, synovial fluid, joint capsule
Describe the SI articular surface of innominate
diarthrosis
• fibrocartilage
• central convex ridge
• heavy ligamentous attachments on posterior and superior surfaces
DEscribe the SI articular surface of sacrum
- diarthosis
- “L” shaped (boot shaped)
- hyaline articular cartilage
- central groove (concavity)
What is the Keystone Effect? How is displacement resisted?
the sacrum forms the keystone of an arch suspended by strong sacroiliac ligaments
• inferior displacement is resisted by the wedge shape of sacrum
• posterior displacement is resisted by the sacroiliac ligaments
• anterior displacement is resisted by the sacroiliac ligaments and the symphysis pubis
Describe the self-locking mechanism of the SI joint.
• form closure …the anatomy of the SI joints
promotes stability
- wedge shape of sacrum
- interlocking groove (sacrum) and ridge (ilium)
- “S” shaped joint surfaces
Posterior Myofascial Sling
Self-locking mechanism …
• force closure … tension in muscles, ligaments and fascia aids in stabilizing the SI joints
- creates lateral to medial pressure from the ilium to the sacrum, compressing SI joints
- creates anterior closure of symphysis pubis
How does the pelvis assist in shock absorption?
Transmit forces between lower extremity and axial skeleton
- slides and pivots to absorb and adapt to forces generated between trunk and lower extremity during locomotion
- decreases stress to lumbar spine and opposite SI joint
What is sacroiliac nutation and counternutation?
Nutation: the sacral base rotates anteriorly on the ilium… or the ilium rotates posteriorly on the sacrum … or both
Counternutation: the sacral base rotates posteriorly on the ilium… or the ilium rotates anteriorly on the sacrum … or both
What motion does sacral nutation create?
- anterior sacral tilt
- posterior iliac rotation
- increased lumbar lordotic curve
What motions does counternutation create?
- posterior sacral tilt
- anterior iliac rotation
- decreased lumbar lordotic curve
What type of movement is available by the pubic symphisis?
• compression
• distraction (separation)
• rotation in sagittal plane with SI joint motion
• gliding (translation) ...
• anterior-posterior
• superior-inferior
* involved in all pelvic motions!What motions occur during locomotion?
Reciprocal motion between the ilium and sacrum
• flexion of the hip and ilium (posterior inferior movement of the PSIS) is accompanied by ipsilateral anterior inferior movement of the sacral base (SI flexion)
• extension of the hip and ilium (anterior superior movement of the PSIS) is accompanied by ipsilateral posterior superior movement of the sacral base
What is involved in the force couple for an anterior pelvic tilt.
*Reminder: the WHOLE pelvis as a joint
• Hip Flexors: iliopsoas and rectus femoris (attach at lesser trochanter)
• Back Extensors: erector spinae
*consider the actions for open and closed chain systems.
What muscles are involved in the force couple of Posterior Pelvic Tilt?
• Hip extensors: gluteus maximus and hamstrings
• Abdominal muscle: rectus abdominis and obliquus externus abdominis
*common structural issue: weak glut max and weak abs –> imbalance causing anterior pelvic tilt
