General arrangement of the pelvic girdle
The Pubic Symphysis
The pubic symphysis joins both pubic bones and constitutes a fibrocartilaginous articulation of the amphiarthrosis type. The articular surfaces are ovoid, of 32 mm by 12 mm on average, with the long oblique axis aligned anterosuperiorly at an inclination of 40°. An intervertebral, wedge-shaped fibrocartilage disc fills the larger joint space anteroinferiorly.
The ligaments are located on the four sides—anterior, posterior, superior, and inferior (or arched).
Mobility (Walheim ) is very low with a craniocaudal displacement of 1 mm for males and 1.6 mm for females; anterior translation of 0.5 mm (males) and 0.9 mm (females); sagittal translation <1 mm and frontal and sagittal rotations <1.5°.
There are small movements during childbirth and sport (football) which can cause pain (pubalgia) and arthropathic deterioration.
In severe trauma of the pelvis, there may be a disruption of the symphysis, sometimes associated with a fracture or a dislocation with sacroiliac instability by rupture of the pelvic ring (Sénégas ).
The sacral cartilage is three times thicker than the coxal cartilage, 2.5 mm versus 1 mm on average (Bogduk ).
The articular surfaces are constituted of the coxa and sacrum in a crescent shape at the posterior concavity whose center of curvature corresponds to the iliac tuberosity and the perforated fossa of the sacrum.
The joint space is oblique anterolaterally from 12° to 20° with respect to the sagittal plane (angle of declination) and oblique at the bottom and within 75–85° with respect to the transverse plane (angle of inclination). The line has a double obliquity which favors the isolation of the sacral corner and the stability of non-coaxial sacroiliac joints. This also allows the asymmetrical movements of the two coxal bones with helical oblique axes. The sacroiliac joint extends over three sacral bone segments (S1–S3).
Classically, the innervation is posterior from the posterior branches of L4–S3 and anterior from the anterior branches of L2–S2 (Klein ). However, contradictory studies describe an exclusively posterior innervation from the dorsal branches S1 and S2 (Bogduk ).
With age, the articular surfaces show erosion and fibrillation of cartilage with a loss of thickness. The capsule and synovium become thicker and more fibrous especially after 50 years (Bogduk ). The sacral cartilage decreases by 1 mm and ilium decreases by 0.5 mm, i.e. 50% of the thickness with restriction of mobility. Ankylosis is one of the first signs of pelvispondylitis.
The Sacrococcygeal Joint
The sacrococcygeal joint is an amphiarthrosis with ventral and dorsal interosseous and sacrococcygeal ligaments, remnant of the tail in humanoid primates. The mobility is 15° in flexion while seated and during the contraction of the pelvic floor muscles (Maigne ).
This joint has no role in the pelvic girdle but is the origin of coccygodynia from its hypermobility (traumatic or postpartum).
Static: The Pelvic Girdle Is Hyperstable
Stability in the Frontal Plane
The pelvis forms an arch whose apex is the sacrum acting as a wedge between the two hip bones. The body weight is transmitted to the lower limbs by means of the sacroiliac joints and coxal bones to the hip joint and joints of the ground reaction force towards the symphysis pubis. The interosseous ligament is very powerful as well as the force of the abductor muscles (Klein ).
Stability in the Transverse Plane
Levers: the sacrum and the two coxal bones.
The articular links: sacroiliac joints, symphysis, and coxofemoral with a closed kinetic chain. A movement in one articulation necessarily causes movement in the other articulation. The sacrum cannot move freely.
The passive brakes of sacral nutation (oscillatory movements within the axis of rotation) are the powerful sacrospinous and sacrotuberal ligaments and the active brakes are the iliac muscles, hip flexors (right femoral, fascia lata tensor, sartorius), piriform muscles, and pelvic floor.
The psoas muscle induces a moment of flexion on the lumbar spine and lumbosacral junction in the sense of nutation (Sturesson ).