Joints

Fig. 1

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 [1]) 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 [2]).

Sacroiliac Joints

The sacroiliac joints are the association of a synovial joint in the anterior part, and a posterior syndesmosis by the interosseous and posterior ligaments. The posterior volume of the interosseous ligament is almost the same size as the anterior articular surface without significant difference in both sexes (Klein [3]) (Fig. 2).

../images/451372_1_En_17_Chapter/451372_1_En_17_Fig2_HTML.jpg — Fig. 2

Ligaments of the sacroiliac joint on a ventral (a) and dorsal (b) view: (1) iliolumbar ligament; (2) Sacroiliac ligament superior; (3) Sacrotubal ligament; (4) Sacrospinous ligament; (5) Superficial posterior sacrio-iliac ligament; (6) Deep sacroiliac ligament

The sacral cartilage is three times thicker than the coxal cartilage, 2.5 mm versus 1 mm on average (Bogduk [4]).

The convex parts refer to the coxal bone and the concave parts to the sacrum (Farabeuf [5], Weils [6]). The aspect is reversed at the bottom third.

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 articular surfaces have an elongated L shape with a long, almost horizontal, upright arm of 5.4 cm ± 0.5 cm, and an almost vertical short arm of 2.9 cm ± 0.6 cm; (Klein [3]), with an open angle backwards of 110° ± 11° (Fig. 3).

../images/451372_1_En_17_Chapter/451372_1_En_17_Fig3_HTML.jpg — Fig. 3

The joints of the pelvic girdle

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 [3]). However, contradictory studies describe an exclusively posterior innervation from the dorsal branches S1 and S2 (Bogduk [4]).

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 [4]). 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 [7]).

This joint has no role in the pelvic girdle but is the origin of coccygodynia from its hypermobility (traumatic or postpartum).

Biomechanical

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 [3]).

Stability in the Transverse Plane

The pelvis is held between the two coxal bones like a nut between the arms of a nut-cracker. Posterior sacroiliac ligaments ensure the power of the damping system according to a second or third class lever with the pressure of the femoral heads (Figs. 4 and 5). The equilibrium is likened to a torsion bar with the role of the lateral rotator pelvitrochanteric muscles, especially the piriformis muscle.

../images/451372_1_En_17_Chapter/451372_1_En_17_Fig4_HTML.png — Fig. 4

Static in the frontal or coronal plane, *C a*/t = constraints of the acetabulum on the femoral heads

../images/451372_1_En_17_Chapter/451372_1_En_17_Fig5_HTML.jpg — Fig. 5

Static in the horizontal or transverse plane: A = support, R = sacral resistance, P = ligament tension and femoral head pressure

In the sagittal plane, the equilibrium is unstable (Fig. 6). The lumbopelvic wedge (Klein [3]) joins the lumbosacral junction, pelvis, and hip. This damping system is linked to the evolution of Homo erectus with:

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 [8]).