Pubic symphysis
Anteriorly, the pelvic ring is closed by the pubic symphysis (symphysis pubica). This joint represents a transitional form between a continuous articulation and a true joint.
The main structural feature of the symphysis is the presence of a special fibrocartilaginous disc located between the articulating surfaces of the pubic bones. Within the thickness of this cartilaginous disc, a small slit-like cavity may occasionally form.
To ensure mechanical stability, the pubic symphysis is additionally reinforced by a ligamentous apparatus:
- Superiorly, the joint is reinforced by the superior pubic ligament (lig. pubicum superius).
- Inferiorly, the pubic arch is supported by the arcuate pubic ligament (lig. arcuatum pubis).
Sacroiliac joints
The posterior part of the pelvic ring is formed by the paired sacroiliac joints (articulationes sacroiliacae).
Specific articular facets—the auricular surfaces located on the lateral masses of the sacrum and the corresponding parts of the ilium—take part in the formation of this joint. According to its biomechanical classification, this joint is classified as a plane joint. Functionally, it is an amphiarthrosis—a slightly movable articulation, which is critically important for the stability of the entire pelvic ring during weight transfer.
The joint capsule here is very strong and tightly stretched. The powerful stabilizing apparatus of the sacroiliac joint includes the following ligaments:
- Internal ligaments of the joint: ventral, dorsal, and interosseous sacroiliac ligaments (ligg. sacroiliaca ventralia, dorsalia et interossea).
- Additional fixation is provided by the iliolumbar ligament (lig. iliolumbale).
Ligaments forming the sciatic foramina
In addition to the ligaments that directly reinforce the joints, large fibrous bands are located in the region of the pelvic ring, playing a crucial topographical role.
These include:
- Sacrotuberous ligament (lig. sacrotuberale).
- Sacrospinous ligament (lig. sacrospinale).
These two powerful ligaments span between the bony structures of the pelvis and bridge the bone notches, turning them into the greater and lesser sciatic foramina. Important neurovascular bundles and muscles subsequently pass through these foramina.
Briefly: answers to topical questions
Which specific bony notches are bridged by the sacrotuberous and sacrospinous ligaments?
The sacrotuberous and sacrospinous ligaments bridge the greater and lesser sciatic notches of the hip bone.
- Greater sciatic notch (incisura ischiadica major) — bridged by the sacrospinous ligament (lig. sacrospinale) and the sacrotuberous ligament (lig. sacrotuberale), forming the greater sciatic foramen.
- Lesser sciatic notch (incisura ischiadica minor) — bridged by the same ligaments, forming the lesser sciatic foramen.
Which muscles and neurovascular bundles pass through the greater sciatic foramen?
The piriformis muscle (m. piriformis) passes through the greater sciatic foramen, dividing it into two compartments that transmit neurovascular bundles.
- Suprapiriform foramen (foramen suprapiriforme) — transmits the superior gluteal neurovascular bundle (superior gluteal artery and veins, superior gluteal nerve).
- Infrapiriform foramen (foramen infrapiriforme) — transmits the inferior gluteal neurovascular bundle, the sciatic nerve (n. ischiadicus), the posterior cutaneous nerve of the thigh, and the pudendal neurovascular bundle.
What passes through the lesser sciatic foramen?
The lesser sciatic foramen transmits muscular and neurovascular structures heading toward the gluteal region and the ischioanal fossa.
- Obturator internus muscle (m. obturatorius internus) — exits the pelvic cavity to the posterior surface.
- Pudendal neurovascular bundle — returns to the pelvic cavity. It includes the internal pudendal artery (a. pudenda interna), internal pudendal veins (vv. pudendae internae), and the pudendal nerve (n. pudendus).
Which continuous joints (synarthroses) form the hip bone itself?
The hip bone itself is formed by continuous cartilaginous joints—temporary synchondroses.
- Temporary synchondroses exist until puberty.
- In the region of the acetabulum, they unite the three component parts of the hip bone: the pubis, ischium, and ilium.
Where is the obturator membrane located and what function does it perform?
The obturator membrane (membrana obturatoria) is located in the region of the obturator foramen on the anterolateral wall of the pelvis.
It serves as the site of origin for the obturator internus muscle (m. obturatorius internus) from its inner surface.
To recall the ligaments of the sacroiliac joint, use spatial orientation: anterior (ventral), posterior (dorsal), and internal (interosseous) — ligg. sacroiliaca ventralia, dorsalia et interossea.
Class and exam questions
What type of joint is the sacroiliac joint?
This is a plane joint that functionally acts as an amphiarthrosis (a slightly movable joint).
What structures form the pubic symphysis?
The pubic symphysis is formed by the surfaces of the pubic bones, between which lies a fibrocartilaginous disc (occasionally with a slit-like cavity), along with the superior pubic and arcuate ligaments.
Which ligaments close the sciatic foramina?
The greater and lesser sciatic foramina are formed by the sacrotuberous (lig. sacrotuberale) and sacrospinous (lig. sacrospinale) ligaments.
What else to study on this topic
- Structure of the fibrocartilaginous disc of the pubic symphysis
- Auricular surfaces of the sacrum and ilium
- Topography of the iliolumbar ligament (lig. iliolumbale)
- Boundaries of the greater and lesser sciatic foramina
- Ventral, dorsal, and interosseous ligaments of the pelvis
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