Topography and X-ray Anatomy of the Pelvic Ring
Sacrum (os sacrum) is the central bony structure forming the posterior wall of the pelvis. On a plain pelvic radiograph, it is visualized in conjunction with other anatomical formations. The sacrum articulates with the iliac bones, forming the paired sacroiliac joint (articulatio sacroiliaca). This joint is topographically closely related to the iliac crest (crista iliaca) and the posterior inferior iliac spine (spina iliaca posterior inferior).
In its lower part, the sacrum connects with the coccyx (os coccygis). The coccyx consists of three to five separate or fused coccygeal vertebrae (vertebrae coccygeae). Their articulation with the sacrum occurs via the fibrocartilaginous sacrococcygeal symphysis (symphysis sacrococcygea). The radiograph also clearly defines other pelvic landmarks surrounding the sacrum: the acetabular roof (acetabulum), the ischial tuberosity (tuber ischiadicum), the obturator foramen (foramen obturatum), as well as the pubic rami and pubic symphysis (symphysis pubica).
Ligamentous Apparatus of the Sacrum and Pelvis
Secure fixation of the sacrum within the pelvic ring is provided by a complex system of ligaments. Anatomical specimens and diagrams distinguish several key structures that hold the bone in place:
- Anterior sacroiliac ligaments (ligamenta sacroiliaca anteriora) are located on the anterior surface and strengthen the joint of the same name.
- Sacrospinous ligament (ligamentum sacrospinale) and sacrotuberous ligament (ligamentum sacrotuberale) are powerful bands running from the sacrum to structures of the ischium.
- Sacrococcygeal ligaments (ligamenta sacrococcygea) stabilize the sacrococcygeal symphysis.
- Iiolumbar ligaments (ligamenta iliolumbalia) connect the lumbar spine to the iliac bone, lying in close proximity to the upper part of the sacrum.
In addition, the anterior longitudinal ligament (ligamentum longitudinale anterius) runs along the anterior surface of the spine and sacrum.
Arterial Blood Supply of the Sacral Region
Blood supply to the structures adjacent to the sacrum is provided by branches of the abdominal aorta (pars abdominalis aortae). The abdominal aorta is located to the left of the spine and, at the level of the fourth lumbar vertebra (L4), divides into the common iliac arteries (a. iliaca communis).
The direct downward continuation of the aorta is the median sacral artery (a. sacralis mediana). This is a relatively slender unpaired vessel that descends strictly along the anterior surface of the sacrum into the lesser pelvis.
Lateral to the sacrum lies the internal iliac artery system (a. iliaca interna), which gives off parietal branches to the pelvic walls. For example, the left superior gluteal artery (a. glutea superior sinistra) enters the suprapiriform foramen, providing blood flow to adjacent areas.
Venous Outflow and the Sacral Plexus
Venous blood from the sacral region drains into the inferior vena cava system (v. cava inferior), which is formed by the union of the right and left common iliac veins at the level of L4–L5.
- Median sacral vein (v. sacralis mediana) is an unpaired tributary collecting blood from the anterior surface of the sacrum. It empties directly into the bifurcation region of the common iliac veins.
- Lateral sacral vein (v. sacralis lateralis) is a paired vessel draining blood from the lateral masses of the sacrum.
- Sacral venous plexus (plexus venosus sacralis) is located on the anterior surface of the bone.
An important feature of all lesser pelvis venous plexuses (including uterine, vaginal, vesical, and rectal) is the absence of valves and the presence of broad anastomoses. This creates conditions for the free redistribution of blood. Additionally, the ascending lumbar veins (v. lumbalis ascendens) originate from the iliac veins, forming cavo-caval anastomoses that link the inferior vena cava system with the azygos and hemiazygos veins of the thoracic cavity.
Remember the topography of the vessels: the median sacral vessels (artery and vein) run unpaired strictly along the center of the sacrum, while the lateral veins are located on the sides, together forming the valveless sacral plexus.
Class and exam questions
Which bones does the sacrum articulate with?
The sacrum articulates with the iliac bones, forming the paired sacroiliac joint (articulatio sacroiliaca). In its lower part, it connects with the coccyx via the sacrococcygeal symphysis.
Which vessels run along the anterior surface of the sacrum?
The median sacral artery (a continuation of the abdominal aorta) and the unpaired median sacral vein descend along the anterior surface. The sacral venous plexus is also located here.
Where does the median sacral vein empty?
The median sacral vein (v. sacralis mediana) is an unpaired tributary and empties into the bifurcation area where the two common iliac veins merge into the inferior vena cava.
Which ligaments fix the sacrum to the ischium?
The sacrum connects to the ischium via two powerful ligaments: the sacrospinous (ligamentum sacrospinale) and sacrotuberous (ligamentum sacrotuberale) ligaments.
What else to study on this topic
- Topography and Branches of the Internal Iliac Artery (a. iliaca interna)
- Venous Plexuses of the Lesser Pelvis and Their Anastomoses
- Formation and Tributaries of the Inferior Vena Cava (v. cava inferior)
- Cavo-Caval Anastomoses via the Ascending Lumbar Vein System
- X-ray Anatomy of the Bones and Joints of the Pelvic Ring
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