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Pelvic diseases (including fractures and bone tumors) account for 2%-4% of orthopedic cases, with a mortality rate of approximately 20% for pelvic fractures with complications. Additionally, pelvic-related diseases also have a relatively high proportion in gynecology, obstetrics, and pediatrics.
Today, using CT as a reference,
let’s briefly study the anatomy of the pelvis.
The pelvis is composed of the sacrum, coccyx, bilateral hip bones, and bone connections, broadly including the fifth lumbar vertebra.
Components of the pelvis: fifth lumbar vertebra (blue), sacrum (green), and bilateral hip bones (red)
1. Fifth Lumbar Vertebra
The fifth lumbar vertebra is an important joint bone regulating pelvic anterior and posterior tilt, with the rotation point located at the articular processes joint between the fourth and fifth lumbar vertebrae.
Relationship between the articular processes joint of L4-L5 and pelvic anterior tilt
The CT appearance of this articular processes joint is shown below:
Axial, coronal, and sagittal views of the L4-L5 articular processes joint
From the above figure, the articular surface of the L4-L5 articular processes joint is sagittally oriented, moving in flexion and extension about the coronal axis. Flexion causes pelvic anterior tilt, extension causes pelvic posterior tilt.
The iliolumbar ligament directly connects the fifth lumbar vertebra to the pelvis, linking the transverse process of L5 to the posterior part of the iliac crest.
Iliolumbar ligament (Baidu image)
2. Sacrum
The fifth lumbar vertebra connects downward to the sacrum, which is formed by the fusion of 5-6 sacral vertebrae.
Axial, coronal, and sagittal views of the sacrum
Sacrum and sacral vertebrae
The spinal canal formed between the sacral body and sacral arch has evolved into the sacral canal, the lower opening of which is called the sacral hiatus.
Volume rendering and axial, coronal, sagittal sectional images of the sacral hiatus
The anterior margin of the upper end of the sacrum is the sacral promontory, a bony protrusion projecting forward. On both sides are the upper edges of the pelvic surface formed by the fusion of the first sacral transverse processes.
Sacral promontory
The sacral anterior and posterior foramina are respectively the anterior and posterior openings formed after fusion of the sacral transverse processes and communicate with the sacral canal via the lateral recesses.
Sacral anterior foramen
Sacral canal lateral recess
Sacral posterior foramen
The sacroiliac joint is a micro-movement joint formed by the auricular surfaces of the sacrum and iliac bone. The joint is reinforced anteriorly and posteriorly by the anterior and posterior sacroiliac ligaments.
Right sacroiliac joint
The sacroiliac joint is a load transmission line, so its stability is especially important.
3. Hip Bone
The hip bone consists of the ilium anterior-superiorly (2/5), ischium posterior-inferiorly (2/5), and pubis anterior-inferiorly (1/5).
Right hip bone (Baidu image)
01
The ilium is composed of the iliac body and iliac wing. The fan-shaped upper part is the iliac wing; the portion connecting to the other two bones is the iliac body. The iliac wing forms the lateral walls of the greater (false) pelvis.
Iliac wing
Surface landmarks of the ilium include the anterior superior iliac spine, anterior inferior iliac spine, and iliac crest.
Anterior superior iliac spine
Anterior inferior iliac spine
Iliac crest
In the standing position, the anterior superior iliac spine and the pubic tubercle lie in the same coronal plane.
Positional relationship between anterior superior iliac spine and pubic symphysis in standing
This positional relationship is also an important reference for assessing anterior and posterior pelvic tilt.
02
The ischium consists of the ischial ramus and its body connected with the other two bones. The enlarged starting part of the ischial ramus is called the ischial tuberosity, which bears body weight when sitting.
Ischial tuberosity
A large notch at the transition between the ischial body and iliac body is called the greater sciatic notch. Below it is the ischial spine, and further down is the lesser sciatic notch.
Greater sciatic notch
Ischial spine
Lesser sciatic notch
The ischial tuberosity attaches to the sacrotuberous ligament and serves as the origin of posterior thigh muscles such as the long head of the biceps femoris, semitendinosus, and semimembranosus. The ischial spine attaches the sacrospinous ligament. The greater sciatic notch, sacrospinous ligament, and lateral margin of the sacrum define the greater sciatic foramen; the lesser sciatic notch, sacrotuberous ligament, and sacrospinous ligament define the lesser sciatic foramen.
03
The pubis consists of the pubic body (connected with the other two bones), superior pubic ramus, and inferior pubic ramus. The superior ramus extends from the body to the upper edge of the pubic symphysis and includes the pubic crest and pubic tubercle; the inferior ramus curves from the symphysis and connects with the ischial ramus, together forming the obturator foramen, which is partly closed by the obturator membrane.
Pubic crest
Pubic tubercle
Obturator foramen
04
The acetabulum is composed of the ischial body, iliac body, and pubic body, located on the lateral aspect of the hip bone, forming the socket of the hip joint. Below the acetabulum is a notch called the acetabular notch.
Acetabulum
The acetabular diameter is about 5.5 cm, and its depth is about 2.3 cm. Based on data, it is basically hemispherical.
Acetabular diameter and depth (left), acetabular notch (right)