Sacrum
The sacrum is the penultimate segment of the vertebral column and also forms the posterior part of the bony pelvis. It transmits the total body weight between the lower appendicular skeleton and the axial skeleton.
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Gross anatomy
The sacrum is an irregularly-shaped bone, shaped roughly like an inverted triangular, with its base superiorly and apex inferiorly. It is curved with an anterior concavity and convex posteriorly.
The sacrum consists of five fused sacral vertebral and costal segments (numbered one-to-five) that form a central sacral body and paired sacral alae (singular ala), which arise laterally from S1. Superiorly, the anterior lip of the S1 is called the sacral promontory. Posteriorly, the spinous processes are fused to form the median sacral crest.
Through the center of the sacral body passes the triangular-shaped sacral canal, which is the continuation of the lumbar vertebral canal. It terminates inferiorly at the sacral hiatus and contains sacral and coccygeal nerve roots, spinal meninges (to the level of S2) and filum terminale. The spinal epidural space extends to the sacral hiatus. First to fourth sacral nerve roots exit the sacrum via four paired anterior and posterior foramen. The fifth sacral nerve root exits via the sacral hiatus.
Articulations
- lumbosacral: superiorly, the sacrum articulates with the inferior endplate of L5 via the L5/S1 intervertebral disk; superior articular processes of S1 articulates with the L5 inferior articular facets to form the L5/S1 facet (zygapophyseal) joints
- sacrococcygeal: inferiorly, the sacrum articulates with the first coccygeal segment
- sacroiliac: laterally, the sacral ala articulate with the ilium
Attachments
Ligamentous
- lumbosacral and iliolumbar ligaments: reinforce lumbosacral articulation
- ligamentum flavum (S1 to L5): closes the spinal/sacral canal posteriorly
- sacroiliac ligaments: reinforce the sacroiliac joint
- sacrotuberous ligament
- sacrospinous ligament
- sacrococcygeal ligaments: superficial part of the posterior sacrococcygeal ligament closes the sacral hiatus
Musculotendinous
- iliacus muscle: anterosuperiorly
- piriformis muscle: anteroinferiorly
- erector spinae muscle: posteriorly
- gluteus maximus: posteriorly
Relations
- anteriorly: fascia of Waldeyer, superior rectal vessels; peritoneum (S1-S2); rectum (S3-S5); median sacral artery; lymph nodes; piriformis muscle; sacral plexus
Arterial supply
- iliolumbar arteries (posterior division of internal iliac artery)
- superior lateral sacral arteries (posterior division of internal iliac artery)
- inferior lateral sacral arteries (posterior division of internal iliac artery)
- median sacral artery (abdominal aorta)
Venous drainage
- via vertebral venous plexus to:
- median sacral vein (to either common iliac vein or IVC)
- lateral sacral vein (to internal iliac vein)
Variant anatomy
Related pathology
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Anatomy: abdominopelvic
- skeleton of the abdomen and pelvis
- muscles of the abdomen and pelvis
- spaces of the abdomen and pelvis
- anterior abdominal wall
- posterior abdominal wall
-
abdominal cavity
-
peritoneum
- peritoneal ligaments
- mesentery
-
peritoneal spaces
-
supramesocolic space
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right supramesocolic space
- right subphrenic space
-
right subhepatic space
- anterior right subhepatic space
- posterior right subhepatic space (Morison pouch)
-
lesser sac
- epiploic foramen (of Winslow)
- left supramesocolic space (left perihepatic space)
-
right supramesocolic space
- inframesocolic space
-
supramesocolic space
- inguinal canal (mnemonic)
- Hesselbach triangle
- scrotal sac
- retroperitoneum
-
peritoneum
- pelvic cavity
- perineum
- abdominal and pelvic viscera
- gastrointestinal tract
- spleen
- hepatobiliary system
-
endocrine system
-
adrenal gland
- adrenal vessels
- chromaffin cells
- variants
- pancreas
- organs of Zuckerkandl
-
adrenal gland
-
urinary system
-
kidney
- renal pelvis
- renal sinus
- avascular plane of Brodel
-
variants
- number
- fusion
- location
- shape
- ureter
- urinary bladder
- urethra
- embryology
-
kidney
- male reproductive system
-
female reproductive system
- vulva
- vagina
- uterus
- uterine tubes
- ovaries
- broad ligament (mnemonic)
- variant anatomy
- embryology
- blood supply of the abdomen and pelvis
- arteries
-
abdominal aorta
- inferior phrenic artery
- celiac artery
- superior mesenteric artery
- middle suprarenal artery
- renal artery (variant anatomy)
- gonadal artery (ovarian artery | testicular artery)
- inferior mesenteric artery
- lumbar arteries
- median sacral artery
-
common iliac artery
- external iliac artery
-
internal iliac artery (mnemonic)
- anterior division
- posterior division (mnemonic)
- variant anatomy
-
abdominal aorta
- portal venous system
- veins
- anastomoses
- arterio-arterial anastomoses
- portal-systemic venous collateral pathways
- watershed areas
- arteries
- lymphatics
- innervation of the abdomen and pelvis
- lumbar plexus
-
sacral plexus
- lumbosacral trunk
- sciatic nerve
- superior gluteal nerve
- inferior gluteal nerve
- nerve to piriformis
- perforating cutaneous nerve
- posterior femoral cutaneous nerve
- parasympathetic pelvic splanchnic nerves
- pudendal nerve
- nerve to quadratus femoris and inferior gemellus
- nerve to internal obturator and superior gemellus
Anatomy: Spine
-
osteology
- vertebrae
- spinal canal
- cervical spine
- thoracic spine
- lumbar spine
- sacrum
- coccyx
-
anatomical variants
- vertebral body
- neural arch
- transitional vertebrae
- ossicles
- ossification centers
- intervertebral disk
- articulations
- ligaments
- musculature of the vertebral column
- muscles of the neck
- muscles of the back
-
suboccipital muscle group
- rectus capitis posterior major muscle
- rectus capitis posterior minor muscle
- obliquus capitis superior muscle
- obliquus capitis inferior muscle
- splenius capitis muscle
- splenius cervicis muscle
- erector spinae group
- transversospinalis group
- quadratus lumborum
-
suboccipital muscle group
-
spinal cord
- gross anatomy
-
white matter tracts (white matter)
- anterolateral columns
- lateral columns
-
dorsal columns
- fasiculus gracilis (column of Goll)
- fasiculus cuneatus (column of Burdach)
- grey matter
- nerve root
- spinal meninges and spaces
- functional anatomy
- spinal cord blood supply
- sympathetic chain