[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Anatomy](https://durgical.com/en/anatomy/) › [Lower limb](https://durgical.com/en/anatomy/region/lower-limb/) › Sacrum [Skeletal system](https://durgical.com/en/anatomy/system/skeletal-system/) · [Lower limb](https://durgical.com/en/anatomy/region/lower-limb/) # Sacrum The sacrum is the triangle-shaped bone at the base of the [spine](https://durgical.com/en/anatomy/region/spine/); it joins the two hip bones to close the back of the [pelvis](https://durgical.com/en/anatomy/region/pelvis/). [See in 3D →](https://durgical.app/?from=durgipedia&lang=en#region=lower_limb&select=sacrum&side=r) Opens in the Durgical app, no sign-up needed. ## What it is A large, triangle-shaped bone made of five fused vertebrae at the base of the spine, wedged between the two hip bones. ## What it does Passes the weight of the upper body to the pelvis through the sacroiliac joints. ## Key points - The sacral hiatus, between the sacral cornua where the S5 laminae are missing, is the entry for a caudal epidural block - The dural sac usually ends at S2, level with the posterior superior iliac spines - Sacral insufficiency fractures in older adults with osteoporosis give an H-shaped uptake (Honda sign) on bone scan - Fractures through the foramina or the central canal (Denis zones II and III) are increasingly likely to injure the sacral nerve roots, with bladder and bowel effects ## Key facts **Joints with** L5 (lumbosacral joint); coccyx; both hip bones (sacroiliac joints) **Landmarks** Five fused vertebrae; promontory; alae; four pairs of anterior and posterior sacral foramina; median sacral crest; sacral hiatus; auricular surface ## Relations **Above** Base with L5 at the lumbosacral disc; the lumbosacral trunk (L4–L5) crosses the ala **In front** Sacral plexus roots from the anterior foramina, [piriformis](https://durgical.com/en/anatomy/piriformis-muscle/) origin, sympathetic trunks and median sacral vessels; the [rectum](https://durgical.com/en/anatomy/rectum/) and mesorectum in front **Behind** Median and lateral sacral crests; erector spinae and [multifidus](https://durgical.com/en/anatomy/multifidus-muscles/); part of the origin of [gluteus maximus](https://durgical.com/en/anatomy/gluteus-maximus-muscle/) **Lateral** Auricular surface forming the [sacroiliac joint](https://durgical.com/en/anatomy/sacroiliac-joint/) with the ilium; the sacrotuberous and sacrospinous ligaments on its lateral border **Below** Apex with the [coccyx](https://durgical.com/en/anatomy/coccyx/) at the [sacrococcygeal joint](https://durgical.com/en/anatomy/sacrococcygeal-joint/) **Contents** Sacral canal: [cauda equina](https://durgical.com/en/anatomy/cauda-equina/) roots, the lower end of the dural sac and the [filum terminale](https://durgical.com/en/anatomy/filum-terminale-internum/) ## How to find it The [skin](https://durgical.com/en/anatomy/skin/) dimples over the posterior superior iliac spines mark S2 and the middle of the sacroiliac joints. The median sacral crest is felt above the natal cleft; the sacral hiatus is the hollow between the two cornua at the top of the cleft, just above the coccyx. The front is felt on rectal examination; the promontory, upper end of the diagonal conjugate, may be reached vaginally. ## Examination - Sacroiliac provocation tests (distraction, thigh thrust, compression, sacral thrust, Gaenslen): three or more positive out of five support the sacroiliac joint as the source of pain - FABER (Patrick) test: with the hip flexed, abducted and externally rotated, pain over the back of the sacroiliac joint points to a sacroiliac source, while groin pain points to the hip ## On imaging Easy to miss on the AP pelvis because of its tilt and overlying bowel gas: the arcuate lines of the sacral foramina are checked for breaks. CT for fractures; MRI or bone scan for insufficiency fractures. Lumbosacral transitional vertebrae are counted from the top of the spine to avoid level errors. ## Common problems - Stress fractures (in older adults with weak bones, or in runners) and fractures from falls or accidents. ## Clinical notes - Caudal epidural block through the sacral hiatus is widely used in children - The sacroiliac joint is a source of low back and buttock pain; sacroiliitis is an early sign of ankylosing spondylitis - A fall onto the buttocks or a high-energy pelvic injury can fracture it; a transverse fracture with kyphosis threatens the sacral roots ## Anatomical variants - Lumbosacral transitional vertebra (sacralised L5 or lumbarised S1), up to about 12% in a meta-analysis; it changes level counting before surgery or injections - Sacral spina bifida occulta is common and usually harmless; the number of foramina varies - The female sacrum is shorter and wider, with a less prominent promontory ## Development Five sacral vertebrae, each with a body, arches and lateral (costal) elements that build the alae, fuse progressively from adolescence until about the mid-twenties. ## Related conditions - [Spondylolisthesis](https://durgical.com/en/conditions/spondylolisthesis/) - [Sacroiliac joint pain](https://durgical.com/en/conditions/sacroiliac-joint-pain/) - [Spina bifida (myelomeningocele)](https://durgical.com/en/conditions/spina-bifida/) - [Pressure injury (pressure ulcer)](https://durgical.com/en/conditions/pressure-injury/) - [Immobility syndrome and hospital-associated deconditioning](https://durgical.com/en/conditions/immobility-syndrome-and-hospital-associated-deconditioning/) - [Incontinence-associated dermatitis](https://durgical.com/en/conditions/incontinence-associated-dermatitis/) - [Pelvic fracture](https://durgical.com/en/conditions/pelvic-fracture/) - [Pilonidal disease](https://durgical.com/en/conditions/pilonidal-disease/) ## Test yourself Think of the answer, then tap the question to see it. ## What is the entry point for a caudal epidural block on the sacrum? The sacral hiatus, between the sacral cornua where the S5 laminae are missing. ## At what level does the dural sac usually end, and what surface landmark marks it? At S2, level with the posterior superior iliac spines (the skin dimples). ## What is the Honda sign of the sacrum on bone scan? H-shaped uptake from a sacral insufficiency fracture, typical of older adults with osteoporosis. ## Which sacral fractures put the sacral nerve roots at risk? Fractures through the foramina or the central canal (Denis zones II and III), with bladder and bowel effects. [Add to my cards (4)](https://durgical.app/estudiar/?add=/en/anatomy/sacrum/) Educational information, not medical advice. For any symptom, see a health professional. ## Its names **Official Latin (TA2)** os sacrum TA2 1071 **Official Spanish (DeCS)** Sacro DeCS D012447 **English (TA2)** Sacrum **Also called** sacral bone · vertebrae sacrales · vertebrae sacrales I-V · vertebrae S1-S5 ## Related - [Sacroiliac ligaments](https://durgical.com/en/anatomy/sacroiliac-ligaments/) - [Coccyx](https://durgical.com/en/anatomy/coccyx/) - [Sacrotuberous ligament](https://durgical.com/en/anatomy/sacrotuberous-ligament/) - [Sacrospinous ligament](https://durgical.com/en/anatomy/sacrospinous-ligament/) - [Fascia lata](https://durgical.com/en/anatomy/fascia-lata/) - [Gluteus maximus](https://durgical.com/en/anatomy/gluteus-maximus-muscle/) - [Iliopsoas fascia](https://durgical.com/en/anatomy/iliopsoas-fascia/) - [Piriformis](https://durgical.com/en/anatomy/piriformis-muscle/) Sources: Terminologia Anatomica (TA2); [DeCS](https://decs.bvsalud.org/es/ths/resource/?id=12823); [Anatomy, Back, Sacral Vertebrae](https://www.ncbi.nlm.nih.gov/books/NBK551653/); Anatomía humana (4.ª ed.), tomo 1, cap. 4: Esqueleto de la columna vertebral. ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/anatomy/sacrum/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/anatomy/sacrum/