Functional Anatomy of the Pelvis

Original Editor - Ewa Jaraczewska

Top Contributors - Ewa Jaraczewska, Jess Bell and Kim Jackson  

Introduction

The pelvis is a complex anatomical structure with many functions, including transmitting body weight to the lower limbs during locomotion, accommodating childbirth in females, and providing support for the abdominal organs. Because of its complexity, it is crucial to understand the anatomy of the pelvis to diagnose and treat pelvic girdle pathologies. It is important to note that our understanding of the pelvis has continued to evolve in recent years, with some recent findings contradicting previously accepted descriptions. This article discusses the key anatomical structures of the pelvis, including the bony structures, articulations, ligaments, muscles, nerves and the vascular supply.

Key Terms

Axes: lines around which an object rotates. The rotation axis is a line that passes through the centre of mass. There are three axes of rotation: sagittal passing from posterior to anterior, frontal passing from left to right, and vertical passing from inferior to superior. The rotation axes of the foot joints are perpendicular to the cardinal planes. Therefore, motion at these joints results in rotations within three planes. Example: supination involves inversion, internal rotation, and plantarflexion.

Bursae: reduces friction between the moving parts of the body joints. It is a fluid-filled sac. There are four types of bursae: adventitious, subcutaneous, synovial, and sub-muscular.

Capsule: one of the characteristics of the synovial joints. It is a fibrous connective tissue which forms a band that seals the joint space, provides passive and active stability and may even form articular surfaces for the joint. The capsular pattern is "the proportional motion restriction in range of motion during passive exercises due to tightness of the joint capsule."

Closed pack position: the position with the most congruency of the joint surfaces. In this position, joint stability increases. The closed pack position for interphalangeal joints is full extension.

Degrees of freedom: the direction of joint movement or rotation; there is a maximum of six degrees of freedom, including three translations and three rotations.

Ligament: fibrous connective tissue that holds the bones together.

Open (loose) pack position: position with the least joint congruency where joint stability is reduced.

Planes of movement: describe how the body moves. Up and down movements (flexion/extension) occur in the sagittal plane. Sideway movements (abduction/adduction) occur in the frontal plane. The transverse plane movements are rotational (internal and external rotation).

Pelvis Structure

The term ‘pelvis’ refers to "the bony pelvic ring and associated stabilizing ligaments". In contrast to this, the 'pelvic cavity' is the anatomical space within the bony ring, which contains the pelvic organs.

The bones of the pelvis (i.e. the sacrum, coccyx, and two innominate bones) form the pelvic ring. The innominate bones close the pelvic ring at the front and sides, while the sacrum and coccyx form the posterior border of the ring.

The sacrum is a large bone located at the terminal part of the vertebral canal. It forms the posterior aspect of the pelvis. Its thickness enables it to support and transmit the body's weight. The sacrum comprises five fused vertebrae. It is configured as an inverted triangular bone that is concave anteriorly and convex posteriorly. Posteriorly, the sacrum articulates with the innominate bones to form the sacroiliac (SI) joints on either side. The sacrum articulates with the coccyx at the sacrococcygeal joint.

The posterior aspect of the sacrum has three bony landmarks:

• Median sacral crest
• Intermediate sacral crests
• Lateral sacral crests

The coccyx is the terminal bone of the spine. It is triangular and consists of three to five segments: the first and largest of these vertebral segments articulates with the sacrum. The last three segments diminish in size and usually form a single piece of bone. The first coccygeal segment has processes called coccygeal cornua, which articulate with the sacral cornua. The anterior surface of the coccyx contains three transverse grooves - these grooves indicate the lines of fusion of the coccygeal segments. The coccyx curves in a concave-shaped direction. Its anterior portion angles into the pelvis.

The two innominate bones consist of three bones fused together: the ischium, ilium, and pubis.

The ischium forms the lower and back part of the hip bone and the posterior and inferior boundary of the obturator foramen. It consists of two main parts: the body of the ischium, which forms the posterior one-third of the acetabulum, and the ramus of the ischium. The ischial ramus extends down from the body, turns anteriorly and unites with the inferior ramus of the pubic bone. The following landmarks are located on the ramus of the ischium:

• Ischial tuberosity
• Ischial spine
• Lesser sciatic notch

The ilium makes up the upper portion of the hip bone and pelvis. It is the largest and uppermost bone of the hip and is an essential part of the pelvic girdle. It is attached to the sacrum via the sacroiliac ligaments. The ilium consists of a flared expanded upper part, which forms the iliac crests, the iliac spines, and a small inferior part, which makes up two-fifths of the acetabulum. The ilium has four borders: superior, anterior, posterior and medial; and three surfaces: gluteal, iliac fossa and sacropelvic.

The following landmarks are located on the ilium:

• Iliac crest
• Anterior superior spine
• Anterior inferior spine
• Posterior superior spine
• Posterior inferior spine
• Iliac fossa
• Greater sciatic notch

The pubis makes up the anteroinferior portion of the pelvic ring and is its most forward-facing bone. The two pubis bones are connected by cartilage, making the pubic symphysis. The pubis has three parts: the pubic body, the superior pubic ramus and the inferior pubic ramus.

The following landmarks are located on the pubis:

• Pubic symphysis
• Pubic tubercle
• Pubic crest
• Pubic body
• Superior pubic ramus
• Inferior pubic ramus

Physiologic Variability

There are differences in the pelvises of biological males and biological females (also known as the android pelvis (male) and the gynaecoid pelvis (female)).

The android pelvis	The gynaecoid pelvis
Male	Female
Narrow	Wide
Heart-shaped pelvic inlet	Round pelvic inlet
Sacrum is less mobile, more curved, less backwards tilt	Sacrum is more mobile, less curved, more backwards tilt
Adapted for better bipedal locomotion	Adapted to facilitate pregnancy and childbirth

Bones, Articulations and Kinematics of the Pelvis

The bones of the pelvis form four articulations:

• Anteriorly, the innominates (pubis) articulate to form the extremely strong pubic symphysis
• Posteriorly, the sacrum articulates with the innominates (ilium) to form two sacroiliac (SI) joints, one on each side
• The sacrum articulates with the coccyx at the sacrococcygeal joint

Bones	Articulations	Characteristics	Key palpation points
Pubis	Pubic symphysis	Articulation between left and right pubis bones. This articulation can resist tensile, shearing and compressive forces. Absorbs the weight and stress from the upper body. Facilitates vaginal childbirth.	The patient is in a supine position. Palpate the iliac crest by finding the top of the pelvis and the bottom of the rib cage. Roll your hands in and push down to feel the superior part of the iliac crest. Follow the iliac crest anteriorly until you reach the next bony landmark, the anterior superior iliac spine (ASIS). Next, place the palm of your hands on the ASIS on both sides and your thumbs on the umbilicus. Move your thumbs down slowly by pressing in. Once you start feeling tissue tension at the muscle insertion, you are approaching the pubic tubercle. The superior aspect of the pubic tubercle will be under your thumb when you push posteriorly. The pubic symphysis is found at the midline between two tubercles and can be recognised by an indentation, which is consistent with the cartilage of the pubic symphysis.
Sacrum Ilium	Sacroiliac (SI) joints	The joints between the sacrum and the ilium are the strongest joints in the body. The SI joint is usually formed with the S1, S2 and S3 sacral segments (please note a complete S3 segment is uncommon in females). The SI joints provide flexibility for transferring intra-pelvic forces between the lumbar spine and lower extremities. There are several sexually dimorphic features, including size, shape, and surface area.	First, palpate the superior aspect of the pelvis and the inferior border of the ribs. Roll your hands in and push down to palpate the iliac crest. Move your thumbs down until you feel the bony peak, which defines the posterior superior iliac spine (PSIS). Next, continue moving your fingers down inferiorly along the iliac bone without crossing the midline. Before you reach a soft area, you can palpate the last part of the bone, which is the posterior inferior iliac spine (PIIS). Place your fingers on the line between the PSIS and PIIS. The area between the sacrum and your fingers defines the sacroiliac joint.
Sacrum Coccyx	Sacrococcygeal joint	The joint between the sacrum and the coccyx. It can move slightly. There are reports describing the occurrence of sacrococcygeal fusion between the first coccygeal segment and sacrum. It may be related to age, genetics and/or environmental factors.

Pelvis Kinematics

Joint	Type of joint	Plane of movement	Motion	Kinematics	Closed pack position	Open pack position
Pubic symphysis	Secondary cartilaginous that acts as a hinge joint as it allows a small amount of movement	Transverse	Small amount of shift Rotation During pregnancy, the mobility of this joint increases.	Shift: 2 mm Rotation: 1 degree	Nutation	Counternutation
Sacroiliac joint	Synovial plane	Saggital Transverse Frontal	Flexion or anterior (nutation) Extension or posterior (counternutation) Axial rotation Lateral flexion	Flexion/extension: 3 degrees Axial rotation: 1.5 degrees Lateral flexion: 0.8 degrees	Nutation	Counternutation
Sacrococcygeal joint	Secondary cartilaginous that acts as a hinge joint as it allows a small amount of movement	Saggital	Only passive flexion/ extension	Small degree of passive motion	Nutation	Counternutation

Ligaments of the Pelvis

Key ligaments	Origin	Insertion	Action/role	Key palpation points
Anterior sacroiliac ligament (ASL)	Pre-auricular surface of the ilium (the area just anterior to the auricular surface)	Third segment of the sacrum	Forms the anteroinferior component of the SI joint capsule Provides pelvis stability
Posterior sacroiliac ligament: Short posterior sacroiliac ligament (SPSL) Long posterior sacroiliac ligament (LPSL)	SPSL: first and second transverse tubercles of the sacrum LPSL: posterior superior iliac spine	SPSL: tuberosity of the ilium LPSL: the third and fourth transverse tubercles of the sacrum	Resists counternutation	First, palpate the superior aspect of the pelvis and the inferior border of the ribs. Roll your hands in and push down to palpate the iliac crest. Move your thumbs down until you feel the bony peak, which defines the posterior superior iliac spine (PSIS). The posterior sacroiliac ligament can be palpated directly below the PSIS.
Interosseous sacroiliac ligament (ISL)	Sacral tuberosity	Iliac tuberosity	The strongest of the ligaments supporting the SI joint Provides for major multidirectional structural stability Prevents anterior and inferior movement of the sacrum
Sacrotuberus ligament	PSIS and PIIS The lateral side of the sacrum below the auricular surface The lateral side of the upper part of the coccyx	Lower margin of the ischial ramus	Provides pelvis stability Prevents tipping or twisting of the sacrum in the pelvis Transforms the greater and the lesser sciatic notches posteriorly into the greater and the lesser sciatic foramina Prevents forward tilting of the sacral apex	Position the patient in prone. Palpate the middle of the thigh to find the hamstrings. Move your fingers superiorly until you reach the horizontal crease marking the buttock line. Push your thumb anteriorly and lift upwards to locate the superior part of the ischial tuberosity. The second hand is placed on the lateral border of the sacrum. The sacrotuberous ligament is located between the hands and can be felt by performing medial and lateral friction.
Sacrospinous ligament	Lower sacral and upper coccygeal segments, anterior to the sacrotuberous ligament	Ischial spine	Provides pelvis stability Prevents tipping or twisting of the sacrum in the pelvis Transforms the greater and the lesser sciatic notches superiorly into the greater and the lesser sciatic foramina Prevents forward tilting of the sacral apex	The sacrospinous ligament CANNOT be palpated externally
Iliolumbar ligament (IL): Superior band (SB) Inferior band(IB)	Superior and inferior bands: Transverse processes of L4/L5	SB: iliac crest blends with the thoracolumbar fascia IB: posterior region of the iliac fossa	Helps to stabilise the lumbosacral spine on the pelvis Restricts motion at the lumbosacral junction, particularly side bending Influences SI joint stability	Position the patient in prone. Palpate from the posterior iliac crest towards the PSIS. Hook your fingers on the anterior surface of the PSIS and move them diagonally, creating friction on the iliolumbar ligament. Because the transverse process of L5 is not palpable due to its deep location, most of the iliolumbar ligament cannot be palpated.
Anterior pubic ligament (APL)	Anterior aspect of the pubic bones	Merges with the periosteum of the pubic bones	Provides attachment points for adductor longus, adductor brevis, rectus abdominis and the pyramidalis muscles
Superior pubic ligament (SPL)	Superior margins of the pubic symphysis	Lateral margin of the pubic crest, near the pubic tubercles	Mixed reports: important in reinforcing the joint vs functionally inconsequential
Inferior pubic ligament (subpubic or arcuate pubic ligament) (IPL)	Arch bridging the inferior parts of the pubic rami	Inferior fibres are attached to the inferior pubic rami Upper fibres blend with the interpubic disc	Forms the upper boundary of the pubic arch Stabilises the symphysis pubis Provides attachment points for the adductor brevis and gracilis muscle
Inguinal ligament (Pouparts’s ligament)	Anterior superior iliac spine	Pubic tubercle	Forms the superior boundary of the femoral triangle Contributes to the floor of the inguinal canal Anchors the external oblique and transversus abdominis muscles to the pelvis.	Place your hands on the superior border of the iliac crest. Next, follow the iliac crest anteriorly until you reach the next bony landmark, the anterior superior iliac spine (ASIS). From the ASIS, move your hands inferiorly and medially along the inguinal ligament, which ends at the pubic tubercle.
Anterior sacrococcygeal ligament	Anterior surface of the sacrum	Anterior surface of the apex of the coccyx	Increases the anteroposterior diameter of the pelvis during labour and defecation Stabilises the sacrococcygeal symphysis
Superficial posterior sacrococcygeal ligament	The margin of the sacral hiatus	Dorsal surface of the coccyx	Closes the posterior aspect of the most distal part of the sacral canal Stabilises the sacrococcygeal symphysis
Deep posterior sacrococcygeal ligament	Dorsal surface of the fifth sacral segment	Dorsal surface of the coccyx	Completes the lower and back part of the sacral canal
Lateral sacrococcygeal ligament	Inferolateral angle of the sacrum	Transverse processes of the coccyx	Stabilises the sacrococcygeal symphysis Connects the transverse process of the coccyx to the lower lateral angle of the sacrum
Intercornual ligament	Cornua of the sacrum	Cornua of the coccyx	Stabilises the sacrococcygeal symphysis

Muscles of the Pelvis

The muscles of the pelvis are organised into superficial, middle, and deep muscle layers.

• Deep layer: levator ani and the coccygeus muscles
• Middle layer: urethral sphincter, deep transverse perineal muscle
• Superficial layer: bulbospongiosus, ischiocavernosus, superficial transverse perineal muscle

The pelvic floor muscles are located on the bottom of the pelvis. There are supported by the obturator internus and the piriformis which help to form the walls in the pelvic floor. The primary role of the pelvic floor muscles is to support the pelvic organs. Pelvic floor muscle weakness can lead to problems with bladder and bowel control.

Muscle	Origin	Insertion	Innervation	Action
Levator ani: Pubococcygeus (PC) Puborectalis (PR) Iliococcygeus (IC)	PC: body of the pubic bone and the anterior aspect of the tendinous arch PR: body of the pubic bone IC: anteriorly at the ischial spines	PC: anococcygeal ligament and the coccyx PR: circles around the anal canal IC: anococcygeal ligament and the coccyx	Nerve to levator ani (S4) Pubococcygeus receives nerve branches from the pudendal nerve (S2- S4)	Stabilises the abdominal and pelvic organs Forms the pelvic diaphragm together with the coccygeus muscle and their associated fascias
Coccygeus	Ischial spines	Lateral aspect of the coccyx and sacrum, along the sacrospinous ligament	Anterior rami of spinal nerves S4/S5	Helps to support the pelvic viscera Assists with flexing the coccyx
Urethral sphincters Internal urethral sphincter (IUS, smooth muscle under autonomic control) External urethral sphincter (EUS, skeletal muscle)	EUS: Ischial rami Inferior pubic rami Adjacent fascia	EUS: Surrounds the middle-lower, membranous part of the urethra. Males only: EUS, which is sometimes called the rhabdosphincter muscle, forms an annular sphincter.	EUS: Pudendal nerve (S2 to S4 nerve roots)	Controls the flow of urine
Deep transverse perineal	Ischial ramus	The fibres of each muscle connect in the midline at the perineal body and cross with the contralateral superficial transverse perineal muscle	Perineal nerve (branch of the pudendal nerve)	Assists in the stability of the perineal body
Bulbospongiosus (a sexually dimorphic muscle that varies in different sexes)	Males: Perineal body Median penile raphe Females: Perineal body	Males: Perineal membrane Dorsal aspect of corpus spongiosum and corpora cavernosa Fascia of bulb of penis Females: Pubic arch Fascia of corpora cavernosa and clitoris	Deep branch of perineal nerve (branch of pudendal nerve) (S2-S4)	Males: Compresses the bulb of the penis during urination and ejaculation Assists in the erection of the penis Supports the perineal body Females: Assists in the erection of the clitoris/bulb of vestibule Supports the perineal body
Ischiocavernosus	Ischial tuberosity and ramus	Crus of clitoris or penis	Deep branch of perineal nerve (branch of pudendal nerve) (S2-S4)	Aids in erection and ejaculation in males and clitoral erection in females
Superficial transverse perineal	Anteromedial surface of the ischial tuberosity	Perineal body	Deep branch of the perineal nerve	Stabilises the perineal body Assists in expelling semen or vaginal secretions during the ejaculatory process.

Innervation of the Pelvis

The pelvic floor is primarily innervated by the sacral plexus, a network of nerves that originates from the nerve roots of L4 down to S4.

Nerve	Origin	Branches	Motor fibres	Sensory fibres
Pudendal	Ventral roots of S2–S4 in the sacral plexus	Inferior rectal nerve Perineal nerve Medial and posterolateral scrotal/labial nerve. Dorsal branch of penis/clitoris	External anal sphincter Females only: The compressor urethrae The sphincter urethrovaginalis	The anal canal The body and glans of the penis or clitoris.
Perineal	Pudendal	Superficial perineal nerve Deep perineal nerve	Ischiocavernosus, superficial transverse perineal, and bulbospongiosus muscles, deep transverse perineal muscles	Skin overlying the urogenital triangle: Males: The skin over the perineum Posterior scrotal skin Females: Labia minora Vaginal vestibule The inferior portion of the vaginal canal Posterior aspect of the labia majora
Ventral (anterior) rami of spinal nerves S4/S5	Sacral plexus	Spinal nerves S1, S2, S3, and S4	Coccygeous (ventral rami of S4/S5) Levator ani (ventral rami of S4)	Sensory innervation through the pudendal nerve

Vascular Supply of the Pelvis

The internal iliac artery is the major blood supplier of the pelvis. Along with the external iliac artery, it branches off the common iliac artery that descends from the abdominal aorta.

Artery	Origin	Branches	Supply
Internal iliac artery	Common iliac artery	Anterior trunk branches (ATB): umbilical artery, obturator artery, middle rectal artery, internal pudendal artery, and inferior gluteal artery. Males only: inferior vesical artery Posterior trunk branches (PTB): iliolumbar, lateral sacral, superior gluteal arteries	ATB: Males and Females: Bladder, distal ureter, ischiococcygeus muscle, iliococcygeus muscle, the skin of the thigh. Males only: The proximal aspect of the ductus deferens and the seminal vesicles, the prostate gland. PTB: The skin over the sacrum

Clinical Relevance

1. 16–30% of chronic mechanical lower back pain cases are thought to be related to sacroiliac joint pain
2. Pain localised to the region of the coccyx is referred to as coccydynia. Possible causes of pain include chronic inflammation and contractions of the attachments to the coccyx.
3. A fall onto the buttocks (e.g. falling off a chair) can fracture the coccygeal vertebrae.
4. Stress incontinence in males occurs when the sphincter mechanism cannot control the amount of pressure exerted from the bladder, resulting in leaking urine. You can learn about male urinary incontinence here.
5. Pain along the distribution of the pudendal nerve is called pudendal neuralgia.
6. A blood clot can obstruct blood flow in the pelvic veins leading to pelvic vein thrombosis. This can be a very serious condition characterised by pelvic and lower abdominal pain.
7. Chronic pelvic pain is commonly defined as non-malignant intermittent or continuous pain in the lower abdomen, pelvis or intrapelvic structures. Learn more about differential diagnosis and management of the chronic pelvic pain here.
8. Pelvic health dysfunction is common in both males and females and includes urinary and faecal incontinence, pelvic organ prolapse (in females) and persistent pelvic pain. Learn about common women’s pelvic health conditions here and male pelvic health in the Introduction to Male Pelvic Health Programme.

Resources

• Becker I, Woodley SJ, Stringer MD. The adult human pubic symphysis: a systematic review. J Anat. 2010 Nov;217(5):475-87.
• Helpful video on communicating about genital anatomy with medical providers for trans and nonbinary patients.