Anatomy of the pelvis

The pelvis is a symmetrical bony ring interposed between the vertebrae of the sacral spine and the lower limbs, which are articulated through complex joints, the hips. It supports the spinal column and connects the upper body to the lower extremities.  Importantly, the pelvis functions as the reservoir for the abdominal organs including the bladder, intestine, and uterus in women or prostate in men.

The pelvis is a symmetrical bony girdle. It consists of three strong bones fused together: the ilium, ischium and pubis. These bones merge to form bilateral concave sockets named acetabulum, which accommodate the femur heads to form the hip joints.

The ilium is the largest flat bone located on either side of the upper portion of the pelvis, including the iliac crest, which is the protruding tip bone of the pelvis that is easily felt with palpation.

The pubis is the smallest bone at the anterior side of the pelvis. Both ends of the pelvis are fused by the symphysis pubis consisting of a cartilaginous flexible tissue.

Males and females differ significantly in the anatomy of the pelvis: Females’ pelvis is wider and the pubis shorter than males’. This is an adaptation to child bearing and birth. In pregnant women the symphysis is extended to allow for the passage of the child through the birth canal.

The sacrum

The sacrum consists of five fused vertebrae (S1 to S5) adopting a triangular shape located in the posterior side of the pelvis. The upper, wider region of the sacrum articulates with the ilium (sacroiliac joint) on each side. In the lower portion it connects to the tail bone or coccyx formed by five small fused bones.

The sacrum is particularly important in forming and stabilising the pelvic ring via the presence of numerous ligaments between bones and allows the connection of muscles of the pelvis and muscles of the hip joint (gluteus maximus, iliacus and piriformis).

At the lower tract of the spinal cord numerous nerves of the cauda equina extend from the foramen of the lumbar spine into the sacral canal. These nerves exit the sacrum through four pairs of cavities (sacral foramina) on each side of the canal or through the sacral hiatus located at the lower extremity of the canal.

Anatomy of the hip

The hip is a major ball-and-socket joint connecting the long bones of the lower limbs (femur) to the pelvis. This joint allows a wide range of movements of the lower limbs and is used when walking, running, climbing, lunging and bending.  Because it bears the body weight, the hip joint is supported by large muscles, strong tendons and ligaments.

The hip joint consists of the round head of the thighbone named femur, which inserts into the pelvis ring in a socked called acetabulum. The femoral head continues distally with the femoral neck and the greater trochanter. The latter is the protruding portion of the femur bone that can be felt laterally at the higher thigh, below the pelvis. The hip joint is protected by articular cartilage, which is a layer of elastic yet tough connective tissue surrounding both the femoral head and the acetabulum. The cartilage facilitates the movement of the joint by reducing the friction of the femoral head rotating inside the socket and importantly it absorbs shock impacts. Overuse of the cartilage is often leading to degeneration or osteoarthritis of the hip.

Anatomy of the femur

The femur or thighbone is the longest and strongest bone in the human skeleton. It extends from the hip to the knee joint. The femoral head is the distal (upper) end of the femur that inserts into the acetabulum of the hip joint. As it descends towards the knee, it is separated from the longest tubular bone, or shaft, by the femoral neck.

The greater trochanter is the protruding extremity of the upper femur that can be felt laterally at the hip. The great trochanter inserts the large gluteus muscle. Below and on the opposite side of the great trochanter is the lesser trochanter, which connects to the ileo-psoas muscle. The femoral shaft contains the bone marrow. At the proximal (lower) end the femur forms two large structures on the outer side, the lateral epicondyle and the lateral condyle whereas at the inner side are the adductor tubercle, the medial epicondyle and the medial condyle. The lateral and medial epicondyles articulate with the tibia and fibula to form the knee joint, including the trochlear groove, which holds the patella (kneecap).

Ligaments of the pelvis and hip

The bones of the pelvis are held together by a large number of ligaments and muscles. The stability and flexibility of the hip joint is provided by two structures: the ligaments, made of strong connective tissue, which connect bones to bones, and the tendons, which connect muscles to bones.

In addition to the labrum and the ligamentum teres, three other ligaments stabilise the femoral head into the acetabulum. They merge to form the capsule of the hip joint:

Iliofemoral ligament is the strongest ligament in the body. It is Y-shaped and extends from the lower front iliac spine of the coccyx to the femur trochanter (intertrochanteric line). This ligament prevents the extension of the femur in a standing position

Ischiofemoral ligament begins at the ischium, posteriorly to the acetabulum

Pubofemoral ligament originates lateral to the pubis adjacent to the iliofemoral ligament.

Ligamentum teres is smaller ligament that connects the higher extremity of the femoral head to the acetabulum. It contains an artery that supplies blood to the femoral head.

The labrum and capsule

The labrum is a cuff made of fibro-cartilaginous rim covering the edge of the acetabular cavity of the pelvis. It consists of two parts, one in contact to the femoral head and the other connecting with the joint capsule. The labrum can be subject to tear and injury.

The capsule of the hip is an essential bundle of strong ligaments that surround the hip to consolidate the joint elements and hold the femoral head in place during movement. The ligaments are the iliofemoral, pubofemoral and ischiofemoral. The capsule produces synovial fluid to keep the hip joint lubricated and facilitate the sliding of the joint components.

Bursas of the hip

A bursa is a fluid-filled sac, functioning as a cushion to absorb shock and facilitate gliding of muscles and bones around the joint. The hip has two main bursas:

The great trochanteric bursa is located on the great trochanter where the large ilio-psoas muscles of the hip joint are attached. This is quite a large bursa and is known for the related pathology, hip bursitis (trochanteric bursitis).

The ilio-psoas bursa is located on the inner side of the hip. Also this bursa is subject to inflammation or bursitis, albeit less commonly.

Muscles of the pelvis

The muscles of the pelvis and hip control the vast range of movement of the legs and torso. On the posterior side they are the glutei and on the anterior side the hip muscles extending into the thighs.

Psoas consists of a pair of deep muscles (psoas major and iliacus) located on each side of the pelvis in the abdomen. They originate in the lowest thoracic vertebra of the lumbar spine. As they descend, they cross to the front of the body along the pelvis and attach to the femur. The main function of the psoas is to: flex the hips, externally rotate the femur and laterally bend the spine. It also controls the movement of the thigh crossing over to the opposite leg.

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Gluteus maximus is the largest muscle forming the glutei comprising the gluteus minimus and gluteus medius. It originates in the sacrum above the iliac crest. The gluteus maximus controls hip extension when the leg is flexed backwards. It is used in running and jumping. It also attaches to the ilio-tibial (IT) band, which is a robust complex of connective fibres running along the external side of the thigh, from the gluteal fascia to the tibia.

The lateral movements of the lower limbs are controlled by smaller muscles:

Piriformis, originates on each side of the sacrum and attaches on the upper femur trochanter. It is the most superior group of deep muscles of the pelvic wall and is used when externally rotating the thigh

Obturator internus begins at the inner pelvis and wraps around the sitting bones to connect to the posterior side of the femur head

Obturator externus originates at the meeting point of the ischium and the pubis and attaches to the femoral head

Gemellus superior and gemellus inferior originate from the ischium and insert on top of the femur

Quadratus femoris is a shorter but wider muscle, running from the ischial tuberosity (the actual “sit bone”) to the femur just below the head.

Tensor fasciae latae begins at the outer iliac crest and descends to insert into the ilio-tibial band or tract. This muscle stabilises the femoral head into the acetabulum and the knee during extension

Rectus femoris is a hip flexor that belongs to the quadriceps muscles of the thigh

Sartorius is a thin and the longest human muscle of the inner thigh. It begins at the front side of the iliac crest and descends diagonally to the knee to insert to the inner tibia. It assists in the external rotation of the hip when sitting with crossed legs.

Muscles of the thigh

The quadriceps is a group of four muscles located on the front side of the thigh. The rectus femoris originates at the pelvis while the vastus medialis, vastus lateralis and vastus intermedialis begin at the femoral head. These muscles extend over the knee joint and connect to the tibia.

The tendon of the quadriceps and specifically of the rectus femoris continues to the knee joint over the patella or kneecap. The function of the quadriceps is the extension of the lower leg while straightening the knee and lifting the patella.

The inner side of the femur is served by four muscles that begin at the pubic ramis and attach to the femur at the higher segment (pectineus), the middle section (adductor brevis and adductor longus) and the lower section (adductor magnus). They form what is known as hamstrings. These muscles are used when flexing and bending the knee.

On the posterior side of the thigh, three muscles, the semimembranosis, semiteninosus and biceps femoris begin at the ischial tuberosity of the pelvis, extend diagonally to cross the knee and attach to the opposite outer side of the tibia. These adductor muscles control the movement of the leg towards the centre of the body to the opposite leg.

The table summarises the complexity of the muscles found in the pelvis, hip and thigh. The muscles in bold are shown in the figures.

Blood vessels of the pelvis

Due to its role in hosting the pelvic organs, the pelvis is highly vascularised by a complex network of arteries and veins, which differ between males and females. Injuries to the pelvis can cause severe bleedings.

Arteries

The abdominal aorta descends towards the pelvis and divides at the 4th lumbar vertebra into the left and right common iliac arteries, which branch further into the external and internal iliac arteries.

The external iliac arteries continue to supply the lower extremities by separating into the inferior epigastric and deep circumflex iliac arteries.

The internal iliac artery remains within the pelvic region to supply blood to pelvic organs, gluteal muscles and perineum. It divides into the anterior and posterior branch and downstream to many more branches.

Anterior branch generates:

Umbilical artery and superior vesical artery: upper bladder

Obturator artery: pelvic muscles, ilium, femur head

Inferior vesical artery and branches (males): prostate, testis, vas deferens ?

Middle rectal artery: rectum

Internal pudental artery and branches: perineum (anal muscles), penis, clitoris

Inferior gluteal artery: pelvic, gluteal muscles (piriformis, coccygeus, levator ani, gluteal)

Uterine artery (females): ureter, uterus, uterine tube, ovary, vagina

Posterior branch generates:

Ilio-lumbar artery: iliacus and psoas major, quadratus lumborum muscles, cauda equina (lowest spine)

Lateral sacral arteries: piriformis muscle and vertebral canal

Superior gluteal artery: gluteal muscles and tensor fasciae latae.

Other arteries include:

Superior rectal artery: rectum and anal canal

Median sacral artery: sacrum and coccyx

Ovarian arteries (females): ovaries

Testicular arteries (males): testis

Veins

Deoxygenated blood flows through the veins in an inverse direction compared to arterial

blood, namely from peripheral organs to the heart. The main veins mostly take the names of the accompanying arteries. The anatomical position of pelvic veins follows the same path of the arteries and mostly take their names. 

Great saphenous vein: is the longest vein in the body. It originates from the lesser saphenous vein at the foot and runs superficially along the medial aspect of the thigh to the inguinal region.

Femoral vein: located in the thigh along the femur. It reaches the pelvis through the inguinal ligament to change name into external iliac vein.

Other important veins include: prevesical veins and pre-sacral veins.

Vessels of the hip and thigh

The internal and external iliac arteries supply most of the blood to the lower extremities. The inferior iliac artery becomes femoral artery in the posterior thigh when it reaches the inguinal ligament. The femoral artery descends further dividing into the anterior and posterior tibial arteries. The main arteries of the hip are:

Deep femoral artery (profunda femoris), the main branch of the femoral artery

Lateral femoral circumflex emerges from the femoral artery to vascularise the hip joint and many thigh muscles

Medial femoral circumflex artery at the head of femur contributing to blood supply to the hip and knee joints

Popliteal artery deviation of the femoral artery around the thigh to become popliteal artery

Lymphatic system in the pelvis

The pelvis is equipped with an extensive lymphatic system made of large numbers of lymph nodes and lymphatic ducts. Lymph nodes are mostly located along blood vessels. Their function is to readily combat potential infections in a region frequently exposed to external environment. They are also responsible of cancer cell dissemination during the metastatic process.

External iliac lymph nodes: along the femoral vein

Internal iliac lymph nodes: perineum and gluteal region

Sacral lymph nodes: between the pelvic organs and internal iliac nodes

Common iliac lymph nodes: receive lymph from other pelvic nodes

Pararectal nodes: in proximity of the internal iliac lymphatic vessels

Superficial inguinal and deep inguinal nodes: region of the trunk and perineum

Lumbar nodes: along the aorta and inferior vena cava

Nerves of the pelvis

The pelvis is predominantly innervated by nerves arising from the sacral plexus, coccygeal plexus and the autonomic nervous system of the pelvic region.

Lumbar plexus

The lumbar plexus is a network of nerve fibres that supplies the skin and the musculature of the lower limbs. It is located in the lumbar spinal region within the psoas major muscle and anterior to the transverse processes of the lumbar vertebrae. The lumbar plexus is formed by the anterior and posterior rami (divisions) of five to six peripheral nerves of the lumbar nerves L1, L2, L3 and L4. In half the population the thoracic spinal nerve 12 contributes to the lumbar plexus. Once exiting from the spinal cord through the intervertebral foramina, these nerves divide into anterior and posterior branches. The posterior branches innervate the paravertebral muscles and the skin of the lumbar back. The anterior branches exit as single nerves into the pelvis. The nerve arising from the T12 contributes to the ilio-hypogastric nerve.

Sacral plexus

The nerves exiting the 4th and 5th lumbar vertebrae form the lumbo-sacral trunk merge with the sacral plexus, which consists of the sacral nerves from the 1st to the 4th sacral vertebrae. The sacral plexus descends along the posterior pelvic wall above the pelvic muscle, piriformis. The nerves of the sacral plexus include:

Sciatic nerve: largest human nerve formed by the anterior branches of nerves of the lumbar vertebrae L4, L5 and sacral vertebrae S1, S2, S3 branches. It exits the sciatic underneath the gluteal muscle and extends along the posterior leg to the lateral foot. The sciatic nerve is two individual nerves bundled together in the same connective tissue sheath; the tibial nerve and the common fibular nerve. They usually separate at the apex of the popliteal fossa, however in 12% of people they separate as they leave the pelvis. The nerve innervates the muscles of the posterior thigh and the hamstring portion of the adductor magnus. It innervates (via its terminal branches) the muscles of the leg and foot. The sciatic nerve has no direct sensory function, but indirectly innervates (via its terminal branches) the skin of the lateral leg, heel and both the dorsal and plantar surfaces of the foot.

Pudental nerve: muscles skin of perineum

Superior gluteal nerve: gluteal muscles

Inferior gluteal nerve: gluteal muscles

Nerve of the quadratus femoris muscle: same hip muscle

Nerve of the obturator femoris muscle: same hip muscle

Nerve of the piriformis muscle: same hip muscle

Perforating cutaneous nerve: skin of lower medial buttocks

Posterior femoral cutaneous nerve: skin perineum and posterior skin of thigh and leg

Pelvic splanchnic nerves: sympathetic nerves of pelvic organs

Coccygeal plexus

The nerves of the coccygeal plexus exit the 4th and 5th vertebrae and include the coccygeal nerves. They innervate the coccygeus and levator ani muscles and the sacro-coccygeal joint. The ano-coccygeal nerves also extend to the skin of the perineum (between anus and genitals).

Obturator nerve

This nerve originates from the lumbar plexus and only passes through the pelvis to innervate downstream, the medial thigh muscle.

Pelvic autonomic system

This comprises the parasympathetic and sympathetic nerves.

The parasympathetic nerves regulate functions such as bowel peristalsis and contraction of the bladder and rectum and penis erection.

The sympathetic nerves have an inhibitory effect on peristalsis but control muscle contraction of the genitals during orgasm.

Sacral sympathetic trunks: These nerves extend from the lumbar sympathetic trunks. They provide sympathetic fibres to the sacral plexus that descend to innervate the low extremities. In addition, they provide fibres to the hypogastric plexus.

Superior hypogastric plexus: It includes sympathetic fibres from the aortic plexus. Once in the pelvic region it divides into the left and right hypogastric nerves.

Inferior hypogastric plexuses: The plexuses are situated on either side of the rectum and bottom of the bladder and consist of sympathetic and parasympathetic fibres.

Pelvic splanchnic nerves (S2–S4): with the hypogastric nerves they form the inferior hypogastric plexuses.