The skeleton of the pelvis is a basin-shaped ring of bones connecting the vertebral column to the femora. It is then connected to two hip bones. Its primary functions are to bear the weight of the upper body when sitting and standing, transferring that weight from the axial skeleton to the lower appendicular skeleton when standing and walking, and providing attachments for and withstanding the forces of the powerful muscles of locomotion and posture. Compared to the shoulder girdle, the pelvic girdle is thus strong and rigid. Its secondary functions are to contain and protect the pelvic and abdominopelvic viscera (inferior parts of the urinary tracts, internal reproductive organs), providing attachment for external reproductive organs and associated muscles and membranes.
As a mechanical structure The pelvic girdle consists of the two hip bones. The hip bones are connected to each other anteriorly at the
pubic symphysis, and posteriorly to the
sacrum at the
sacroiliac joints to form the pelvic ring. The ring is very stable and allows very little mobility, a prerequisite for transmitting loads from the trunk to the lower limbs. As a mechanical structure the pelvis may be thought of as four roughly triangular and twisted rings. Each superior ring is formed by the iliac bone; the anterior side stretches from the acetabulum up to the
anterior superior iliac spine; the posterior side reaches from the top of the acetabulum to the
sacroiliac joint; and the third side is formed by the
palpable iliac crest. The lower ring, formed by the rami of the
pubic and
ischial bones, supports the acetabulum and is twisted 80–90 degrees in relation to the superior ring. An alternative approach is to consider the pelvis part of an integrated mechanical system based on the
tensegrity icosahedron as an infinite element. Such a system is able to withstand omnidirectional forces—ranging from weight-bearing to childbearing—and, as a low energy requiring system, is favoured by
natural selection. The pelvic inclination angle is the single most important element of the human body posture and is adjusted at the hips. It is also one of the rare things that can be measured at the assessment of the posture. A simple method of measurement was described by the British orthopedist
Philip Willes and is performed by using an
inclinometer.
As an anchor for muscles The
lumbosacral joint, between the sacrum and the last
lumbar vertebra, has, like all vertebral joints, an
intervertebral disc,
anterior and
posterior ligaments,
ligamenta flava,
interspinous and
supraspinous ligaments, and synovial joints between the articular processes of the two bones. In addition to these ligaments the joint is strengthened by the
iliolumbar and lateral lumbosacral ligaments. The iliolumbar ligament passes between the tip of the transverse process of the fifth lumbar vertebra and the posterior part of the iliac crest. The lateral lumbosacral ligament, partly continuous with the iliolumbar ligament, passes down from the lower border of the transverse process of the fifth vertebra to the ala of the sacrum. The movements possible in the lumbosacral joint are flexion and extension, a small amount of lateral flexion (from 7 degrees in childhood to 1 degree in adults), but no axial rotation. Between ages 2–13 the joint is responsible for as much as 75% (about 18 degrees) of flexion and extension in the lumbar spine. From age 35 the ligaments considerably limit the range of motions. The three extracapsular ligaments of the
hip joint—the
iliofemoral,
ischiofemoral, and
pubofemoral ligaments—form a twisting mechanism encircling the neck of the
femur. When sitting, with the hip joint flexed, these ligaments become lax permitting a high degree of mobility in the joint. When standing, with the hip joint extended, the ligaments get twisted around the femoral neck, pushing the head of the femur firmly into the
acetabulum, thus stabilizing the joint. The
zona orbicularis assists in maintaining the contact in the joint by acting like a buttonhole on the femoral head. The intracapsular ligament, the
ligamentum teres, transmits blood vessels that nourish the femoral head.
Junctions The two hip bones are joined anteriorly at the
pubic symphysis by a
fibrous cartilage covered by a
hyaline cartilage, the interpubic disk, within which a non-synovial cavity might be present. Two ligaments, the
superior and
inferior pubic ligaments, reinforce the symphysis. Both
sacroiliac joints, formed between the auricular surfaces of the sacrum and the two hip bones. are
amphiarthroses, almost immobile joints enclosed by very taut joint capsules. This capsule is strengthened by the
ventral,
interosseous, and
dorsal sacroiliac ligaments. The
iliolumbar ligament is a strong ligament which connects the tip of the transverse process of the fifth lumbar vertebra to the posterior part of the inner lip of the iliac crest. It can be thought of as the lower border of the
thoracolumbar fascia and is occasionally accompanied by a smaller ligamentous band passing between the fourth lumbar vertebra and the iliac crest. The
lateral lumbosacral ligament is partly continuous with the iliolumbar ligament. It passes between the transverse process of the fifth vertebra to the
ala of the sacrum where it intermingle with the anterior sacroiliac ligament. The joint between the sacrum and the coccyx, the
sacrococcygeal symphysis, is strengthened by a series of ligaments. The
anterior sacrococcygeal ligament is an extension of the
anterior longitudinal ligament (ALL) that run down the anterior side of the
vertebral bodies. Its irregular fibers blend with the
periosteum. The
posterior sacrococcygeal ligament has a deep and a superficial part, the former is a flat band corresponding to the
posterior longitudinal ligament (PLL) and the latter corresponds to the
ligamenta flava. Several other ligaments complete the
foramen of the
last sacral nerve.
Shoulder and intrinsic back The inferior parts of
latissimus dorsi, one of the muscles of the upper limb, arises from the posterior third of the iliac crest. Its action on the shoulder joint are internal rotation, adduction, and retroversion. It also contributes to respiration (i.e. coughing). When the arm is adducted, latissimus dorsi can pull it backward and medially until the back of the hand covers the buttocks.
Abdomen The muscles of the
abdominal wall are subdivided into a superficial and a deep group. The superficial group is subdivided into a lateral and a medial group. In the medial superficial group, on both sides of the centre of the abdominal wall (the
linea alba), the
rectus abdominis stretches from the cartilages of ribs V-VII and the
sternum down to the
pubic crest. At the lower end of the rectus abdominis, the
pyramidalis tenses the
linea alba. The lateral superficial muscles, the
transversus and
external and
internal oblique muscles, originate on the rib cage and on the pelvis (iliac crest and
inguinal ligament) and are attached to the anterior and posterior layers of the
sheath of the rectus. Flexing the trunk (bending forward) is essentially a movement of the rectus muscles, while lateral flexion (bending sideways) is achieved by contracting the obliques together with the quadratus lumborum and intrinsic back muscles. Lateral rotation (rotating either the trunk or the pelvis sideways) is achieved by contracting the internal oblique on one side and the external oblique on the other. The transversus' main function is to produce abdominal pressure in order to constrict the abdominal cavity and pull the diaphragm upward.
Hip and thigh The muscles of the
hip are divided into a dorsal and a ventral group. The dorsal hip muscles are either inserted into the region of the
lesser trochanter (anterior, or inner, group) or the
greater trochanter (posterior, or outer, group). Anteriorly, the
psoas major (and occasionally
psoas minor) originates along the spine between the rib cage and pelvis. The
iliacus originates on the
iliac fossa to join psoas at the
iliopubic eminence to form the
iliopsoas, which is inserted into the lesser trochanter. The iliopsoas is the most powerful hip flexor. The posterior group includes the
gluteus maximus,
gluteus medius, and
gluteus minimus. The maximus has a wide origin stretching from the posterior part of the iliac crest and along the sacrum and coccyx and has two separate insertions: a proximal, which radiates into the
iliotibial tract, and a distal, which inserts into the
gluteal tuberosity on the posterior side of the femoral shaft. It is primarily an extensor and lateral rotator of the hip joint, but, because of its bipartite insertion, it can both adduct and abduct the hip. Medius and minimus arise on the external surface of the ilium and are both inserted into the greater trochanter. Their anterior fibers are medial rotators and flexors while the posterior fibers are lateral rotators and extensors. The
piriformis has its origin on the ventral side of the sacrum and is inserted on the greater trochanter. It abducts and laterally rotates the hip in the upright posture and assists in extension of the thigh. It presses the
head of the femur into the
acetabulum and flexes, medially rotates, and abducts the hip. The anterior thigh muscles form the
quadriceps, which is inserted on the
patella with a common tendon. Three of the four muscles have their origins on the femur, while
rectus femoris arises from the anterior inferior iliac spine and is thus the only of the four acting on two joints. The posterior thigh muscles have their origins on the
inferior ischial ramus, with the exception of the short head of the
biceps femoris. The
semitendinosus and
semimembranosus are inserted on the tibia on the medial side of the knee, while biceps femoris is inserted on the
fibula, on the knee's lateral side.
In pregnancy and childbirth In later stages of
pregnancy the
fetus's head aligns inside the pelvis. Also joints of bones soften due to the effect of pregnancy hormones. These factors may cause
pelvic joint pain (
symphysis pubis dysfunction, SPD). As the end of pregnancy approaches, the ligaments of the sacroiliac joint loosen, letting the pelvis outlet widen somewhat; this is easily noticeable in the
cow. During
childbirth (unless by
Cesarean section) the fetus passes through the maternal
pelvic opening. ==Clinical significance==