The pelvis (: pelves or pelvises) is the lower part of an anatomical trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis or pelvic skeleton).
The pelvic region of the trunk includes the bony pelvis, the pelvic cavity (the space enclosed by the bony pelvis), the pelvic floor, below the pelvic cavity, and the perineum, below the pelvic floor. The pelvic skeleton is formed in the area of the back, by the sacrum and the coccyx and anteriorly and to the left and right sides, by a pair of hip bones.
The two hip bones connect the spine with the lower limbs. They are attached to the sacrum posteriorly, connected to each other anteriorly, and joined with the two femurs at the hip joints. The gap enclosed by the bony pelvis, called the pelvic cavity, is the section of the body underneath the abdomen and mainly consists of the reproductive organs and the rectum, while the pelvic floor at the base of the cavity assists in supporting the organs of the abdomen.
In mammals, the bony pelvis has a gap in the middle, significantly larger in females than in males. Their offspring pass through this gap when they are born.
Structure The pelvic region of the trunk is the lower part of the trunk, between the abdomen and the thighs. It includes several structures: the bony pelvis, the pelvic cavity, the pelvic floor, and the perineum. The bony pelvis (pelvic skeleton) is the part of the skeleton embedded in the pelvic region of the trunk. It is subdivided into the pelvic girdle and the pelvic spine. The pelvic girdle is composed of the appendicular hip bones (ilium, ischium, and pubis) oriented in a ring, and connects the pelvic region of the spine to the lower limbs. The pelvic spine consists of the sacrum and coccyx. the pelvic cavity, typically defined as a small part of the space enclosed by the bony pelvis, delimited by the pelvic brim above and the pelvic floor below; alternatively, the pelvic cavity is sometimes also defined as the whole space enclosed by the pelvic skeleton, subdivided into: the greater (or false) pelvis, above the pelvic brim the lesser (or true) pelvis, below the pelvic brim the pelvic floor (or pelvic diaphragm), below the pelvic cavity the perineum, below the pelvic floor
Pelvic bone
The pelvic skeleton is formed posteriorly (in the area of the back) by the sacrum and the coccyx, and laterally and anteriorly (forward and to the sides) by a pair of hip bones. Each hip bone consists of three sections: ilium, ischium, and pubis. During childhood, these sections are separate bones, joined by the triradiate cartilage. During puberty, they fuse together to form a single bone.
Pelvic cavity
The pelvic cavity is a body cavity that is bounded by the bones of the pelvis and which primarily contains reproductive organs and the rectum.
A distinction is made between the lesser, or true, pelvis, inferior to the terminal line, and the greater, or false, pelvis above it. The pelvic inlet, or superior pelvic aperture, which leads into the lesser pelvis, is bordered by the promontory, the arcuate line of ilium, the iliopubic eminence, the pecten of the pubis, and the upper part of the pubic symphysis. The pelvic outlet, or inferior pelvic aperture, is the region between the subpubic angle, or pubic arch, the ischial tuberosities, and the coccyx.
Ligaments: obturator membrane, inguinal ligament (lacunar ligament, iliopectineal arch) Alternatively, the pelvis is divided into three planes: the inlet, midplane, and outlet.
Pelvic floor
The pelvic floor has two inherently conflicting functions: One is to close the pelvic and abdominal cavities and bear the load of the visceral organs; the other is to control the openings of the rectum and urogenital organs that pierce the pelvic floor and make it weaker. To achieve both these tasks, the pelvic floor is composed of several overlapping sheets of muscles and connective tissues.Thieme Atlas of Anatomy (2006), p. 137
The pelvic diaphragm is composed of the levator ani and the coccygeus muscle. These arise between the symphysis and the ischial spine and converge on the coccyx and the anococcygeal ligament, which spans between the tip of the coccyx and the anal hiatus. This leaves a slit for the anal and urogenital openings. Because of the width of the genital aperture, which is wider in females, a second closing mechanism is required. The urogenital diaphragm consists mainly of the deep transverse perineal, which arises from the inferior ischial and pubic rami and extends to the urogenital hiatus. The urogenital diaphragm is reinforced posteriorly by the superficial transverse perineal.Platzer (2004), p. 106
The external anal and urethral sphincters close the anus and the urethra. The former is surrounded by the bulbospongiosus, which narrows the vaginal introitus in females and surrounds the corpus spongiosum in males. The ischiocavernosus muscle squeezes blood into the corpora cavernosa penis and clitoridis.Thieme Atlas of Anatomy (2006), p. 136
Variation
Modern humans are to a large extent characterized by bipedal locomotion and large brains. Because the pelvis is vital to both locomotion and childbirth, the process of natural selection is constrained by two conflicting demands: a wide birth canal and locomotion efficiency, a conflict referred to as the "obstetrical dilemma". The female pelvis, or gynecoid pelvis, has evolved to its maximum width for childbirth—a wider pelvis would make human females unable to walk. In contrast, human male pelvises are not constrained by the need to give birth and therefore are more optimized for bipedal locomotion.
The principal differences between male and female true and false pelves include: The female pelvis is larger and broader than the male pelvis, which is taller, narrower, and more compact.Thieme Atlas of Anatomy, (2006), p. 113 The female pelvis is lighter and thinner than the male pelvis. The female inlet is larger and oval in shape, while the male sacral promontory projects further (i.e. the male inlet is more heart-shaped). The sides of the male pelvis converge from the inlet to the outlet, whereas the sides of the female pelvis are wider apart. The angle between the inferior pubic rami is acute (70 degrees) in males but obtuse (90–100 degrees) in females. Accordingly, the angle is called the subpubic angle in males and the pubic arch in females. Additionally, the bones forming the arch are more concave in females but straight in males. The distance between the ischia bones is small in males, making the outlet narrow, but large in females, who have a relatively large outlet. The ischial spines and tuberosities are heavier and project farther into the pelvic cavity in males. The greater sciatic notch is wider in females. The iliac crests are higher and more pronounced in males, making the male false pelvis deeper and narrower than in females. The male sacrum is longer, narrower, and straighter, and has a pronounced sacral promontory. The female sacrum is shorter, wider, and more curved posteriorly, and has a less pronounced promontory. The acetabula are farther apart in females than in males. In males, the acetabulum faces more laterally, while it faces more anteriorly in females. Consequently, when males walk the leg can move forwards and backwards in a single plane. In females, the leg must swing forward and inward, from where the pivoting head of the femur moves the leg back in another plane. This change in the angle of the femoral head gives the female gait its characteristic (i.e. swinging of hips).
Development Each side of the pelvis is formed as cartilage, which ossifies as three main bones which stay separate through childhood: ilium, ischium, pubis. At birth the whole of the hip joint (the acetabulum area and the top of the femur) is still made of cartilage (but there may be a small piece of bone in the greater trochanter of the femur); this makes it difficult to detect congenital hip dislocation by X-raying.
There is preliminary evidence that the pelvis continues to widen over the course of a lifetime.
Functions 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.Thieme Atlas of Anatomy (2006), p. 112
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.Holm (1980), pp. 425–6
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.Levin (2003), A Different Approach to the Mechanics of the Human Pelvis: Tensegrity (See conclusions.)
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. Palastanga (2006), pp. 331–2