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Bony pelvis: want to learn more about it?

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Bony pelvis

Bony pelvis (Pelvis ossea)

The bony pelvis is a complex basin-shaped structure that comprises the skeletal framework of the pelvic region and houses the pelvic organs.

It is usually divided into two separate anatomic regions: the pelvic girdle and pelvic spine. The pelvic girdle, also known as the hip bone, is composed of three fused bones: the ilium, ischium and the pubic bone. The pelvic spine is the posterior portion of the pelvis below the lumbar spine, composed of the sacrum and coccyx. The two pelvic bones are connected anteriorly by the pubic symphysis, while posteriorly they articulate with the pelvic spine to form the sacroiliac joints.

The pelvis plays several important functions in the human body. First of all, the pelvis carries the entire weight of the upper body, stabilizes it and transmits it to the lower limbs, allowing various actions to occur (e.g. sitting, standing, bipedal gait). Also, it houses and protects the abdominopelvic viscera and provides the attachment point for muscles and reproductive organs. Lastly, the bony pelvis provides a comfortable environment for the fetus during pregnancy. The integrity, biomechanical properties and anatomical features of the female pelvis are important for carrying out the labor.

Key facts about the pelvis
Bones Hip bone (ilium, ischium, pubis), sacrum
Joints Sacroiliac, pubic symphysis, lumbosacral, sacrococcygeal, hip joint
Types of pelvis Gynaecoid android, anthropoid, platypelloid
Function Weight-bearing and ambulation, landmarks, labor and delivery

This article will focus on the anatomy and function of the bony pelvis.

Hip bone

The hip bone is an irregularly shaped bone, also known as the pelvic girdle. It consists of three bones; ilium, ischium and pubis. These three bones are also known as the innominate bones, pelvic bones or coxal bones. They develop separately from each other and, in children, are connected only by cartilage. However, they completely fuse during puberty to comprise the complex and compact hip bone.

The hip bone has two surfaces (lateral and medial) and is bounded by four margins (anterior, posterior, superior and inferior).

The lateral surface houses the most prominent landmark of the bone, the acetabulum. It is a socket shaped articular surface via which the hip bone articulates with femur and makes the hip joint. The acetabulum has a “C” shaped acetabular margin that is accentuated by the cartilaginous acetabular labrum and completed inferiorly by the transverse acetabular ligament. The acetabulum is the principal unison point of the three bones, that have a characteristic arrangement within it;

  • The ilium extends superiorly to the hip joint, therefore, it comprises the superior portion of the acetabulum.
  • The ischium is the massive posteroinferior part of the hip bone thus it fits into the posteroinferior third of the acetabulum.
  • The pubic bone builds the anteroinferior third of acetabulum and is the anteroinferior part of the bone that articulates with the contralateral hip bone.

Another important landmark of the hip bone is the obturator foramen. It is a large opening located anteroinferior to the acetabulum, bounded by the ischium and pubis. The obturator foramen serves as the communication between pelvic and thigh regions through which neurovascular structures pass.

Ilium

The ilium is a blade-shaped bone found superior to the hip joint. It consists of the two main parts: the body and ala (wing). The body of ilium is a smaller, inferior, part that contributes to the formation of the acetabulum. The superior part, called ala, is a large, flat portion of the bone that has four borders and three surfaces.

The ilium has four major protruding areas that are commonly used as reference landmarks when locating other structures. The most prominent of these are the four iliac spines, that include:

  • The anterior superior iliac spine (ASIS) is located at the anterior end of the iliac crest. It serves as a point of attachment for the inguinal ligament and it can be easily palpated.
  • The anterior inferior iliac spine (AIIS) is located anterior to the supra-acetabular groove and acetabular margin. The AIIS provides points of attachment for the rectus femoris and proximal part of the iliofemoral ligament. It is separated from the ASIS by a short, vertical slope.
  • The posterior superior iliac spine (PSIS) is located at the posterior end of the iliac crest. This spine cannot be palpated but is commonly represented by a dimple at the medial gluteal region. The PSIS is laterally related to the iliac tuberosity and sacropelvic surface.
  • The posterior inferior iliac spine (PIIS) is located inferior to the PSIS.

Borders

The ilium features four distinctive borders; superior (iliac crest), anterior, posterior and medial.

The superior border of ilium is called the iliac crest. This is a rough, crescentic surface that starts at the posterior superior iliac spine posteriorly and arches forward to end at the anterior superior iliac spine anteriorly. The iliac crest has inner and outer lips as well as an intermediate zone between the lips. There is also an iliac tubercle located 5 cm above and behind the anterior superior iliac spine.

The anterior border of ilium extends from anterior superior iliac spine to the acetabulum. It features anterior inferior iliac spine just superior to its acetabular end. The part of the border between the spines is concave anteriorly.

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The posterior border of ilium begins at the posterior superior iliac spine and extends to the posterior border of ischium. It features a posterior inferior iliac spine. The course of this border is irregular; the part between the spines is concave posteriorly, while the part from the inferior spine to the ischial border first runs horizontally then posteroinferiorly to comprise the superior border of greater sciatic notch. This notch is completed inferiorly by posterior ischial border and ischial spine. The sacrospinous and sacrotuberous ligaments respectively enclose the notch superiorly and posteroinferiorly, converting it into the greater sciatic foramen. This foramen acts as a conduit for seven nerves, three pairs of blood vessels and one muscle, namely:

Contents of the greater sciatic foramen
Nerves Sciatic nerve;
Sacral plexus branches: superior gluteal, inferior gluteal, pudendal, posterior femoral cutaneous, nerve to quadratus femoris, nerve to obturator internus
Vessels Internal iliac artery branches: superior gluteal, inferior gluteal and internal pudendal arteries
Muscle Piriformis muscle

Surfaces

The four borders of the ilium bound its three bony surfaces; gluteal, sacropelvic and iliac (internal). The gluteal surface represents the posterolateral face of the bone. It is bounded superiorly  by the outer lip of the iliac crest and inferiorly by the inferior gluteal line. It features three gluteal lines that provide attachment points for the gluteal and thigh muscles:

  • Inferior gluteal line that runs just above the acetabular margin.
  • Posterior gluteal line just above and anterior to the greater sciatic notch and both posterior iliac spines.
  • Anterior gluteal line that travels obliquely along the gluteal surface from the tubercle of the iliac crest toward the posterior gluteal line.

The iliac surface is the anteromedial facet of the bone. It is marked by the iliac fossa. The fossa is bounded superiorly by the inner lip of the iliac crest, inferiorly by the arcuate line and posteriorly by the anterior borders of the iliac tuberosity and auricular surface of ilium. On the right side, the iliac surface is in relation to the terminal ileum, cecum and vermiform appendix. However, on the left side, it supports the distal descending colon and the proximal portion of the sigmoid colon.

The sacropelvic surface begins at the posterior border of iliac fossa and continues to the posterior extent of the ilium. It houses an anterior auricular surface and a posterior iliac tuberosity. The auricular surface is an ear-shaped surface via which the ilium articulates with sacrum to form the sacroiliac joint. The Iliac tuberosity provides a point of attachment for ligaments and muscles of the back and the lower limb.  There is also a pelvic surface found anteroinferior to the auricular surface, contributing to the lateral wall of the lesser pelvis.

Muscle attachments

The robust ilium provides numerous points of attachment for muscles of the trunk and lower limbs. The iliac crest is a point of insertion for the latissimus dorsi, external oblique and internal oblique, tensor fasciae latae and quadratus lumborum muscles. The iliac surface functions as a point of origin for iliacus, which arises from the upper two-thirds of the iliac fossa. Sartorius arises from the anterior superior iliac spine while rectus femoris arises from both anterior inferior iliac spine and supraacetabular margin. The gluteal surface of ilium provides origin points for gluteus maximus, medius and minimus muscles.

Ischium

The ischium is found posteroinferiorly to the hip joint. It is an ‘L’ shaped bone continuous superiorly with the ilium and anteriorly with the pubic bone. Ischium consists of two parts - the body and ramus. The body of ischium is rough and broad with three bony surfaces, which include the medial (pelvic), lateral (femoral) and posterior surfaces.

The ramus (Lat. branch) of the ischium is a roughly cylindrical, branch-like part of the ischium. It extends anteromedially from the inferior aspect of the body to meet with the inferior pubic ramus. Together, these bony rami form the inferior border of obturator foramen.

Surfaces

The medial surface is relatively smooth and unremarkable. However, it forms the lateral wall of the ischioanal (ischiorectal) fossa along with the obturator internus muscle and its fascia.

The femoral surface is angled anteroinferiorly and laterally toward the proximal shaft of the femur. Anteriorly, it is limited by the posterior margin of the obturator foramen and laterally by the lateral border of the ischial tuberosity.

The posterior surface is relatively smooth on its upper aspect and is continuous superiorly with the gluteal surface of ilium. Its upper border holds a small, conical projection pointed posteromedially known as the ischial spine. It is the attachment site for the sacrospinous ligament. Immediately below the ischial spine is a small ‘C’ shaped concavity known as the lesser sciatic notch, which is the anterior border of lesser sciatic foramen. This foramen is posteriorly bounded by the sacrotuberous ligament and anteriorly by the sacrospinous ligament. It serves as a passageway for the pudendal nerve and nerve to obturator internus, internal pudendal vessels and tendon of obturator internus muscle.

Muscle attachments

The ischium provides numerous points of attachment for pelvic and lower limb muscles. The medial surface provides attachment for both transverse perinei, obturator internus and externus, piriformis, coccygeus and levator ani muscles.

The lateral and posterior surfaces give attachment to most of the proximal lower limb muscles such as adductor magnus, long head of biceps femoris, semitendinosus, semimembranosus and quadratus femoris muscles.

Pubic bone

The pubic bone or pubis is the last and smallest of the hip bones to be discussed. It has a small body located medially and two rami (superior and inferior) extending posterolaterally. The rami of the pubic bone resemble the oblique lines of the letter ‘K’ from an anterior view.

Parts and surfaces

The body of pubic bone has three surfaces that include: anterior (external), posterior (internal) and medial (symphyseal) surfaces. The surfaces are confluent, except at the pubic crest (located anterosuperior part of body of pubic bone) which marks the transition from external to internal surfaces.

The anterior surface is directed inferolaterally, showing a smooth area for the attachments of the adductors of the thigh. Facing posterosuperiorly, the posterior surface comprises the anterior wall of the lesser pelvis. The symphyseal surface faces medially to articulate with its contralateral counterpart, giving rise to pubic symphysis joint. The bones are connected with the symphyseal cartilage interposed between them. Pubic symphysis can be palpated in the midline of the mons pubis.

The pubic crest separates the anterior and posterior surfaces of the bone. It features the pubic tubercle on its lateral end, precisely 2.5 cm lateral to the pubic symphysis. The pubic tubercle serves as a point of attachment for the inguinal ligament and can be used clinically (in conjunction with the anterior superior iliac spine) to locate the rings of the inguinal canal. The curved area below the pubic symphysis is known as the pubic arch. It is one of the sexually dimorphic areas of the pelvis – meaning that its shape differs between the male and female sexes.

The superior pubic ramus starts at the pubic tubercle and extends posterolaterally and upwards to the acetabulum. It has three borders and three surfaces. The anterior surface or pectineal surface extends from the pubic tubercle to the iliopubic ramus. It is limited by the round obturator crest anteriorly and by pecten pubis (pectineal line) posteriorly. The pectineal line is continuous with the arcuate line of the ilium. Together these two lines form the linea terminalis or pelvic brim, a line that separates the greater from the lesser pelvis.

The obturator surface of superior ramus faces posteroinferiorly and is bounded anteriorly by the obturator crest and inferiorly by its own sharp inferior border. Finally, the pelvic surface faces posterosuperiorly and is relatively smooth compared to the anterior surface. It is limited by the pecten pubis above and the inferior border below.

The inferior pubic ramus projects posteroinferiorly and laterally from the superolateral angle of the pubic body. It has two surfaces (anterolateral and posteromedial), separated by the two margins (anterior and medial). The inferior ramus continues inferolaterally to unite with the ischial ramus to complete the obturator foramen.

The anterolateral surface of the inferior pubic ramus is directed towards the thigh and is continuous superiorly with the body of the pubis. On the other hand, the posteromedial surface faces the lesser pelvis, providing the attachment for the crus of the penis (males) or clitoris (females). The medial border outlines obturator foramen, providing a point of attachment for the fascia lata and the membranous layer of the superficial perineal fascia.

Muscle attachments

Similar to the other innominate bones, the pubic bone also provides points of attachment for several anterior abdominal, pelvic and lower limb muscles. The superior border of pubic bone provides attachment points for rectus abdominis, pectineus and conjoint tendon (union of aponeuroses of internal oblique and transversus abdominis). The external surface of pubic bone supports gracilis, adductor brevis, obturator externus and adductor longus muscles.

Sacrum

The sacrum is a triangular bone that consists of five fused sacral vertebrae. It has anterior (pelvic) and posterior (gluteal) surfaces, a base, two alae and an apex. The pelvic surface is smooth and concave, while the gluteal surface is rough, irregular and convex. The gluteal surface is marked by numerous rudimentary spinous processes that form a median sacral crest.  Its base is formed by the first sacral vertebra, is directed superiorly and articulates with the last lumbar vertebra.

The base of the sacrum features a slight projection at the midline known as the sacral promontory, which helps in defining the pelvic diameters. The fused transverse processes of the first two sacral vertebrae form the wings or alae of the sacrum. The lateral surface of each ala is flattened to facilitate articulation with the ipsilateral ilium, thus forming the sacroiliac joint. The sacrum tapers down toward the apex where it articulates with the base of the coccyx.

Joints

The bones of the pelvis articulate with each other via four joints. Posterior to anterior, these are the lumbosacral, sacroiliac, sacrococcygeal, hip, and pubic symphysis joints.

  • The lumbosacral joint is a symphysis (secondary cartilaginous) joint between the fifth lumbar vertebra and the base of the sacrum. It allows flexion, extension, lateral flexion and minimal rotation of the torso with respect to the pelvis and lower limbs.
  • The sacroiliac joint is a synovial joint between the ala of sacrum and the auricular surface of ilium. This joint allows very little mobility through slight gliding and rotation movements. In women, the ligaments of the joint soften during pregnancy, enabling the increase of pelvic diameter during childbirth.
  • The sacrococcygeal joint is an amphiarthrodial joint between the fifth sacral and first coccygeal segments. It allows flexion and extension of the coccyx. However, these movements are rather passive, occurring during childbirth and defecation.
  • The pubic symphysis is a secondary cartilaginous joint between the medial surfaces of the pubic bones. The surfaces are lined with a layer of hyaline cartilage and connected by the fibrous symphyseal cartilage interposed between them. Usually, there are no movements on this joint, except in pregnancy when the ligaments and cartilage soften, allowing the increase of pelvic diameters during labor.

Greater and lesser pelvis

Observed as a whole, the bony pelvis bounds a basin-shaped pelvic cavity. This bony framework opens superiorly toward the abdomen via a wide superior pelvic aperture (pelvic inlet, or pelvic brim). This aperture has an entirely bony outline; it is bordered by the linea terminalis (a continuous ring made up by the pectineal and arcuate lines) and the sacral promontory (posteriorly).

The inferior opening of the pelvis is called the inferior pelvic aperture (pelvic outlet). Its borders include bony and ligamentous elements;  posterolaterally it is bordered by the sacrotuberous ligaments and anterolaterally by the ischiopubic rami. The pelvic outlet is sealed by the muscles of the pelvic floor and the perineum.

The pelvic inlet and outlet divide the pelvis into the following compartments:

  • The greater (false) pelvis is located superiorly to the pelvic inlet and contains the distal part of the intestines. It is bordered by the ala of the ilium laterally and the 4th - 5th lumbar vertebrae and the base of the sacrum posteriorly. The anterior border of the greater pelvis is completed by the inferior part of the anterior abdominal wall. Superiorly, the greater pelvis communicates with the peritoneal cavity. Inferiorly, the greater cavity communicates with the lesser cavity through the pelvic inlet.
  • The lesser (true) pelvis is found between the pelvic inlet and pelvic outlet and contains the internal genitalia, perineum and distal organs of the urinary tract. It is bounded on either side by the ilioischial complex and posteriorly by the sacrococcygeal complex. The pubic bones, pubic symphysis and their rami form the anteroinferior border.

Learn everything about the pelvis with our articles, video tutorials, quizzes and labeled diagrams.

Differences between the male and female pelvis

The shape of the pelvic girdle varies between sexes and among individuals and races. The morphological differences are based on the different pelvic diameters and mainly have obstetric implications (i.e. childbirth). Less commonly, these differences can be used in forensic anthropology to help identify fossils or as part of criminal investigations. These variations have been studied and categorized into four main types outlined below.

The fundamental differences in the pelvis of the two sexes are based on the shape of the pelvic inlet, the angle of the ischiopubic rami and the projection of ischial spines. The female pelvis tends to have a wider (circular) inlet, wider ischiopubic rami and shorter, blunted ischial spines. All these features make the delivery process more favorable when compared to the male pelvis. The latter is characterized by a heart-shaped inlet with narrow ischiopubic rami and conical, medially projecting ischial spines that are more likely to cause obstruction of labor.

Summary of the different types of pelvises
Gynaecoid Common among females in Western civilization
Inlet slightly oval along the transverse axis
Ischial spines blunted and do not protrude into the cavity
Sacrum is broad and has a deep concavity

Ideal for delivery


Android Most common male type pelvis
Inlet heart-shaped (large sacral promontory)
Ischial spines conical and protrude into the cavity
Sacrum is slightly curved
Ideal for the attachment of bulky muscles

Anthropoid Features of both the android and gynaecoid pelvis.
Inlet is oval in the anteroposterior axis
Ischial spines are blunt
Sacrum is long, narrow and less curved than in the gynaecoid pelvis
Can facilitate delivery but high risk of obstructed labor

Platypelloid Called a contracted pelvis.
Inlet and ischial spines similar to those seen in the gynaecoid pelvis
Sacrum is slightly curved.

Explore our study units to learn more about the contents of the male and female pelvises respectively.

Functions

The main functions of the pelvic girdle are to support the weight of the upper body and aid in walking, protect the lower abdominal and pelvic viscera and facilitate natural childbirth. Additionally, there are certain points on the pelvis that can be felt (palpated) externally that clinicians use as landmarks for different clinical procedures.

Weight-bearing and ambulation

The pelvic girdle is a thick, robust structure that is designed to support the weight of the upper body. The weight is transferred from the axial skeleton to the lower appendicular skeleton via the pelvis while standing and walking. Additionally, the thick bones provide points of attachment for some of the largest muscles within the body that are needed for adequate posture and locomotion.

Support

The bony pelvis also provides anchoring points for the smaller muscles and ligaments of the pelvic floor and the perineum.

Along with these structures, the bony pelvis holds in place and protects the organs located in the pelvic cavity including the urinary bladder, pelvic colon, reproductive organs, and rectum. Also, the structures of the pelvic floor are designed to maintain the continence of the anus and urinary tract.

Labor and delivery

After about nine months of growth and development in the uterus, the fetus is ready to be born. This is an exciting, yet risky, series of events that require very specific and coordinated actions. The details of the mechanisms of labor require that the fetal parts are smaller than the pelvic diameters and that the presenting fetal parts engage (enter) the pelvis appropriately. If the fetal head is too big, the pelvic diameters are too small, or the fetal part is not oriented appropriately (long axis of the head should be in the transverse plane) then it may be impossible for vaginal delivery to take place.

Therefore, it is important for managing obstetricians to determine if the expectant mother’s pelvis is adequate to deliver the fetus. Additionally, the size of the fetus should also be monitored and compared with the pelvic diameters to establish the risk of cephalopelvic disproportion at the time of delivery.

Bony pelvis: want to learn more about it?

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