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Anatomy of the Pelvis

Niall is a Master of Osteopathy working from private clinics in Yorkshire. He provides strength & conditioning and anatomy learning online.

General Anatomy of the Pelvis

The pelvis is made up of four bones, the two large innominate bones, the sacrum which is wedge-shaped and fits between the two innominates posteriorly. As well as the coccyx, a small "tail" like bone attaching to the underside of the sacrum. The joining of the sacrum and innominate bones forms the two sacroiliac joints of the pelvis, hence the joint's name.

The sacrum is a solid bone, which was once thought to be a continuation of the spinal joints. It's thought that it was once separated into multiple vertebral joints with an intervertebral disc in between each. Evolution seems to have found more use for a stable bone used to support the rest of the pelvis and act as an anchorage for ligaments and muscles. The bones have therefore fused over time.

The innominate bones are interesting, as before the age of about 15, each innominate is actually made up of three bones which begin to fuse together during the early teenage years. The ilium, the posterior-most bone which articulates with the sacrum. The ischium which forms part of the acetabular fossa where the acetabulum head sits to form the hip joint. The third bone is the pubis, which forms the rest of the acetabular fossa and joins the medial anterior cartilage that makes the pubic symphysis. The pubic symphysis has an important role in pregnancy which will be discussed later.

The ligaments of the pelvis, are amongst the strongest in the human body. They form what can be described as a "basket weave" formation, in order to create strength and tensegrity within the structure. This is part of the "forced closure" method that the pelvis adopts in order to keep itself secure. This will be explored further on.

The pelvis is one of the largest muscular attachment points in the human body, partly due to its central position and a crucial link between the upper and lower body. The muscles also contribute to "forced closure", anchoring themselves in a way that, when contracted, push the bones of the pelvis together. The largest muscle in the body, gluteus maximus attaches directly into the pelvis.

The bones of the pelvis, with addition of the coccyx sat directly below the sacrum

The bones of the pelvis, with addition of the coccyx sat directly below the sacrum

Formed and Forced Closure

When observing the bones of the pelvis, you'll notice that they fit together in a very congruent way. They fit almost like a jigsaw, giving the structure support and a good foundation for keeping itself locked in place. However, it is also maneuverable during times when the body needs it to have a little freedom of movement, such as pregnancy. This jigsaw effect of the bones interlocking is what's known as formed closure and is the first phenomenon of the pelvis.

The second and equally important phenomenon is that of forced closure. This concerns itself with the structures that act on the pelvic bones to "force" them closed. After all, it doesn't take much to break a jigsaw back into its original pieces. These structures are the muscles and ligaments. They act as the glue and sticky tape that hold the jigsaw together and prevent it from moving if accidentally nudged or pushed. As described earlier on in the article, the ligaments of the pelvis form a basket weave pattern which further forces the pelvis into its sturdy, closed position.

A Visual Representation of Formed and Forced Closure

A fantastic graphic highlighting how the bones' congruent shape + the forces acting on them through the ligaments and muscles equal a stable pelvis.

A fantastic graphic highlighting how the bones' congruent shape + the forces acting on them through the ligaments and muscles equal a stable pelvis.

The Ligaments

The pelvis is made up of multiple ligaments. I have grouped them together in a table below to highlight their role. Labelled images are also below which help to visulaise the positioning and structural support they offer. Groupings I've developed are: Structural, ovarian (attach to the ovaries), utero (attach to the uterus), fallopian (attach to the fallopian tubes).

Obviously, some ligaments are found in the female pelvis but not the male. Some, namely the suspensory ligament, amongst others, are not true ligaments. They're blends of fascias and other ligaments that form an independent structure.

The broad ligament is a sheet of ligament that spans across the pelvis, much like the diaphragm of the chest. Its function is to stabilise and protect certain organs as specified in the table

Structural ligamentsOvarianUteroFallopian

Iliolumbar

Broad ligament (Mesovarium)

Broad ligament (Mesometrium)

Broad ligament (Mesosalpinx)

Sacrospinous

Ovarian

Round

 

Sacrotuberous

Suspensory

Cardinal

 

Posterior sacroiliac

 

Pubocervical

 

Anterior sacroiliac

 

Uterosacral

 

The broad ligament

The broad ligament

Structural ligaments

Structural ligaments

Bony Prominences

It’s worth noting the most prominent of the pelvis’ bony prominences before discussing the muscles. These prominences include;

  • Ischial tuberosity (sitting bone)
  • Posterior superior iliac spine (PSIS)
  • Posterior inferior iliac spine (PIIS)
  • Anterior inferior iliac spine (AIIS)
  • Anterior superior iliac spine (ASIS)
  • Iliac crest
  • Pubic ramus

The Muscles

The pelvis offers one of the largest muscular attachment bases of the human body. The strongest and largest muscles attach their tendons directly into the bony landmarks of the pelvis in order to lever off it. Below is a table with the muscle along with their origin and insertions. These contribute to forced closure, core stability, the pelvic floor and human movement and biomechanics.

MuscleOriginInsertion

Gluteus Maximus

Ilium, lumbar fascia, sacrum, sacrotuberous ligament

Gluteal tuberosity of femur

Gluteus Minimus

Outer ilium between anterior and inferior gluteal lines

Greater trochanter

Gluteus Medius

Outer ilium between posterior and middle gluteal lines

Greater trochanter

Piriformis

Anterior Sacrum

Medial greater trochanter

Biceps Femoris

Long head: Ischial tuberosity. Short head: Linea Aspera and supracondylar ridge

Styloid process fibula, LCL and lateral tibial condyle

Semitendinosus

Ischial tuberosity

Shaft of tibia

Semimembranosus

Ischial tuberosity

Medial tibial condyle and fascia of popliteus

Gracilis

Ischiopubic ramus

Shaft of tibia

Sartorius

Just below anterior superior iliac spine

Shaft of tibia

Rectus Femoris

Anterior inferior iliac spine

Tibial tuberosity boa ligamentum patellae

Rectus Abdominis

Pubic symphysis and pubic crest

Costal cartilage of ribs 5,6,7 and xiphoid process

Internal abdominal oblique

Lumbar fascia, inguinal ligament, iliac crest

Costal margin, aponeurosis rectus sheath, pubic crest

External abdominal oblique

Lower 8 ribs

Iliac crest, inguinal ligament, pubic tubercle, aponeurosis of rectus sheath

Transversus abdominis

Costal margin, lumbar fascia, iliac crest, inguinal ligament

Aponeurosis rectus sheath, pubic crest

Quadratus Lumborum

Transverse processes of L1-4, iliolumbar ligament, iliac crest

12th rib

Iliocostalis

Iliac crest, sacrum, lumbar vertebrae

Transverse processes of cervical spine, ribs

Latissimus dorsi

Spinous processes of all thoracic, lumbar and sacral vertebrae, including and below T7, lumbar fascia, iliac crest, last 4 ribs and scapula

Bicipital groove of humerus

Tensor fascia lata

Just above anterior superior iliac spine

Iliotibial band

Muscles of the Pelvic Floor

The pelvic floor is a bowl-shaped collection of muscles, ligaments and fascia. It can be described as one of the bodies diaphragms. The pelvic floor muscles include; Coccygeus, iliococcygeus, puborectalis, pubococcygeus. The function of the pelvic floor is to help assist with child birth, prevent incontinence and support organs within the pelvis.

Muscles of the pelvic floor

Muscles of the pelvic floor

The Male and Female Pelvis

The pelvis of males and females differ for obvious reasons. The male pelvis tends to be slimmer, with a smaller inner area. The females on the other hand tend to be wider with a larger inner area. However, female pelvis’ differ amongst themselves significantly. This can have either a positive or negative effect during pregnancy depending on shape. The different shapes are shown below along with a male pelvis. Try to picture how the different shapes would affect a baby being delivered.

Male vs female pelvis

Male vs female pelvis

Four alternative female pelvic shapes. Android is considered the most masculine of the female pelvis and is least desirable for childbirth

Four alternative female pelvic shapes. Android is considered the most masculine of the female pelvis and is least desirable for childbirth

Pregnancy

Pregnancy is a phenomenal process which would only be possible with the bodies fascinating ability to change at different stages of its life. No place is better to observe this than the pelvis during pregnancy and labour.

During pregnancy, the female body secretes a hormone called “relaxin” into the body. This allows the ligaments of the body to express further extensibility, which is why pregnant women tend to become slightly hypermobile. There is, however, a reason for this. As discussed previously, the pelvis fits together in a congruent fashion to increase tensegrity. Relaxin, allows the forced closure of the muscles and ligaments to reduce, reducing the tight hold on the pelvic joints. This makes labour possible.

A fascinating photo was taken of a lady during labour which highlights just how much the bones separate to allow the baby to be delivered. Notice in the photo below just how high the sacrum has risen, and how far the innominates separate! The process of relaxin release can, however, be linked to injury and pain during pregnancy. It’s the main reason many women suffer from sacroiliac dysfunction, low back pain and pubic symphysis disorder.

Movement of the pelvis during

Movement of the pelvis during

Comments

JC Scull from Gainesville, Florida on July 09, 2020:

Excellent article. Very detailed and informative.

Deborah Demander Reno from First Wyoming, then THE WORLD on April 08, 2020:

Very informative article. Thanks for the detail on musculature origin and insertion points. As a massage therapist and yoga instructor, it serves as a great reminder.

Thank you for writing.

Namaste