Pelvic musculoskeletal anatomy
Accompanies the illustrations:
- Pelvic musculoskeletal anatomy
- Pelvic organ anatomy
What are the bones, muscles, and ligaments of the pelvis?
Pelvic girdle and pelvic connective tissue
The pelvic girdle is bowl-shaped arrangement of bones that connects the spine to the legs and contains the pelvic muscles and organs. The pelvic bones are made up of the sacrum and coccyx (bottom of the spine) and the 3 hip bones – pubis, ilium, and ischium. The pubis is the bone felt behind the mons pubis (see Vulval anatomy). The ilium is the bone felt when a hand is placed on the hip. The ischium is the bone that presses against a surface when sitting down. Strong connective tissue, called fascia, runs between bones and provides attachment points for muscles and support for pelvic organs.
Pelvic bones, from front to back
Pubic tubercle – part of the pubis felt at lower abdomen, where the abdominal wall muscles connect in front
Pubic symphysis (not pictured) – where the two pubic bones meet in the middle and are held together with a band of cartilage
Superior ramus of pubis – part of the pubic bone that goes from the pubic tubercle towards the top of the hip joint where it connects with the ilium; the pelvic floor muscles connect to the internal part of this bone
Inferior ramus of pubis – part of the pubic bone that goes from the pubic symphysis downwards to the ischium and the ‘sit bone’ (ischial tuberosity)
Ischial spine – a pointed area on the inside of the bony pelvis that is the connection point for 2 important ligaments – tendinous arch of the pelvis that goes forward to the pubis, and the sacrospinous ligament that goes back to the sacrum. The pelvic muscles attach onto these ligaments.
Ischial tuberosity (not pictured) – the lowest part of the bony pelvis, also called the ‘sit bone’, can be felt through the buttock
Sacrum – a triangular bone at the bottom of the spine that connects to the pelvic bones, the piriformis (hip rotator) muscle connects to it
Coccyx / tailbone – a small triangular bone at the bottom tip of the sacrum, where the pelvic floor muscles connect in back
Tendinous arch of the pelvis – also called the ‘white line’, this band of connective tissue is the attachment point for many pelvic muscles
Sacrospinous ligament (not pictured) – triangular band of connective tissue that runs between the ischial spine and the sacrum, the coccygeous muscle lies in front of it and the pudendal nerve travels behind it
Sacrotuberous ligament (not pictured) – triangular band of connective tissue that runs between the ischial tuberosity and the sacrum, the fibres intermingle with the sacrospinous ligament
Superior and inferior fascia of the urogenital diaphragm – connective tissue that runs over and joins together the muscles that lie underneath the vulva
Perineal body (not pictured) – area of connective tissue that sits between the vagina and anus, it forms the centre of the figure-of-8 shape made by the perineal muscles that all connect onto it
Pelvic floor muscles
Four muscles make up the ‘pelvic floor’, also called the levator ani. They meld into one another and work together to support the pelvic organs.
Pubococcygeous – hammock-like muscle that extends from the pubic bone to the coccyx and goes around the lower vagina and rectum
Puborectalis (not pictured) – U-shaped muscle that connects to the pubic tubercle on both sides and forms a sling around the rectum
Iliococcygeous – triangular muscle that connects to the back of the pubic bone, the tendinous arch of the pelvis, and the coccyx
Coccygeous – triangular muscle that forms the back part of the pelvic floor and travels from the sacrospinous ligament to the coccyx
Hip rotator muscles
There are many muscles that rotate the hips. The 2 found inside the bony pelvis are the piriformis and obturator internus. They help with outward rotation of the hip. These are higher up in the pelvis than the levator ani.
Obturator internus – muscle that sits above the iliococcygeous and connects the top of the femur (thigh bone) to the inner pubic bone, ischium, and tendinous arch of the pelvis; the obturator and pudendal nerves travel along it
Piriformis – muscle that sits above the coccygeous and travels from the sacrum to the top of the femur (thigh bone)
Perineal muscles – muscles underneath the vulva
The perineal muscles form a figure-of-8 shape around the vagina and the anus. Another line of muscle goes horizontally though the middle of the 8 to connect with the ischial bone on either side.
Bulbocavernosus (not pictured) – forms the top part of the 8, lies underneath the labia, supports the vestibular bulbs, and joins onto the perineal body
External urethral sphincter (not pictured) – circle of muscle around the urethra that allows control over passage of urine, it interconnects with the fascia of the urogenital diaphragm and nearby muscles
Superficial and deep transverse perineal muscles – muscles that travel side to side through the middle of 8, and connect onto the perineal body
External anal sphincter – forms the bottom part of the 8, a circle of muscle around the anus that allows control over bowel motions, it connects onto the perineal body
The pelvis – everything is interconnected
The pelvis has a complex job to do – it lets us stand and walk, have sex and give birth, empty out our waste, and meanwhile keep our pelvic organs on the inside. The organs sit extremely close together, so a problem in one often affects the others. The blood vessels, nerves, muscles, and connective tissues all interlace with one another and the major organs.
Pain sensation from the pelvis travels through nerves that meet in the sacral plexus on their way to the spinal cord. At the spinal cord, these nerves interact with each other as they transmit messages up to the brain. Pain in one area may cause the nearby muscles to contract, then those muscles also become sources of pain. For this reason, pain that starts in one place can end up being felt in other places. For example, chronic back pain can spread to the pelvis, since the involved nerves run through the pelvic floor muscles. Problems with the hip joint can cause pain with sex. Surgeries for pelvic organ prolapse can trigger widespread vaginal pain. Bowel and bladder dysfunction often go together, and a flare of one makes the other worse.
It is important to look at the whole body when thinking about pelvic pain, sexual pain, and bowel and bladder dysfunction. Sometimes what seems like a simple solution, like removing an organ surgically, turns out to NOT fix the problem. When considering treatment of pelvic symptoms, try non-surgical options like lifestyle changes, medications, and physiotherapy first. When surgery is recommended, talk to your doctor about possible medium and long-term complications, like pain, bowel and bladder issues, sexual function, and how likely it is the symptoms will persist or come back after surgery. An important goal of treatment is to maintain and improve all the important and interrelated pelvic functions.