Sacro-iliac joint (SIJ) is a common joint in humans. It connects the pelvis to the lower back. SIJ plays an important role in maintaining posture and stability of body during standing, walking, sitting and other activities.
The sacrum is the triangular bone at the top of your spine that sits between your ribcage and hips. The sacrum supports most of your weight when it’s not being supported by muscles or bones supporting it from above.
The ilium is a thin, fibrous band of tissue that runs along the front part of your hip bone. It attaches to the pubic bone and forms part of the pelvic floor.
Skeletal anatomy: sacrotuberous ligament (SL)
The sacro-iliac joint (SIJ), also known as sacrotuberous ligament or simply SIJ, is a common joint in humans that connects the pelvis to the lower back. It helps maintain balance and stability of the body while standing, walking, sitting and other activities.
The SIJ is composed of three parts: the sacrum (triangular bone at the top of your spine), the ilium (fibrous band that extends from hipbone to pubic bone) and labia majora (the two folds of skin that cover each side). These are collectively referred to as the sacro-iliac joint.
The pelvis is a basin-shaped structure made of bone surrounded by muscles and covered by layers of skin. The bones that make up the pelvis include four innominate bones (ilium, ischium, pubis, and the sacrum) that are fused together. The SIJs are located where the ilium joins with either the pubic symphysis (pubis) or the sacrum.
The ischium, the largest of the four bones that make up the pelvis, forms the lower rear part of the pelvis. The ischium is located on the back of the pelvis. It’s the widest and strongest part of the pelvic bone.
Formed by three bones called sacral vertebrae, the sacrum sits at the base of the spine. The sacrum is a triangular bone that is wider at the base than it is at the top, and its broadest part attaches to the ilium.
The iliac bone is located on each side of the pelvis. The paired iliac bones join at the top end (hip joint) and are separated in the middle of their upper surface by a depression (the pit of the stomach).
The iliac bone is a large, irregular bone that forms the principal part of the hip bone and the upper part of the pelvis. Each ilium has three parts: the wing (which forms most of the upper part of the hip bone), the spine (upper and back part), and the trough (or fossa, the lowest part).
In addition to these bones, small areas of cartilage (cristae) are located within some of these joints.
The articular disc is a small, triangular layer of hyaline cartilage that acts as a cushion for the joint.
The disc sits between the head of the femur and the acetabulum of the pelvis. The head of the femur and the acetabulum never touch. They are separated by a narrow gap filled with hyaline cartilage, which absorbs much of the force when these bones move in relation to one another.
The acetabulum is a deep, cup-shaped socket in the pelvis that forms the hip joint. The acetabulum is more than twice as deep as it is wide. At the bottom of the acetabulum is a shallow depression called the pelvis.
The head of the femur (caput femoris) is largest and most superior aspect of the thighbone. It’s slightly rounded, and broader from front to back than from side to side. Paired, the two femurs attach to common pelvic bone.
The long, slender thighbone, the femur, connects to the hipbone and extends downward to the knee. At its upper end, near the hip joint (acetabulum) of the pelvis, the head of the femur joins with the acetabular fossa. The head is a rounded projection that fits into an indentation (acetabulum) in the pelvis.
The sacrum is the large, triangular bone formed where the five sacral vertebrae fuse together. It’s wider at its upper edge than its lower edge. The base of the sacrum attaches to the back of the pelvis, and its other end (upper surface) articulates with the last lumbar vertebra.
There is also a small dimple (the inferior sacral aperture) that connects with the tailbone.
The last lumbar vertebra, also called the lumbosacral vertebra or the fifth lumbar vertebra, consists of a single bone. The lower end (base) of the vertebra is attached to the sacrum and the upper end (apex) to the coccyx. The lumbar region contains nerves and spinal cord.
The last of the individual bones in the spinal column is the coccyx, or tailbone. Attached to the pelvis at the base of the sacrum, the coccyx is a small, triangular bone made up of three to five small, irregular pieces that form part of the muscular wall and tissues of the rectum.
The sacral and lumbar vertebrae are strong yet flexible. Their large, flat surfaces (articular facets) enable them to form the major part of the arches of the vertebral column.
The lumbar region consists of five connected vertebrae (the lowest part of the spine) and the sacrum, an elongated bone that forms at the base of the pelvis and is fused with five lumbar vertebrae. The lumbar vertebrae are larger than those in the neck and upper back. The lumbar region and the chest are separated by the floating ribs, five pairs of ribs (12 ribs in all) that are attached to the spine and thoracic spinal column.
The last two pairs of ribs are known as the floating ribs because they aren’t connected or attached to either the spine or the sternum.
The uppermost part of the spine is made up of the seven cervical vertebrae. They sit in the neck, just below the base of the skull. The first vertebra forms part of the foramen magnum that forms part of the skull.
The other six vertebrae are linked together by strong muscles and connective tissue. They’re smaller than the lumbar and lower back vertebrae, but they have the same features.
The spinal cord is a tubular collection of nerve fibers that carry messages between the brain and the rest of the body. The spinal cord begins in the brain and runs from there down the center of the spine. Nerve fibers in the spinal cord transmit messages to and from the brain along this tube.
The spinal cord is protected by the vertebrae, bones that help form the flexible yet strong spinal column.
The spinal cord begins between the second and third lumbar vertebrae and extends downward until it ends at about the beginning of the second sacral vertebra. The spinal cord contains nerve fibers that transmit messages back and forth between the brain and the rest of the body. The spinal cord is protected by the bony vertebrae that surround it.
The bones of the spine are divided into four regions. These include the cervical (neck), thoracic (chest), lumbar (lower back), and sacral (pelvis) regions.
The spinal cord extends from the brain and continues down to about the waist, ending at the lower part of the second lumbar vertebra. The spinal cord is protected by a series of bony blocks (vertebrae) that give the spine its flexibility.
These bones are arranged into different regions, or sections. The first seven vertebrae are numbered C1 through C7, with C1 being the topmost bone, at the base of the skull.
Sources & references used in this article:
Load application to the sacrotuberous ligament; influences on sacroiliac joint mechanics by A Vleeming, JP Van Wingerden, CJ Snijders… – Clinical …, 1989 – Elsevier
The sacrotuberous ligament: a conceptual approach to its dynamic role in stabilizing the sacroiliac joint by A Vleeming, R Stoeckart, CJ Snijders – Clinical Biomechanics, 1989 – Elsevier
A functional-anatomical approach to the spine-pelvis mechanism: interaction between the biceps femoris muscle and the sacrotuberous ligament by JP Van Wingerden, A Vleeming, CJ Snijders… – European Spine …, 1993 – Springer
Anatomical and surgical considerations of the sacrotuberous ligament and its relevance in pudendal nerve entrapment syndrome by M Loukas, RG Louis, B Hallner, AA Gupta… – Surgical and Radiologic …, 2006 – Springer
Anatomy in practice: the sacrotuberous ligament. by SJ Woodley, E Kennedy… – New Zealand Journal of …, 2005 – search.ebscohost.com