The sacrum is a large bone located at the terminal part of the vertebral canal, where it forms the posterior aspect of the pelvis. It is remarkably thick, which aids in supporting and transmitting the weight of the body. In this article we will look at the anatomy of the sacrum – its gross structure, muscular and ligamentous attachments, neighbouring neurovasculature, as well as some relevant clinical notes.
The sacrum is formed by the fusion of the five sacral vertebrae. It has an inverted triangular, concave shape. The bone consists of a base, apex and four surfaces:
Internally, the central canal of the vertebral column continues along the core of the sacrum and ends at the 4th sacral foramina, as the sacral hiatus.
The dorsal surface of the sacrum is coarse and rugged. This can be attributed to the fusion of the sacral vertebrae, which give rise to three bony ridges (or crests). In the midline of the dorsal surface, there is a central ridge of bone, called the median sacral crest. It is formed by the fusion of the spinous processes of the first three sacral vertebrae. It gives attachment to the supraspinous ligament.
The intermediate sacral crests are formed by the fusion of the sacral articular processes.The posterior sacroiliac ligaments are attached along this crest. It should be noted that the superior articulating process of S1 and the inferior articulating process of S5 are not fused. Therefore, the former articulates with the inferior articulating process of L5, while the latter – also known as the sacral cornu – articulates with the coccygeal cornu (superior articulating process of coccyx).
Finally, the transverse processes of the five sacral bones fuse to form the lateral sacral crests, which offers a point of attachment to the posterior sacroiliac ligaments as well as the sacrotuberous ligament. The fusion of these processes is not complete, allowing the sacral nerve fibres to enter and leave the central canal by way of the four pairs of posterior sacral foramina.
The pelvic surface of the sacrum is less remarkable than the dorsal surface. In the adult, the surface is marked by four transverse lines – the remnants of the fused sacral intervertebral discs (fusion of the sacral vertebrae begins at age 20). Superiorly, there is an anterior projection of bone, known as the sacral promontory. It forms the posterior margin of the pelvic inlet and as a result, it is serially continuous with the margin of the ala of the sacrum, arcuate line of the ilium, and the pectin pubis and pubic crest of the pubic bone.
Both the anterior and posterior surfaces of the sacrum serve as points of origin or attachment for several lower limb and back muscles. They can be divided into the those that are associated with the anterior surface, and those associated with the posterior surface.
Anterior Surface:
The sacrum contributes to the sexual dimorphism of the pelvis. Firstly, the sacral promontory is less prominent in females than in males. This results in an oval-shaped pelvic inlet in females and a heart-shaped pelvic inlet in males.
The sacrum is a fundamental anatomical structure in the human body. Its thick, substantial nature provides crucial support and stability to the body, while the different surfaces and muscular attachments offer a range of functions. In addition, the sacrum is integral in the sexual dimorphism of the pelvis, which demonstrates its importance in human anatomy.
The sacrum is formed by the fusion of the five sacral vertebrae and has an inverted triangular, concave shape. It has four distinct surfaces: Base, Apex, Auricular surfaces and Anterior and Posterior surfaces. Each surface has unique anatomical features and functions, which are essential for the proper functioning of the pelvic girdle and vertebral column.
The base of the sacrum articulates superiorly with the fifth lumbar vertebra and its associated intervertebral disc. This allows the sacrum to remain stable during movement.
The apex of the sacrum abuts the coccyx inferiorly, providing support and stability.
The auricular surfaces are located laterally on the sacrum, and are shaped like the outer ear - hence the name. Each articulates with the auricular surface of the ilium.
The anterior and posterior surfaces provide attachment to pelvic ligaments and muscles. The posterior surface has two main muscles - Multifidus lumborum and the Erector spinae. Multifidus lumborum is the deepest muscle, arising from the sacrum and attaching to the transverse processes of the superior vertebrae. This muscle is essential in providing stability to the spine. The Erector spinae is partly a continuation of Multifidus lumborum and partly arises from the posterior sacrum and the sacrospinous ligament, allowing it to help in achieving the extension and lateral bending of the head and vertebral column.
The sacrum is in close proximity to several important structures. As it is a part of the pelvic girdle and vertebral column, the sacrum is bordered by the spinal cord, autonomic ganglia and vasculature. The central canal of the sacrum is home to the sacral fibres of the cauda equina, a bundle of spinal nerves that arises from the terminus of the spinal cord. The proximal parts of these fibres are contained within the dural sac, which terminates at about the level of S2 and is joined by the arachnoid and dura mater at the level of S2 and continues inferiorly through the sacrum as the coccygeal ligament to its attachment at the coccyx. The sacral part of the two sympathetic trunks run along the pelvic surface of the sacrum, medial to the sacral foramina. Each trunk has four ganglia in this region. The median and lateral sacral arteries are closely related to the sacrum. The Median sacral artery has a midline relation to the sacrum and arises from the abdominal aorta posteriorly before its bifurcation. It runs along midline to the coccyx while supplying the posterior rectum, glomus coccygeum (coccygeal gland), and anastomosing with the lateral sacral arteries and supply meninges and sacrum along the way. The Lateral sacral arteries are a pair of bilateral vessels that runs along the medial border of the sacral foramina, arising from the posterior division of internal iliac artery and giving rise to superior and inferior branches. These arteries also supply the meninges, sacrum and surrounding muscles.
The dorsal surface of the sacrum is coarse and rugged due to the fusion of the sacral vertebrae resulting in three bony ridges (or crests). The median sacral crest is in the midline of the dorsal surface and, is formed by the fusion of the spinous processes of the first three sacral vertebrae. It gives attachment to the supraspinous ligament while the intermediate sacral crests are formed by the fusion of the sacral articular processes and offer a point of attachment to the posterior sacroiliac ligaments. The transverse processes of the five sacral bones fuse to form the lateral sacral crests, which provides attachment to the posterior sacroiliac ligaments as well as the sacrotuberous ligament. These ridges are important for stabilizing the sacrum and providing movement.
The distal attachment of the sacrum is to the lesser trochanter of the femur. This allows it to flex the thigh at the hips and stabilize the hip joint.
The sacrum is a large, triangular-shaped, fused vertebra comprised of five fused bones and is located at the bottom of the vertebral column. It attaches both superiorly and inferiorly to the pelvic bones. There are three main surfaces to the sacrum: the anterior, lateral, and posterior.
The pelvic surface of the sacrum is less remarkable than the dorsal surface. In the adult, the surface is marked by four transverse lines - the remnants of the fused sacral intervertebral discs (fusion of the sacral vertebrae begins at age 20). Superiorly, there is an anterior projection of bone known as the sacral promontory, which forms the posterior margin of the pelvic inlet and is serially continuous with the margin of the ala of the sacrum, arcuate line of the ilium, and the pectin pubis and pubic crest of the pubic bone.
The anterior and posterior surfaces of the sacrum serve as points of origin or attachment for several lower limb and back muscles. Muscles associated with the anterior surface include the Piriformis, which originates from S2 – S4 level of the pelvic surface and due to its attachment at the trochanter of the femur, is able to externally rotate, abduct, extend, and stabilize the hip joint. The Coccygeus muscle inserts on the lower sacrum, gives support to the contents of the pelvic cavity, and due to its attachment to the coccyx, is able to flex the bone. The Iliacus, although primarily arising from the iliac fossa, also has fibres originating at the ala of the sacrum. Its distal attachment to the lesser trochanter of the femur allows it to flex the thigh at the hips and stabilise the hip joint.
On the posterior surface, the Multifidus lumborum is the deepest muscle arising from the sacrum, with some of its fibres covering the upper two sacral foramina. This muscle attaches to the transverse processes of the superior vertebrae and is therefore able to help stabilise the spine. The Erector spinae partly arise from the posterior sacrum and the sacrospinous ligament. It is essential in achieving extension and lateral bending of the head and vertebral column.
In addition to muscles, the sacrum lies in close proximity to several important structures. The central canal of the sacrum is home to the sacral fibres of the cauda equina, a bundle of spinal nerves that arise from the terminus of the spinal cord. The proximal parts of these fibres are contained within the dural sac, which terminates at about the level of S2. The filum terminale, a continuation of the pia mater from the conus medullaris of the spinal cord, is joined by the arachnoid and dura mater at the level of S2 and continues inferiorly through the sacrum as the coccygeal ligament to its attachment at the coccyx.
The two sympathetic trunks run along the pelvic surface of the sacrum, medial to the sacral foramina. Each trunk has four ganglia in this region. The median sacral artery, which is the continuation of abdominal aorta that arises posteriorly before its bifurcation, runs along midline to the coccyx while supplying the posterior rectum, glomus coccygeum (coccygeal gland), and anastomosing with the lateral sacral arteries and supplying the meninges and sacrum along the way. The lateral sacral arteries are a pair of bilateral vessels that run along the medial border of the sacral foramina. They arise from the posterior division of internal iliac artery and give rise to superior and inferior branches. Like the median artery, they also supply the meninges, sacrum, and surrounding muscles.
The fusion of these processes is not complete, allowing the sacral nerve fibres to enter and leave the central canal by way of the four pairs of posterior sacral foramina.
In conclusion, the sacrum has an essential function in the human body due to its strong connection between the vertebral column and the pelvis. Physiologically, it has numerous neurovascular relations, which are important in providing support, stability, and mobility to the body.
Fusion of the sacral vertebrae is an important process in the development of the spine. This process can be expected to commence around six to seven years of age, and be completed by the time the individual reaches eighteen to twenty years of age. This timeline is highly consistent for the majority of individuals, although some may experience minor variations in the duration of the fusion.
This process is not only noteworthy in terms of the individual's physical development, but also for its relevance to certain pelvic disorders. In particular, there is sexual dimorphism associated with the pelvis, which means that there are certain differences between men and women. These differences are of special clinical interest when considering the diagnosis and management of certain pelvic disorders.
For example, in women the angle of the pubic arch is much wider than in the male population. This angle is especially important for childbirth, as it dictates the amount of space available for the baby to pass through. Furthermore, women's pelvises tend to be shallower and more bowl-shaped than men, in order to provide greater stability during the birthing process.
In contrast, men's pelvises are more ‘heart-shaped’, and typically contain greater bone mass than women’s, which aids in providing the male body with greater stability and strength. Furthermore, the pubic arch of men is usually narrower and more sharply angled than in the female, which is required in order to facilitate the demands of activities such as jumping and running.
The differences between the male and female pelvis have a clear clinical relevance, especially when it comes to assessing and diagnosing any pelvic issues. For example, if a woman presents with pelvic instability, the clinician may first assess the patient’s pubic arch angle in order to ascertain whether it is wide enough to accommodate childbirth. Depending on the results of this assessment, the physician may suggest additional treatments such as massage or physiotherapy in order to reduce the pain and improve the overall stability of the pelvis.
Likewise, if a man presents with pelvic pain, the clinician may assess the shape of the pubic arch in order to determine whether the angle is too sharp for activities such as running and jumping. Depending on the results of this assessment, the physician may suggest treatments such as stretching exercises, strengthening of particular muscles, or even the use of orthotics.
Overall, fusion of the sacral vertebrae is an essential process in the normal development of the human spine. Furthermore, the sexual dimorphism of the pelvis is of special clinical relevance when assessing and treating certain pelvic disorders. Through knowledge of the normal differences between the male and female pelvis, clinicians can more accurately diagnose and treat any pelvic issues, in order to ensure the patient’s optimal health and safety.
