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The sacrum is a large bone at the base of the spine that supports the weight of the entire body. It is located in the pelvic cavity, where it serves as an attachment point for numerous muscles and ligaments. The sacrum has two surfaces: a coarse dorsal surface and a relatively smooth pelvic surface. The muscles that attach to the sacrum can be divided into those associated with the anterior surface and those associated with the posterior surface. These muscles include the piriformis, coccygeus, iliacus, gluteus maximus, and multifidus lumborum. The sacrum also serves as an attachment site for ligaments such as the sacroiliac and sacrotuberous ligaments.
| Characteristics | Values |
|---|---|
| Muscles | Piriformis, Coccygeus, Iliacus, Multifidus lumborum, Gluteus maximus, Iliopsoas, Quadratus lumborum, Biceps femoris |
| Ligaments | Sacroiliac, Sacrotuberous, Supraspinous, Posterior sacroiliac, Interosseous sacroiliac, Anterior sacrococcygeal, Posterior sacrococcygeal, Lateral posterior sacrococcygeal |
| Arteries | Median sacral artery, Lateral sacral arteries |
| Nerves | Sacral spinal nerves (S1-S5), Spinal nerves, Cauda equina |
| Vertebrae | L5, S1 |
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What You'll Learn
Piriformis, gluteus maximus, and coccygeus muscles
The piriformis muscle is located partially on the posterior wall of the pelvis and partially posterior to the hip joint. It originates from several locations in the pelvis, passes through the greater sciatic notch, and inserts on the superior aspect of the greater trochanter. The piriformis is intimately associated with the gluteus medius, gluteus maximus, obturator internus, and gemelli muscles. The piriformis externally rotates the thigh along with the superior and inferior gemelli, quadratus femoris, and obturator internus and externus.
The gluteus maximus is supplied by the inferior gluteal nerve, which leaves the pelvis inferior to the piriformis. The gluteus maximus is also associated with the piriformis syndrome, which is a cause of pain in the buttocks.
The coccygeus muscle inserts on the lower sacrum. It gives support to the contents of the pelvic cavity and due to its attachment to the coccyx, is able to flex the bone.
The piriformis, gluteus maximus, and coccygeus muscles are all associated with the sacrum and/or the sacroiliac joint. The piriformis muscle may have one or two attachments to the sacrum or capsule of the hip joint in some cases. The gluteus maximus is supplied by the inferior gluteal nerve, which leaves the pelvis inferior to the piriformis. The coccygeus muscle inserts on the lower sacrum and provides support to the pelvic cavity.
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Iliacus and multifidus lumborum muscles
The iliacus muscle is part of the hip muscle group and is one of the muscles associated with the anterior surface of the sacrum. Although it primarily arises from the iliac fossa, it also has fibres originating at the ala of the sacrum.
The multifidus lumborum is a deep muscle that arises from the sacrum. It is part of the transversospinalis muscle group and is the deepest muscle in this group. The multifidus lumborum attaches to the transverse processes of the superior vertebrae, helping to stabilise the spine. It is an important stabiliser of the lumbar spine, working together with the transversus abdominis and pelvic floor muscles. The multifidus muscle is continuously active in upright postures and is probably active in all anti-gravity activity. It is divided into four regions based on the location of the spine: the cervical region, the thoracic region, the lumbar region, and the sacral region. The multifidus muscle spans the whole length of the vertebral column but is most developed in the lumbar area. It is also located beneath the erector spinae group, which includes the spinalis, longissimus, and iliocostalis muscles.
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Sacroiliac and sacrotuberous ligaments
The sacrum is a triangular bone in the vertebral column that forms the base of the spine. It is a crucial structure as it supports the weight of the entire body. The anterior and posterior surfaces of the sacrum serve as attachment sites for several muscles and ligaments.
The sacroiliac joint is surrounded by over 40 muscles. The main muscle groups that affect the joint are the back muscles, such as the erector spinae, quadratus lumborum, and multifidus lumborum, and hip muscles, such as the iliopsoas. The gluteus maximus and piriformis are also associated with the sacroiliac joint.
The sacroiliac ligaments attach to the lateral sacral crests formed by the fusion of the transverse processes. The sacrotuberous ligament is a thick fibrous band that connects the ilium, sacrum, and coccyx to the ischial tuberosity. It is formed by the merge of the posterior sacroiliac ligaments and fibres that extend from the posterior margin of the ilium and the base of the coccyx. The sacrotuberous ligament is pierced by the coccygeal branches of the inferior gluteal artery, the perforating cutaneous nerve, and branches of the coccygeal plexus.
The sacrotuberous ligament is essential for stabilising the sacroiliac joint and strengthening the connection between the bony pelvis and the vertebral column. It also assists in pelvic stability, preventing the anterior tipping of the sacrum by controlling sacral nutation. It is one of the most vital ligaments of the sacroiliac joint, with critical functional links between the spine, pelvic girdle, and lower extremity muscles, such as the gluteus maximus and hamstring muscles.
The sacrotuberous ligament is usually thin and cannot be palpated externally. However, it can become thicker and tighter due to aggressive physical activity or injury, leading to ligament ossification and pressure on the pudendal nerve. This can cause pain in the back of the thigh, and in severe cases, the pain may extend to the calf and heel.
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Supraspinous and sacrospinous ligaments
The supraspinous ligament is a cord-like fibrous band that connects the tips of the spinous processes from the seventh cervical vertebra (C7) to the sacrum. It is thicker and broader in the lumbar region than in the thoracic region, and it becomes indistinct inferior to L4. The supraspinous ligament functions to prevent separation of the spinous processes during flexion and resists hyperflexion of the spine. It also helps maintain the upright position of the head.
The median sacral crest, formed by the fusion of the first three sacral spinous processes, serves as the attachment point for the supraspinous ligament. The auricular surfaces of the sacrum, located laterally and shaped like the outer ear, articulate with the auricular surface of the ilium. The anterior and posterior surfaces of the sacrum provide attachment sites for pelvic ligaments and muscles.
The sacrospinous ligament, also known as the small or anterior sacrosciatic ligament, is a thin, triangular ligament in the human pelvis. The base of the ligament is attached to the outer edge of the sacrum and coccyx, while the tip attaches to the spine of the ischium, a bony protuberance on the pelvis. Its fibres are intermingled with the sacrotuberous ligament, which passes behind it. The sacrospinous ligament covers the coccygeus muscle and its presence in the greater sciatic notch creates an opening called the greater sciatic foramen. The ligament's main function is to prevent rotation of the ilium past the sacrum.
Vaginal or uterine prolapse in women can be treated with sacrospinous fixation surgery, where the apex of the vagina is sutured to the sacrospinous ligament to provide sturdy support. The coccygeus muscle, which the sacrospinous ligament closely covers, supports the contents of the pelvic cavity and can flex the bone due to its attachment to the coccyx.
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Erector spinae and quadratus lumborum muscles
The sacrum is a large bone that forms an ideal base to support the weight of the entire body. Both the anterior and posterior surfaces of the sacrum serve as attachment points for several lower limb and back muscles.
The erector spinae is a group of muscles that arise partly from the posterior sacrum and the sacrospinous ligament. It is essential in achieving extension and lateral bending of the head and vertebral column. It is also activated during the flexion-relaxation phenomenon, along with the quadratus lumborum. The erector spinae muscles exert a force of 100 N during extension.
The quadratus lumborum is a deep back muscle that originates from the iliac crest and inserts on the transverse processes of lumbar vertebrae L1-L5 and the 12th rib. It is an integral part of the thoracolumbar fascia, a myofascial system that covers the posterior area of the human body, involving parts of the lower and upper limbs. The quadratus lumborum is described as a lumbar spine extensor, stabilizer, and lateral tiltor, and an accessory muscle for inspiration. It exerts a force of 10 N during extension, which is relatively small, and it participates with less than 10% of the force required for a coronal inclination.
The erector spinae and quadratus lumborum muscles work together with the multifidus muscle to antagonize the abdominal muscles. They help create an antagonist force to the muscles of the anterolateral abdomen wall.
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Frequently asked questions
The iliacus, piriformis, and coccygeus attach to the anterior sacrum.
The gluteus maximus, erector spinae, and multifidus lumborum attach to the posterior sacrum.
The muscles attaching to the sacrum help to stabilise the hip joint and spine, as well as support the weight of the entire body.
The sacrum is a triangular bone formed by the fusion of sacral vertebrae. It is part of the pelvic girdle and vertebral column, lying in close proximity to several important structures.
