Introduction
Sacroiliac joint abnormalities resulting in lower back pain is growing in prevalence amongst the aging population and is an issue that currently remains largely unresolved. The sacroiliac joint is essential for effectively transferring tensile loads from the lumbar spine to the legs. Although an individual’s lower back pain may be a result of sacroiliac joint dysfunction, it is crucial to understand that a single anatomical structure does not function in isolation. Any mechanical loads placed upon the body are distributed amongst a continuous network of fascia, ligaments, and musculature.
With this concept in mind, it is easier to understand how the pelvis, sacrum, and spine are functionally interrelated to the arms, legs, and head through muscular, fascial, and ligamentous interconnections. Analyzing the issue of lower back pain through functional anatomical and biomechanical models rather than using standard topographic anatomy schemata allows for a more holistic and successful approach to resolving an individual’s pain rather than simply treating the symptoms. The following paper examines the functional anatomy of the sacroiliac joint, current diagnoses and treatments, and provides a recommendation of how sacroiliac joint dysfunction can undergo a gold standard of treatment that corrects the functional anatomy of the joint rather than adapting to the dysfunction.
General Anatomy
In order to perform normal gait movement, the body utilizes three main muscles to perform the contralateral arm and leg movements associated with walking and running: the obliques, the latissimus dorsi, and the gluteus maximus. During left weight bearing, the left internal oblique and right external oblique pull the left rib cage down and forward, while the left latissimus dorsi and right gluteus maximus pull the left shoulder down and forward. Under normal conditions, this pattern is reversed for right weight bearing. In the lower portion of this system, the back functional line is the term used to describe how the latissimus dorsi and gluteus maximus are connected. The back functional line is a myofascial chain that includes the latissimus dorsi, contralateral gluteus maximus, and vastus lateralis (krause 1). All three muscles – in conjunction with the thoracolumbar myofascial tissue – help to form the posterior muscular X-shape that allows for normal, contralateral gait movements. The main bony structure that supports normal gait movement as well as the transfer of weight between the upper extremity, trunk, and lower extremity is the pelvis. Other bones that serve as bony interfaces include the shaft of the humerus and the shaft of the femur.
In Depth Anatomy of the Pelvis: Bones and Joints
The pelvis is a basin-shaped combination of bones that serves as the connection between the superior and inferior portions of the body. The pelvic girdle also supports the weight of the upper extremity and serves as a means to transfer weight to the lower extremity. It is formed by two hipbones that are connected anteriorly at the pubic symphysis and posteriorly by the sacrum. Each of the hipbone complexes are made up of three bones: the ilium, the ischium, and the pubis. By young adulthood, these three bones have fused together to become a single bone. These three portions of the hipbone unite together at the acetabulum– the socket that forms the hip joint that then houses the greater trochanter of the femur, creating the acetabulofemoral joint.
Osteologically, the pelvic bone divided into two sections: the greater pelvis and lesser pelvis. The greater pelvis – also known as the false pelvis – is the superior portion of the pelvic girdle. It provides support for the lower portion of the abdominal viscera. The lesser pelvis – also known as the true pelvis – is the inferior portion of the pelvic girdle. The lesser pelvis sits within the greater pelvis and contains the pelvic cavity and viscera. The greater and lesser pelvis are conjoined at the pelvic inlet.
There are four articulations within the pelvic girdle: two sacroiliac joints, the sacrococcygeal symphysis, and the pubic symphysis. The sacroiliac joints are an articulation between the ilium of the hipbones and the sacrum, the sacrococcygeal symphysis is an articulation between the sacrum and the coccyx, and the pubic symphysis is an articulation between the two anterior inferior portions of the hip bones.
In Depth Anatomy of the Pelvis: Ligaments and Muscles
The pelvis is held together by three vertebropelvic ligaments: the iliolumbar ligament, the sacrospinous ligament, and the sacrotuberous ligament. The iliolumbar ligament is a thick and strong connective tissue that originates from the tip of the transverse process of the fifth lumbar vertebra and stretches out to the posterior portion of the iliac crest. It stabilizes and strengthens the lumbosacral joint as well as restricts the rotational movement of the lumbosacral joint. The sacrospinous ligament is a triangular-shaped band of connective tissue that attaches from the ischial spine of the ischial bone to the lateral side of the sacrum and coccyx. It divides the sciatic notch into the greater sciatic foramen and lesser sciatic foramen. The sacrotuberous ligament is a fan-shaped band of connective tissue that spans from the sacrum and the upper coccyx to the tuberosity of the ischial tuberosity.
There are three main groups of muscles that can be located within the pelvis: muscles that cross the lumbosacral joint and attach from the trunk to the pelvis, muscles that cross the hip joint and attach from the pelvis to the thigh or leg, and muscles that are located solely within the pelvis – also known as pelvic floor muscles.
There are nine muscles that cross the lumbosacral joint attach from the trunk to the pelvis: the erector spinae group, transversospinalis group, quadratus lumborum, latissimus dorsi, rectus abdominis, external abdominal obliques, internal abdominal obliques, and the psoas minor. The erector spinae attaches at the sacrum and extends vertically up the spine to attach the angles of ribs on each side of the vertebral column in all three sections – cervical, thoracic, and lumbar. Together, the erector spinae muscle group contralaterally rotates and anteriorly tilts the pelvis and elevates the same side of the pelvis at the lumbosacral joint. The rotatores and multifidus of the transversospinalis muscle group are housed within the laminar grove of the thoracic and lumbar spine. The rotators attach from the transverse processes of the thoracic and lumbar spine to segments of one or two vertebrae superior to the inferior attachment site. The multifidus attaches from the sacrum, posterior superior iliac spine, the transverse processes of the thoracic spine, and the mamillary processes of the lumbar spine to the spinous process of the vertebral segments that are three or four vertebrae superior to the inferior attachment. At the pelvis, the rotatores and the multifidus help to anteriorly tilt, ipsilaterally rotate, and elevate the same side of the pelvis at the lumbosacral joint. The quadratus lumborum attaches from the inferomedial border of the 12th rib the transverse processes of L1-L4 to the posteromedial iliac crest. At the pelvis, the quadratus lumborum elevates the same side of the pelvis and anteriorly tilts the pelvis at the lumbosacral joint. The latissimus dorsi attaches from the spinous processes of T7-L5, the posterior sacrum, and the posterior iliac crest – via the thoracolumbar fascia – to the lowest three or four ribs. It also attaches from the inferior angle of the scapula to the medial lip of the bicipital grove of the humerus. At the pelvis, the latissimus dorsi anteriorly lifts the pelvis at the lumbosacral joint. The rectus abdominis attaches from xiphoid process of the sternum and costal cartilages of ribs 5-7 to the crest and symphysis of the pubic bone. In the pelvis, the rectus abdominis posteriorly tilts the pelvis at the lumbosacral joint. The external abdominal oblique attaches from the lower eight ribs to the anterior iliac crest, abdominal aponeurosis, pubic bone, and inguinal ligament. In the pelvis, the external abdominal oblique posteriorly tilts and ipsilaterally rotates the pelvis and elevates the same side of the pelvis at the lumbosacral joint. The internal abdominal oblique attaches from the lower three ribs and the abdominal aponeurosis to the iliac crest, inguinal ligament, and thoracolumbar fascia. At the pelvis, the internal abdominal oblique posteriorly tilts and contralaterally rotates the pelvis. It also elevates the same side of the pelvis at the lumbosacral joint. The psoas minor attaches from T12-L1 to the pelvis bone. At the pelvis, the psoas minor posteriorly tilts the pelvis at the lumbosacral joint.
There are ten muscles that cross the hip joint and attach from the pelvis to the thigh or leg: the tensor fascia latae, the rectus femoris, the sartorius, the iliopsoas, the adductor group, the hamstring group, the gluteus maximus, the gluteus medius, the gluteus minimus, and the deep lateral rotator group. The tensor fasciae latae attaches from the anterior iliac crest and anterior superior iliac spine to the iliotibial band. At the pelvis, the tensor fascia latae flexes, abducts, and medially rotates the thigh at the hip joint and anteriorly tilts and depresses the same side of the pelvis at the hip joint. The rectus femoris attaches from the anterior inferior ilia spine to the patella. At the pelvis, the rectus femoris flexes the thigh and anteriorly tilts the pelvis at the hip joint. The sartorius attaches from the anterior superior iliac spine to the pes anserine tendon at the tibia. At the pelvis, the sartorius flexes, abducts, and laterally rotates the thigh. It also anteriorly tilts the pelvis and depresses the same side of the pelvis at the hip joint. The psoas major of the iliopsoas attaches from T12-L5 to the lesser trochanter of the femur. The iliacus of the iliopsoas attaches from the internal surface of the ilium to the lesser trochanter of the femur. At the pelvis, both the psoas major and iliacus flex and laterally rotate the thigh and anteriorly tilt the pelvis at the hip joint. The adductor group attaches from the ischium and pubic bone to the linea aspera, pectineal line, adductor tubercle of the femur, and the pes anserine tendon at the tibia. At the pelvis, the adductor group adducts, flexes, and medially rotates the thigh and anteriorly tilts and ipsilaterally rotates the pelvis at the hip joint. The adductor magnus also posteriorly tilts the pelvis at the hip joint. In the hamstring group, the biceps femoris attaches from the ischial tuberosity and linea aspera of the femur to the head of the fibula. The semitendinosus attaches from the ischial tuberosity to the pes anserine tendon of the tibia. The semimembranosus attaches from the ischial tuberosity to the posterior surface of the medial condyle of the tibia. At the pelvis, the hamstring group extends the thigh and posteriorly tilt the pelvis at the hip joint. The gluteus maximus attaches from the posterior iliac crest, the sacrum, and the coccyx to the gluteal tuberosity and the iliotibial band. At the pelvis, the gluteus maximus extends, laterally rotates, abducts (upper fibers), and adducts (lower fibers) the thigh and posteriorly tilts and contralaterally rotates the pelvis at the hip joint. The gluteus medius attaches from the external ilium to the greater trochanter of the femur. At the pelvis, the entire gluteus medius abducts the thigh and depresses the same side of the pelvis at the hip joint. The anterior fibers also flex and medially rotate the thigh and anteriorly tilt and ipsilaterally rotate the pelvis at the hip joint. The posterior fibers extend and laterally rotate the thigh and posteriorly tilt and contralaterally rotate the pelvis at the hip joint. The gluteus minimus attaches from the external ilium to the greater trochanter of the femur. The entire gluteus minimus abducts the thigh and depresses the same side of pelvis at the hip joint. The anterior fibers also flex and medially rotate the thigh and anteriorly tilt and ipsilaterally rotate the pelvis at the hip joint. The posterior fibers extend and laterally rotate the thigh and posteriorly tilt and contralaterally rotate the pelvis at the hip joint. The deep lateral rotator group attaches from the sacrum and the pelvic bone to the greater trochanter of the femur. The entire deep lateral rotators laterally rotate the thigh at the hip joint and contralaterally rotate the pelvis at the hip joint.
The pelvic floor is made up of three layers of muscle: the superficial perineal layer, the deep urogenital diaphragm layer, and the pelvic diaphragm. All three layers have a multitude of functions that differ according to gender. The overall goals of the pelvic floor muscles is to stabilize the sacroiliac and symphysis pubis joints and to create a stable floor for the visceral contents of the abdominopelvic cavity.
In Depth Anatomy of the Pelvis: Movement of the Pelvis
Because the pelvis links the trunk of the upper portion of the body to the lower extremities, it must cooperate with the motions of both sections while simultaneously providing structure, stability, and support to the entire body. Due to these relationships, motion of the pelvis can be further categorized into primary and secondary movements. Primary movement refers to the occasional scenario in which movement is initiated at the pelvis with the spine and thighs assisting with the movement. Secondary movement refers to when movement is initiated by the spine or trunk – and occasionally at the thighs – with the pelvis assisting with the movement. Secondary movement occurs much more frequently than primary movement. The same four pelvic motions occur in each category, the only difference is the movement’s origin of initiation.
There are four movements of the pelvis: the forward tilt, the backward tilt, the lateral tilt, and rotation. These changes in pelvic positioning are enabled by the motions of the lumbar spine and the hip joints. The forward tilt is the movement of the pelvis along the sagittal plane about the mediolateral axis which allows the pelvis to bilaterally turn the pubis symphysis downward so that the posterior portion of the sacrum turns upward. This motion can also be described as increased inclination. The backward tilt is the movement of the pelvis along the sagittal plane about the mediolateral axis which allows the pelvis to bilaterally turn the pubis symphysis forward and upward so that the posterior portion of the sacrum is turned slightly downward. This motion can also be described as decreased inclination. The lateral tilt is the rotation of the pelvis along the frontal plane about the anteroposterior axis which allows the one iliac crest of the pelvis to be raised while the other iliac crest of the pelvis is simultaneously lowered. Rotation of the pelvis occurs along the transverse plane about the longitudinal axis and is named in concurrence with the direction of the movement.
Anatomical Effects of Gender upon the Pelvis
– Women have three more ligaments https://www.ncbi.nlm.nih.gov/books/NBK493215/
– https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1472875/
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In Depth Anatomy of the Sacroiliac Joint
The sacroiliac joint is the articulation between the sacrum of the lower spine and the ilium of the pelvic bones. There are two sacroiliac joints in concurrence with the both sides of the sacrum. The anterior of the sacroiliac joint is classified as a diarthrodial joint – meaning that is freely moveable. However, the posterior of the sacroiliac joint is classified as a synarthrosis joint – meaning that it is only slightly moveable. There is a large difference in range of motion between the anterior and posterior portions of the sacroiliac joint due to the difference between the amount of ligamentous support between the anterior and posterior portions of the pelvis.
There is one ligament on the anterior side of the pelvis and five ligaments on the posterior side of the pelvis.
On the anterior side of the pelvis is the anterior sacroiliac ligament. The anterior sacroiliac ligament connects the anterior surface of the lateral part of the sacrum to the margin of the auricular surface of the ilium and the preauricular sulcus. However, the fibrous capsule of the anterior sacroiliac ligament is relatively weak and thin when compared to the posterior ligaments, making it the most susceptible to injury and a common source of pain because of its thinness.
The posterior portion utilizes the interosseous sacroiliac ligament, the posterior sacroiliac ligament, the iliolumbar ligament, the sacrotuberous ligament, and the sacrospinous ligament. The interosseous sacroiliac ligament is located between the sacrum and ilium and helps to maintain joint congruency. It is located where a majority of the weight transfer from the spine to the hip and other lower extremities occurs. The posterior sacroiliac ligament is a continuation of the interosseous sacroiliac ligament and provides support to the interlocking mechanism on the front surfaces of the bones of the joint. It does this by bringing the ilium closer to the sacrum so that it aids the sacrum in absorbing the weight of the spine superiorly. The iliolumbar ligament assists with weight transfer as well as the supporting functions of the interosseous sacroiliac and posterior sacroiliac ligaments. The sacrotuberous ligament is a large ligament that extends from the sacrum to the ischial tuberosities – also known as the sitting bones. The sacrospinous ligament is a smaller ligament that also connects the sacrum to the ischial tuberosities, however it does not cover as much of the joint as the sacrotuberous ligament. Both of the sacrotuberous and sacrospinous ligaments limit flexion of the sacrum.
There are thirty-five muscles that attach to the sacrum which mainly provide support and stability rather than producing movement. However, there are five main muscles: the piriformis, gluteus maximus (superior deep fibers), coccygeus and levator ani, paraspinals, and hamstrings. The piriformis attaches from the anterior sacrum to the greater trochanter of the femur. The superior deep fibers of the gluteus maximus connect the lateral sacrum to the iliac bone. Although the coccygeus and levator ani are technically pelvic floor muscles, their contractive forces cross the sacroiliac joint. The coccygeus attaches from the sacrum and coccyx medially to the ischial spine of the pelvic bone laterally. The levator ani attaches from the coccyx to surrounding soft tissue. Because it attaches to the coccyx, the levator ani exerts pull on the sacrum because by adulthood the sacrum and coccyx have often become fused. Since the piriformis, the gluteus maximus, and the coccygeus cross the sacroiliac joint, they are some of the first muscles that tighten when the sacroiliac joint undergoes an injury.
The paraspinal muscles refer to the erector spinae muscle group and the transversospinalis group. Although they do not cross the sacroiliac directly across the mediolateral axis, the paraspinal muscles can still decrease sacroiliac motion. Within the sacroiliac region, the multifidus of the longissimus of the erector spinae group and transversospinalis group run vertically and cross from the lumbar region onto the pelvis to attach to the sacrum. The hamstrings can indirectly support and effect the sacroiliac joint because of its part in the superficial back line of myofascial tissue. The superficial back line runs from the plantar fascia to the gastrocnemius, up into the hamstrings, sacrotuberous ligament, sacrum, thoracolumbar fascia, erector spinae musculature, and then into the epicranius of the head. Therefore, if the hamstrings are tight, their tension will create a pulling force that will be exerted through the sacrotuberous ligament and onto the sacrum.
Functional Anatomy of the Sacroiliac Joint
The main function of the sacroiliac joint is to transmit forces received from the lumbar spine and sacrum into the pelvis, the hips, and down into the lower extremities. This transmission of tensile forces is vital as it prevents the pelvis from fracturing under heavy loads or from other bipedal functions.
To do this, the upper part of the gluteus maximus attaches to the ilium and pulls the ilium posteriorly into nutation – or sacral flexion. The lower portion attaches to the sacrum and pulls the sacral apex anteriorly into counternutation (anterior tilt). Together, the upper and lower portions of the gluteus maximus pull the entire pelvis posteriorly to extend the pelvis and raise the trunk of the torso. The latissimus dorsi and contralateral gluteus maximus then work together to create a force that induces nutation on the gluteal side and counternutation on the latissimus dorsi side in order to induce forward locomotion.