Sacrum, Sacroiliac & Hip Joints

Main Anatomy Index | Muscles of the hip and thigh

The Sacrum

General

• In older adults this large, triangular wedge-shaped bone is composed of five fused vertebrae.
• The triangular shape of the sacrum resulted from the rapid shrinkage in the size of the lateral masses. The inferior half is not weight bearing and therefore diminished rapidly in bulk.
• The sacrum (L. scared bone) provides strength and stability to the pelvis and transmits the weight of the body to the pelvic girdle through the sacroiliac joints.
• On the pelvic and dorsal surfaces, there are typically four pairs of foramina between its vertebral components for the exit of the ventral and dorsal divisions of the sacral nerves.
• Observe that the anterior (pelvic) sacral foramina are larger than the posterior (dorsal) ones.

Components of the Sacrum

• The base of the sacrum is formed by the superior surface of S1 vertebra. Its superior articular processes articulate with the inferior processes of the massive L5 vertebra.
• The projecting anterior edge of the body of the first sacral vertebra is called the sacral promontory (L. promontorium, mountain ridge). This is an important obstetrical landmark.
• The sacrum supports the vertebral column and forms the posterior part of the bony pelvis. It is tilted so that it articulates with L5 vertebra and the lumbosacral angle.
• The sacrum is often wider in proportion to the length in the female than in the male, but the body of the first sacral vertebra is usually larger in males.
• The pelvic (ventral) surface of the sacrum is smooth and concave. There are four transverse lines on the pelvic surface of old adult sacra, which indicate where fusion of the five sacral vertebrae occurred. Fusion starts after the 20^th year.
• The dorsal surface of the sacrum is rough, convex, and marked by prominent longitudinal ridges.

• The central one, called the median sacral crest, represents the fused reduced spinous processes of the superior three of four sacral vertebrae (S5 has no spinous process).
• The median sacral crest can be palpated in the median plane, just inferior to the "small of the back."

• The intermediate sacral crests represent the fused articular processes, and the lateral sacral crests are the tips of the transverse processes of the fused sacral vertebrae.

• The clinically important features of the dorsal surface of the sacrum are the inverted U-shaped sacral hiatus (L. aperture) and the sacral cornua (L. horns). The sacral hiatus results from the absence of laminae and the spinous process of S5 vertebral (sometimes S4 also). The sacral hiatus leads into the sacral canal, the inferior end of the vertebral canal. Its depth varies, depending on how much of the laminae and spinous process of L4 are present.
• The sacral hiatus contains fatty connective tissue, the filum terminale, S5 nerve, and the coccygeal nerve.

• The sacral cornua, representing the inferior articular processes of S5 vertebra, project inferiorly on each side of the hiatus and are a helpful guide to its location.
• The sacral hiatus and cornua can be palpated in the superior end of the natal cleft.

• The lateral surface of the sacrum looks somewhat like an auricle (L. external ear); because of this shape it is called the auricular surface. This is the site of the sacroiliac joint, located between the sacrum and the ilium.

• The coccyx articulates (from Co1) with the apex of the sacrum. Also, the coccygeal cornua articulates with the sacral cornua.

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Sacroiliac Joint

• The sacroiliac articulations are strong synovial joint between the articular surfaces of the sacrum and ilium.
• These surfaces have irregular elevations and depressions, which result in a partial interlocking of the bones. The strong articular capsule is attached close to the articulating surfaces of the sacrum and ilium.
• The sacrum is suspended between the iliac bones, and the bones are firmly held together by the interosseous and posterior sacroiliac ligaments. These are the strongest ligaments in the body.
• Hyaline cartilage on the sacrum.
• Fibrocartilage on the ilium.

The Interosseous Sacroiliac Ligaments

• These massive, very strong ligaments unite the iliac and sacral tuberosities. They consist of short, strong bundles of fibres that blend with and are supported by the thick firm posterior sacroiliac ligaments.

The Posterior (Dorsal) Sacroiliac Ligaments

These ligaments are composed of:

• Strong, short transverse fibres joining the ilium and the first and second tubercles of the lateral crest and the sacrum.
• Long vertical fibres uniting the third and fourth transverse tubercles of the sacrum to the posterior iliac spines. These ligaments blend with the sacrotuberous ligaments.

The Anterior (Ventral) Sacroiliac Ligaments

• This thin wide sheet of transverse fibres is located on the anterior and inferior aspects of the sacroiliac joint. It covers the abdominopelvic surface of this articulation. Replacement of most or all of these ligaments by bone often begins after 50 years of age.
• The iliolumbar, sacrotuberous, and sacrospinous ligaments are accessory ligaments of the sacroiliac joints. These articulations are covered posteriorly by the massive erectory spine and gluteus maximus muscles. The skin dimples indicating the posterior superior iliac spines are located at the middle of the sacroiliac joints.

Functions and Movements of the Sacroiliac Joints

• These joints are strong weight bearing synovial joints of an irregular plane type.
• They differ from other synovial joints in that they possess very little mobility.
• This provides for stability and is related to their responsibility for transmitting the weight of most of the body to the hip bones.
• Because the articular surfaces of the sacrum and ilium are irregular, they fit together securely and are not easily dislocated. This arrangement lessens the strain on the supporting ligaments of the joints.
• Movement of the sacroiliac joints is limited to a slight gliding and rotary movement, except when a considerable force is applied as occurs during a jump from a height. In this case the force is transmitted via the vertebral column to the superior end of the sacrum, which tends to rotate anteriorly. This rotation is counterbalanced by the interlocking articular surfaces and the strong supporting ligaments, especially the strong sacrotuberous and sacrospinous ligaments. This allows the force to be transmitted to each ilium and lower limb.

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The Pubic Symphysis

• The pubic symphysis (symphysis pubis) is a median, secondary cartilaginous joint between the bodies of the two pubic bones.
• Each articular surface is covered by a thin layer of hyaline cartilage, which is connected to the cartilage of the other side by a thick fibrocartilaginous interpubic disc. This disc is generally thicker in women than in men; it also contains a small cavity that is also larger in women and increases in size during pregnancy.
• These sex differences in the interpubic disc permit more mobility of the bones and increase the diameter of the pelvic cavity for passage of the foetus during childbirth.
• The articular surface of each pubic bone is irregularly ridged and grooved but the irregularities of the two sides fit tightly together to form a strong joint.

The ligaments of the pubic symphysis

• The ligaments joining the pubic bones are thickened superiorly and inferiorly to form the superior pubic ligament and the arcuate pubic ligament, respectively.
• The superior pubic ligament connects the superior pubic rami along their superior surfaces. It extends as far laterally as the pubic tubercles.
• The arcuate pubic ligament is a thick arch of fibres that (1) connects the inferior borders of the joint, (2) rounds off the subpubic angle, and (3) forms the superior border of the pubic arch.
• The decussating tendinous fibres of the rectus abdominis and external oblique muscles also strengthen the joint anteriorly.

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The Hip Joint

• This is a multilaxial ball and socket type of synovial joint between the head of the femur and the acetabulum of the hip bone.

Articular Surfaces of the Hip Joint

• The globular head of the femur articulates with the cup-like acetabulum of the hip bone.
• The wider superior part of the articular surface is the weight bearing area. Thus it is the ilium that bears the weight.

• The rim of the acetabulum is defective inferiorly at the acetabular notch, which is bridged by the transverse acetabular ligament. The head of the femur forms about two-thirds of a sphere and is covered with hyaline cartilage, except over the roughened fovea or pit, to which the ligaments of the head of the femur is attached.
• More than half of the femoral surface is contained within the acetabulum.
• The articular or lunate surface of the acetabulum is horseshoe-shaped.
• The acetabulum has a centrally located nonarticular fossa, which is occupied by a fatpad that is covered with synovial membrane.
• This nonarticular bone is paper thin and translucent.

The Acetabular Labrum

• The depth of the acetabulum is increased by this fibrocartilaginous labrum (L. lip). It is attached to the bony rim of the acetabulum and to the transverse acetabular ligament.
• The labrum deepens the socket for the femoral head and its free thin edge clasps the head beyond the widest diameter. This helps to hold it firmly in the acetabulum (i.e., preventing dislocation)

The Articular Capsule of the Hip Joint

• The fibrous capsule is strong and dense. Proximally, it is attached to the edge of the acetabulum, just distal to the acetabular labrum, and to the transverse acetabular ligament.
• Distally, the fibrous capsule is attached to the neck of the femur as follow: anteriorly to the intertrochanteric line and the root of the greater trochanter and posteriorly to the neck proximal to the greater trochanter and posterior to the neck proximal to the intertrochanteric crest.

• The fibrous capsule forms a cylindrical sleeve that encloses the hip joint and most of the neck of the femur.
• Most of its fibre take a spiral course from the hip bone to the lateral potion of the intertrochanteric line of the femur, but some deep fibres from an orbicular zone (zona orbicularis) and pass circularly around the neck of the femur. These fibres form a collar around the neck of the femur, which constricts the capsule and helps to hold the femoral head in the acetabulum.
• Some deep longitudinal fibres of the fibrous capsule form retinacula, which are reflected superiorly along the neck of the femur as longitudinal bands that blend with the periosteum.
• The retinacula contain blood vessels that supply the head and neck of the femur.

• Four main groups of longitudinal capsular fibres or intrinsic ligaments are given names according to the region of the hip bone which they attach to the femur.
• These intrinsic ligaments are thickened parts of the fibrous capsule that strengthen the hip joint.

Ligaments of the Hip Joint

• The Iliofemoral Ligament is a very strong band that covers the anterior aspect of the hip joint. It is Y-shaped and attached proximally to the anterior inferior iliac spine and the acetabular rim.
• The iliofemoral ligament is attached distally to the intertrochanteric line of the femur.
• The capsule of the hip joint is taut and the iliofemoral ligament is tense in full extension of the joint. This strong ligament has an important role in preventing overextension of the hip joint during standing (i.e., it helps to maintain the erect posture.
• It screws the head of the femur into the acetabulum and thereby maintains the integrity of the joint.

• The Pubofemoral Ligament arises from the pubic part of the acetabular rim and the iliopubic eminence and blends with the medial part of the iliofemoral ligament.
• It strengthens the inferior and anterior parts of the fibrous capsule of the hip joint.
• The pubofemoral ligament tightens during extension of the hip joint and becomes tense during abduction. Although it is relatively weak, this ligament tends to prevent overabduction of the thigh at the hip joint.

• The Ishiofemoral Ligament reinforces the fibrous capsule to the hip joint posteriorly. It arises from the ischial portion of the acetabular rim and spiral superolaterally to the neck of the femur, medial to the base of the greater trochanter. Its anatomical construction tends to screw the femoral head medially into the acetabulum during extension of the thigh at the hip joint, thereby preventing hyperextension of it.

• The Ligament of the Head of the Femur is an intracapsular ligament about 3.5 centimetres long, is weak and appears to be of little importance in strengthening the hip joint.
• Its wide end is attached to the margins of the acetabular notch and to the transverse acetabular ligament, and its narrow end is attached to the fovea or pit in the femur.
• Usually it contains a small artery to the head of the femur, which is a branch of the obturator artery. The ligament of the head of the femur (LHF) is stretched when the flexed thigh is adducted or laterally rotated. It is located inside the fibrous capsule of the hip joint and is surrounded by a sleeve of synovial membrane.

Stability of the Hip Joint

• The hip joint is very strong and stable articulation. It is surrounded by powerful muscles and a dense fibrous capsule, which is strengthened by strong intrinsic ligaments, particularly the iliofemoral ligament, unites the articulating bones.
• Its stability is largely the result of the adaptation of the articulating surfaces of the acetabulum and the femoral head to each other.
• Anterior aspect-fewer muscles, strong ligaments.
• Posterior aspect-stronger muscles, fewer ligaments.
• Position of weakness is when the hip joint is flexed, adducted and medially rotated.

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