Sacroiliac & Hip Joints
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Index | Muscles
of the hip and thigh
Last updated 30 March 2006
- 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
- 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
- Observe that the anterior (pelvic) sacral foramina are larger than the posterior
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
- 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
- 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 20th 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
- 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
- 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.
- 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
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
- Long vertical fibres uniting the third and fourth
transverse tubercles of the sacrum to the posterior iliac
spines. These ligaments blend with the sacrotuberous
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
- 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.
- 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
- 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.
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,
- 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
- 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
- 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
- More than half of the femoral surface is contained within
- The articular or lunate surface of the acetabulum
- The acetabulum has a centrally located nonarticular
fossa, which is occupied by a fatpad that is covered with
- This nonarticular bone is paper thin and translucent.
- 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
- 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
- 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
- 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.
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
- 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.
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.