III. Syndesmology. 5i. Articulations of the Pelvis. Gray, Henry. 1918. Anatomy of the Human Body.

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5i. Articulations of the Pelvis

The ligaments connecting the bones of the pelvis with each other may be divided into four groups: 1. Those connecting the sacrum and ilium. 2. Those passing between the sacrum and ischium. 3. Those uniting the sacrum and coccyx. 4. Those between the two public bones.

1. Sacroiliac Articulation (articulatio sacroiliaca).—The sacroiliac articulation is an amphiarthrodial joint, formed between the auricular surfaces of the sacrum and the ilium. The articular surface of each bone is covered with a thin plate of cartilage, thicker on the sacrum than on the ilium. These cartilaginous plates are in close contact with each other, and to a certain extent are united together by irregular patches of softer fibrocartilage, and at their upper and posterior part by fine interosseous fibers. In a considerable part of their extent, especially in advanced life, they are separated by a space containing a synovia-like fluid, and hence the joint presents the characteristics of a diarthrosis. The ligaments of the joint are:

The Anterior Sacroiliac.		The Posterior Sacroiliac.
The Interosseous.

The Anterior Sacroiliac Ligament (ligamentum sacroiliacum anterius) (Fig. 319).—The anterior sacroiliac ligament consists of numerous thin bands, which connect the anterior surface of the lateral part of the sacrum to the margin of the auricular surface of the ilium and to the preauricular sulcus.

FIG. 319– Articulations of pelvis. Anterior view. (Quain.) (See enlarged image)

The Posterior Sacroiliac Ligament (ligamentum sacroiliacum posterius) (Fig. 320).—The posterior sacroiliac ligament is situated in a deep depression between the sacrum and ilium behind; it is strong and forms the chief bond of union between the bones. It consists of numerous fasciculi, which pass between the bones in various directions. The upper part (short posterior sacroiliac ligament) is nearly horizontal in direction, and passs from the first and second transverse tubercles on the back of the sacrum to the tuberosity of the ilium. The lower part (long posterior sacroiliac ligament) is obique in direction; it is attached by one extremity to the third transverse tubercle of he back of the sacrum, and by the other to the posterior superior spine of the ilium.

FIG. 320– Articulatios of pelvis. Posterior view. (Quain.) (See enlarged image)

The Interosseous Sacroiliac Ligament (ligamentum sacroiliacum interosseum).—This ligament lies deep to the poserior ligament, and consists of a series of short, strong fibers connecting the tubeosities of the sacrum and ilium.

2. Ligaments Connecting the Sacrum and Ischium (Fig. 320).

The Sacrotuberous.

The Sacrospinous.

The Sacrotuberous Ligament (ligamentum sacrotuberosum; great or posterior sacrosciatic ligament).—The sacrotuberous ligament is situated at the lower and back part of the pelvis. It is flat, and triangular in form; narrower in the middle than at the ends; attached by its broad base to the posterior inferior spine of the ilium, to the fourth and fifth transverse tubercles of the sacrum, and to the lower part of the lateral margin of that bone and the coccyx. Passing obliquely downward, forward, and lateralward, it becomes narrow and thick, but at its insertion into the inner margin of the tuberosity of the ischium, it increases in breadth, and is prolonged forward along the inner margin of the ramus, as the falciform process, the free concave edge of which gives attachment to the obturator fascia; one of its surfaces is turned toward the perineum, the other toward the Obturator internus. The lower border of the ligament is directly continuous with the tendon of origin of the long head of the Biceps femoris, and by many is believed to be the proximal end of this tendon, cut off by the projection of the tuberosity of the ischium.

Relations.—The posterior surface of this ligament gives origin, by its whole extent, to the Glutæus maximus. Its anterior surface is in part united to the sacrospinous ligament. Its upper border forms, above, the posterior boundary of the greater sciatic foramen, and, below, the posterior boundary of the lesser sciatic foramen. Its lower border forms part of the boundary of the perineum. It is pierced by the coccygeal nerve and the coccygeal branch of the inferior gluteal artery.

The Sacrospinous Ligament (ligamentum sacrospinosum; small or anterior sacrosciatic ligament).—The sacrospinous ligament is thin, and triangular in form; it is attached by its apex to the spine of the ischium, and medially, by its broad base, to the lateral margins of the sacrum and coccyx, in front of the sacrotuberous ligament with which its fibers are intermingled.

Relations.—It is in relation, anteriorly, with the Coccygeus muscle, to which it is closely connected; posteriorly, it is covered by the sacrotuberous ligament, and crossed by the internal pudendal vessels and nerve. Its upper border forms the lower boundary of the greater sciatic foramen; its lower border, part of the margin of the lesser sciatic foramen.

These two ligaments convert the sciatic notches into foramina. The greater sciatic foramen is bounded, in front and above, by the posterior border of the hip bone; behind, by the sacrotuberous ligament; and below, by the sacrospinous ligament. It is partially filled up, in the recent state, by the Piriformis which leaves the pelvis through it. Above this muscle, the superior gluteal vessels and nerve emerge from the pelvis; and below it, the inferior gluteal vessels and nerve, the internal pudendal vessels and nerve, the sciatic and posterior femoral cutaneous nerves, and the nerves to the Obturator internus and Quadratus femoris make their exit from the pelvis. The lesser sciatic foramen is bounded, in front, by the tuberosity of the ischium; above, by the spine of the ischium and sacrospinous ligament; behind, by the sacrotuberous ligament. It transmits the tendon of the Obturator internus, its nerve, and the internal pudendal vessels and nerve.

3. Sacrococcygeal Symphysis (symphysis sacrococcygea; articulation of the sacrum and coccyx).—This articulation is an amphiarthrodial joint, formed between the oval surface at the apex of the sacrum, and the base of the coccyx. It is homologous with the joints between the bodies of the vertebræ, and is connected by similar ligaments. They are:

The Anterior Sacrococcygeal.		The Posterior Sacrococcygeal.
The Lateral Sacrococcygeal.		The Interposed Fibrocartilage.
The Interarticular

The Anterior Sacrococcygeal Ligament (ligamentum sacrococcygeum anterius).—This consists of a few irregular fibers, which descend from the anterior surface of the sacrum to the front of the coccyx, blending with the periosteum.

The Posterior Sacrococcygeal Ligament (ligamentum sacrococcygeum posterius).—This is a flat band, which arises from the margin of the lower orifice of the sacral canal, and descends to be inserted into the posterior surface of the coccyx. This ligament completes the lower and back part of the sacral canal, and is divisible into a short deep portion and a longer superficial part. It is in relation, behind, with the Glutæus maximus.

The Lateral Sacrococcygeal Ligament (ligamentum sacrococcygeum laterale; intertransverse ligament).—The lateral sacrococcygeal ligament exists on either side and connects the transverse process of the coccyx to the lower lateral angle of the sacrum; it completes the foramen for the fifth sacral nerve.

A disk of fibrocartilage is interposed between the contiguous surfaces of the sacrum and coccyx; it differs from those between the bodies of the vertebræ in that it is thinner, and its central part is firmer in texture. It is somewhat thicker in front and behind than at the sides. Occasionally the coccyx is freely movable on the sacrum, most notably during pregnancy; in such cases a synovial membrane is present.

The Interarticular Ligaments are thin bands, which unite the cornua of the two bones.

The different segments of the coccyx are connected together by the extension downward of the anterior and posterior sacrococcygeal ligaments, thin annular disks of fibrocartilage being interposed between the segments. In the adult male, all the pieces become ossified together at a comparatively early period; but in the female, this does not commonly occur until a later period of life. At more advanced age the joint between the sacrum and coccyx is obliterated.

Movements.—The movements which take place between the sacrum and coccyx, and between the different pieces of the latter bone, are forward and backward; they are very limited. Their extent increases during pregnancy.

4. The Pubic Symphysis (symphysis ossium pubis; articulation of the pubic bones) (Fig. 321).—The articulation between the pubic bones is an amphiarthrodial joint, formed between the two oval articular surfaces of the bones. The ligaments of this articulation are:

The Anterior Pubic.		The Posterior Pubic.
The Superior Pubic.		The Arcuate Pubic.
The Interpubic Fibrocartilaginous Lamina.

The Anterior Pubic Ligament (Fig. 319).—The anterior pubic ligament consists of several superimposed layers, which pass across the front of the articulation. The superficial fibers pass obliquely from one bone to the other, decussating and forming an interlacement with the fibers of the aponeuroses of the Obliqui externi and the medial tendons of origin of the Recti abdominis. The deep fibers pass transversely across the symphysis, and are blended with the fibrocartilaginous lamina.

The Posterior Pubic Ligament.—The posterior pubic ligament consists of a few thin, scattered fibers, which unite the two pubic bones posteriorly.

The Superior Pubic Ligament (ligamentum pubicum superius).—The superior pubic ligament connects together the two pubic bones superiorly, extending laterally as far as the pubic tubercles.

The Arcuate Pubic Ligament (ligamentum arcuatum pubis; inferior pubic or subpubic ligament).—The arcuate pubic ligament is a thick, triangular arch of ligamentous fibers, connecting together the two pubic bones below, and forming the upper boundary of the pubic arch. Above, it is blended with the interpubic fibrocartilaginous lamina; laterally, it is attached to the inferior rami of the pubic bones; below, it is free, and is separated from the fascia of the urogenital diaphragm by an opening through which the deep dorsal vein of the penis passes into the pelvis.

The Interpubic Fibrocartilaginous Lamina (lamina fibrocartilaginea interpubica; interpubic disk).—The interpubic fibrocartilaginous lamina connects the opposed surfaces of the pubic bones. Each of these surfaces is covered by a thin layer of hyaline cartilage firmly joined to the bone by a series of nipple-like processes which accurately fit into corresponding depressions on the osseous surfaces. These opposed cartilaginous surfaces are connected together by an intermediate lamina of fibrocartilage which varies in thickness in different subjects. It often contains a cavity in its interior, probably formed by the softening and absorption of the fibrocartilage, since it rarely appears before the tenth year of life and is not lined by synovial membrane. This cavity is larger in the female than in the male, but it is very doubtful whether it enlarges, as was formerly supposed, during pregnancy. It is most frequently limited to the upper and back part of the joint; it occasionally reaches to the front, and may extend the entire length of the cartilage. It may be easily demonstrated when present by making a coronal section of the symphysis pubis near its posterior surface (Fig. 321).

FIG. 321– Symphysis pubis exposed by a coronal section. (See enlarged image)

Mechanism of the Pelvis.—The pelvic girdle supports and protects the contained viscera and affords surfaces for the attachments of the trunk and lower limb muscles. Its most important mechanical function, however, is to transmit the weight of the trunk and upper limbs to the lower extremities.

It may be divided into two arches by a vertical plane passing through the acetabular cavities; the posterior of these arches is the one chiefly concerned in the function of transmitting the weight. Its essential parts are the upper three sacral vertebræ and two strong pillars of bone running from the sacroiliac articulations to the acetabular cavities. For the reception and diffusion of the weight each acetabular cavity is strengthened by two additional bars running toward the pubis and ischium. In order to lessen concussion in rapid changes of distribution of the weight, joints (sacroiliac articulations) are interposed between the sacrum and the iliac bones; an accessory joint (pubic symphysis) exists in the middle of the anterior arch. The sacrum forms the summit of the posterior arch; the weight transmitted falls on it at the lumbosacral articulation and, theoretically, has a component in each of two directions. One component of the force is expended in driving the sacrum downward and backward between the iliac bones, while the other thrusts the upper end of the sacrum downward and forward toward the pelvic cavity.

The movements of the sacrum are regulated by its form. Viewed as a whole, it presents the shape of a wedge with its base upward and forward. The first component of the force is therefore acting against the resistance of the wedge, and its tendency to separate the iliac bones is resisted by the sacroiliac and iliolumbar ligaments and by the ligaments of the pubic symphysis.

FIG. 322– Coronal section of anterior sacral segment. (See enlarged image)

FIG. 323– Coronal section of middle sacra segment. (See enlarged image)

If a series of coronal sections of the sacroiliac joints be made, it will be found possible to divide the articular portion of the sacrum into three segments: anterior, middle, and posterior. In the anterior segment (Fig. 322), which involves the first sacral vertebra, the articular surfaces show slight sinuosities and are almost parallel to one another; the distance between their dorsal margins is, however, slightly greater than that between their ventral margins. This segment therefore presents a slight wedge shape with the truncated apex downward. The middle segment (Fig. 323) is a narrow band across the centers of the articulations. Its dorsal width is distinctly greater than its ventral, so that the segment is more definitely wedge-shaped, the truncated apex being again directed downward. Each articular surface presents in the center a marked concavity from above downward, and into this a corresponding convexity of the iliac articular surface fits, forming an interlocking mechanism. In the posterior segment (Fig. 324) the ventral width is greater than the dorsal, so that the wedge form is the reverse of those of the other segments—i. e., the truncated apex is directed upward. The articular surfaces are only slightly concave.

FIG. 324– Coronal section of posterior sacral segment. (See enlarged image)

Dislocation downward and forward of the sacrum by the second component of the force applied to it is prevented therefore by the middle segment, which interposes the resistance of its wedge shape and that of the interlocking mechanism on its surfaces; a rotatory movement, however, is produced by which the anterior segment is tilted downward and the posterior upward; the axis of this rotation passes through the dorsal part of the middle segment. The movement of the anterior segment is slightly limited by its wedge form, but chiefly by the posterior and interosseous sacroiliac ligaments; that of the posterior segment is checked to a slight extent by its wedge form, but the chief limiting factors are the sacrotuberous and sacrospinous ligaments. In all these movements the effect of the sacroiliac and iliolumbar ligaments and the ligaments of the symphysis pubis in resisting the separation of the iliac bones must be recognized.

During pregnancy the pelvic joints and ligaments are relaxed, and capable therefore of more extensive movements. When the fetus is being expelled the force is applied to the front of the sacrum. Upward dislocation is again prevented by the interlocking mechanism of the middle segment. As the fetal head passes the anterior segment the latter is carried upward, enlarging the antero-posterior diameter of the pelvic inlet; when the head reaches the posterior segment this also is pressed upward against the resistance of its wedge, the movement only being possible by the laxity of the joints and the stretching of the sacrotuberous and sacrospinous ligaments.


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