1.1 Anatomy of the pelvic ring

10.1055/b-0035-121620

1.1 Anatomy of the pelvic ring

Marvin Tile, James F Kellam

1 Introduction

This chapter describes the important surgical anatomy necessary to understand injuries to the pelvic ring and their treatment, especially operative treatment. A more detailed account of pelvic anatomy is best found in anatomy texts [1].

The pelvis is a ring structure made up of three bones: the sacrum and the two innominate bones. In turn, the innominate bones are formed by the fusion of the three separate ossification centers from the ilium, the ischium, and the pubis. They meet at the triradiate cartilage, which fuses by the age of 16 years. The three bone components of the pelvis have no inherent stability; if all soft tissues were removed from the pelvis, then it would fall apart ( Fig 1.1-1 ). Yet, in vivo it is able to withstand major forces; therefore, the soft tissues must confer stability to the pelvic girdle while the bone provides structure. Stability and structure to allow weight transference are the essential anatomical features of the pelvis.

2 Structural stability

The pelvic ring is formed by the connection of the sacrum to the innominate bones at the sacroiliac joints and the symphysis pubis. Because the major weight-bearing forces are transmitted from the hip joint through the iliac bone, across the sacroiliac joint into the sacrum and up the spine, it must be assumed that the major stabilizing structures are posterior ( Fig 1.1-2 ). The anterior joint (the symphysis pubis) acts like a strut, preventing collapse of the pelvis, rather than a major weight-bearing, stabilizing structure. Absence of this anterior strut, as in patients with congenital bladder exstrophy ( Fig 1.1-3 ) or trauma victims ( Fig 1.1-4 ), only minimally affects this weight-bearing function. Many mammals possess a posterior bone strut to provide posterior stability ( Fig 1.1-5 ). In humans this function is accomplished by the strong posterior sacroiliac, sacrospinous, and sacrotuberous ligaments.

2.1 Posterior pelvic stability

2.1.1 Sacroiliac joints

The adjacent iliac and sacral surfaces of the sacroiliac joint are divided into two parts: a lower one, the articular surfaces, and an upper one, the tuberosities ( Fig 1.1-6 ). The articular surface of the sacrum is covered with hyaline cartilage, and the adjacent surface of the ilium is covered with fibrocartilage; however, the articulation is not truly a synovial joint. Embryonically, the sacroiliac joints develop not as other synovial joints do, as clefts in a continuous rod of condensed mesenchyme, but by the direct contact of the ilium and sacrum posteriorly. In the upright position the body weight pushes the sacrum down between the wings of the ilium causing approximately 5° of dorsoventral rotation as the innominate bones move backward and the anterior pubic rami swing upward [2]. This motion is markedly restricted by the posterior ligamentous complex, the strongest being the interosseous ligaments [3].

**Fig 1.1-2a–b** The major posterior stabilizing structures are ligaments. a Ligaments from anterior aspect of pelvis. b Ligaments from posterior aspect of pelvis.

**Fig 1.1-3** X-ray of a 39-year-old woman with congenital bladder exstrophy. Note the total lack of anterior stability because of excessive widening of the symphysis pubis. Despite this, the sacroiliac joints have remained intact and show no evidence of instability.

**Fig 1.1-4** AP x-ray of the pelvis of a 21-year-old woman who had previously lost a portion of the superoinferior pubic rami in an open fracture of the pelvic ring at 12 years old. The superior ramus fracture entered the right acetabulum. No posterior instability is noted despite the pelvic ring gap. The patient has no symptoms referable to the sacroiliac joints or the gap anteriorly. She was seen for an unrelated medical condition.

**Fig 1.1-5** The skeleton of an elephant shows the symphysis ischium, a solid bone strut posteriorly.

**Fig 1.1-6a–b** The adjacent surfaces of the sacroiliac joint, indicating the area of hyaline cartilage on the articular surface and fibrocartilage over the iliac and tuberosity.

2.1.2 Interosseous sacroiliac ligaments

The interosseous sacroiliac ligaments, the strongest in the body, unite the tuberosities of the ilium and sacrum, and confer stability on the posterior sacroiliac complex ( Fig 1.1-7 ).

2.1.3 Posterior sacroiliac ligaments

Two distinct bands are described:

The short posterior sacroiliac ligament consists of a number of fibers that pass obliquely from the tubercle or ridge of the sacrum to the posterior superior and posteroinferior spine of the ilium.
The long posterior sacroiliac ligament comprises longitudinal fibers that run from the posterior superior iliac spine to the lateral portion of the sacrum, intermingling with the origin of the sacrotuberous ligament, and covering the short ligament ( Fig 1.1-8 ).

2.1.4 Anterior sacroiliac ligaments

Anterior sacroiliac ligaments are strong, flat bands, comprising transverse and oblique fibers that pass from the anterior surface of the sacrum to the adjacent anterior surface of the ilium ( Fig 1.1-9 ).

2.1.5 Sacrotuberous ligament

The sacrotuberous ligament is an extremely strong, broad band extending from the lateral portion of the entire dorsum of the sacrum and the posterior surfaces of the posterior superior and inferior iliac spines to the ischial tuberosity. In some areas it covers, and in others it is contiguous with, the sacrospinous ligaments. The medial border extends as a falciform border to the ischial tuberosity, where it is continuous with the obturator fascia. Laterally, at its superior origin, it gives attachment to the gluteus maximus. The sacrotuberous ligament forms a portion of the pelvic outlet ( Fig 1.1-10 , Fig 1.1-11 ).

**Fig 1.1-7** Cross-section through the sacroiliac joints shows the direction of the interosseous sacroiliac ligaments.

**Fig 1.1-8** The posterior sacroiliac ligaments are much stronger and hold the posterior ilium and sacrum together.

**Fig 1.1-9** The anterior sacroiliac ligament is a broad, thin structure crossing the anterior aspect of the joint.

**Fig 1.1-10a–b** a Sacrospinous/sacrotuberous exterior ligaments. b Sacrospinous/sacrotuberous intrapelvic ligaments.

2.1.6 Sacrospinous ligament

The sacrospinous ligament is a strong triangular sheet arising from the lateral margin of the sacrum and the coccyx, deep to the sacrotuberous ligament, and passing to the ischial spine. It divides the ischial area into the greater sciatic notch and the lesser sciatic foramen. Its pelvic surface covers and is adherent to the coccygeus muscle ( Fig 1.1-10 ).

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