Pelvic Girdle

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36
Pelvic Girdle


The pelvic girdle is an attachment point for the vertebral column and lower limbs (Fig. 36.1). It is also involved in balance and weight transfer by transmitting the weight of the head and neck, trunk, and upper limbs to both lower limbs.


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FIGURE 36.1 Pelvis—anterior view.


The left and right sides of the pelvic girdle are identical and are composed of two hip bones, which are formed by fusion of the ilium, ischium, and pubis.



The sacrum is in the midline of the posterior pelvic girdle. This triangular bone has multiple foramina for the sacral nerves to pass through. It unites the posterior pelvic girdle via the sacro-iliac joints.


At the inferior margin of the sacrum is the coccyx, which is made up of small, usually fused bones.


The sacrococcygeal joint is a cartilaginous joint between the sacrum and coccyx. It is heavily reinforced by ligaments on its anterior, lateral, and posterior surfaces. The sacrum and coccyx stabilize the pelvis and provide attachment points for the ligaments and muscles of the pelvis, lower part of the back (see Chapter 28), and thigh (see Chapters 40 and 42).


On the lateral surface of the pelvic girdle is the acetabulum—a cup-shaped depression where the head of the femur articulates with the pelvic girdle.


Two major ligaments join the sacrum to the ilium and ischium:



Neurovascular structures from within the pelvic girdle pass through the greater and lesser sciatic foramina to the gluteal region, posterior compartment of the thigh, and perineum:



The obturator foramen is inferior to the acetabulum (see Fig. 36.1), and most of it is closed by a membrane of flat connective tissue, the obturator membrane.


Muscles


The pelvic girdle is lined by muscles that support the trunk and move the lower limbs. These muscles are described in Chapters 28, 35, 40, and 42. The pelvic girdle does not have any mobile joints, so there are no intrinsic muscles (muscles that move pelvic bones). However, several muscles span different parts of the pelvis and support the pelvic viscera and perineum (see Chapter 38).


The muscles of the pelvis are described according to their location. The obturator internus muscle covers part of the lateral pelvic wall. It passes from the internal surface of the obturator membrane of the pelvis, and its fibers converge to form a tendon that leaves the pelvis laterally through the lesser sciatic foramen, inserts onto the greater trochanter of the femur, and assists in lateral rotation of the thigh.


The posterior pelvic wall is partly covered by the piriformis muscle. This muscle originates from the anterior surface of the sacrum, extends laterally through the greater sciatic foramen to insert onto the greater trochanter of the femur, and rotates the femur laterally. The nerves of the sacral plexus are medial to the origin of the piriformis.


The pelvic diaphragm (pelvic floor) is formed from the levator ani and coccygeus muscles (see Chapter 38).


Nerves


The nerve supply to the pelvis is provided by voluntary (somatic) and involuntary (autonomic) nerves (Fig. 36.2). The somatic nerves are branches of the sacral plexus of nerves, which is formed by joining of the L4 and L5 lumbar spinal nerves (lumbosacral trunk) and the sacral spinal nerves S1 to S4. Nerve fibers from this plexus intertwine in various combinations to form the following nerves:



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FIGURE 36.2 Nerves of the pelvis—anterior view.


Several other nerves that originate from the sacral plexus do not leave the pelvis. These nerves are the nerve to the piriformis muscle, sacral branches to the levator ani muscles, coccygeal nerve, and inferior anal nerves.


The pelvic splanchnic nerves

Jun 11, 2016 | Posted by in ANATOMY | Comments Off on Pelvic Girdle
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