WebLecture-Pelvis-Lec24

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I. Orientation of Pelvis

II. Osteology

III. Joints

IV. Ligaments

V. Pelvic Musculature

VI. Perineum

WebLecture Slide 24

This is a posterior view of the pelvis including those structures that have just been introduced. The model shown is one rarely seen in dissection, as additional muscles of the hip joint (e.g. gluteal muscles) and soft tissue structures of the pelvis and perineum cloud the view. The relationships of these structures are instructive however, and so we provide this very basic model. Note that both the piriformis and the obturator internus muscles take their origin within the pelvis but exit to find their site of attachment outside of the pelvis. The route they take is important to understanding their biomechanical advantage and their relationship to other structures. The obturator externus muscle performs the same action (laterally rotates the thigh), but originates from the anterior aspect of the pelvis. Note too the sacrospinous and sacrotuberous ligaments joining the sacrum to bony landmarks of the ischium (the ischial spine and ischial tuberosity, respectively). Knowledge of the relationships of the structures in this model lays a solid foundation for the more complex anatomy of the perineum.

The figure also provides a more complete picture of the proximal course of the pudendal nerve. Although the pelvic diaphragm has yet to be described, the course of the pudendal nerve can be appreciated from this posterior view. Roots of the nerve leave the pelvic foramen of sacral vertebrae 2, 3 and 4 on the pelvic surface and join to form the nerve proper. Once formed, the nerve leaves the pelvis via the greater sciatic foramen but after crossing the sacrospinous ligament, the nerve enters the perineum using the lesser sciatic foramen. It is by this route that the pudendal nerve reaches its targets without penetrating the muscular pelvic diaphragm. The nerve gives off three major branches which find their target organs.

Quicktime Movie 24

Note that the sacrotuberous and sacrospinous ligaments are not in place but that the piriformis passes through the greater sciatic (notch) foramen while the obturator internus passes through the lesser sciatic (notch) foramen. Note also the linear course of the piriformis muscle and the turn in the course of the obturator internus muscle.

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