HBF-III LEC 05 Gross Anatomy Overview Pelvic Cavity Notes 2025 PDF

Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 1 of 16 Dr. Dennis Goebel LEARNING OBJECTIVES 1. Summarize the bones that form the pelvis. 2. Describe the anatomical features of the bony pelvis: linea terminalis, pelvic inlet/outlet and pelvic conjugat...

Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 1 of 16 Dr. Dennis Goebel LEARNING OBJECTIVES 1. Summarize the bones that form the pelvis. 2. Describe the anatomical features of the bony pelvis: linea terminalis, pelvic inlet/outlet and pelvic conjugates, orientation in the anatomical position, gender differences, ligaments and membranes, and muscles of the pelvic wall. 3. Describe the pelvic diaphragm: composition, pelvic diaphragm muscles, and relationship to the urogenital diaphragm. SESSION OUTLINE I. The Bony pelvis A. Pelvic bone 1. Ilium 2. Ischium 3. Pubis B. Sacrum and coccyx II. Features of the bony pelvis A. Linea terminalis 1. Regional bony features that define the lineal terminalis B. Pelvic inlet, outlet and defining pelvic conjugates 1. Pelvic inlet 2. Pelvic outlet 3. Gynecological conjugates C. Orientation of the pelvis D. Gender differences in the bony pelvis 1. Female bony pelvis 2. Male bony pelvis Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 2 of 16 Dr. Dennis Goebel E. Ligaments and membranes of the bony pelvis 1. Iliolumbar, anterior and posterior sacroiliac ligaments 2. Superior and inferior pubic ligaments 3. Obturator membrane 4. Sacrospinous and sacrotuberous ligaments 5. Defining the greater and lesser sciatic foramen F. Muscles of the pelvic wall 1. Obturator internus muscle a. Specialized fascial thickenings of the obturator internus muscle i. Tendinous arch a. Attachments of the tendinous arch b. Defining boundary between the pelvis and the perineum 2. The piriformis muscle III. The pelvic diaphragm A. Composition of the pelvic diaphragm 1. Relationship of the pelvic diaphragm to the anococcygeal ligament 2. Formation of the median raphe 3. Formation of the urogenital hiatus 4. Defines the boundary between the true pelvis and the perineum B. Pelvic diaphragm muscles 1. Levator ani a. Puborectalis muscles i. attachments ii. Defines the anorectal flexure iii. Function b. Pubococcygeus muscles i. Anteriorly the pair forms the urogenital hiatus ii. Posterior to the anal canal forms part of the medial raphe c. Iliococcygeus muscles 2. Coccygeus muscles 3. Innervation of the pelvic diaphragm C. Relationship of the pelvic diaphragm with the urogenital (UG) diaphragm Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 3 of 16 Dr. Dennis Goebel I. The Bony pelvis is defined by four sets of bones: pelvic bone (2), sacrum and coccyx. A. The Pelvic bone (also known as the innominate bone, Os coxae, or hip bone) is comprised of 3 fused bones, the ilium, Ischium and pubis. The fusion of these three bones occurs in the region of the acetabulum (socket for the head of the femur). For descriptive reasons (you’ll see these terms again) you will be responsible for the identification of the following features for each bone. 1. Ilium: (See Figure 1) a. Body b. Iliac crest c. Anterior superior and anterior inferior iliac spines d. Posterior superior and posterior inferior iliac spines e. Arcuate line (part of Linea terminalis, see below) f. Articular surface of the Ilium with the sacrum (not labled in figure) g. Iliac fossa h. Acetabulum i. Greater sciatic notch 2. Ischium: (See Figure 1) a. Body b. Ischial tuberosity c. Ischial spine i. Greater sciatic notch (superior to the spine; ilium) ii. Lesser sciatic notch (inferior to the spine; ischium) d. Ramus of ischium (makes up half of the ischiopubic rami) e. Acetabulum f. Obturator foramen (forms half) Figure 1: Gray’s 5.21 Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 4 of 16 Dr. Dennis Goebel 3. Pubis: (See Figure 1 on previous page) a. Body of the pubis b. Pubic symphysis c. Pubic crest (part of Linea terminalis, see below) d. Pectineal line (part of Linea terminalis, see below) e. Pubic tubercle f. Inferior pubic ramus (makes up half of the ischiopubic rami) g. Superior pubic ramus i. Acetabulum j. Obturator foramen (forms half) B. Sacrum and Coccyx : These were covered in the Musculo-skeletal unit, however, please review the following structures of the sacrum: the articular surface, ala, promontory (part of Linea terminalis, see below), the margin of ala (part of Linea terminalis, see below) and the anterior and posterior sacral foramen. II. Features of the Bony Pelvis A. Linea terminalis (Terminal line) is a sharp marginal line that defines the boundary of the “true pelvis” (below this line) from the “false pelvis” (above this line). See Figure 3 on next page. The linea terminalis also defines the “pelvic inlet”. 1. Regional bony features that define the linea terminalis.(listed anterior to posterior) are: the pubic crest (pubis), pectineal line (pubis), arcuate line (ilium), margin of ala (sacrum), and the promontory (sacrum). See Figure 2. Figure 2: Gray’s 5.28 Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 5 of 16 Dr. Dennis Goebel Figure 3: Moore 3.1 B. Pelvic inlet/outlet and defining pelvic conjugates: 1. The pelvic inlet is defined by the linea terminalis (Figure 2), and marks the transition from the abdominal cavity (defined as the false pelvis superior to it), and the true pelvis below). See Figure 2 on previous page and Figure 3 above. 2. The pelvic outlet: is defined by the circular dimension measured from the inferior aspect of the pubic symphysis and the tip of the coccyx. This region is bordered by the coccyx, medial boundary of the sacrotuberous ligament, ischial tuberosity, ischiopubic rami and pubic symphysis (see Figure 4 on the next page). Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 6 of 16 Dr. Dennis Goebel Figure 4: Gray’s 5.32 3. Gynecological conjugates are used to determining the dimensions of the birth canal prior to delivery. There are several methods to determine this (which you will learn in the clinics), but all measure the anterior/posterior dimensions for the pelvic inlet and outlet. The pelvic inlet is calculated by measuring from the superior surface of the pubic symphysis to the sacral promontory. The pelvic outlet is determined by measuring from the inferior aspect of the pubic symphysis to the tip of the coccyx (Figure 5). Figure 5: N332d Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 7 of 16 Dr. Dennis Goebel C. Orientation of pelvis: The pelvis is orientated such that the anterior-superior iliac spines lie in the same vertical plane as the pubic tubercles (Figure 6). Figure 6: Gray’s 5.26 D. Gender differences in the bony pelvis: Although there are many subtle differences between the male and female pelvis, the following are consistent for each gender. 1. The female bony pelvis: (See Figure 7A) a. The pelvic inlet and outlet are wider and with the shape of the inlet usually circular/oval (classified as gynocoid). c. The subpubic angle is larger in the female (apx. 80-85 degrees) b. The sacral ala is wider and the promontory is not as pronounced as it is in the male. c. The orientation of the neck of the femur is ~15-20 degrees anterior to the alignment in the male. This, and the widening of the female pelvic-inlet and -outlet, moves the lower limbs away from the birth canal. 2. The male bony pelvis: (See Figure 7B) a. The pelvic inlet is narrower than the female and usually heart-shaped (classified as Android). b. The subpubic angle is smaller in males (50-60 degrees). c. The orientation of the neck of the femur is positioned~15-20 degrees posterior to the female. Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 8 of 16 Dr. Dennis Goebel d. The male pelvic bone is much denser and taller than the female. e. The male pelvic bones display a more pronounced sacral promontory and ischial spine than the female. E. Ligaments and membranes of the bony pelvis: 1. The iliolumbar, anterior & posterior sacroiliac ligaments have been previously described in the musculoskeletal unit. Please review. 2. Superior and inferior pubic ligaments are two very important ligaments that maintain the integrity of the right and left pelvic bones (Figure 8). Figure 8: N330 Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 9 of 16 Dr. Dennis Goebel 3. Obturator membrane: Occupying most of the obturator foramen is a tendinous membrane called the obturator membrane. The obturator membrane does not fully enclose the foramen and has a small opening at its superior margin, which accommodates the passage of the obturator nerve, artery and vein from the pelvis into the adductor compartment of the thigh (Figure 9). 4. The sacrotuberous and sacrospinous ligaments. a. The sacrotuberous ligament attaches to the posterior aspect of the sacrum and inserts onto the ischial tuberosity (Figure 9). Recall from the MSK unit that Its posterior surface is firmly attached to the inferior aspect (free edge) of the anterior surface of the gluteus maximus. b. The sacrospinous ligament is positioned anteriorly to the sacrotuberous ligament and attaches from the ischial spine to the lower lateral margin of the sacrum. Note also, that the sacrospinous ligament divides the greater sciatic foramen (superior to sacrospinous ligament) from the lesser sciatic foramen (inferior to sacrospinous ligament (Figure 9). 5. Defining the greater and lesser sciatic foramen a. Greater sciatic foramen: The greater sciatic foramen provides communication between the true pelvis and gluteal region (see Figures 9 &10). This foramen accommodates the outflow of blood vessels (superior/inferior gluteal, internal pudendal artery & vein), nerves (sciatic, pudendal, superior/inferior gluteal nerves, the nerve to the obturator internus, quadratus femoris, posterior femoral cutaneous and perforating cutaneous N.), and the exit of the piriformis muscle from the pelvis into the gluteal region (see Figure 10 on the next page). Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 10 of 16 Dr. Dennis Goebel b. Lesser sciatic foramen: The lesser sciatic foramen (see Figure 9 on the previous page and Figure 10 below) provides communication between the gluteal region and the perineum. This foramen accommodates the passage of the pudendal nerve, internal pudendal vessels and the nerve to the obturator internus (more on this later). The only structure from the pelvis that exits out through the lesser sciatic foramen is the tendon of obturator internus muscle (Figure 10). Figure 10: Gray’s 5.30 F. Muscles of the pelvic wall: There are two pairs of muscles (rt & left) that line the wall of the pelvis. They are the obturator internus and the piriformis muscles (Figure 10). 1. The obturator internus is a broad muscle that firmly attaches to the internal surface of the obturator membrane and to the superior pubic and inferior ischiopubic rami. Its tendon exits the perineum by making a 90-degree bend, around the lesser sciatic notch, and then passes out through the lesser sciatic foramen into the gluteal region. The tendon inserts on the greater trochanter of the femur (Figure 10). a. The obturator internus muscle gives rise to two specialized fascial thickenings: the tendinous arch and the fascia forming the pudendal canal (the latter, to be described in a separate session on the perineum). i. The tendinous arch is a thickened caliginous structure that is derived from the obturator internus fascia. Its function is to suspend part of the levator ani muscles off the pelvic wall to form the pelvic diaphragm (more on this in section III below). Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 11 of 16 Dr. Dennis Goebel a. The tendinous arch attaches to the superior pubic rami (just lateral to the pubic tubercle) and extends, in an arc, on the surface of the obturator internus muscle, to the ischial spine (see Figure 11on the next page). b. The tendinous arch bisects the obturator internus muscle, and with its attachment to the pelvic diaphragm (See section III below), forms the division between the true pelvis above, and the perineum below (see Figure 10 on previous page). Therefore, the obturator internus muscle can be seen in both the true pelvis (superior to the tendinous arch, see Figure 11 and 12, on the next page) and in the perineum (inferior to the tendinous arch). See Figure 10 on the previous page (blue dotted line), and Figure 14 on page 12/14 of these notes. 2. The piriformis muscle has its origin on the pelvic (anterior) surface of the sacrum. It exits out the greater sciatic foramen and attaches to the greater trochanter of the femur. Both the piriformis and the obturator internus muscles laterally rotate the femur. The piriformis is innervated by a small branch from the ventral lumbar-sacral plexus (see Figures 11&12 on the next page). III. The pelvic Diaphragm A. The Pelvic diaphragm is comprised of two paired muscle groups that form the majority of the floor of the true pelvis and the roof of the perineum below. Each half is formed by the levator ani (made up of three distinct muscles: see section B below) and the coccygeus muscle (see Figure 11 on the next page). 1. The rt & left halves of the pelvic diaphragm are fused (at midline) posterior to the anal canal, with each half attaching to the anococcygeal ligament, a central ligament that suspends the anal canal to the tip of the coccyx (see Figure 14 on page 12 of these notes). 2. The fusion of the two halves of the pelvic diaphragm forms the median raphe (see Figure 14 on page 12). 3. Anterior to the anal canal, the pelvic diaphragm splits, with each side sweeping laterally to allow midline passage of the urethra in the male, and the urethra and vagina in the female. This opening is called the urogenital hiatus (see Figure 12 on next page). 4. As mentioned above, the pelvic diaphragm, defines the boundary between the region of the true pelvis (located superior to the diaphragm) and the perineum (which is located inferior to the diaphragm). With the aid of the urogenital diaphragm (shown in Figures 16 & 17), these two diaphragms form the inferior physical boundary of the peritoneal cavity and prevent abdominal and pelvic viscera from escaping into the perineum. Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 12 of 16 Dr. Dennis Goebel Figure 11:Gray’s 5.34 Figure 12: N338 Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 13 of 16 Dr. Dennis Goebel B. Pelvic Diaphragm Muscles: 1. The levator ani (see Figures 11 &12 on previous page) is comprised of three separate muscles (per side) that have their origin from the internal surface of the body and superior rami of the pubis and by the tendinous arch. They are: puborectalis, pubococcygeus and the iliococcygeus muscles (Figure 11). a. The puborectalis (paired), (can be seen best from the perineum) consist Figure 13: of a thin strap muscle, positioned mostly inferior to the pubococcygeus muscle (see Figures 14 & 15 on the next page). i. It attaches to the pubis (just lateral to the pubic symphysis) and wraps around the anal-rectal junction posteriorly, where it fuses with its partner to form a sling (see Figures 13 and Figure 11 on the previous page). ii. Its function is important in maintaining the anorectal flexure at the anorectal junction (Figure 13). iii. The puborectalis muscle is normally in a contracted state, which places an 80 degree bend in the alignment of the rectum with the anal canal (Figure 13). This flexure maintains fecal continence. b. The pubococcygeus (paired) are suspended off of body of the pubis and the anterior half of the tendinous arch. i. Posterior to the anal canal, the right and left pubococcygeus muscles fuse at midline to form part of the median raphe, which attaches to superior surface of the anococcygeal ligament (see Figure 14 on the next page). c. The Iliococcygeus (paired) have their origin from the posterior half of the tendinous arch and the ischial spine. They insert onto the superior surfaces of the anococcygeal ligament and tip of the coccyx (Figures 12 on the previous page, & Figures 14 & 15 on the next page). 2. Coccygeus muscle (paired, Figure 12 on previous page), is a thin triangular muscle that has its origin from the ischial spine and attaches to the coccyx and to lateral margin of the sacrum at the level of SV5. It attaches onto the superior surface of the sacrospinous ligament. The two coccygeus muscles together with the rt & left levator ani muscles, form the pelvic diaphragm. 3. Innervation of pelvic diaphragm (levator ani and coccygeus) is provided by ventral sacral rami (located on the floor of the pelvis) and from the inferior rectal N. (a branch of the pudendal nerve). Details will be forthcoming in a subsequent presentation. Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 14 of 16 Dr. Dennis Goebel Median Anococcygeal lig. raphe (cut) Figure 14:N337b Figure 15:N338a (modified) Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 15 of 16 Dr. Dennis Goebel C. Relationship of Pelvic Diaphragm with the Urogenital (UG) Diaphragm: The UG- diaphragm (will be described in detail in a separate presentation to follow) is a tendinous membrane that occupies the space between the two ischiopubic rami (IPR) up to the ischial tuberosities. It forms a physical boundary to close off the opening made by the urogenital hiatus of the levator ani (see Figure 14 on previous page and Figure 16). In addition, it accommodates the passage of the male and female urethra and on its inferior surface, and serves as the foundation for the attachment of the male and female external genitalia (more on this in a in a subsequent presentation). The fascia of the levator ani fuses with the superior fascia of the UG diaphragm (on its superior surface) along the margin of the urogenital hiatus to prevent abdominal and pelvic cavity contents from gaining access to the perineum (Figure 16). Lateral to the fusion of the fascia (inferior to the levator ani, and superior to the UG diaphragm), is a potential space called the anterior recess (of the ischioanal fossa) (see Figure 17 on last page of these notes). The anterior recess is occupied by a portion of the ischioanal fat pad. Probing the anterior recess, the levator ani will be positioned medially, the UG diaphragm inferiorly and the obturator internus muscle and ischiopubic rami are located laterally (see Figure 17). * The take home message here is that, the pelvic diaphragm (e.g., levator ani muscles) and the UG diaphragm are two separate structures, whereby the pelvic diaphragm is positioned superior to the UG diaphragm (see Figures 16 & 17). Figure 16: N336b Gross Anatomy: Bony Pelvis & Pelvic Diaphragm Page 16 of 16 Dr. Dennis Goebel Figure 17: DJG Sources for Figures in the notes: Drake, Vogl and Mitchell, Gray's Anatomy for Students, 3rd Ed., Churchill & Livingstone, Philadelphia, © 2014. Moore, Clinically Oriented Anatomy, 3rd Edition, Williams & Wilkens, Baltimore, © 1992. Netter, Atlas of Human anatomy, 6th Ed., Saunders, Philadelphia, © 2014.