Pelvis and Foetal Skull PDF

# Gynaecoid Pelvis ## Bones of the Pelvis and Position - **Innominate Bone:** - Consists of 3 bones that fuse in early adulthood: - **Ilium:** - Upper portion shaped like a fan. - **External Surface:** Curved, roughened, gluteal muscles attached. - *...

# Gynaecoid Pelvis ## Bones of the Pelvis and Position - **Innominate Bone:** - Consists of 3 bones that fuse in early adulthood: - **Ilium:** - Upper portion shaped like a fan. - **External Surface:** Curved, roughened, gluteal muscles attached. - **Inner Surface:** Concave, smooth, forms the iliac fossa. - **Superior Ridge:** Iliac Crest. - **Spines:** - Anterior Superior Iliac Spine - Posterior Superior Iliac Spine - Anterior Inferior Iliac Spine - Posterior Inferior Iliac Spine - Forms the upper 2/5 of the acetabulum. - Forms the upper part of the greater sciatic notch. - Inner aspect is the ilio-pectineal line which fuses with the pubis at the ilio-pectineal eminence. - **Ischium:** - Lower posterior portion, sickle shaped. - Forms the lowest 2/5 of the acetabulum. - Forms the side wall of the true pelvis and the lower boundary of the obturator foramen. - Becomes continuous with the inferior ramus of the pubis. - **Ischial Tuberosity:** Lateral bony prominence below the acetabulum. - **Ischial Spine:** Protrudes inwards, separating the greater sciatic notch from the lesser sciatic notch. - **Pubis:** - Anterior portion, flat and square shaped body with superior and inferior rami. - Articulates with its fellow at the symphysis pubis. - Fuses with the ilium at the ilio-pectineal eminence. - Inferior ramus joins the ischium forming the pubic arch. - Upper surface is the pubic crest and the pubic tubercle. - Laterally the superior ramus forms the medial 1/5 of the acetabulum. - Forms the upper boundary of the obturator foramen. - **Sacrum:** - Dense triangular shaped bone with a central canal. - Composed of 5 fused sacral vertebrae. - **Inner Surface:** Smooth and concave, forming the hollow of the sacrum. - **4 Transverse Ridges:** Where vertebral bodies have fused. - **Sacral Promontory:** Body of the first sacral vertebra which projects anteriorly. - **Outer/Posterior Surface:** Rough and convex, for attachment of muscles and ligaments. - **Median Sacral Crest:** Formed by fused spinous processes. - **Sacral Hiatus:** Gap formed because the 5th spinous process is not fused, covered by dura mater. - **Sacral Cornua:** Lie on either side. - **4 Pairs of Foramina:** Formed from fused intervertebral notches for passage of sacral nerves. - **Sacral Alae (Wings):** Formed from fused transverse processes. - **Sacral Canal:** Central canal runs longitudinally, ends at the sacral hiatus lined by the meninges. - Contains the cauda equina and the cerebrospinal fluid. - **Articulates Superiorly:** 5th lumbar vertebra (sacro-lumbar joint) - **Articulates Inferiorly:** Coccyx (sacro-coccygeal joint) - **Articulates Laterally:** Ilium (sacro-iliac joint) - **Coccyx:** - Small triangular shaped bone. - Composed of 4 fused coccygeal vertebrae. - **Inner Surface:** Smooth. - **Outer Surface:** Rough, attachment for muscles and ligaments. - **Articulates Superiorly:** Sacrum. - Moves posteriorly in labour, increasing pelvic outlet diameter under the effect of relaxin. ## Joints of the Pelvis - **Right and Left Sacro-Iliac Joints:** Synovial joint between sacrum and ilium. - Slight movements, increased under the influence of relaxin. - **Symphysis Pubis:** Thick pad of cartilage between pubic bones. - **Sacro-Coccygeal:** Cartilage between sacrum and coccyx. - **Lumbo-Sacral:** Between 5th lumbar vertebra and sacrum. ## Ligaments of the Pelvis - **Anterior, Posterior, Interosseous Sacro-Iliac:** Span articular surfaces of sacroiliac joints. - **Sacro-Tuberous:** From posterior superior iliac spines and lateral borders of sacrum and coccyx to ischial tuberosities. - **Sacro-Spinous:** Sides of sacrum and coccyx to ischial spines. - **Anterior, Posterior, Superior, Inferior Pubic:** Strengthen disc of cartilage of the symphysis pubis. - **Sacro-Coccygeal Ligaments:** Small, allow considerable mobility in late pregnancy. - **Inguinal (Poupart's):** From anterior superior iliac spine to pubic tubercle. - **Obturator Membrane:** Fibrous layer attached to the edges of the obturator foramen, except anteriorly where vessels and nerves pass. ## The Pelvis as a Whole - It is important to understand the pelvis as a whole and its regions. - This enables one to understand how the pelvis facilitates the passage of the foetal head during labour. ## Divisions of the Pelvis - **False Pelvis:** Lies above the brim. - **Posterior:** Lumbar Vertebrae. - **Lateral:** Iliac fossae. - **Anterior:** Anterior abdominal wall. - Not important in obstetrics. - **True Pelvis:** Lies below the brim. - Serves to support the enlarged uterus during pregnancy. - Forms the bony canal through which the foetus must pass. - **Divided into:** - Pelvic Inlet/Brim - Pelvic Cavity - Pelvic Outlet ## Functions of the Pelvis - **Girdle for Attachment of Lower Limbs:** Transmits weight to legs. - **Forms Part of Wall of Abdominal Cavity.** - **Contains and Protects Pelvic Viscera.** - **Extensive Attachment to Muscles.** - **Supports Trunk in Sitting Position.** - **Bony Canal for Passage of Foetus.** ## Planes and Measurements of a Gynaecoid Pelvis - **Pelvic Planes:** Imaginary flat surfaces passing across the pelvic canal at different levels. - **Pelvic Inclination:** When standing upright, the female pelvis is not at right angles to the spine. - The plane of the brim, cavity and outlet are not horizontal, but slope at a different angle to the horizontal: - **Pelvic Brim:** 60° to the horizontal. - **Pelvic Cavity:** 30°. - **Pelvic Outlet:** 11°. - **Note:** - Pelvic inclination must be taken into account when trying to determine whether the foeal head will enter the pelvic brim (engagement). - It may be increased in some African women causing delay in the foetal head entering the pelvic brim. - Because the plane of the pelvic outlet is at a lesser angle than the plane of the pelvic brim, the foetus turns at an acute angle as it passes through the pelvic canal. - **Axis of the Pelvic Canal:** An imaginary line drawn at right angles through the centre of each plane. - This represents the anatomical axis of the pelvic canal (the curve of carus). - Theoretically, this is the path followed by the centre of the foetal head as it descends through the canal during labour. - **Note:** - In manual manipulation, the hand must follow the direction of the axis. - When traction is applied with forceps or vacuum, the direction of traction must follow the axis. ## Pelvic Inlet/Brim - **Shape:** Circular/oval, ± 40cm in circumference. - **Boundaries:** Sacral promontory, sacral alae, sacro-iliac joint, ilio-pectineal line, ilio-pectineal eminence, superior ramus of pubis, symphysis pubis. ### Measurements - **Anterior-Posterior Diamerers:** - **Anatomical/True Conjugate:** From centre of sacral promontory to summit of symphysis pubis = 12cms. - **Obstetrical Conjugate:** (Assessed by X-ray) From centre of sacral promontory to 1.25cms down on the posterior surface of symphysis pubis = 11cms. - **Diagonal Conjugate:** (on vaginal examination) From lower border of symphysis pubis to centre of sacral promontory = 12.5cms (minus 1.5cms for pubic bone = obstetrical conjugate of 11cms) - **Oblique Diameter:** From sacro-iliac joint to ilio-pectineal eminence on the opposite side = 12cms. - **Right Oblique Diameter:** From right sacro-iliac joint. - **Left Oblique Diameter:** From left sacro-iliac joint. - **Transverse Diameter:** Between the widest points of the ilio-pectineal line, posterior to the ilio-pectineal eminences = 13cms. - **Sacro-Cotyloid Diameter:** From centre of sacral promontory to ilio-pectineal eminence = 9.5cms. - **Right Sacro-Cotyloid Diameter:** To right ilio-pectineal eminence. - **Left Sacro-Cotyloid Diameter:** To left ilio-pectineal eminence. ## Pelvic Cavity - **Shape:** Circular, extends from brim to outlet in a curve. - **Boundaries:** - **Posterior Wall:** Hollow of sacrum = 12cms. - **Anterior Wall:** Posterior surface of pubis = 4cms. - **Lateral Walls:** Greater sciatic notch, ilium, body of ischium, obturator foramen. ### Measurements - **Anterior Posterior Diameter:** From midway down sacral hollow to midway down inner surface of symphysis pubis = 12cms. - **Oblique Diameter:** From greater sciatic notch to the obturator foramen = 12cms. - **Transverse Diameters:** - **Between the widest points in the lateral walls:** = 12cms. - **Between the ischial spines:** = 10cms. ## Pelvic Outlet - **Shape:** Diamond-shaped. - **Boundaries:** Tip of coccyx, sacro-tuberous ligament, ischial tuberosity, pubic arch, lower border of symphysis pubis. - **Note:** This outlet is not all on the same plane and the size varies, depending on the movement of the coccyx (backwards) during labour. ### Measurements of the Outlet - **Anterior-Posterior Diameter:** From sacro-coccygeal joint to lower border of symphysis pubis = 13cms. - **Oblique Diameter:** From sacrotuberous ligament to junction of ischial ramus and inferior pubic ramus = 12cms. - **Transverse Diameter:** Between ischial tuberosities = 11cms (tissue lessens this diameter by at least 1.2cms) ## Pelvic Floor Muscles - **Definition:** - Area of soft tissue that fills the pelvic outlet. - Comprised of muscle and pelvic fascia. - Shaped like a hammock. - Provides strong support for pelvic contents. - Pierced by: - Urethra - Vagina - Anus ### Structures Which Make Up Pelvic Floor - **From Within Out:** - **Pelvic Peritoneum** - **Extra Peritoneal Fat and Connective Tissue:** Fills in spaces between organs & makes up supporting ligaments of uterus and cervix. - **Deep Structures:** Levator ani muscles (elevate the anus). - **Middle Layers** - **Superficial Structures:** 4 pairs of superficial perineal muscles ### Deep Structures - Levator Ani Muscles - 2 powerful muscles lying one on either side of the pelvic cavity. - 3-5 mm thick and made up of 3 parts: #### Pubo-Coccygeus - **Origin:** Posterior surface of public bones, sweeps posterior below the bladder, on either side of the urethra, past the lower third of the vagina, past the anal canal. - **Insertion:** Into the wall of the anal canal and ano-coccygeal body. - As the muscle passes posteriorly muscle fibres criss-cross between the vagina and anal canal to form part of the perineal body. Some fibres form slings or loops around the vagina and anus. #### Ilio-Coccygeus - **Origin:** Fascia of obturator internus muscle. - **Insertion:** Ano-coccygeal body and coccyx. #### Ischio-Coccygeus - **Origin:** From circumference of pelvis, converge towards the midline, have concave upper surface, and convex lower surface. - **Insertion:** - Perineal body. - Anal canal. - Ano-coccygeal body - Coccyx - Lower end of sacrum - **Do not form part of the perineal body.** ### Middle Structures - **Deep Transverse Perineal Muscles** - **Origin:** Inf. Ramus of the ischium. - **Insertion:** Midline into the central tendon ### Superficial Structures - **4 pairs of superficial muscles.** - **Position:** - Lie in the pelvic outlet. - Inferior to the anterior part of levator ani muscle. - Form the superficial ½ of the perineal body. - Surround the anal and vaginal orifices (most liable to injury during delivery). #### External Anal Sphincter - **Muscle:** Which encircles the anal canal. - **Anteriorly:** It enters into the perineal body. - **Posteriorly:** Some fibres are attached to the tip of the coccyx. - **Function:** To occlude the lumen of the anal canal, forming the external anal sphincter #### Superficial Transverse Perineal Muscles - **Origin:** Ischial tuberosities. - **Pass transversely inwards:** And meet in the centre of perineum. - **They consist of a deep and superficial portion.** #### Bulbo-Cavernosus - **Origin:** Central part of perineum, passes anteriorly on either side of vagina. - **Insertion:** Corpora cavernosa of clitoris. - **Function:** - Forms part of the perineal body. - Helps support the lower part of the vagina. #### Ischio Cavernosus - **Origin:** Ischial tuberosities, pass upwards and inwards along the pubic arch. - **Insertion:** Corpora cavernosa of clitoris. - **Function:** Cause engorgement of clitoris. ### Perineal Body - **Fibro-muscular pyramid** - **Situation:** Between lower third vagina anteriorly, anal canal posteriorly, ischial tuberosities laterally. - **Structure:** Deeper half of perineal body. - Made of muscle fibres of the pubo-coccygeal muscle which criss-cross between vagina and anal canal. - Superficial half of the perineal body made up of superficial transverse perineal muscles, bulbocavernosus and the external anal sphincter. - **Function:** Acts as 'shock absorber'. - **Measurement:** 4cms in each direction ## The Triangular Ligament (Uro-genital Membrane) - Fills in space between: - The triangular area between the bulbocavernosus, ischiocavernosus and the transverse perineal muscles. - The deep transverse perineal muscle and the urethral sphincter lie between the layers of triangular ligament. - **Function:** Helps suspend the urethra and vagina in their normal position. ## Urethral Sphincter - Muscle fibres that occlude the lumen. - **Perforations:** Pierced by the urethra, vagina and anal canal. - **Structure:** Embedded into the pelvic floor, above, below and between the pelvic floor muscles, there are layers of pelvic fascia. This is made up of loose areolar tissue which is used like packing material in these spaces. - **The tissue that fills the triangular space between the bulbocavernosus, the ischiocavernosus **and the transverse perineal muscles is known as the triangular ligament.** ## The Fetal Skull - **The fetal skull is made up of the following:** - **Face:** Composed of 14 small bones which are firmly united and non-compressible. - The face is small in the neonate and extends from the orbital ridges and the root of the nose to the junction of the chin and the neck. - **Cranium (Vault):** - **The skull, which is made up of the following bones:** - 2 Frontal - 2 Parietal - 2 Temporal (upper section) - 1 Occipital - **These bones of the cranium are separated by sutures and fontanelles, making the cranium compressible.** - **Base:** - The base is comprised of bones which are firmly united to protect the vital centres in the medulla (brain). The brain rests on the base. - Divided into anterior, middle and posterior fossa. ## Cranial Bones are Developed From Membrane. - At term the bones of the vault are not completely ossified - they are pliable and there are membranous strips between two neighbouring bones (called sutures). - There are larger membranous areas where 3 or more bones meet (called fontanelles). ## Sutures - Cranial joints and are formed where bones adjoin. - **The Lambdoidal Suture:** Separates the occipital bone from the two parietal bones and becomes a fixed joint. - **The Sagittal Suture:** Lies between the two parietal bones and becomes a fixed joint. - **The Coronal Suture:** Separates the frontal bones from the parietal bones, passing from one temple to another and becomes a fixed joint. - **The Frontal Suture:** Runs between the 2 halves of the frontal bone. It becomes obliterated when the 2 bones fuse - usually before the age of 8 years. ## Fontanelles - Formed where 2 or more sutures meet. - **The Posterior or Lambda:** - Situated at the junction of the lambdoidal and sagittal sutures. - Small, triangular in shape and can be recognised vaginally because a suture leaves from each of the 3 angles. - It is normally closed by 6/52. - **The Anterior Fontanelle or Bregma:** - Found at the junction of the sagittal, coronal and frontal sutures. - Broad, kite or diamond shaped and recognisable vaginally because a suture leaves from each of the 4 corners. - It measures 3-4cms long and 1.5-2cms wide and normally closes by the time the child is 18 months old. - Pulsations of cerebral vessels can be felt through it. - **Note:** The sutures and fontanelles, because they consist of membranous spaces, allow for a degree of overlapping of the skull bones during labour and delivery. ## Fetal Skull ### Regions of the Fetal Skull: - **Occiput:** Lies between the foramen magnum and the posterior fontanelle. - Part below the occipital protuberance is the suboccipital region. - The protuberance itself can be seen and felt as a prominent point on the posterior aspect of the skull. - **Vertex:** Bounded by the posterior fontanelle, the 2 parietal eminences and the anterior fontanelle. (95% of babies born head first, present by the vertex). - **Sinciput (brow):** Extends from the anterior fontanelle and the coronal suture to the orbital ridges. - **Face:** Extends from the orbital ridges and the root of the nose to the junction between the chin and the neck. ### Diameters of the Fetal Skull at Term: - **Important due to the relationship between the fetal skull and the mother's pelvis.** - **Some diameters are more favourable for easy passage through the pelvic canal and this will depend on the attitude of the head.** #### Transverse Diameters: - **Biparietal diameter:** 9.5cms - between the 2 parietal eminences. - **Bitemporal diameter:** 8.2cms - between the furthest points of the coronal suture at the temples. #### A-P Diameters: - **Suboccipito-Bregmatic Diameter:** 9.5cms. Measured from below the occipital protruberance to the centre of the anterior fontanelle (bregma) - **Suboccipito-Frontal Diameter:** 10cms. Measured from below the occipite! protuberance to the centre of the frontal suture. - **Occipito-Frontal Diameter:** 11.5cms. - **Mentovertical Diameter:** 13.5cms. From the mentum (point of the chin) to the highest point on the vertex (slightly closer to the posterior fontanelle than to the anterior fontanelle) - **Submentovertical Diameter:** 11.5cms. From the point where the chin joins the neck to the highest point on the vertex. - **Submento-Bregmatic Diameter:** 9.5cms. From the point where the chin joins the neck to the centre of the bregma. ### Circumference of Fetal Skull at Term - Normal size (occipito frontal measurement) = 33 - 37cms. - **< 33cms:** S.G.A. (small for gestational age), preterm or microcephaly. - **> 37cms:** L.G.A. (large for gestational age), post - term or hydrocephaly. ### Obstetric Significance of Cephalic Presentations - **The diameters of the head are those which are at right angles to the curve of carus**. - **There are always 2 - an antero-posterior and a transverse diameter**. ## Moulding of the Fetal Skull - **Moulding** is the term applied to the change in shape of the foetal skull that takes place during its passage through the birth canal. - **This alteration in shape is possible because the bones of the vault, not being well ossified, permit a slight degree of over-riding at the sutures.** - **Moulding is present on every baby's head except elective caesarian section deliveries.** ### During Moulding: - The engaging diameter is compressed and shortened by 1.25cm. - The diameter at right angles to it is elongated. #### Vertex Presentation (L.O.A.) - **Suboccipito-Bregmatic Diameter:** Reduced. - **Mento-Vertical Diameter:** Lengthened. #### Preterm babies: - Moulding can be excessive as soft skull bones and wide sutures afford little protection to the delicate brain substance. #### Post Mature babies: - Sutures are almost closed and head does not mould well. - Hardness of the head rather than the increased size tends to make labour more difficult. ## Vertex Presentation - When the head is well flexed, the suboccipito bregmatic diameter and the biparietal diameter present. - These two diameters are 9.5cms, therefore the presenting area is circular and measures 29cms in circumference. - This is the most favourable shape for dilating the cervix and the diameter that distends the vaginal orifice is the suboccipito frontal diameters - 10cms. - When the head is not well flexed, the presenting diameters are the occipito frontal - 11.5cms and the biparietal - 9.5cms (eg O.P.P.). - The diameter distending the vaginal orifice will be the occipito frontral 11.5cms - Circumference = 31cms. ## Brow Presentation - When the head is partially extended, the mentovertical diameter - 13.5cms and the bitemporal diameter - 8.2cms present. - Vaginal delivery is not possible. - Circumference is 34.5cms - egg shape. ## Face Presentation - When the head is completely extended, the presenting diameters are the (submento bregmatic) 9.5cms and the bitemporal - 8.2cms. - The submento vertical diameter 11.5cms will distend the vaginal orifice.