Fetal skull and maternal pelvis

Fetal skull is to some extent compressible and made mainly of thin pliable tabular (flat) bones forming
the vault. This is anchored to the rigid and incompressible bones at the base of the skull.

1. Fetal Skull and Maternal Pelvis

2. FETAL SKULL Fetal skull is to some extent compressible and made mainly of thin pliable tabular (flat) bones forming the vault. This is anchored to the rigid and incompressible bones at the base of the skull .

3. AREAS OF SKULL: The skull is arbitrarily divided into several zones of obstetrical importance . Vertex : It is a quadrangular area bounded anteriorly by the bregma and coronal sutures behind by the lambda and lambdoid sutures and laterally by lines passing through the parietal eminences . Brow : It is an area bounded on one side by the anterior fontanel and coronal sutures and on the other side by the root of the nose and supraorbital ridges of either side . Face : It is an area bounded on one side by root of the nose and supraorbital ridges and on the other, by the junction of the floor of the mouth with neck.

4. Sinciput is the area lying in front of the anterior fontanel and corresponds to the area of brow and the occiput is limited to the occipital bone. Flat bones of the vault are united together by non-ossified membranes attached to the margins of the bones. These are called sutures and fontanels. Of the many sutures and fontanels, the following are of obstetric significance.

5. SUTURES The sagittal or longitudinal suture lies between two parietal bones . The coronal sutures run between parietal and frontal bones on either side. The frontal suture lies between two frontal bones. The lambdoid sutures separate the occipital bone and the two parietal bones.

6. Importance: ( 1) It permits gliding movement of one bone over the other during molding of the head, a phenomenon of significance while the head passes through the pelvis during labor . ( 2) Digital palpation of sagittal suture during internal examination in labor gives an idea of the manner of engagement of the head ( asynclitism or synclitism ), degree of internal rotation of the head and degree of molding of the head.

7. FONTANELS Wide gap in the suture line is called fontanel . Of the many fontanels (6 in number ), two are of obstetric significance: ( 1) Anterior fontanel or bregma and (2) Posterior fontanel or lambda. Anterior fontanel : It is formed by joining of the four sutures in the midplane . The sutures are anteriorly frontal, posteriorly sagittal and on either side, coronal. The shape is like a diamond. Its anteroposterior and transverse diameters measure approximately 3 cm each. The floor is formed by a membrane and it becomes ossified 18 months after birth. It becomes pathological, if it fails to ossify even after 24 months .

8. Importance: Its palpation through internal examination denotes the degree of flexion of the head. it facilitates molding of the head. As it remains membranous long after birth, it helps in accommodating the marked brain growth; the brain becoming almost double its size during the first year of life. Palpation of the floor reflects intracranial status—depressed in dehydration, elevated in raised intracranial tension. Collection of blood and exchange transfusion, on rare occasion, can be performed through it via the superior longitudinal sinus. Cerebrospinal fluid can be drawn, although rarely, through the angle of the anterior fontanel from the lateral ventricle.

9. Posterior fontanel It is formed by junction of three suture lines — sagittal suture anteriorly and lambdoid suture on either side. It is triangular in shape and measures about 1.2 × 1.2 cm (1/2" × 1/2"). Its floor is membranous but becomes bony at term. Thus, truly its nomenclature as fontanel is misnomer. It denotes the position of the head in relation to maternal pelvis.

10. Sagittal fontanel It is inconsistent in its presence. When present, it is situated on the sagittal suture at the junction of anterior two-third and posterior one-third. It has got no clinical importance.

11. DIAMETERS OF SKULL The engaging diameter of the fetal skull depends on the degree of flexion present . Biparietal diameter —9.5 cm (3 ¾"): It extends between two parietal eminences. Whatever may be the position of the head, this diameter nearly always engages. Super- subparietal —8.5 cm (3 ½"): It extends from a point placed below one parietal eminence to a point placed above the other parietal eminence of the opposite side. Bitemporal diameter —8 cm (3 ¼"): It is the distance between the anteroinferior ends of the coronal suture. Bimastoid diameter — 7.5 cm (3 "): It is the distance between the tips of the mastoid processes. The diameter is incompressible and it is impossible to reduce the length of the bimastoid diameter by obstetrical operation.

12. CIRCUMFERENCES: Circumference of the plane of the diameter of engagement differs according to the attitude of the head. MOLDING: It is the alteration of the shape of the fore-coming head while passing through the resistant birth passage during labor . There is, however, very little alteration in size of the head, as volume of the content inside the skull is incompressible although small amount of cerebrospinal fluid and blood escape out in the process. During normal delivery, an alteration of 4 mm in skull diameter commonly occurs.

13. Mechanism: There is compression of the engaging diameter of the head with corresponding elongation of the diameter at right angle to it. Thus , in well flexed head of the anterior vertex presentation , the engaging suboccipitobregmatic diameter is compressed with elongation of the head in mento -vertical diameter which is at right angle to suboccipitobregmatic . During the process, the parietal bones tend to overlap the adjacent bones, viz. the occipital bone behind , the frontal bones in front and the temporal bones at the sides. In first vertex position, the right parietal bone tends to override the left one and this becomes reverse in second vertex position. Molding disappears within few hours after birth .

14. Grading : There are three gradings . Grade-1—the bones touching but not overlapping, Grade-2— overlapping but easily separated and Grade-3—fixed overlapping.

15. Mechanism of formation While the head descends to press over the dilating cervix or vulval ring, the overlying scalp is free from pressure, but the tissues in contact with the full circumference of the girdle of contact is compressed. This interferes with venous return and lymphatic drainage from the unsupported area of scalp → stagnation of fluid and appearance of a swelling in the scalp. Caput usually occurs after rupture of the membranes .

16. Importance: It signifies static position of the head for a long period of time. Location of the caput gives an idea about the position of the head occupied in the pelvis and the degree of flexion achieved. In left position, the caput is placed on right parietal bone and in right position , on left parietal bone. With increasing flexion , the caput is placed more posteriorly .

17. CAPUT SUCCEDANEUM It is the formation of swelling due to stagnation of fluid in the layers of the scalp beneath the girdle of contact . The girdle of contact is either bony or the dilating cervix or vulval ring . The swelling is diffuse, boggy and is not limited by the suture line. It may be confused with cephalhematoma . It disappears spontaneously within 24 hours after birth.

18. PELVIS From the obstetrical standpoint, it is useful to consider the bony pelvis as a whole rather than separately. For descriptive purpose, an articulated pelvis is composed of four bones—two innominate bones, sacrum and coccyx. These are united together by four joints—two sacroiliac joints, sacrococcygeal joint and the symphysis pubis.

19. The pelvis is anatomically divided into a false pelvis and a true pelvis, the boundary line being the brim of the pelvis. The bony landmarks on the brim of the pelvis from anterior to posterior on each side are —upper border of symphysis pubis, pubic crest, pubic tubercle, pectineal line, iliopubic eminence, iliopectineal line, sacroiliac articulation, anterior border of the ala of sacrum and sacral promontory.

20. FALSE PELVIS The false pelvis is formed by the iliac portions of the innominate bones and is limited above by the iliac crests. It has got little obstetric significance except that its measurements can to a certain extent, predict the size and configuration of the true pelvis . Its only obstetric function is to support the enlarged uterus during pregnancy . Its boundaries are: posteriorly—lumbar vertebrae, laterally—iliac fossa and anteriorly—anterior abdominal wall.

21. TRUE PELVIS This part of the pelvis is chiefly of concern to the obstetricians, as it forms the canal through which the fetus has to pass. It is shallow in front, formed by symphysis pubis and measures 4 cm (1 ½") and deep posteriorly , formed by the sacrum and coccyx and measures 11.5 cm (4 ½"). For descriptive purpose , it is divided into inlet , cavity and outlet . The pelvic measurements given in the text are average when measured radiologically and vary within a limited degree in different countries. The conversion of centimeters into inches is approximate.

22. INLET As the inlet is the brim of the pelvis, the circumference of the inlet is formed by the bony landmarks mentioned previously. Shape: It is almost round ( gynecoid ) with the anteroposterior diameter being the shortest. Other different shapes of the inlet are anthropoid , android and platypelloid . Plane: It is an imaginary flat surface bounded by the bony points mentioned as those of the brim. It is not strictly a mathematical plane and is, therefore, often referred to as superior strait . Inclination: In the erect posture, the pelvis is tilted forward. As such, the plane of the inlet makes an angle of about 55° with the horizontal and is called angle of inclination. Another way of measuring the inclination radiographically is to take the angle between the plane of the inlet and the front of the body of the fifth lumbar vertebra. The angle is normally about 135 °.

23. When the angle of inclination is increased due to sacralization of fifth lumbar vertebra, it is called high inclination . High inclination has got obstetric significances: 1. There is delay in engagement because the uterine axis fails to coincide with that of inlet 2. It favors occipitoposterior position 3. There is difficulty in descent of the head due to long birth canal and flat sacrum interfering with internal rotation. The angle of inclination may be lessened in case of lumbarization of first piece of sacral vertebra and is called low inclination . It has got no obstetric significance . It actually facilitates early engagement .

24. Sacral angle: It is the angle formed by the true conjugate with the first two pieces of the sacrum. Normally, it is greater than 90°. A sacral angle of lesser degree suggests funnelling of the pelvis. Axis: It is a mid-perpendicular line drawn to the plane of the inlet. Its direction is downward and backward . When extended, the line passes through the umbilicus to coccyx. It is important that the uterine axis should coincide with the axis of the inlet so that the force of the uterine contractions will be spread in the right direction, to force the fetus to pass through the brim.

25. Diameters: The measurements of the diameters are all approximate and minor variation is the rule rather than the exception . Anteroposterior ( Syn : true conjugate, anatomical conjugate , conjugate vera ) : It is the distance between the midpoint of the sacral promontory to the inner margin of the upper border of symphysis pubis. It measures 11 cm (4 ¼ " ) . It is not the shortest diameter of the inlet in the anteroposterior plane. In practice, the true conjugate cannot be estimated directly . However, its measurement is inferred by subtracting 1.2 cm (½") from the diagonal conjugate thus allowing for the inclination, thickness and height of the symphysis pubis.

26. Obstetric conjugate: It is the distance between the midpoint of the sacral promontory to prominent bony projection in the midline on the inner surface of the symphysis pubis. The point is somewhat below its upper border. It is the shortest anteroposterior diameter in the anteroposterior plane of the inlet. It measures 10 cm (4 " ) . It cannot be clinically estimated but is to be inferred from the diagonal conjugate—1.5–2 cm (¾") to be deducted or by lateral radiopelvimetry .

27. Diagonal conjugate: It is the distance between the lower border of symphysis pubis to the midpoint on the sacral promontory. It measures 12 cm (4 ¾ " ) . It is measured clinically during pelvic assessment in late pregnancy or in labor. Obstetric conjugate is computed by subtracting 1.5–2 cm from the diagonal conjugate depending upon the height, thickness and inclination of the symphysis pubis . The internal fingers are removed and the distance between the marking and the tip of the middle finger gives the measurement of diagonal conjugate. For practical purpose, if the middle finger fails to reach the promontory or touches it with difficulty , it is likely that the conjugate is adequate for an average size head to pass through.

28. T ransverse diameter: It is the distance between the two farthest points on the pelvic brim over the iliopectineal lines. It measures 13 cm (5 ¼ " ). The diameter usually lies slightly closer to sacral promontory and divides the brim into anterior and posterior segment. The head negotiates the brim through a diameter, called available or obstetrical transverse . This is described as a diameter which bisects the anteroposterior diameter in the midpoint. Thus the obstetrical transverse is either equal or less than the anatomical transverse.

29. Oblique diameters: There are two oblique diameters—right and left. Each one extends from one sacroiliac joint to the opposite iliopubic eminence and measures 12 cm (4 ¾ " ) . Right or left denotes the sacroiliac joint from which it starts. Sacrocotyloid —9 . 5 cm (3 ¾"): It is the distance between the midpoint of the sacral promontory to iliopubic eminence. It represents the space occupied by the biparietal diameter of the head while negotiating the brim in flat pelvis.

30. CAVITY Cavity is the segment of the pelvis bounded above by the inlet and below by plane of least pelvic dimensions . Shape : It is almost round. Plane : The plane extends from the midpoint of posterior surface of symphysis pubis to the junction of second and third sacral vertebrae. It is called plane of greatest pelvic dimensions . It is the most roomy plane of the pelvis and is almost round in shape.

31. Axis : It is the mid-perpendicular line drawn to the plane of the cavity. Its direction is almost downward. Diameters : Anteroposterior (12 cm or 4 ¾"): It measures from the midpoint on the posterior surface of the symphysis pubis to the junction of second and third sacral vertebrae. Transverse (12 cm or 4 ¾"): It cannot be precisely measured as the points lie over the soft tissues covering the sacrosciatic notches and obturator foramina.

32. OUTLET Obstetrical outlet: It is the segment of the pelvis bounded above by the plane of least pelvic dimensions and below by the anatomical outlet . Its anterior wall is deficient at the pubic arch; its lateral walls are formed by ischial bones and the posterior wall includes whole of the coccyx . Shape : It is anteroposteriorly oval. Plane : The plane is otherwise known as plane of least pelvic dimensions or narrow pelvic plane . The plane extends from the lower border of the symphysis pubis to the tip of ischial spines and posteriorly to meet the tip of the fifth sacral vertebra.

33. Diameters : Transverse— Syn : Bispinous (10.5 cm or 4 1/5 " ) : It is the distance between the tip of two ischial spines. Anteroposterior (11 cm or 4 ¼ " ) : It extends from the inferior border of the symphysis pubis to the tip of the sacrum. Posterior sagittal (5 cm or 2 " ) : It is the distance between the tip of the sacrum and the midpoint of bispinous diameter . Axis : It is represented by a line joining the center of the plane with the sacral promontory. Its direction is almost vertical.

34. Anatomical Outlet : It is otherwise known as bony outlet . It is bounded in front by the lower border of the symphysis pubis; laterally by the ischiopubic rami, ischial tuberosity and sacrotuberous ligament and posteriorly by the tip of coccyx. Thus, it consists of two triangular planes with a common base formed by a line joining the ischial tuberosities . The apex of the anterior triangle is formed by the inferior border of the pubic arch and that of the posterior triangle by the tip of the coccyx. Shape : It is diamond-shaped. Plane : It is formed by a line joining the lower border of the symphysis pubis to the tip of the coccyx. It forms an angulation of 10° with the horizontal. Axis : It is a mid-perpendicular line drawn to the plane of the outlet. Its direction is downward and forward

35. Diameters : Anteroposterior: It extends from the lower border of the symphysis pubis to the tip of the coccyx. It measures 13 cm or 5 ¼" with the coccyx pushed back by the head when passing through the introitus in the second stage of labor; with the coccyx in normal position, the measurement will be 2.5 cm less. Transverse — Syn : Intertuberous (11 cm or 4 ¼"): It measures between inner borders of ischial tuberosities . Posterior sagittal diameter (8.5 cm or 3 ½"): It is the anteroposterior distance between the sacrococcygeal joint and the midpoint of transverse diameter of outlet (TDO). It is clinically measured by the distance between the sacrococcygeal joint and anterior margin of the anus. Subpubic angle: It is formed by the approximation of the two descending pubic rami. In normal female pelvis, it measures 85°. Pubic arch: Arch formed by the descending rami of both the sides is of obstetric importance. Normally, it measures 6 cm in between the pubic rami at a level of 2 cm below the apex of the subpubic arch. Clinically , it is assessed by placing 3 fingers side by side.

36. MIDPELVIS Midpelvis is the segment of the pelvis bounded above by the plane of greatest pelvic dimensions and below by a plane known as midpelvic plane . Midpelvic plane: The midpelvic plane extends from the lower margin of the symphysis pubis through the level of ischial spines to meet either the junction of S4 and S5 or tip of the sacrum depending upon the configuration of the sacrum. If the plane meets the tip of the fifth sacrum, it coincides with the plane of least pelvic dimensions. If the plane meets the junction of S4 and S5, the plane becomes a wedge posteriorly

37. Diameters: Transverse diameter — Syn : bispinous (10.5 cm) : It measures between the two ischial spines. Anteroposterior diameter (11.5 cm): It extends from the lower border of the symphysis pubis to the point on the sacrum at which the midpelvic plane meets. Posterior sagittal diameter (4.5 cm): It extends from the midpoint of the bispinous diameter to the point on the sacrum at which the midpelvic plane meets.

38. PELVIC AXIS Anatomical (curve of Carus ): Anatomical pelvic axis is formed by joining the axes of inlet , cavity and outlet. It is uniformly curved with the convexity fitting with the concavity of the sacrum. The fetus does not, however, transverse the uniform curved path. Obstetrical: It is through this axis that the fetus negotiates the pelvis. It is not uniformly curved. Its direction is first downward and backward up to the level of ischial spines and then directed abruptly forward

39. PELVIC JOINTS Symphysis pubis: It is a secondary fibrocartilaginous joint. It has got no capsule and no synovial cavity. The articular surfaces are covered with hyaline cartilage. Due to softening of the ligaments during pregnancy, there is considerable amount of gliding movement. Sacroiliac articulation: It is a synovial joint and is an articulation between the articular surface of the ilium and sacrum. The articulating surfaces are not alike. It has got a capsule and a synovial cavity. Engagement to diagnose, it is better to palpate gently with two hands facing down over the abdomen than to prod around with Pawlik’s grip, which in non-experienced hands is painful. Sacrococcygeal joint is a synovial hinge joint. It allows both flexion and extension. Extension increases the anteroposterior diameter of the outlet.

40. THANK YOU