Bony pelvis and fetal skull Dr Manal Behery

2014

Objects

Fetal headPelvic anatomyPelvic shapes

Fetal head

LandmarksSuturesFontanellesDiameters

Fetal head

From an obstetrical point of view it’s the most important part:largest least compressible part of the fetus.most frequent presenting part

Landmarks

Head is divided into 3areas (1) the sinciput or brow portion;(2) the vertex, or top of the head between the 2 fontanelles; (3) Base large, ossified, firmly united, and noncompressible

SuturesMembrane-occupied spaces between the cranial bones

1-Sagittal suture:- lies btw the parietal bones-extends in an AP direction btw the

fontanelles -divides the head into right and left sides

2-lambdoid suture: extends from the posterior fontanelle laterally separate the occipital from the parietal bones.

3-coronal suture: extends from the anterior fontanelle laterally separate the parietal and frontal bones.

4- frontal suture: lies between the frontal bones extends from the anterior fontanelle to the glabella (the prominence between the eyebrows).

Clinical importance of suturesPosition of fontanelle & sagittal suture can identify attitude and position of vertex.By plapating the sagittal suture during labour, degree of internal rotation & molding of the head can be noticed.In deep transverse arrest, this sagittal suture lies transversely at the level of the ischial spines.

Moulding…Reshaping of the fetal skull:

Obliteration of the sutures.

Overlapping of the bones of the vault:

One parietal bone overlaps the other.

Both overlap the occipital bone.

It accounts for diminution of the biparietal diameter and suboccipitobregmatic diameters by 0.5-1 cm. 0r even more.

A: Well flexed HeadB: Partially Flexed HeadC: Deflexed HeadD: Face PresentationE: Brow presentation

The anterior fontanelle (bregma) :diamond shaped area(2 × 3 cm) of

unossified membrane formed by the junction of 4 suture.

The posterior fontanelle: It is the triangular depressed area at

the junction of 3 suture:closes at 6 to 8 weeks of life Y- or T-shaped

ROT LOT

LOAROA

LOP

ROP

LOP

Transverse Diameters of the Fetal Skull

Biparietal Diameter

9.5 cm Between the 2 parietal eminences

Bitemporal Diameter

8.5 cm.

Bimastoid Diameter

7.5 cm. Between the 2 mastoid processes (Not reducible nor destroyable even by destructive procedures

Supra-subparietal 8.25 - 9 cm.

Asynclitic head

presenting AP diameter when the head is deflexed >>> OP

-presenting AP diameter in a brow presentation -longest AP diameter of the head

-4. Submentobregmatic (9.5 cm):presenting AP diameter in face presentations

Length Presentation

1-Suboccipito-bregmatic 9.5 cm. Flexed vertex

2-Suboccipito-frontal 10.5 cm. Partially deflexed vertex

3-Occipito-frontal 11.5 cm. Deflexed vertex

4-Mento-vertical 13.75-14 cm. Brow

5-Submento-bregmatic 9.5 cm. Face

6-Submento-vertical 11.5 cm. Face Not fully extended

6

Engaging Diameters of Fetal Skull

Well Flexed Head

Circle of 9.5 cm.The engaging Diameter is the

Suboccipito-Bregmatic diameter

A deflexed Head

An oval The longer occipito-frontal

diameter Of 11.5 cm. Is exposed.

Greater Deflexion of the Head

An ovalThe longer mento vertical

diameter of 13.75-14 cm. is exposed

Full Extension of the Head

A circle of 9.5 cm.The engaging dimeter is the submento-vertical diameter

Female bony Female bony pelvispelvis

False Pelvis

bordered by:lumbar vertebrae posteriorlyiliac fossa bilaterallyabdominal wall anteriorly.

supports the abdominal contents after 1st trimester helps support the

gravid uterus.

Pelvic outlet

Ischial Tuberosity Ischial Spine

SP

The Planes of the pelvis

Plane of the pelvic inlet.Plane of the cavity: Plane of greatest Pelvic DimensionsPlane of the mid pelvis (plane of obstetric outlet)Plane of the Anatomical outlet

(Inlet (Pelvic brim)

passing with the boundaries of pelvic brim and making an angle of 55o with the horizon (angle of pelvic inclination).

Pelvic inclination: The plane of the pelvic inlet makes an angle of 55 degree with the horizon in the standing position"

The consequences of walking upright…

When a women stands erect:The pelvic inlet makes an angle of about 55° with the horizon.The pelvic outlet makes an angle of 15° with the horizon

If the angle made by the inlet is greater than 55° this may make the descent of the fetal head in the pelvis difficult.

Diameters of the pelvic inlet

The True Conjugate = 11 cmThe Obstet. Conjugate = 10.5cmThe Diagonal Conjugate = 12 cm

(4) External conjugate:From: The upper anterior margin of the symphysis pubis.To: The depression below the tip of the 5th lumbar spine (20 cm).

(5) Anatomical transverse diameter:Between the farthest points on iliopectineal lines (= 13 cm).It lies 4 cm infornt of the sacral promontory, 7 cm behind the symphysis pubis.

(6) Obstetric transverse diameter:

It bisects the true conjugate (12.5 cm)

ATDOTD

TC

(7) The oblique diameters: (12 cm)

The right extends from the right joint to the left eminence and vice versa.

(8) Sacrocotyloid diameter: (9-9.5 cm)

The right diameter ends in the right eminence & vice versa.

The pelvic Cavity

Boundaries:Above: pelvic brim.

Below: plane of least pelvic dimensions.

Anteriorly: syrnphysis pubis.

Posteriorly: sacrum.

bordered by: the posterior midpoint of the pubis anteriorly the upper part of the obturator foramina laterallythe junction of the 2nd and 3rd sacral vertebrae posteriorly.

The fetal head rotates to the anterior position in this plane

2- pelvic cavity The plane of greatest diameter:

The pelvic Outlet(A)Anatomical outlet: Lozenge

-shaped, bounded by:Lower border of symphysis pubis.

Pubic arch.

Ischial tuberosities

Sacrotuberous & sacrospinous ligaments.

Tip of the coccyx.

The Plane of the Outlet

Anterior Sagittal Plane

Posterior Sagittal Plane

Diameters of the pelvic outlet

(1)Anteroposterior

Anatomical anteroposterior (11 cm)

b Obstetric anteroposterior (13 cm)

(2) Transverse diametersBituberous (11 cm), between the inner aspects of ischial tuberositiesBispinous (10.5 cm), between the tips of ischial spines.

Diameters of the pelvic outlet

Interspinous diam. = 10.5 cm.

Obstet. Ant. Post diam= 13 cm.

Anato. Ant. Post diam= 11 cm.

(3) Anterior sagittal diameter = 6-7 cm

. (4) Posterior sagittal diameter = 7-10 cm

Diameters of the pelvic outlet

Diameter

Inlet

Cavity

outlet

Anteroposterior diameter

11

12.5

13 Transverse diameter

13

12.5

11

Diameters of the pelvic

The pelvic axis:

Anatomical axis (Curve of Carus):

The Obstetric Pelvic Axis

This represents the path that the presenting part must follow for delivery to occur:

The upper part moves downward approximately in a straight line till the level of the ischial spine.The trajectory then changes to become a curvilinear path directed forward and downward

The Fetal Head Has Five Fifths…

0 : Head Not Palpable1 : Sinciput felt – Occiput Not Felt2 : Sinciput felt – Occiput Just Felt3 : Sinciput easily felt – Occiput Felt4 : Sinciput High – Occiput easily Felt5 : Complete above pelvic brim

fift

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bove t

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rim

-5

0

+5

Pelvic Shapes

Gynecoid PelvisThe classic female type.

Found in approximately 50% of women.

Characteristics:

Android Pelvis

The typical male type

Found in less than 30% of women

Characteristics: 1. Triangular inlet with a flat

Anthropoid Pelvis

Resembles ape pelvis.

Found in approximately 20% of women

Characteristics: ,

Platypelloid Pelvis Flattened gynecoid pelvis.

Found in only 3% of women