Anatomy of pelvis

1. Mudaliar
and
Menon’s
Clinical
Obstetrics
13TH EDITION

2. Chapter 1
PELVIS
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3. BOUNDARIES OF A TRUE PELVIS
•The pelvis is an important structure from the
obstetric point of view, as it forms the canal
through which the fetus has to pass.
•The pelvis is divided by the linea terminalis into
two parts:
oThe upper part known as pelvis major or false
pelvis
oThe lower part called pelvis minor or true pelvis
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4. ABOVE
• Promontory and alae of the sacrum, linea
terminalis and the upper margin of the pubic
bones
• The linea terminalis is formed by the upper
border of the sacral vertebra, the arcuate line of
the ilium and the pectineal line of the pubis
BELOW • The pelvic outlet
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5. STRUCTURE
OF THE
PELVIS
• The pelvic cavity is cylindrical in shape.
• Extent: Inlet lies above the outlet below.
• Shape: Bent cylinder with the posterior wall
deeper than the anterior wall.
• The depth of the posterior wall is 10 cm.
• The anterior wall is 5 cm.
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6. AXIS OF THE
BIRTH
CANAL
•The upper part is directed downwards and
backwards.
•The lower part curves downwards and forwards.
•Called the curve of Carus.
•The curve of Carus is an imaginary line joining
the midpoints of the AP diameters of the inlet,
cavity and the outlet.
•This line runs downwards and backwards in the
upper half of pelvis, then turns downwards and
forwards in the lower half of the pelvis.
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7. •The descent of the fetal
head follows the curve of
Carus.
•At this level, the
contraction of the pelvic
floor muscles direct the
fetal head downwards
and forwards until
delivery.
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8. WALLS
OF THE
PELVIS
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Posteriorly The anterior surface of the
sacrum
Laterally The inner surface of the ischial
bones and the sacrosciatic
notches and ligaments
Anteriorly The pubic bones, the ascending
superior rami of the ischial
bones, and the obturator
foramina

9. MUSCLES
OF THE
TRUE PELVIS
•The pelvic diaphragm is
a musculo-aponeurotic
part separating the
pelvis above from the
perineum and vulva
below. This is formed by
the levator ani and the
coccygei muscles.
•Three orifices, namely,
the urinary meatus, the
vulval outlet and the
anus, pierce this
diaphragm.
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Location Muscles forming the
walls
Sides The pyriformis
Posteriorly The coccygeus muscles
Laterally The obturator internus
Inferiorly The levator ani

10. PLANES AND
DIAMETERS
OF THE PELVIS
•The plane of the pelvic inlet (superior
strait)
•The plane of the pelvic outlet (inferior
strait)
•The plane of the least pelvic dimensions
(midpelvis)
•The plane of the greatest pelvic dimensions
in the cavity (no obstetrical significance)
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11. BOUNDARIES AND DIAMETERS OF THE PELVIC INLET
The three boundaries are:
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1 Posterior Sacral promontory and alae of the sacrum
2 Lateral Linea terminalis
3 Anterior Horizontal rami of the pubic bones and
symphysis pubis

12. The three diameters at
the brim are:
•Anteroposterior
•Transverse
•Right and left oblique
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13. There are three anteroposterior
diameters:
•The obstetric conjugate
•The diagonal conjugate
•The anatomical conjugate (conjugate vera)
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14. DIAMETERS
OF THE
INLET
1. ANTERO–POSTERIOR
a) Obstetric conjugate
• Middle of sacral promontory to middle of
the posterior margin of the pubic symphysis
• Measures 10 cm
b) Diagonal conjugate
• Subpubic angle to middle of the sacral
promontory
• Measures 12 cm
Subtracting 1.5–2 cm from the diagonal
conjugate gives the obstetric conjugate.
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15. c) Anatomic conjugate
• Middle of sacral promontory to the
upper portion of the inner surface of
the symphysis pubis
• Measures 11 cm
• No obstetric significance
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16. TRANSVERSE DIAMETER
•Widest distance between the linea terminalis on both
sides.
•Measures 13 cm.
OBLIQUE DIAMETER
•Extends from the right sacroiliac joint to the
iliopectineal eminence on the opposite side.
•It is occupied by the suboccipito bregmatic diameter of
the fetal head in occipitoanterior position.
•Measures about 13 cm.
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18. SACROCOTYLOID DIAMETER
•It is the distance from the midpoint of the sacral
promontory to the ilio-pectineal eminence on the same
side
•Measures 9 cm
POSTERIOR SAGITTAL DIAMETER:
•It is that part of the AP diameter which lies posterior to
the transverse diameter
•This diameter increases from the pelvic brim to the
outlet.
•At the brim, the posterior sagittal diameter measures 5
cm
Clinical importance at the pelvic brim
The cardinal movement of engagement occurs at the
pelvic brim
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19. PELVIC OUTLET
•Diamond-shaped
•Lithotomy
◦ Produces upward gliding movement
of the sacroiliac joint and thus
increases the transverse diameter of
outlet (ITD) by 1.5 to 2 cm
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Posteriorly By the tip of the
coccyx
Laterally By the ischial
tuberosities
Anteriorly By the pubic arch formed by
inferior rami of the ischium
and the pubis as they converge
towards the symphysis pubis
Boundaries

20. Pubic Symphysis
Pubic Arch
Ischial Tuberosity
Sacrotuberous Ligament
Pelvic Outlet
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21. PELVIC
OUTLET
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• Anterior
• Posterior
• Anterior apex–subpubic angle
Base – bituberous diameter
Sides – pubic rami, ischial
tuberosity
• Posterior apex–
sacrococcygeal joint
Base – bituberous diameter
Sides – sacrotuberous
ligament
2. Triangular planes

22. DIAMETERS
OF THE PELVIC
OUTLET
Anteroposterior diameter
•Inferior margin of the pubic symphysis to the
posterior aspect of tip of sacrum
•Measures 12 cm
Transverse diameter
• Distance between the inner edges of 2
ischial tuberosities
• Measures 10.5–11 cm
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23. Posterior sagittal diameter:
•Extends from the middle of the transverse
diameter to the tip of the sacrum
•The posterior sagittal diameter of the outlet
usually exceeds 7 cm
Anterior sagittal diameter:
•Extends from the lower border of the
symphysis pubis to the centre of the
bituberous diameter
•Measures 6 cm
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DIAMETERS
OF THE PELVIC
OUTLET

24. WASTE
SPACE OF
MORRIS
•Normally, the width of the pubic arch is such
that a round disk of 9.4 cm (diameter of a well-
flexed head) can pass through the pubic arch at
a distance of 1 cm from the inferior border of
the symphysis pubis.
•This distance is known as the waste space of
Morris.
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25. •Angle formed by approximation of
the two descending pubic rami
forming the pubic arch
•In a normal gynecoid pelvis, this
angle should be >90o
•If this angle is smaller, the
transverse diameter of the outlet is
also smaller
•Clinically, the subpubic angle
should admit two fingers
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SUBPUBIC
ANGLE

26. CAVITY
Extends from the inlet to the outlet
Plane of greatest pelvic dimension
Plane of least pelvic dimension
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27. •This plane has no obstetrical significance
•Roomiest part.
•It passes through the junction of the second and third sacral vertebrae, and
laterally through the ischial bones over the middle of the acetabulum and
posterior surface of the symphysis pubis
•It is nearly circular
•The anteroposterior diameter measures 12.5 cm
•The transverse diameter measures 12.75 cm
PLANE OF GREATEST PELVIC DIMENSION
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28. PLANE OF
LEAST PELVIC
DIMENSION
•Midpelvis
•Important plane of pelvis
•The arrest of labor can take place here
•Extends from the apex of the subpubic arch
through the ischial spines to the sacrum (s4 &
s5)
Boundaries
1. Lower border of the pubic symphysis
2. White line
3. Ischial spine
4. Sacrospinous ligament
5. Sacrum
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29. DIAMETERS
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Transverse diameter or
interspinous diameter
This is the distance between the two ischial
spines and is the smallest diameter of the
pelvis. It measures 10 cm.
Antero-posterior diameter It extends from the lower border of the
symphysis pubis to the junction of the 4th and
5th sacral vertebrae. This distance should be a
minimum of 11.5 cm.
Posterior sagittal diameter It extends from the mid point of the inter
ischial diameter to the junction of the 4th and
5th sacral vertebrae. This measures 6 cm.

30. MID-CAVITY
ASSESSMENT
In a mid-cavity contraction:
•The ischial spines are prominent
•The sacrum is not curved and is flat
•The pelvic side walls are converging
•The sacrosciatic notch does not allow 2
fingers
•The subpubic arch is narrow and does not
admit 2 fingers
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31. PELVIC JOINTS
Pubic symphysis
Sacroiliac joint
Sacrococcygeal joint
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32. •There is relaxation of the sacroiliac joints during pregnancy due to hormonal
changes.
•Marked mobility of the pelvis—upward gliding movement of the sacroiliac
joint.
•Relaxation of the symphysis pubis—starts in early pregnancy but increases
during the last three months and regresses after delivery.
•When vaginal delivery is conducted in the dorsal lithotomy position,
displacement of the sacroiliac joint is greatest and increases the diameter of
the outlet by 1.5–2 cm.
•In shoulder dystocia, McRobert’s maneuver is successful due to the mobility of
the sacroiliac joint.
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CLINICAL SIGNIFICANCE OF THE PELVIC JOINTS

33. CALDWELL– MOLLOY
CLASSIFICATION
Four types of pelvises:
• Gynacoid
• Anthropoid
• Platypelloid
• Android
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34. 1. GYNECOID PELVIS
CHARACTERISTICS
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Inlet • Since the transverse diameter is only slightly
greater than or equal to the anteroposterior
diameter, the inlet is slightly oval or round.
• The posterior sagittal diameter at the inlet is
slightly less than the anterior sagittal diameter.
• The sides of the posterior segment are well
rounded and wide.
Midpelvis • In the midpelvis, the side walls of the pelvis
are straight, and the spines are not
prominent. The transverse diameter at the
ischial spines is 10 cm or more.
Outlet • At the outlet, the pubic arch is wide.
Sacrum • The sacrosciatic notch is well-rounded.

35. UNIVERSITIES PRESS PVT. LTD.
2. ANDROID PELVIS: CHARACTERISTICS
Inlet The posterior sagittal diameter is less than the
anterior sagittal diameter. This restricts the use of
posterior space. The sides of the posterior
segment are not rounded, and the anterior pelvis
is narrow and triangular.
Midpelvis The side walls are usually convergent, and the
ischial spines are prominent.
Outlet The subpubic arch is narrowed.
Sacrum It is set forward in the pelvis and is usually
straight, with little or no curvature. The
sacrosciatic notch is narrow and highly arched.

36. UNIVERSITIES PRESS PVT. LTD.
3. ANTHROPOID PELVIS: CHARACTERISTICS
Inlet The anteroposterior diameter is greater than the
transverse diameter. The anterior segment is narrow
and pointed.
Midpelvis The sidewalls are often convergent, and the ischial
spines are likely to be prominent.
Outlet The subpubic arch is narrowed but well shaped.
Sacrum It usually has six segments and is straight. The
sacrosciatic notch is large.

37. •This pelvis is the rarest of the pure varieties and is found in less
than 3% of women.
•The characteristics of the platypelloid pelvis are:
Transverse oval inlet
Very wide rounded subpubic angle
Very wide flat posterior segment
Narrow sacrosciatic notch
Average sacral inclination
Very wide subpubic arch
Straightside walls
Very wide interspinous and intertuberous diameters
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4. PLATYPELLOID PELVIS: CHARACTERISTICS

38. CLINICAL
SIGNIFICANCE
Small gynecoid pelvis
•In this type, the diameters are proportionately reduced,
but the shape is normal.
•Hence, there is a delay at every stage of labour due to
the lack of space.
•Powerful uterine contractions are required to push the
presenting part downward. Can cause CPD.
Android pelvis
•With this type of pelvis, the occipitoposterior position is
common.
•Due to the funnel shape of the pelvis, progressive
difficulty is faced, rotation fails to occur, and transverse
arrest is common.
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39. CLINICAL
SIGNIFICANCE
Anthropoid pelvis
•In this pelvis, persistent occipito posterior position is
common.
Platypelloid pelvis
•In this type of pelvis, there is asynclitic engagement.
•Face presentation can occur.
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40. •Rachitic flat pelvis
•Nagele’s pelvis
•Robert’s pelvis
•Kyphoscoliosis
•Obliquely contracted pelvis
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ABNORMALITIES
OF THE PELVIS