Clinical anatomy of pelvis


                  
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Pelvimetry

There are three anatomical cavities of the lesser pelvis

  • Inlet

  • Midcavity

  • Outlet

 

 

The inlet is placed at the level of the pelvic brim which extends from the superior surface of the symphysis pubis, the pubic crest, the pectineal line and the pectin pubis to the iliopubic eminence and finally ends posteriorly at the sacral promontory.

 

The midcavity is placed at the level of ischial spines.

The outlet is placed at the inferior aperture of the lesser pelvis and its boundaries are osteofascial. It runs anteriorly from the inferior surface of the pubic symphysis to the inferior surfaces of the ischiopubic rami and then to the margin of the sacrotuberous ligaments and finally to the posterior aspect at the coccyx.

The inlet cavity can be measured by both clinical assessment and X ray pelvimetry.

The dimensions under the Xray pelvimetry are as follows

Anteroposterior diameter of the inlet this is measured from the superior surface of the pubic symphysis to the sacral promontory.

Transverse diameter measures the widest portion between the lateral walls of the inlet.

Oblique diameter measures the diameter from the iliopubic eminence tot the sacroiliac joint of the other side.

 

In clinical assessment of the inlet dimensions, the diagonal conjugate diameter is the single most important measurement to be taken. It measures the diameter from the inferior surface of the pubic symphysis to the sacral promontory. This is best assessed by vaginal (pelvic) examination with the use of the index finger. The examine first of all measures his own index finger length and determines during examination whether he can feel for the sacral promontory or not.

  1. If the examining finger just touches the sacral promontory then the diagonal conjugate diameter is equal to the length of he examining finger.

  2. If the examining finger does not reach the sacral promontory then the diagonal conjugate diameter is less then the length of the examining finger.

 

The midcavity can also be assessed by Xray and clinical pelvimetry. The dimensions are as follows

 

In cm

                                   Trans          Oblique          *AP

Inlet                              12.5             11.5           10

Mid-pelvis                     11.5            11.5           11.5

Outlet                            10               11.5           12.5

 

*AP- anteroposterior

Midcavity
The most used methods of clinical assessment of the dimensions of the midcavity are

1.      Morphology of ischial spines. The spines may be heavy and long or small. If heavy then they should decrease the transverse diameter of the midcavity and probably cause transverse arrest of the head of the baby during labour.

2.      Sacropspinous diameter is measure by insinuating two fingers into the interval between the ischial spine and the lateral border of the sacrum during pelvic examination. The width of the two examining fingers are initially noted to determine the approximate diameter of the sacrospinous interval.

Outlet cavity

Outlet dimensions are assessed by both Xray and clinical pelvimetry as follows

        Anteroposterior diameter

        Oblique diameter

        Transverse diameter

Clinically the following are important measurements of the outlet cavity.

1.      Intertubersichial diameter which measures the diameter between the ischial tuberosities. It is best assessed with the knuckle of the examining hand pressed firmly in the interval between the ischial tuberosities. It should be about 8cm or above.

2.      Subpubic angle is assessed by the use of the angle between the index finger and the thumb in determining the angle between the two inferior rami of the pubis.

The following problems may be identified by the use of clinical assessment in labor

  • 1.      General contraction of all 3 types of cavities- possible justominor gynecoid pelvis

  • 2.      Contraction only at the outlet which may point to an android pelvis

  • 3.      Contraction at the midcavity with heavy spines which may be indicative of anthropoid pelvis.

 

  • Android pelvis is associated with outlet dystocias (difficulty labour caused by outlet problem) and engagement of the fetal head may be delayed by encroachment of  the sacral promontory. Occipitolateral and occipitoposterior positions are common (see below).

  • Gynecoid pelvis is the normal female pelvis but it may be generally contracted in the gynaecoid pelvis of a small woman which is called the justominor variety.

  • Anthropoid pelvis is associated with transverse arrest of the head of the fetus in labour because of the usual prominence of the ischial spines.

  • Flat pelvis is always associated with severe with severe labour difficulty and its dimensions of the inlet are heavily contracted.

 

Pelvic types (After Caldwell and Molloy)

Gynecoid

Android

Anthropoid

 

Gynaecoid

  1. Inlet: Rounded or slightly heart shaped. Ample anterior and posterior segments.

  2. Sacrum: Curved, average length

  3. Sacrosciatic notch: Medium width

  4. Side walls (lateral view): Straight, divergent or convergent.

  5. Side walls (lateral view): Straight, divergent or convergent

  6. Interspinous diameter: Wide

  7. Pubic arch: Curved

  8. Subpubic angle: Wide.

  9. Intertuberous diameter: Wide.

 

 

Android

  1. Inlet: Wedge shaped or rounded triangle. Posterior segment usually flat, and narrow and pointed.

  2. Sacrum: Straight with forward inclination

  3. Sacrosciatic notch: Narrow

  4. Side wall: Usually convergent

  5. Side walls (lateral view): Usually convergent

  6. Interspinous diameter: Shortened

  7. Pubic arch: Straight

  8. Subpubic angle: Very narrow.

 

 

Anthropoid

  1. Inlet: Anteroposterior ovoid with length of anterior and posterior segment increased: Transverse diameter reduced.

  2. Sacrum: Normally curves, but long and narrow.

  3. Sacrosciatic notch: Wide, shallow

  4. Side walls: Straight

  5. Side walls (lateral view): Often straight

  6. Interspinous diameter: Shortened.

  7. Pubic arch: Slightly curved

  8. Subpubic angle: Narrow

  9. Intertuberous diameter: Often shortened.

 

 

Platypelloid

  1. Inlet: Transverse ovoid; increased transverse and anteroposterior diameter of both segments.

  2. Sacrum: Curved short.

  3. Sacrosciatic notch: Slightly narrowed.

  4. Side walls: Straight or slightly divergent

  5. Side walls: (lateral view): straight or divergent

  6. Interspinous diameter: Increased

  7. Pubic arch: Increased

  8. Subpubic angle: Wide.

  9. Intertuberous diameter: Wide.

 

 

 

 

DIGITAL LOCATOR OF MALE PELVIS: Layer 1 Skin of pelvis- anterior abdominal wall and perineum). Layer 2- Superficial aspect of pelvis.  Layer 3 - intermediate dissection of male pelvis. Layer  4 - deep dissection of male pelvisLayer 5- deepest layer of male pelvis. Layer 6- layer of pelvic floor of male pelvis. Layer  7 - layer of pelvic bones DIGITAL LOCATOR OF FEMALE PELVIS: Layer 1 Skin of pelvis- anterior abdominal wall and perineum). Layer 2- Superficial aspect of pelvis.  Layer 3 - intermediate dissection of male pelvis. Layer  4 - deep dissection of male pelvisLayer 5- deepest layer of male pelvis. Layer 6- layer of pelvic floor of male pelvis. Layer  7 - layer of pelvic bones.

 

 

 

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