What is female pelvis?
A female pelvis is a bony canal that assists the fetal passage for being born.
Composition of bony pelvis:
4 bones:
- 2 innominate bones
- 1 Sacrum
- 1 coccyx
1] Innominate bone:
Each innominate bone consists of 3 parts.
Ilium:
- It is the large flared out part.
- Crest is the upper edge of ilium.
- Bony prominence of ilium at front is called anterior superior & inferior iliac spine each of which are 2 in number.
- 2 similar points at the back is called posterior superior & inferior iliac spine.
- Iliac fossa is the concave anterior surface of ilium.
Ischium:
- It is the thick lowest part.
- Ischial tuberosity is the prominence on which the body rests while sitting.
- Ischial spines are posterior prominence of ischium which is responsible for estimation of fetal station at the time of labor.
Pubis:
- It is the anterior bone.
- Superior & inferior rami are the body of pubis.
- Inferior ramus of each side merges to form the pubic arch.
- Fusion of pubic arch occurs at Ramus of ischium.
- 2 pubic bones meet at symphysis pubis.
- Greater & lesser sciatic notch are the curves on the lower border of innominate bones.
2] The sacrum:
- Wedge shaped bone formed by the fusion of 5 fused vertebrae.
- Anterior concave surface of sacrum is called as hollow of sacrum.
- Upper border of first sacral vertebra is called sacral promontory.
- Ala are the wings of sacrum.
- Posterior rough surface receives the muscle attachments.
- Nerve supply from cauda equina emerge to supply pelvic organs.
3] Coccyx:
- It is triangular bone consist of 4 fused vertebrae.
Pelvic Joints:
- The symphysis pubis-1
- The sacroiliac joints-2
- The sacroccygeal joint-1
Pelvic ligaments:
Each pelvis joint is held together by following ligaments:
- Interpubic ligament
- Sacroiliac ligaments
- Sacrococcygeal ligament
- Sacrotuberous ligament
- Sacrospinous ligament
Caldwell- Moloy classification of pelvis:
Gynecoid pelvis:
It is the typical female pelvis & is most suitable for labor & delivery.
Features:
- Inlet is round
- Sacrum is concave & sacral promontory is not prominent.
- Sacrosiatic notch admits 2 fingers.
- Pelvic side walls are straight & parallel.
- Ischial spines are not prominent.
- Subpubic angle >90 degrees
- Interischial diameter admits knuckles.
Android Pelvis:
It is a typical male pelvis.
Features:
- Inlet is heart shaped.
- Sacrum is flat & sacral promontory is prominent.
- Sacrosiatic notch does not admit 2 fingers & is narrow.
- Pelvic side walls converge downwards
- Ischial spines are prominent
- Subpubic angle < 80 degrees.
Anthropoid pelvis:
Features:
- Oval shaped.
- Sacrum is curved & sacral promintory not prominent
- Sacrosiatic notch is wide & deep.
- Pelvic side walls are straight.
- Ischial spines are not prominent.
- Subpubic angle = 80 degrees
Platypelloid Pelvis:
- Flat pelvis & is transversely oval.
- Sacrum is flat & sacral promontory is prominent.
- Sacrosiatic notch is wide & shallow.
- Pelvic side walls are straight.
- Ischial spines are not prominent.
- Subpubic angle > 90 degrees.
Anatomical position of pelvis:
- Both Anterior superior iliac spine & upper end of Symphysis pubis lie in same coronal plane.
- Pubis is directed upward & forward.
- Sacral canal facing upward.
- Tip of coccyx should correspond lower border of symphysis pubis.
- Angle of pelvic inlet is 50-60 degrees with horizontal.
- Angle of pelvic outlet- 10 degrees with horizontal.
Landmarks of Pelvis:
- Upper border of symphysis pubis
- Pubic crest
- Pubic tubercle
- Pectineal line
- Iliopectineal eminence
- Iliopectineal line
- Sacroiliac joint
- Ala of sacrum
- Sacral promontory
Parts of pelvis:
False Pelvis:
- Lies above pelvic brim
- Bounded by :
Posteriorly: Lumbar vertebrae
Laterally: Iliac fossae
Anteriorly: Abdominal walls.
True pelvis:
- Bonded by:
Posteriorly: Sacrum
Laterally: Ischium & Sacrosiatic notch
Anteriorly: Pubic bone, obturator foramen & ischiopubic rami.
- Divided into 3 planes:
Pelvic inlet
Pelvic cavity
Pelvic outlet.
Pelvic inlet:
- Boundaries:
Lateral: Pectineal lines.
Anterior : Pubic ramus & Symphysis pubis.
- Shape: Round
- Diameters:
- True conjugate:Distance between Sacral promomtory & upper border of symphysis pubis-11 cm
- Obstetric Conjugate:Distance between sacral promontory & the most prominent part of symphysis pubis- 10 cm.
- Diagonal conjugate: Distance between sacral promontory to inferior border of symphysis pubis- 12cm.
Oblique diamter: Distance between sacroiliac joint of one side to ileopectineal eminence of other-12 cm.
Pelvic cavity:
- No diameters beacuse of absence of bony projection & joining of various ligaments.
- Shape: Truncated cylinder.
- Plane of greatest pelvic dimensions:
At the level of second & third vertebrae of savrum.
- Plane of least pelvic dimensions:
At the level of ischial spine & fourth & fifth vertebrae of sacrum.
- Interischial diameter:At the level of ischial spine : 10 cm.
Pelvic outlet:
- Boundaries:
Anterior: Subpubic arch, ischiopubic rami
Lateral: Sacrosiatic ligaments, ischial tuberosities
Posterior: Tip of sacrum
- Shape: Diamomd
- Diameters:
Anterioposterior:Distance between tip of coccyx to inner border of sympohysis pubis- 12-13 cm
Transverse diameter: Distance between 2 ischial tuberosities- 10 cm
Waste space of Morris:
- When the head is being delivered, it passes through the pelvic outlet between ischiopubic rami behind subpubic angle. The space between subpubic angle & head circumference of fetus < 1 cm.
- In case of android pelvis the subpubic angle is narrow & the space > 1 cm. This is called Waste Space of Morris.
Clinical implications of pelvis:
- The head engages at transverse diameter of inlet which is greater.
- The fetal head rotates to accommodate under anterio-posterior diameter of outlet which is largest.
- Malrotation or arrest usually occurs at interspinous diameter.
- The direction of fetus is along the plevic axis.
- Relaxation of pelvic joints increases the diameters of pelvis which helps in descend & delivery.
- Backache is common due to relaxation of sacroiliac joints.
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