Pelvis (soft and bony) and its obstetrical importance

 Pelvis :

The pelvis is a basin like structure which connects the spine to lower limbs.
It is skeletal ring formed by 2 innominate (or) hip bone and the sacrum and the coccyx.

- Types of pelvic bones - 4
   2 Hip bones (ilium, ischium, pubic)- Innominate
   1 Sacrum
   1 coccyx
- Pelvic Joints - 4
  2 Sacro-iliac joints
  1 Pubic symphysis Joints
  1 Sacro coccygeal joints

Types - 1. True pelvis (Bony)
             2. False pelvis (Soft)

1. True Pelvis (Bony pelvis) :

The true pelvis is the bony canal through which the fetus passes through during birth.
It has 3 Parts - (a) Brim (inlet)
                        (b) Cavity
                        (c) Outlet
Borders of Brim, Cavity and Outlet - (BCO)

Brim :
  • Posterior- by sacrum
  • Lateral - Pectineal lines
  • In front - Upper margin of pubic symphysis and pubic crest.
Cavity :
  • Anterior - Pelvic surface of the bodies of pubic bone, pubic rami and pubic symphysis.
  • Posterior - Sacrum and coccyx
  • Lateral - ischium

Outlet :

  • Anterior - Lower margin of pubic symphysis.
  • Lateral - Ischieal tuberosity
  • Posterior - Hip of coccyx.

Diameters :

                                                    At inlet (cm)           Cavity (cm)                   At outlet (cm)
Antero-posterior                              11                             12                                  13
Oblique                                            12                             12                                  12
Transverse                                        13                            12                                  11

2. False Pelvis (Soft pelvis) :

It is formed by Upper flared out portion of the ilium.

Borders :
  • Laterally by - Iliac fossa
  • Posteriorly by - 5th lumbar vertebrae
  • Anteriorly by - abdominal wall and inguinal ligament.

It protects the abdominal organs, It has no obstetrical importance except that it provide certain landmark for external pelvimetry.

                                                    Male                                               Female
Bones                            taller,heavier,thicker                            lighter, less dense
Iliac fossa                          more cocave                                            Shallow
Asis                                         Inturned                                       Straight forward
Obstructed foramen                large,oval                                     Small , traingular
Pelvic inlet                          Heart shaped                                       Round (longer)
Subpubic angle                Narrow, 50-60 degree                       Wide,80-90degree
Ischial spine                          Inturned                                            Outturned
Symphysis pubis                  Elongated                                            Short
and body

Obstetrical Importance :
  • Adaptation of the fetal head in the pelvis during parturation.
  • Evalution of the adequacy of the pelvis during obstetrical evalution.
  • Angled conjugate- evaluated by per vaginal evalution.
  • The size of the subpubic arch.
  • Pelvis transmits the weight of the trunk to the legs acting as a bridge between femur.
  • Affords protection to the pelvic organs to lesser extent to the abdominal context.

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