External pelvic dimensions
Measurement of external pelvic dimensions is performed using a pelvimeter and includes four main distances of vital importance in obstetric practice. Memorizing the numerical values is simplified by a mnemonic rule: each successive value is approximately 2 cm larger than the preceding one.
- Distantia spinarum — the distance between the anterior superior iliac spines (spinae iliacae anteriores superiores), normally 25–26 cm.
- Distantia cristarum — the distance between the most distant points of the iliac crests (cristae iliacae), normally 28–29 cm.
- Distantia trochanterica — the distance between the greater trochanters of the femurs (trochanteres majores), normally 30–31 cm (31–32 cm according to some sources).
- Conjugata externa — external conjugate, measured from the upper margin of the pubic symphysis to the suprapacral fossa (upper angle of Michaelis rhombus, between the spinous process of L5 and the sacrum), normally 20–21 cm.
Michaelis rhombus
Michaelis rhombus (sacral rhombus) — an area on the posterior surface of the sacrum visible as a skin relief. Its boundaries are:
- upper angle — the depression between the spinous process of L5 and the sacrum;
- lateral angles — the posterior superior iliac spines (spinae iliacae posteriores superiores), the so-called "dimples of Venus";
- lower angle — the tip of the sacrum.
Normally, the horizontal and vertical diagonals of the rhombus are approximately equal, measuring about 11 cm. A symmetrical rhombus indirectly indicates normal pelvic structure. A sharp flattening or elongation of the rhombus indicates anomalies: a significant decrease in the horizontal dimension indicates a transversely contracted pelvis.
Conjugata vera (true conjugate)
Conjugata vera — the smallest anteroposterior diameter of the pelvic inlet, the distance from the sacral promontory (promontorium) to the most projecting point on the posterior surface of the pubic symphysis into the pelvic cavity. It cannot be measured directly and is calculated by indirect methods:
- Subtract 9 cm from conjugata externa: 20 − 9 = 11 cm (normal).
- Using the vertical dimension of Michaelis rhombus — it is approximately equal to the true conjugate.
- Via conjugata diagonalis applying the Solovlev index.
Conjugata diagonalis and Solovlev index
Conjugata diagonalis — diagonal conjugate, the distance from the lower margin of the pubic symphysis to the sacral promontory. Measured during a vaginal examination: the obstetrician inserts the index and middle fingers into the vagina, advancing them to the promontory while marking the level of the lower margin of the symphysis with the thumb.
- If the fingers do not reach the promontory, the diagonal conjugate is greater than 12.5 cm, and the pelvis is normal.
- If the promontory is reached, the distance is measured with a ruler and the true conjugate is calculated.
Solovlev index — the circumference of the wrist joint divided by 10; reflects bone thickness. True conjugate = conjugata diagonalis − Solovlev index. For example, with a wrist circumference of 15 cm, the index is 1.5; with a diagonal conjugate of 12.5 cm, the true conjugate will be 11 cm.
Conjugata lateralis and pubic angle
Conjugata lateralis (Kerner's lateral conjugate) — the distance between the anterior superior and posterior superior iliac spines on the same side. Normally exceeds 14 cm. A decrease to 12 cm or less indicates severe pelvic constriction and is an indication to consider a Cesarean section, as natural vaginal delivery is obstructed.
Pubic angle (angulus pubicus) is the angle between the inferior rami of the pubic bones. In normal females, it is 90–100°. An acute pubic angle is associated with a contracted pelvis and complicates the passage of the fetal head.
Key points
- External pelvic dimensions: spinarum 25–26, cristarum 28–29, trochanterica 30–31, conjugata externa 20–21 cm.
- Conjugata vera ≈ conjugata externa − 9 cm ≈ vertical dimension of Michaelis rhombus.
- Michaelis rhombus is normally symmetrical, diagonals are about 11 cm.
- Conjugata diagonalis is measured by palpation through the vagina from the symphysis to the promontory.
- Solovlev index is 1/10 of the wrist circumference, reflecting bone thickness.
- Conjugata vera = conjugata diagonalis − Solovlev index.
- Conjugata lateralis is normally greater than 14 cm; values of ≤12 cm indicate a Cesarean section.
- Pubic angle is normally 90–100°; its reduction indicates a contracted pelvis.