Assessment of pelvic bone parameters and thickness
To objectively assess the capacity of the lesser pelvis, it is necessary to consider not only external distances but also the massiveness of the bone skeleton.
- Soloviev's index: measured with a tape measure at the level of the wrist joint. The normal value is 14 cm. If the circumference exceeds this value, the bones are considered massive, which makes the actual volume of the lesser pelvis smaller than the calculated values.
- Transverse diameter of the pelvic outlet: the normal value is 11 cm. To find the true value, 1–1.5 cm is added to the measured figure to account for soft tissue thickness. Measurement is performed using a specialized pelvimeter.
- Pubic angle: examined by placing the physician's thumbs along the pubic rami. In normal joint anatomy, the angle is 90°.
- Oblique diameters: measured when deformation is suspected. Paired distances are evaluated: from the anterior superior iliac spine of one side to the posterior superior iliac spine of the opposite side, from the symphysis to the right and left posterior superior iliac spines, and from the supra-sacral fossa to the anterior superior iliac spines. Normally, symmetric values are equal. A difference of more than 1 cm indicates pelvic asymmetry.
In cases where additional clarifying diagnostics, assessment of deformations, or verification of the correspondence between the fetal head and the mother's pelvis are necessary, X-ray pelvimetry is strictly prescribed according to indications.
Diagnostic value of the Michaelis rhombus
Sacral rhombus (Michaelis rhombus) serves as an important indirect landmark for normal body build and correct structure of the bony pelvis.
The boundaries of the shape are formed by the following bony landmarks:
- Upper angle: spinous process of the last lumbar vertebra.
- Lower angle: area of the sacrococcygeal joint.
- Lateral angles: right and left posterior superior iliac spines.
Under physiological norms, depressions (dimples) are clearly contoured in all four angles. The normal dimensions of the rhombus measured with a tape are 11 cm along the longitudinal axis and 10 cm along the transverse axis.
External obstetric examination (palpation)
Abdominal palpation is performed with the pregnant woman in a specific position: she lies on her back with legs slightly flexed at the knee and hip joints. The physician stands on the right side of the woman, facing her.
During the examination, the following tasks are addressed:
- The condition of the anterior abdominal wall is assessed, including the tone of the rectus muscles, the presence of hernia protrusions, or diastasis.
- The dimensions of the uterus and its functional tone (presence of tension upon palpation) are determined.
- In the third trimester, the fetal attitude, lie, position, presentation, and part leading are recorded.
Leopold maneuvers are used for a systematic examination:
- First Leopold maneuver: the palms of both hands are placed on the uterine fundus, with fingertips touching. This maneuver helps determine the height of the uterine fundus, estimate gestational age, and identify which fetal part is located in the fundus. Most easily palpated there are the buttocks, which differ from the head by lesser density and the absence of a distinct spherical shape.
- Second Leopold maneuver: aimed at determining the fetal position. For this, the lateral parts of the uterus are palpated to locate the back and small parts (limbs).
Topographic variants of fetal presentation and position
Based on palpation and the location of fontanelles relative to the bony landmarks of the pelvis, various presentation variants are distinguished:
| Parameter | Variant A | Variant B |
|---|---|---|
| Fetal lie | Longitudinal | Longitudinal |
| Presentation | Cephalic | Cephalic |
| Position and variety | Second position, anterior variety | First position, posterior variety |
| Sagittal suture | In the left oblique diameter | In the left oblique diameter |
| Small fontanelle | Right anterior | Left posterior |
Rule for memorizing key numbers: 14 cm — wrist circumference (Soloviev's index), 11 cm — vertical of the Michaelis rhombus and pelvic outlet size, 10 cm — horizontal of the Michaelis rhombus.
Class and exam questions
Why is Soloviev's index assessed and how does it affect pelvic calculations?
Soloviev's index shows the thickness of the pelvic bones by measuring the wrist circumference (normally 14 cm). Exceeding this value indicates that the bones are massive, so the actual volume of the lesser pelvis will be smaller than the calculated external parameters.
What difference in pelvic oblique diameters is considered pathological?
Normally, paired oblique diameters are absolutely equal. A difference of more than 1 cm between them indicates asymmetry of the bony pelvis.
What are the anatomical landmarks and normal dimensions of Michaelis' rhomboid?
The upper angle of the rhombus is the last lumbar vertebra, the lower is the sacrococcygeal joint, and the lateral are the posterior superior iliac spines. Normally, the vertical size is 11 cm, and the transverse size is 10 cm.
What does the first Leopold maneuver allow to determine?
The first Leopold maneuver allows assessment of the height of the uterine fundus, estimation of gestational age, determination of the fetal lie, and identification of which large part (head or buttocks) is located in the uterine fundus.
What else to study on this topic
- Technique of performing the third and fourth Leopold maneuvers
- Measurement of the main external pelvic dimensions (Distantia spinarum, cristarum, trochanterica, Conjugata externa)
- Classification of anatomically and clinically narrow pelvis
- Methodology for performing vaginal obstetric examination in labor
- Determination of fetal head descent through the planes of the lesser pelvis
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