Anatomical picture of the generally evenly contracted pelvis
Generally evenly contracted pelvis (pelvis aequaliter angusta) represents a classical variant of anatomical contraction of the bony ring. The main distinguishing feature of this pathology is that there is an absolute symmetrical shortening of all dimensions without exception by two centimeters or more compared to standard obstetrical parameters.
Despite such a significant reduction in metric parameters, the spatial geometry and physiological shape of the normal female pelvis remain completely unchanged. Such an anatomical picture is traditionally detected in patients characterized by a regular, proportional, yet petite physique. An important detail is that the pelvic bones in such women, as well as the bone structures of the rest of the skeleton, are usually characterized as very thin. It is precisely because of this natural thinness of the bone tissue that the true capacity of the pelvic cavity has its own specific features.
Foreign approach to the classification of contracted pelves
In international obstetrical practice, it is customary to highlight somewhat different emphases when systematizing contracted pelves. The foreign classification is based on the precise localization of the contraction relative to specific planes of the lesser pelvis.
The following main types of pathological contractions are distinguished:
- Isolated contraction of the pelvic inlet plane.
- Contraction of the lesser pelvis cavity.
- Contraction of the pelvic outlet plane.
- General pelvic contraction. This variant represents a complex combination of all the above-mentioned contractions at different levels and, according to foreign statistical data, is encountered most frequently in clinical practice.
To diagnose inlet contraction, foreign specialists use strict digital criteria. The pathology is reliably diagnosed if the conjugate of the inlet is less than 10 cm and the transverse diameter is less than 12 cm. Notably, the frequency of developing serious obstetric complications increases sharply in cases where the patient exhibits a simultaneous reduction in both of these critically important dimensions.
Transversely contracted pelvis: differential diagnosis
For a full understanding of the topic, it is necessary to clearly distinguish the generally evenly contracted pelvis from the transversely contracted one. In transverse contraction, a completely different, specific picture is observed: the transverse dimensions of the lesser pelvis are reduced by a value from 0.6 to 1.0 centimeters or more.
Unlike uniform contraction, the direct dimensions in the inlet plane, as well as in the narrow part of the lesser pelvis cavity, behave extremely variably—they can be either pathologically reduced or compensatorily increased. Because of such pronounced disproportion, the shape of the pelvic inlet is deformed, losing its normal contours and acquiring a rounded or longitudinally oval shape.
Key anatomical features of the transversely contracted type include:
- Low degree of flaring of the iliac wings.
- Formation of a pathologically narrow pubic arch.
Degrees of contraction and diagnostic methods
Assessment of the degree of contraction of the transversely contracted pelvis is based exclusively on objective data from instrumental methods, primarily X-ray pelvimetry. The main criterion is the transverse diameter of the lesser pelvis inlet plane:
- Grade I contraction: the dimension ranges from 12.4 to 11.5 cm.
- Grade II contraction: the dimension ranges from 11.4 to 10.5 cm.
- Grade III contraction: the dimension is critically reduced and is less than 10.5 cm.
Comprehensive diagnosis of this pathology includes several sequential stages. During external pelvic measurement, the obstetrician records a reduction in external transverse dimensions, a narrowing of the transverse dimension of Michaelis rhombus (this indicator drops below 10 cm), as well as a reduction in the transverse dimension of the pelvic outlet plane. During bimanual vaginal examination, an acute pubic angle and abnormal approximation of the ischial spines are determined by palpation. If necessary, the final diagnosis is confirmed instrumentally using X-ray pelvimetry or magnetic resonance imaging (MRI).
Briefly: answers to topical questions
What are the features of the biomechanism of labor in a generally equally contracted pelvis?
Features of the biomechanism of labor in a generally evenly contracted pelvis include the central position of the fetal head and its advancement strictly along the pelvic axis.
- Advancement — the fetal head occupies a central position and advances strictly along the pelvic axis.
- Head molding — when passing through the planes of the lesser pelvis, the head experiences strong compression, elongates sharply in length, and takes on a pronounced dolichocephalic shape.
- Caput succedaneum (caput succedaneum) — a large caput succedaneum forms in the area of the small fontanelle.
- Course of labor — labor is prolonged.
What complications for the mother and fetus are typical in labor with a generally evenly contracted pelvis?
With a generally evenly contracted pelvis, the appearance of early bearing-down efforts is characteristic, which leads to serious complications for the mother and fetus.
- Diagnostic errors — due to the large caput succedaneum, an error in determining the station of the fetal head is possible.
- Early bearing-down efforts — their appearance with a high-standing head is extremely dangerous.
- Unfavorable prognosis — occurs when contraction is combined with a large fetus, extension presentations, or occiput posterior presentation.
Also, as with any contracted pelvis, postpartum hemorrhage, endometritis, and fistula formation are possible for the mother, and traumatic damage to the cranial bones and cerebral hemorrhages for the fetus.
What are the normal external dimensions of the female pelvis with which indicators are compared during contraction?
Normal external dimensions of the female pelvis include three transverse and one direct dimension.
- Interspinous distance (distantia spinarum) — the distance between the anterior superior iliac spines, normally 25–26 cm.
- Intercristal distance (distantia cristarum) — the distance between the most distant points of the iliac crests, equals 28–29 cm.
- Intertrochanteric distance (distantia trochanterica) — the distance between the greater trochanters of the femurs, is 31–32 cm.
- External conjugate (conjugata externa) — the direct dimension of the pelvis, normally equal to 20–21 cm.
Normally, the difference between adjacent transverse dimensions is 3 cm.
For quick memorization: Generally evenly contracted — shape preserved, all dimensions minus 2 cm. Transversely contracted — oval shape, transverse dimensions minus 0.6–1.0 cm.
Class and exam questions
By how many centimeters are the dimensions reduced in a generally and uniformly contracted pelvis?
In this pathology, absolutely all pelvic dimensions are shortened by 2 centimeters or more compared to standard values.
Does the shape of the pelvis change in a generally and uniformly contracted pelvis?
No, the physiological shape of the normal female pelvis is completely preserved; only a proportional reduction in its scale occurs.
What criteria for pelvic inlet narrowing are used in the foreign classification?
Contraction is diagnosed if the direct dimension is less than 10 cm and the transverse is less than 12 cm. The risk of complications increases with a reduction in both parameters.
How is a transversely contracted pelvis diagnosed during a vaginal examination?
Upon palpation, the doctor determines a characteristic acute pubic angle and abnormal approximation of the ischial spines.
What else to study on this topic
- Detailed analysis of the foreign classification of contracted pelves and their frequency of occurrence.
- Criteria and risks of complications in isolated contraction of the lesser pelvis inlet.
- Anatomy of the transversely contracted pelvis: changes in direct and transverse dimensions.
- Methodology for conducting and interpreting X-ray pelvimetry data in a contracted pelvis.
- Technique of external pelvic measurement and assessment of Michaelis rhombus.
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