Biomechanism in the uniformly contracted pelvis
With this form of contraction, the fetal head occupies a strictly central position in the pelvic cavity. Its descent occurs exclusively along the axis of the birth canal. Due to the uniform contraction of all planes, the head experiences immense compression from all sides. As a result, it becomes sharply elongated, acquiring a characteristic dolichocephalic shape.
An extensive caput succedaneum forms in the area of the posterior fontanelle (caput succedaneum). This phenomenon is of critical clinical significance, as it requires the physician to be extremely careful during vaginal examination. A high risk arises of error in determining the station of the head. The caput succedaneum can be so large that it descends all the way to the pelvic outlet plane. Meanwhile, the head itself with its large circumference is still located high — in the wide part of the pelvic cavity or even higher.
The onset of early bearing-down efforts in such a situation is extremely dangerous and inevitably leads to complications for both mother and fetus. Overall, labor is prolonged. The prognosis becomes extremely unfavorable if pelvic contraction is combined with a large fetus, any extension presentations, or occiput posterior position.
Features of labor in the rachitic flat pelvis
The specifics of the biomechanism in this case are dictated by the reduction of the anteroposterior diameters of the pelvic inlet plane. The fetal head encounters serious resistance: anteriorly it is obstructed by the pubic bones, and posteriorly by the sacral promontory. Because of this, it is physically unable to engage in either of the oblique diameters of the pelvis.
Engagement occurs in such a way that the sagittal suture is located strictly in the transverse diameter of the pelvis. The head remains above the pelvic inlet for a long time, trying to adapt. It undergoes slight extension, as a result of which the anterior and posterior fontanelles are set at the same level. The extended head can fit into the pelvic inlet only due to the fact that its anteroposterior diameter (12 cm) is smaller than the transverse diameter of the pelvic inlet (13 cm).
Passage through the inlet plane has its own characteristics. Through the anteroposterior diameter of the inlet (which is the smallest in this pelvic shape), the head squeezes through with its small transverse diameter (8.5 cm). At the same time, the large transverse diameter of the head (9.5 cm) shifts away from the true conjugate (conjugata vera) and finds space in the more spacious lateral section of the inlet plane.
Asynclitic engagement of the head
If, in a flat pelvis, the extended head positioned in the transverse diameter cannot descend into the cavity, asynclitism occurs—non-axial engagement.
Anterior asynclitism (Naegele's) In this case, the head engages with the anterior parietal bone, while the posterior parietal bone rests against the sacral promontory (promontorium). The sagittal suture is thereby deflected toward the promontory. Under the action of labor forces (contractions) and the constant pressure of the promontory, a complex molding biomechanism is triggered: the posterior parietal bone slips past the anterior one. This allows the transverse diameter of the head to be reduced. As soon as the head dimensions adapt to the contracted pelvic dimensions, the posterior parietal bone slips off the promontory, and the head successfully descends into the pelvic cavity.
Posterior asynclitism (Litzmann's) This variant is less common and is more unfavorable. The posterior parietal bone engages into the pelvic inlet first, and the sagittal suture is deflected anteriorly, toward the pubic symphysis. Fetal descent stops because the anterior parietal bone is securely retained above the pubic symphysis. Vaginal delivery with this type of engagement is generally impossible.
Summary: triad of the rachitic flat pelvis
To sum up, we can distinguish the classic triad of signs characteristic of head engagement in the rachitic flat pelvic inlet:
- Prolonged station of the head with the sagittal suture in the transverse diameter of the pelvic inlet.
- Extension of the head (fontanelles are at the same level).
- Asynclitic (non-axial) engagement of the head to overcome the obstruction.
Briefly: answers to topical questions
What dimensional changes during external pelvimetry are characteristic of a rachitic flat pelvis?
During external pelvimetry of a rachitic flat pelvis, the ratio of transverse diameters changes and the external conjugate decreases.
Characteristic changes:
- Convergence of transverse diameters — the difference between distantia spinarum and distantia cristarum becomes less than normal, or they become equal (26 cm each).
- Inversion of dimensions — with pronounced deformations, the distance between the anterior superior iliac spines becomes greater than that between their iliac crests.
- Reduction of the external conjugate — conjugata externa is 17 cm.
Which pelvic shapes are included in the modern classification of frequently occurring forms of an anatomically contracted pelvis?
The modern classification of frequently occurring forms of an anatomically contracted pelvis includes three main groups.
- Transversely contracted pelvis — the most common form.
- Flat pelvis — includes simple flat (Litzmann/non-rachitic), rachitic flat pelvis, and pelvis with a reduced anteroposterior diameter of the wide part of the cavity.
- Uniformly contracted pelvis — characterized by a proportional reduction of all dimensions.
The triad of signs in a rachitic flat pelvis is easily remembered by the acronym PRA (in Russian): Transverse position of the sagittal suture, Extension of the head, Asynclitic engagement.
Class and exam questions
Why is it easy to make a mistake in determining the head station in a uniformly contracted pelvis?
Due to the formation of a large caput succedaneum. It can descend all the way to the pelvic outlet plane, creating a false impression of a low head station, while its large segment is still located high in the pelvic cavity.
How does the head overcome the contracted anteroposterior diameter in a rachitic flat pelvis?
The head engages with the sagittal suture in the transverse diameter and extends slightly. Through the smallest anteroposterior diameter, it passes with its small transverse diameter (8.5 cm), while the large transverse diameter (9.5 cm) shifts to the more spacious lateral section.
What is the difference in prognosis between anterior and posterior asynclitism?
Anterior (Naegele's) asynclitism allows the head to mold: the posterior parietal bone slips past the anterior one, slides off the sacral promontory, and labor continues. In posterior (Litzmann's) asynclitism, the anterior parietal bone is arrested above the pubic symphysis, descent stops, and vaginal delivery is impossible.
What else to study on this topic
- Technique of differential diagnosis of caput succedaneum during vaginal examination
- Features of labor course in occiput posterior position
- Biomechanism of labor in various extension presentations of the fetus
- Risks and management of labor in the combination of a contracted pelvis and a large fetus
- Topography and clinical significance of the axis of the birth canal
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