Flat Pelves. Rachitic Flat Pelvis
This type of narrow pelvis is characterized by reduction of the anteroposterior diameter of the pelvic inlet. The external conjugate is shortened; the upper portion of lumbar rhomboid is reduced. As for internal constitution (structure) of the pelvis, it is caused by special changes of the sacrum: the sacrum is slightly flattened; the corpus and the apex of the sacrum are usually deviated backward, while the base of the sacrum is moved up forward.
That’s why the anteroposterior diameter of the pelvic inlet is reduced, while anteroposterior diameters of other planes are enlarged, although they may be normal in sizes. On vaginal examination one can find protruding sacral promontory, which can be easily reached by an examiner’s finger. Exostoses are common in rachitic pelvis.
Typical sizes of rachitic flat pelvis are: interspinous diameter – 28 cm, intercristal diameter – 28 cm, intertrochanteric diameter – 30 cm, external conjugate –17 cm (less than 20 cm), diagonal conjugate, true conjugate are less than normal (Fig. 201).
a. b.
Fig. 201. Rachitic flat pelvis: a - front view; b - side view.
Mechanism of labor in rachitic flat pelvis is changed because of the shortened anteroposterior diameter of the pelvic inlet.
The 1st moment is when the sagittal suture aligns with transverse diameter of the pelvic inlet for a long time. Since the anteroposterior diameter is shortened, the head fails to engage and preserves this position for a few hours.
Moderate deflexion of the vertex occurs during this moment, as a result of which the large (anterior) fontanelle becomes a leading point (denominator), and bitemporal diameter of the head becomes the engaging diameter. The typical sign of this mechanism is anterior asynclitism, which occurs more frequently than posterior asynclitism. In anterior asynclitism the posterior parietal bone rests against anteriorly displaced promontory where it is retained, while the anterior parietal bone descends gradually into the pelvis (Fig. 202).
Fig. 202. Anterior asynclitism in flat pelvis
The sagittal suture is now situated nearer the promontory (Fig. 203).
Fig. 203. Sagittal suture is near the promontory
In this position (with the sagittal suture aligned with the transverse diameter, closer to the promontory, and anterior fontanelle below the posterior one) the head remains engaged until it is sufficiently molded, after which the posterior parietal bone slips off the promontory and the asynclitism disappears. The next moments are the same as in normal pelvis.
The 2nd moment is internal rotation of the head, with occiput turned to the symphysis pubis, and face turned to the promontory. The 3rd moment is deflexion of the head, the 4th is internal rotation of the shoulders and external rotation of the head, the 5th moment is lateral flexion of the trunk and shoulders delivery, the 6th moment is expulsion of the trunk.
After the head has passed the contracted inlet plane of the pelvis, the expulsion of the fetus becomes very rapid because the pelvic cavity is normal or enlarged, while the pelvic outlet is wider than normal. Complications are: fetal hypoxya due to precipitate delivery, intracranial injuries of the fetus, injuries of the perineum and other soft tissues of the birth canal.
Simple Flat Pelvis.Simple flat (platypellic) pelvis is characterized by reduction of all anteroposterior diameters of the pelvis. This occurs due to displacement of the sacrum: it is pushed in the pelvic cavity towards the symphysis pubis. Thus all anteroposterior diameters of the pelvic planes are less than in the normal pelvis. In a simple flat pelvis the Michaelis’s rhomboid is shortened in the vertical direction so that its upper and lower angles are obtuse, while the lateral ones are acute.
Typical sizes of simple flat pelvis are: interspinous diameter – 26 cm, intercristal diameter – 28 cm, intertrochanteric diameter – 30 cm, external conjugate – less than 20 cm, diagonal and true conjugates are less than in normal pelvis (Fig.204).
Mechanism of Labor in Simple Flat Pelvis. The head enters the pelvis in the same way as in flat rachitic pelvis: the sagittal suture aligns with the transverse diameter of the pelvic inlet, the engaging diameter of the fetal head is bitemporal diameter, and the leading point (the denominator) is anterior (large) fontanelle. Further it descends into the pelvic cavity and is delivered as in the occiput presentation. But sometimes the head fails to perform the internal rotation because the anteroposterior diameters of the midpelvic and outlet pelvic planes are also reduced.
The head enters the pelvic cavity and sometimes even reaches the pelvic floor, while the sagittal suture remains aligned with the transverse diameter. This condition is termed as lower transverse situation of the sagittal suture. In some cases the head rotates with its occiput anteriorly as it reaches the pelvic floor and labor then ends spontaneously. The rotation of the head on the pelvic floor is usually complicated by lacerations of the perineum. The duration of labor in simple flat pelvis is marketly longer than in rachitic pelvis, but more preferable for the fetus. Typical complications are: secondary uterine inertia, fetal hypoxia, maternal lacerations.
The presenting parietal bone of the delivered head bears a large swelling: the shape of the head may be oblique, named a brachicephalic shape (Fig.205).
Fig. 204. Simple flat pelvis
Fig. 205. Brachicephalic shape of the head
Flat pelvis with reduction of anteroposterior diameter of the 2nd plane of the pelvis. Anteroposterior diameter of the second plane of the pelvis is the only shortened size in this pelvis. This type of pelvis is a result of failed development of skeleton at the age of 12-15 years due to hyperandrogenic and hypoestrogenic condition. There is absence of the sacral curvature, the sacrum is significantly flat, thus the anteroposterior diameter of the 2nd plane is shortened. The diagnosis of this pelvis is rather difficult.
The external diameters of the pelvis are not changed. The general constitution is normal. There are no marked diseases in the anamnesis. An experienced obstetrician may pay attention to the flat sacrum during vaginal examination. Sonography measurement of internal pelvic sizes helps to make the diagnosis.
Mechanism of labor at this pelvis is not well studied. The 1st moment is the same as in normal pelvis. But the 2nd moment (internal rotation of the head) is usually failed due to shortened anteroposterior diameter; cephalo-pelvic disproportion is often present in this pelvis.
Generally Contracted Flat Pelvis. This shape of pelvis is characterized by shortening of all diameters of the pelvis with prevalence of anteroposterior diameters (Fig.206). The mechanism of labor is usually similar to that at other flat pelves, but then the engagement of the head fails. Labor at generally contracted pelvis is very difficult.
Fig. 206. Generally contracted flat pelvis
Date added: 2022-12-25; views: 428;