Mechanism of deformity formation
The pathogenesis is based on rickets suffered in childhood. This disease leads to a marked decrease in bone tissue density, making the bones excessively pliable. In this condition, the pelvis experiences enormous static and dynamic loads. On the one hand, the mass of the trunk presses vertically on it through the spinal column. On the other hand, there is constant tension from the powerful muscular-ligamentous apparatus. Under the influence of these multidirectional forces, the pliable bones deform, and their normal spatial arrangement is irreversibly disrupted for the rest of life.
Sacral deformity and dimensional changes
With this pathology, the sacrum undergoes a pathological rotation around its sagittal axis. The bone itself becomes noticeably shortened and flattened, completely losing its normal physiological concavity characteristic of a healthy female pelvis.
The changes affect key planes:
- Pelvic inlet: Under the pressure of the spine, the sacral promontory protrudes strongly forward and comes as close as possible to the pubic symphysis. This leads to a critical reduction in the anteroposterior diameter of the inlet, which is the main obstacle at the onset of labor.
- Pelvic outlet: In contrast to the narrowed inlet, the sacrococcygeal joint along with the coccyx is displaced posteriorly. As a result of this displacement, the anteroposterior diameter of the outlet increases.
- Lower segment of the sacrum: Due to strong tension of the sacrospinous and sacrotuberous ligaments, the last sacral vertebra and the coccyx acquire a characteristic shape—they are hooked anteriorly. It is important to note that this specific curvature does not prevent the increase in the anteroposterior diameter of the outlet and the passage of the fetus.
Accessory (false) promontory
A specific marker of the rachitic pelvis is the formation of the so-called accessory (false) promontory. It arises as a result of pathological ossification of the cartilage tissue located between the 1st and 2nd sacral vertebrae.
The obstetric significance of this phenomenon is extremely high. The resulting bone protrusion significantly projects into the pelvic cavity. During labor, it can become a serious mechanical obstacle to the normal descent of the fetal presenting part (head), requiring special attention from the physician when assessing the biomechanism of labor.
Changes in the iliac bones
Since the deformed sacrum is displaced deep into the pelvic ring, the iliac bones are forced to change their spatial configuration. The wings of the iliac bones undergo severe flattening, and their crests turn outward, making the pelvis visually flatter.
These anatomical changes are easily detected by the obstetrician during standard external pelvimetry. The flat rachitic pelvis is characterized by a specific change in the ratio of transverse dimensions. The difference between the interspinous distance (distantia spinarum) and the intercristal distance (distantia cristarum) becomes significantly less than the physiological norm. In pronounced cases of deformity, these two dimensions become completely equal to each other, serving as a striking diagnostic sign.
Briefly: answers to topical questions
How do distantia trochanterica and conjugata externa change in a flat rachitic pelvis?
In a flat rachitic pelvis, distantia trochanterica is 31 cm, and conjugata externa decreases to 17 cm.
What are the features of labor biomechanics in a flat rachitic pelvis?
The features of the biomechanism of labor in a flat rachitic pelvis are due to the reduction of the anteroposterior dimensions of the pelvic inlet plane. Engagement of the head is characterized by the following triad of signs:
- Prolonged standing of the head — with the sagittal suture in the transverse diameter of the pelvic inlet.
- Extension of the head — slight extension allows the smallest anteroposterior dimension to pass through with the small transverse dimension.
- Asinclitic engagement — extra-axial engagement of the head (more frequently anterior Nägele's asinclitism, less frequently unfavorable posterior Litzmann's asinclitism).
How does the shape of the sacral rhombus (Michaelis rhombus) change in a flat rachitic pelvis?
In a flat rachitic pelvis, the shape of the sacral rhombus (Michaelis rhombus) undergoes characteristic changes due to pathological deformation and displacement of the sacrum. Upon visual assessment, it is noted that the upper half of the rhombus becomes flattened. The longitudinal (vertical) dimension of the rhombus decreases significantly, as a result of which its upper angle becomes obtuse.
How does a simple flat pelvis differ from a flat rachitic pelvis?
The main difference lies in the nature of changes in the anteroposterior dimensions and the morphology of the sacrum.
| Sign | Simple flat pelvis | Rachitic flat pelvis |
|---|---|---|
| Direct dimensions | Reduced in all planes of the lesser pelvis | The direct dimension of the pelvic inlet is reduced, the direct dimension of the pelvic outlet is increased |
| Sacrum | Shape and curvature remain normal | The sacrum is shortened and flattened, with rotation around the sagittal axis |
| Transverse dimensions | Remain normal | The difference between distantia spinarum and distantia cristarum decreases or these dimensions become equal |
To memorize the geometry of a flat rachitic pelvis, imagine it being heavily pressed from above: the sacrum is pushed inward (inlet narrowed), the iliac wings flare outward (transverse dimensions become equal), and the coccyx points backward (outlet widened).
Class and exam questions
Why does the direct dimension of the pelvic inlet decrease in a flat rachitic pelvis?
Due to bone softening under the pressure of the spine, the sacral promontory shifts forward and approaches the pubic symphysis, narrowing the pelvic inlet.
What happens to the transverse dimensions during external pelvimetry?
Due to the outward flaring of the iliac crests, distantia spinarum and distantia cristarum converge in value or become completely equal, losing their physiological difference.
Why is a false promontory dangerous during labor?
This bony prominence, formed by the ossified cartilage between the 1st and 2nd sacral vertebrae, protrudes into the pelvic cavity and can physically block the descent of the fetal head.
Does the hook-shaped curvature of the coccyx hinder the birth of the fetus?
No. Despite the fact that the last sacral vertebra and the coccyx are curved anteriorly due to ligamentous traction, the total direct dimension of the pelvic outlet remains increased.
What else to study on this topic
- Biomechanism of labor in a flat rachitic pelvis
- Diagnostics of pelvic contraction: vaginal examination
- Degrees of pelvic contraction and choice of delivery management
- Complications for mother and fetus in a clinically narrow pelvis
- Differences between the flat rachitic pelvis and the simple flat pelvis
Master the entire topic in the "Obstetrics and Gynecology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Flat Rachitic Pelvis" and 973 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 21 video lessons on the subject
Other topics in the "Obstetrics" section
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.