Simple flat pelvis and its anatomy
During the formation of a simple flat pelvis, the sacrum is displaced anteriorly toward the pubic symphysis. This results in the narrowing of anteroposterior diameters across absolutely all planes: the pelvic inlet, the wide and narrow parts of the pelvic cavity, and the pelvic outlet.
- The shape and natural curvature of the sacrum remain normal.
- The angle of the pubic arch remains sufficiently wide.
- Specific sign: sometimes an accessory sacral promontory forms between the first and second sacral vertebrae. It protrudes into the pelvic cavity more than the true promontory and is located closer to the symphysis.
Rachitic flat pelvis
This form of deformity occurs as a consequence of rickets suffered in childhood. The pathological process is based on dystrophic changes in bone tissue characteristic of this condition. Due to impaired mineralization, the pelvic ring becomes deformed, requiring special attention during labor management.
Diagnosis and pelvimetry
To detect a flat pelvis, the physician uses external obstetric examination (pelvimetry) and internal vaginal examination.
- External measurements: a reduction in the external conjugate (Conjugata externa) and the anteroposterior diameter of the pelvic outlet is recorded. Transverse dimensions remain within normal limits.
- Vaginal examination: palpation clearly reveals shortening of the diagonal conjugate (Conjugata diagonalis).
Differential diagnosis of contracted pelvis types
The flat pelvis must be differentiated from other anomalies, particularly the generally evenly contracted pelvis. The latter is a clinical manifestation of infantilism (developing in childhood or puberty). In this condition, all main dimensions, including Conjugata externa, are proportionally reduced. In some cases, the capacity of such a pelvis remains sufficient and does not obstruct the biomechanism of labor.
The most important visual criterion is the Michaelis rhombus:
- Normal pelvis: regular shape, normal dimensions.
- Simple flat pelvis: shape altered due to a reduction in the vertical dimension, outline may resemble a triangle.
- Rachitic flat pelvis: upper half is flattened, longitudinal dimension significantly reduced, upper angle is obtuse.
- Generally evenly contracted pelvis: shape is regular, but all dimensions are symmetrically reduced.
- Asymmetrically contracted pelvis: pronounced asymmetry (one half is smaller than the other, diagonals are unequal).
Comparative table of external dimensions
Pelvimetry data allow for precise differentiation of pelvic contraction types.
| Pelvic shape | D. spinarum | D. cristarum | D. trochanterica | C. externa | C. diagonalis |
|---|---|---|---|---|---|
| Normal | 25–26 cm | 28–29 cm | 30–31 cm | 20 cm | 13 cm |
| Simple flat | 26 cm | 29 cm | 30 cm | 18 cm | 11 cm |
| Rachitic flat | 26 cm | 26 cm | 31 cm | 17 cm | 11 cm |
| Generally evenly contracted | 24 cm | 26 cm | 28 cm | 18 cm | 13 cm |
| Transversely contracted | 24–25 cm | 25–26 cm | 28–29 cm | 20 cm | 11 cm |
Briefly: answers to topical questions
What clinical and anatomical causes underlie the formation of a generally evenly contracted pelvis as opposed to a flat pelvis?
The formation of the generally evenly contracted and flat pelvis is caused by various pathogenetic factors, including infantilism, rickets, and anatomical bone displacements.
| Characteristic | Generally evenly contracted pelvis | Flat pelvis |
|---|---|---|
| Clinical causes | Manifestation of infantilism (occurs in childhood or puberty) | Rickets suffered in childhood (for rachitic flat pelvis) |
| Anatomical causes | Regular miniature physique, thin skeletal bones | Displacement of the sacrum toward the pubic symphysis, dystrophic changes in bone tissue |
What is the biomechanism of labor in a simple flat pelvis?
The biomechanism of labor in a simple flat pelvis includes specific engagement of the fetal head at the pelvic inlet and variable passage through the pelvic cavity.
- At the pelvic inlet plane — the mechanism is similar to a rachitic flat pelvis: prolonged standing of the head with the sagittal suture in the transverse diameter, its extension, and asynclitic engagement.
- In the pelvic cavity — birth by occipital presentation is possible.
- Persistent transverse arrest — occurs when internal rotation is obstructed, with the head descending to the pelvic outlet plane with the sagittal suture in the transverse diameter.
For rapid differential diagnosis of contracted pelves, use the visualization of the Michaelis rhombus: - Normal — ideal rhombus. - Generally evenly contracted — ideal, but miniature. - Simple flat — compressed vertically (resembles a triangle). - Rachitic flat — "flattened" from above (upper angle is obtuse). - Asymmetrically contracted — skewed (diagonals are unequal).
Class and exam questions
Which pelvic dimensions change in a simple flat pelvis?
Exclusively the anteroposterior diameters in all planes of the lesser pelvis (inlet, wide and narrow parts, outlet) are reduced. Transverse and oblique dimensions retain their normal values.
What is an accessory sacral promontory?
This is a specific bony projection that sometimes forms between the first and second sacral vertebrae in a simple flat pelvis. It is located closer to the pubic symphysis than the true promontory.
How does Michaelis' rhombus change in a flat rachitic pelvis?
Its upper half becomes flattened, and the longitudinal dimension decreases significantly. As a result, the upper angle of the rhombus becomes obtuse.
What is the difference between a generally uniformly contracted pelvis and a flat pelvis?
In a generally uniformly contracted pelvis (a consequence of infantilism), all pelvic dimensions are proportionally reduced, while Michaelis' rhombus retains its regular shape. In a flat pelvis, only the anteroposterior dimensions are shortened.
What else to study on this topic
- Infantilism as an etiological factor in the development of a generally uniformly contracted pelvis.
- Transversely contracted pelvis: features of external dimension changes during pelvimetry.
- Asymmetrically contracted pelvis: causes of pronounced lumbosacral rhombus asymmetry.
- Pathogenesis of bone tissue dystrophic changes in rickets.
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