Diagnosis of asymmetry and obliquely displaced pelvis
Detecting asymmetry of the bone ring plays a key role in obstetrics practice. An obliquely displaced pelvis can be suspected as early as the visual assessment of the pregnant woman's gait: a characteristic rolling ("waddling") gait often indicates this pathology or excessive mobility of the pelvic joints.
To accurately confirm asymmetry, measurement of oblique diameters is used. The physician determines the distance from the anterior superior iliac spine on one side to the posterior superior iliac spine on the opposite side. Normally, this value is 22.5 cm. If a difference is revealed when comparing two opposite oblique diameters, this reliably indicates asymmetric pelvic deformation.
Additional information is provided by assessing Michaelis rhomboid, located on the posterior surface of the sacrum. In women with a harmoniously developed pelvis, its contours are visible as clearly as possible. With sharp deformations and oblique displacements, the rhomboid loses its symmetry and acquires irregular shapes depending on specific structural features.
History taking and external examination
The diagnosis of any anatomically contracted pelvis is based on a comprehensive approach. The main methods include thorough medical history taking, external examination, obstetrical and vaginal examinations, and pelvimetry.
Special attention is paid to the medical history. Risk factors include tuberculosis or rickets suffered in childhood, as well as injuries to the lower extremities and pelvic bones. In multiparous women, the history of previous labors is of crucial diagnostic significance. Pelvic contraction may be indicated by:
- Difficult passage of the fetal head (even if the baby was not large).
- Traumatic brain injuries in the newborn.
- Cases of intrapartum or postnatal fetal death.
- Need for operative delivery (Cesarean section).
Important nuance: if a woman has previously successfully given birth to an average-weight baby, this indicates the absence of significant contraction, even if current pelvimetry shows a slight deviation from the norm.
During the external examination, which is initially performed in an upright position, attention is paid to height (145–155 cm is a reason to suspect contraction) and traces of past diseases. Rickets in the past is indicated by an "olympic forehead" (wide nasal bridge and protruding frontal tubercles), saber shins. Shortening of one leg, joint ankylosis, and spinal deformities (scoliosis, lordosis, kyphosis) are also recorded.
Assessment of abdominal shape and Michaelis rhomboid
The shape of the abdomen at the end of pregnancy when examined in profile is an important visual marker. Because with a contracted pelvis the fetal head cannot press against the pelvic inlet before labor, the uterine fundus stands high, does not fit under the costal arch, and strongly presses against the anterior abdominal wall.
- In primiparae, the abdomen acquires a pointed shape, tapering upward, since the muscles are still able to retain the uterus.
- In multiparae, the abdominal wall is overstretched, the uterus deviates anteriorly, and the abdomen becomes pendulous.
If the pregnant woman is lying on her back, a characteristic protrusion of the abdominal wall is noticeable above the pubic symphysis — this is the contour of the fetal head located above the pelvic inlet.
Returning to Michaelis rhomboid: its parameters have direct diagnostic significance. The transverse diameter of the rhomboid reflects the transverse parameters of the pelvis (when they are contracted, it becomes smaller), and the vertical diameter correlates with the direct diameters of the pelvic planes.
Other forms of anomalies: kyphotic pelvis and tumors
In addition to the obliquely displaced pelvis, other specific deformations of the bone ring also occur.
Kyphotic pelvis usually forms in childhood due to tuberculous lesions of the spine. If kyphosis is localized in the cervical or thoracic spine, it is balanced by lumbar lordosis and does not interfere with labor. However, kyphosis of the lower sections (hump) changes pathobiomechanics: the sacrum rotates around the transverse axis and shifts posteriorly, the upper parts of the iliac bones diverge, and the lower parts and ischial tuberosities converge. The pelvis becomes funnel-shaped (dimensions decrease from top to bottom). The direct diameter of the inlet plane increases, but the outlet dimensions (especially transverse ones) are markedly contracted, creating an obstacle for the fetal head in the narrow part of the cavity and the outlet plane.
Pelvis contracted by tumors and exostoses is characterized by a reduction in the lumen due to formations on the inner surface of the bones. Exostoses are formed from ossified exchondroses and are most often localized in the area of the sacral promontory (promontorium), symphysis, or sacroiliac joints. Osteosarcomas (bone tissue tumors) are capable of reaching large sizes, occupying the entire pelvic cavity.
Pelvimetry and additional methods
External pelvimetry is used to objectify the data. The doctor evaluates not only the absolute values of the external pelvic dimensions, but also their relationship. Classical parameters are measured: distantia spinarum, distantia cristarum, distantia trochanterica, and conjugata externa.
To clarify the diagnosis, lateral conjugate diameters are measured (the distance between the anterior superior and posterior superior iliac spines on one side). Normally they are 14–15 cm; when the pelvis is contracted, they decrease to 13 cm.
If standard methods are insufficient, additional instrumental examinations are used if necessary: X-ray pelvimetry, computed tomography (CT) pelvimetry, or magnetic resonance imaging (MRI).
Briefly: answers to topical questions
What are the main diseases and pathologies leading to the formation of an obliquely displaced pelvis?
The main diseases and pathologies leading to the formation of an obliquely displaced pelvis are conditions that disrupt the symmetry of the musculoskeletal system in childhood. These include:
- Scoliosis — disrupts the symmetry of the musculoskeletal system.
- Coxitis — disrupts the symmetry of the musculoskeletal system.
- Unilateral hip dislocation — disrupts the symmetry of the musculoskeletal system.
- Shortening of one of the lower extremities — disrupts the symmetry of the musculoskeletal system.
- Poorly healed pelvic and leg fractures — disrupt the symmetry of the musculoskeletal system.
- Functional disorders of one leg — disrupt the symmetry of the musculoskeletal system.
What are the normal vertical and transverse dimensions of Michaelis rhomboid in centimeters?
Normally, the longitudinal (vertical) and transverse dimensions of Michaelis rhomboid are 11 and 10 centimeters, respectively. These parameters are measured with a centimeter tape and are an indirect sign of correct body build and normal pelvic dimensions.
- Longitudinal diameter — equals 11 cm, reflects the direct pelvic dimensions.
- Transverse diameter — equals 10 cm, reflects the transverse pelvic dimensions.
When the direct or transverse dimensions of the pelvic planes decrease, the corresponding dimensions of the rhomboid also become smaller.
For quick memorization of the effect of kyphosis on the pelvis: "The higher the hump, the easier the labor." Kyphosis in the cervical or thoracic region is compensated by lordosis and does not affect the pelvis, while kyphosis in the lower region narrows the pelvic outlet.
Class and exam questions
How does gait help suspect an obliquely displaced pelvis?
A pregnant woman may exhibit a "waddling" (rolling) gait. This occurs due to asymmetry of the bone ring or excessive mobility of the joints of the pelvic bones.
What does a pendulous abdomen in a pregnant woman indicate before labor?
In multiparous women with a contracted pelvis, the fetal head does not press against the pelvic inlet. Because of this, the high-standing uterus presses against the overstretched abdominal muscles, causing it to become pendulous.
How do pelvic dimensions change in lower spinal kyphosis?
A funnel-shaped pelvis is formed: the direct diameter of the inlet increases, and the outlet dimensions (especially transverse ones) sharply decrease due to the convergence of the ischial tuberosities.
Does a decrease in pelvimetry dimensions always mean problems in labor?
No. If a woman has a history of successful childbirth with an average-weight baby, this indicates the absence of contraction of the true pelvis, despite measurement data.
What else to study on this topic
- Biomechanics of kyphotic pelvis formation and sacral displacement
- Impact of exostoses and osteosarcomas on the biomechanics of labor
- Assessment of Michaelis rhombus: correlation of transverse and conjugate dimensions with pelvic shape
- Classical external pelvimetry dimensions (d. spinarum, d. cristarum, d. trochanterica, c. externa)
- Instrumental diagnostic methods: X-ray pelvimetry, CT, and MRI
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