Introduction

The pelvic inlet, also referred to as the superior pelvic aperture or upper pelvic narrow, is the anatomical limit between the true pelvis inferiorly and the false pelvis superiorly. There are morphologic, genetic, and hormonal differences related to reproduction that differentiate the male and female pelvis. In obstetrics, the pelvic inlet is the entrance to the birth canal. The fetal cephalic extremity must position itself and adapt adequately to compare the smaller diameter with the largest diameter of the space delimited by the anatomical line of the maternal pelvic inlet. The shape of the inlet depends on the general shape of the pelvis, according to the traditional classification of Caldwell and Moloy.[1] The dimensions of its anteroposterior, oblique, and transverse diameters vary according to the morphological type of the pelvis. The proportions of the shape of the internal pelvic spaces correspond to the proportion of the sacral area of Michaelis.

Radiological evidence shows that the subject's posture changes the intrapelvic space. The position taken by the subject influences the values of the transversal and anterior-posterior diameters. This evidence is instrumental in facilitating fetal entry into the true pelvis and favoring the dilating phase of labor.[2] Evaluating the diameters of the endopelvic spaces and their adaptability (mobility) would be beneficial for diagnosing the "contracted pelvis" and for avoiding the adverse effects on maternal and newborn health that protracted labor and operative birth can entail.

Structure and Function

Bony Anatomy

The pelvic inlet involves 3 of the 4 elements of the bony pelvis. The pelvic brim is formed by contributions from the first sacral segment, the ilium, and the pubis, but not the ischium. The pelvic inlet is delineated by a bony crest that defines its limit (the pelvic brim). The pelvic brim includes the promontory of the sacrum and continues inferolaterally towards the ilium, traversing the rounded edge that separates the sacral bases from the anterior face of the sacrum itself: the linea terminalis. The pelvic brim continues laterally, crossing the sacroiliac joint, passes along the iliopectineal line in its initial portion as the linea innominata or arcuate line of the ilium (also called the "innominate bone"), continues along the pectineal line of the pubis (crest) where it passes along the superior-posterior edge of the pubis and the pubic tubercle, and ends just lateral to the pubic symphysis. See Image. Pelvis Anatomy.

Along the internal face of the ilium, the arcuate line ends behind the anterior angle that separates the superior and inferior portions of the auricular surface, which is articulated with the sacrum. Tracing an imaginary line that continues posteriorly, the pelvic brim continues towards the posterior superior iliac spine after passing through the inferior iliac tuberosity. In this way, by analogy, the pubic tubercle and the pubic symphysis are the anterior junction bridge of the 2 halves of the pelvic inlet, while the posterior superior iliac spine (PSIS) and the space between the ilia (sacroiliac joints and anterior and posterior ligaments) are the extremes of the posterior bridge.

Visceral Structures

The sigma crosses the upper pelvic narrow with the root of the mesosigma (at the level of the left sacroiliac joint), the ureter, and the vas deferens. Sometimes even the cecum and the appendix descend into the pelvis from the right iliac fossa, near the right sacroiliac joint. The bladder dome tends to protrude beyond the upper strait when the bladder is full, along with the medial vesicoumbilical ligament (vestiges of the urachus) and the medial vesicoumbilical folds (vestiges of the umbilical arteries). The parietal peritoneum is the deepest parietal structure that crosses the upper pelvic cavity.

Embryology

The pelvic skeleton forms via mesenchymal condensation and endochondral ossification. The first ossification center develops in the ilium during the early fetal period. Multiple sites ossify and continue to develop after birth, continuing into adolescence. The fetal pelvis, including subpubic angle, width, and depth of the sciatic notch, varies with biological sex. However, the growth of the ischium and pubis and the ischiopubic indices do not change with age. Sex differences in pelvic morphology may be recognized in fetuses at 9 weeks from fertilization (or 11 weeks from the last menstrual period) after the sex chromosomes are activated.[3]

The connections and articulations of the cartilage in the pelvis are essential for pelvic ring formation in a limited period: between 54 and 60 days after fertilization, at around 8 weeks of embryonic development, or 10 weeks of gestation. The normal period is necessary for effective fetal movement, which may induce mechanical forces and affect normal skeletal development. Fetal movement may also explain some variations in pelvic shape observed later in life. Observations made later in the fetal period indicate that the most common pelvic inlet shape is similar between sexes: android in 56% of male and 54% of female fetuses. It suggests the importance of environmental factors in shaping pelvic morphology.[3]

Blood Supply and Lymphatics

The division of the iliac vessels from the common iliac vessels to the external and internal iliac vessels occurs anterior to the sacroiliac joint. The internal iliac vessels descend into the true pelvis, while the external iliac vessels run along the medial belly of the psoas muscle and parallel to the pelvic brim. More inferiorly, the obturator vessels similarly traverse the edge of the pelvic brim.

The structure running most closely along the pelvic brim is the umbilical artery, located above the obstructive vascular-nervous bundle in contact with the osseous-ligamentous plane of the pelvic inlet. The middle sacral artery originates from the abdominal aorta in the angle formed by the emergence of the common iliac arteries, then passes over the sacral promontory to descend to the coccyx. The retropubic branch of the epigastric artery (a branch of the external iliac artery) bypasses the pelvic brim in its anterior hemicycle. The same happens for the ovarian vessels (but not the testicular ones that enter the inguinal canal from the internal orifice) and the round ligament of the uterus with its vessels.

Nerves

The obturator nerve must cross the pelvic inlet and the fourth and fifth lumbar roots of the lumbosacral plexus. The lateral-vertebral chain of the orthosympathetic system passes anterior to the sacral bases.

The superior hypogastric plexus runs anterior to the abdominal aorta and sacral promontory to descend and mix its fibers with the inferior endopelvic hypogastric plexus. The union of the superior and inferior hypogastric plexuses provides innervation to the gastrointestinal and genital organs in the pelvis.[4]

Muscles

A few myofascial structures insert on the bony edge of the pelvic inlet, yet they are bypassed by many vascular, nervous, and visceral structures. These structures may be impacted by compressive forces while the fetal head is about to engage inside the pelvis in the last week of gestation.

Muscle Structures

On the edge of the pelvis (the pelvic brim), the lower portion of the iliopsoas muscle is inserted posteriorly, straddling the sacroiliac joint before the muscle belly moves forward toward the pubis, sliding parallel to the pelvic brim with its medial border. It is covered by the band that below continues, after passing the arcuate line, on the upper insertion of the obturator internus muscle. The pectineus muscle originates anteriorly with respect to the pubopectineal crest of the pelvic inlet. The psoas minor muscle, inconstant, can insert directly on the pectineal line and its ligament.[5]

Ligaments and Bands

The anterior longitudinal ligament, which runs anteriorly along the entire length of the vertebral column, continues with the sacral periosteum at the level of the promontory. A small bundle of the anterior iliolumbar ligaments, which is part of the fibrous capsule of the sacroiliac joint, has a curvilinear inferior course that is inserted anteriorly on the arched line.[6] The inguinal ligament has its main anteroinferior insertion on the pubic tubercle. It continues with the lacunar ligament, forming a robust thickening as the iliopectineal ligament, which is found on the continuation of the innominate line of the ilium.

The inguinal ligament is the continuation of the abdominal transverse muscle fascia, which originates posteriorly from the lumbar region as a continuation of the thoracolumbar fascia. The iliolumbar ligaments thicken and continue with the thoracolumbar fascia on the anterior side of the lateral aspect of the spine.

Physiologic Variants

Morphology of the Pelvic Inlet

The general shape of the pelvis was defined in a historical publication of the 1930s by Caldwell and Moloy. The shape of the circumference of the pelvic cavity, the internal space of the pelvis, depends on the overall shape of the pelvis itself.

The shape of the female pelvis has been classified into 4 types by Caldwell and Moloy, who report the following proportions in a sample of 147 cases: gynoid, 41.4%; android, 32.5%; anthropoid, 23.5%; and platypelloid, 2.6%. Each has peculiar characteristics regarding the width of the sub-pubic angle, the height of the pelvis, the transverse diameters of the 3 pelvic planes (inlet, mid pelvis, outlet), and the shape of the circumference of the upper pelvic narrow.[1]

The inlet of the gynoid pelvis appears to be ovoid, with a transverse diameter just greater than the anterior-posterior diameter. The android pelvis has a heart-shaped configuration, with the greatest transverse diameter located toward the sacrum. The obstetric conjugate of the anthropoid pelvis is much greater than the transverse diameter. In the platypellic pelvis, the inlet is wide and very narrow. The shape depends on both genetic factors and environmental factors such as nutrition and lifestyle. Food deficits or infectious diseases lead to pelvic deformations, which, fortunately, are now part of the past. Severe forms of adolescent scoliosis can affect pelvic morphology, as well as congenital hip dislocation, and probably also fetal posture in utero (unpublished personal observations). A 1996 Abitbol study correlates pelvic morphology with factors such as hormone levels, age at the onset of upright posture and walking, and intense sports activity in adolescence.[7]

The Orientation of the Pelvic Inlet

The pelvic inlet has an inclination of about 55 to 60 degrees with respect to the anatomical horizontal plane. Major or minor angles mean pelvic retroversion, anteversion, and posterior or anterior pelvic tilt. The general inclination of the pelvis is maintained by the balance of tension between different muscular and fascial anatomical components. The thoracolumbar fascia plays a fundamental role in the statics and dynamics of the pelvis and its relationship with the shoulder girdle, which are essential for maintaining posture, walking, and coordinated limb movements. According to Cottingham (1988), the front tilt corresponds to a general orthosympathetic tone, while the posterior pelvic inclination corresponds to a relative parasympathetic tone.[8][9]

The muscles that maintain the inclination are mainly those of the abdominal "box": the anterior and lateral abdominal muscles (which contribute with their bands to the formation of the inguinal ligament), the paravertebral muscles, the psoas major and the piriformis muscles; the thoracoabdominal diaphragm and the pelvic floor. In the hamstrings muscle group, the biceps femoris muscle is mentioned for its proximal insertion on the ischial tuberosity in continuity with the sacrotuberous ligament, which is 1 of the main thickenings of the sacral-lumbar-thoracic fascia complex.

Clinical Significance

Pelvic External Morphology and Pelvic Inlet

The traditional external pelvimetry of Baudelocque was based on the hypothesis that the external contours and dimensions of the pelvis reflect its internal anatomy. Current literature supports this correlation.[10] One can trace and measure the oblique diameter, the transverse diameter, and the anterior-posterior or conjugate diameter for the pelvic inlet. The shape of the inlet is regulated by the relative proportions between the transverse diameter and the obstetric conjugate.[11]

The sacral diamond area corresponds externally to the pelvic inlet: the transverse and longitudinal diameters of the Michaelis sacral area exhibit a specific reciprocal relationship for each type of pelvis, matching the shape of the pelvic inlet. The transverse diameter of the sacral rhombus-shaped area of Michaelis is the distance between the 2 posterior superior iliac spines (PSIS).[12] The longitudinal diameter is described between the tip of the spinous process of the fifth lumbar vertebra at the top and the upper limit of the intergluteal groove, corresponding to the fourth-fifth sacral segment.

The 2 diameters of the sacral diamond area are usually equal. However, the longitudinal dimension is sometimes larger than the transverse (the opposite is uncommon), and they intersect at their midpoint. Occasionally, the transverse diameter, which intersects the longitudinal diameter at the level of the second sacral segment, divides the latter into 2 unequal parts: the upper part of the longitudinal diameter is shorter than the lower portion.

The biomechanical dynamics differ depending on pelvic morphology, despite the same underlying principle. The fascia anatomy of the sides of the sacral diamond area, which regulates its shape and movement, corresponds to the fascial thickenings that are part of the sacral complex of the thoracolumbar fascia, which surrounds the sacroiliac joints both posteriorly and, from the iliolumbar ligaments, anteriorly. The biomechanical properties of the bands would have repercussions from the inside to the outside and vice versa.[6] The shape of the posterior muscular and adipose tissues seems to correspond with the general pelvic morphology. The classification is as follows: the gynecoid pelvis corresponds to a round buttocks shape, the platypelloid pelvis to a triangular shape, the anthropoid pelvis to a square shape, and the android pelvis to a trapezoidal gluteal form.[13] 

The diameters of the sacral area were used to detect cephalopelvic disproportion, which was followed by complicated labor and dystocia, resulting in a correlation between the transverse diameter and complications during childbirth.[10][12][14][15]

Diameters of the Pelvic Inlet

The pelvic inlet is an irregular circumference; 3 diameters can be defined. The anteroposterior (or "conjugate") diameter is the distance between the pubic symphysis and the sacral promontory. Three distances are:

• The anatomical conjugate or true: Measured between the sacral promontory and the upper edge of the pubic symphysis, and measures an average of 11.0 cm
• The obstetric conjugate: Measured from the sacral promontory to the point bulging the most on the back of the symphysis pubis, located about 1 cm below its upper border. It measures 10.5 cm on average; it is the lesser anteroposterior diameter.
• The diagonal conjugate: Measured between the sacral promontory and the lower edge of the pubic symphysis, measuring an average of 12.5.

The anteroposterior diameter can also be evaluated with external pelvimetry using various pelvic tubes. According to the classical explanation of Baoudeloque, the external conjugate is measured from the upper edge of the pubic symphysis anteriorly to the first sacral segment posteriorly ("the depression 1 cm under the spinous process of the fifth lumbar vertebra"); its dimensions are about 20 cm.[10]

The oblique diameter is the largest of the 3; it is the distance between the arched line near the sacroiliac joint posteriorly and the pubopectineal line anteriorly. This is the diameter of the fetal head; it measures about 12 cm. The transverse diameter is the distance between the 2 innominate lines at their widest point, measuring approximately 13 cm. The entrance of the fetal head into the pelvic excavation normally takes place through a rotation from the oblique diameter to the transversal diameter. External pelvimetry can also evaluate the transverse diameter by measuring the distance between the 2 upper posterior iliac spines (PSIS). The average distance between the 2 PSIS is about 11 cm.[10]

Other Issues

Endopelvic Biomechanical Dynamics

A longitudinal study published in 2016 in PNAS shows that pelvic morphology in both genders is not static but changes with age. Pelvic morphology appears to change every 2 decades, driven by remodeling.[16] For women, the pelvic form, which has the largest internal diameters, is present between 20 and 40 years: the age of sexual maturity and pregnancy, with a lower risk.

Two MRI studies have evaluated the endopelvic diameters of subjects in different postures: supine, hands and knees, and kneeling squats. Significant changes are evident in the position: a decrease in the diameter of the conjugate of about 3 mm, an increase in the anteroposterior diameters of the midplane, and outlet diameters of 4 and 5 mm, while the transverse diameters increase by about 15 mm. The study was conducted on 2 groups of women: pregnant and non-pregnant. The results were the same. The pelvic inlet showed less difference than the outlet and the mid-pelvis; this may indicate that, during positional changes, the inlet alters its shape and inclination more on the horizontal plane than its transverse diameter.[2][17]

In clinical obstetrics and osteopathic practice, assessing the quality of posterior pelvic movement is often considered a general indicator of the feasibility of vaginal delivery. The diameters of the sacral area are the transverse diameter that lies between the 2 posterior superior iliac spines (PSIS) and the longitudinal diameter stretched between the spinous apophysis of the fifth lumbar vertebra and the beginning of the intergluteal groove (the "diamond test," taught for decades in the osteopathic practice). They should increase their amplitude when the pregnant subjects go from standing upright to squatting. Depending on possible amplitude restriction of the range of motion, a joint (sacroiliac, hip, or pubis) or fascial/ligamentous dysfunction could be hypothesized.

Pelvimetry and Obstetrics: Past and Present

The classical external pelvimetry, whose foremost representative was the French physician Baudelocque in the 18th century, was abandoned with the advent of modern technology. It had been an attempt to explain the reasons for dystocia without offering any means to solve the problem.

In the first decades of the twentieth century, radiologic examination shed new light on static pelvimetric evaluation by enabling direct measurement of internal diameters; however, this method proved disappointing. MRI has made measurements easier and more precise due to reproducible scans. The ability to record motion in images has made it possible to observe live childbirth and confirm that changes in the parturient's position alter the internal pelvic diameters.[2] This information was intuitively known and used by midwives for several centuries.

The evidence from radiological studies supports the development of a clinical test of external pelvimetry. The current qualitative test in pelvimetry, commonly recommended in obstetric and osteopathic settings, has practical limitations, including a lack of standardization, difficulty for most pregnant women in late gestation to assume a smooth squatting position, and the absence of reference curves and cutoff values.

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References

1.

SWENSON PC. Anatomical variations in the female pelvis; the Caldwell-Moloy classification. Radiology. 1947 May;48(5):527. [PubMed: 20240130]

2.

Reitter A, Daviss BA, Bisits A, Schollenberger A, Vogl T, Herrmann E, Louwen F, Zangos S. Does pregnancy and/or shifting positions create more room in a woman's pelvis? Am J Obstet Gynecol. 2014 Dec;211(6):662.e1-9. [PubMed: 24949546]

3.

Okumura M, Ishikawa A, Aoyama T, Yamada S, Uwabe C, Imai H, Matsuda T, Yoneyama A, Takeda T, Takakuwa T. Cartilage formation in the pelvic skeleton during the embryonic and early-fetal period. PLoS One. 2017;12(4):e0173852. [PMC free article: PMC5383024] [PubMed: 28384153]

4.

Röthlisberger R, Aurore V, Boemke S, Bangerter H, Bergmann M, Thalmann GN, Djonov V. The anatomy of the male inferior hypogastric plexus: What should we know for nerve sparing surgery. Clin Anat. 2018 Sep;31(6):788-796. [PubMed: 29577446]

5.

Siccardi MA, Tariq MA, Valle C. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): Aug 8, 2023. Anatomy, Bony Pelvis and Lower Limb: Psoas Major. [PubMed: 30571039]

6.

Willard FH, Vleeming A, Schuenke MD, Danneels L, Schleip R. The thoracolumbar fascia: anatomy, function and clinical considerations. J Anat. 2012 Dec;221(6):507-36. [PMC free article: PMC3512278] [PubMed: 22630613]

7.

Abitbol MM. The shapes of the female pelvis. Contributing factors. J Reprod Med. 1996 Apr;41(4):242-50. [PubMed: 8728076]

8.

Cottingham JT, Porges SW, Richmond K. Shifts in pelvic inclination angle and parasympathetic tone produced by Rolfing soft tissue manipulation. Phys Ther. 1988 Sep;68(9):1364-70. [PubMed: 3420170]

9.

Cottingham JT, Porges SW, Lyon T. Effects of soft tissue mobilization (Rolfing pelvic lift) on parasympathetic tone in two age groups. Phys Ther. 1988 Mar;68(3):352-6. [PubMed: 3279437]

10.

Liselele HB, Tshibangu CK, Meuris S. Association between external pelvimetry and vertex delivery complications in African women. Acta Obstet Gynecol Scand. 2000 Aug;79(8):673-8. [PubMed: 10949233]

11.

Betti L, Manica A. Human variation in the shape of the birth canal is significant and geographically structured. Proc Biol Sci. 2018 Oct 24;285(1889) [PMC free article: PMC6234894] [PubMed: 30355714]

12.

Liselele HB, Boulvain M, Tshibangu KC, Meuris S. Maternal height and external pelvimetry to predict cephalopelvic disproportion in nulliparous African women: a cohort study. BJOG. 2000 Aug;107(8):947-52. [PubMed: 10955423]

13.

Cuenca-Guerra R, Quezada J. What makes buttocks beautiful? A review and classification of the determinants of gluteal beauty and the surgical techniques to achieve them. Aesthetic Plast Surg. 2004 Sep-Oct;28(5):340-7. [PubMed: 15666052]

14.

Bansal S, Guleria K, Agarwal N. Evaluation of sacral rhomboid dimensions to predict contracted pelvis: a pilot study of Indian primigravidae. J Obstet Gynaecol India. 2011 Oct;61(5):523-7. [PMC free article: PMC3257337] [PubMed: 23024521]

15.

Kakoma JB. Cesarean section indications and anthropometric parameters in Rwandan nulliparae: preliminary results from a longitudinal survey. Pan Afr Med J. 2016;24:310. [PMC free article: PMC5267785] [PubMed: 28154665]

16.

Fischer B, Mitteroecker P. Covariation between human pelvis shape, stature, and head size alleviates the obstetric dilemma. Proc Natl Acad Sci U S A. 2015 May 05;112(18):5655-60. [PMC free article: PMC4426453] [PubMed: 25902498]

17.

Michel SC, Rake A, Treiber K, Seifert B, Chaoui R, Huch R, Marincek B, Kubik-Huch RA. MR obstetric pelvimetry: effect of birthing position on pelvic bony dimensions. AJR Am J Roentgenol. 2002 Oct;179(4):1063-7. [PubMed: 12239066]

Disclosure: Marco Siccardi declares no relevant financial relationships with ineligible companies.

Disclosure: Onyebuchi Imonugo declares no relevant financial relationships with ineligible companies.

Disclosure: Tafline Arbor declares no relevant financial relationships with ineligible companies.

Disclosure: Cristina Valle declares no relevant financial relationships with ineligible companies.