BONY PELVIS

The bony pelvis is a rigid, basin-shaped ring of bones connecting the vertebral column to the lower limbs. It functions primarily to bear the weight of the upper body, protect pelvic viscera (internal organs), and provide attachment points for muscles of the trunk and lower extremities.

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1. Osteology - Bone Structure of the Pelvis

Component Bones

The pelvic girdle is formed by the fusion of three major bones: the two hip bones laterally and anteriorly, and the sacrum posteriorly. The coccyx forms the terminal segment.

The Hip Bone (Os Coxae) - Detailed Landmarks

A. Ilium

The largest and most superior component of the hip bone. It forms the superior aspect of the acetabulum and extends superiorly to form the iliac fossa.

• Iliac crest: The superior curved border; serves as an attachment site for abdominal muscles and fascia.
• Anterior Superior Iliac Spine (ASIS): The anterior termination of the iliac crest; a palpable landmark and attachment for the inguinal ligament.
• Anterior Inferior Iliac Spine (AIIS): Located below the ASIS; origin of the rectus femoris muscle.
• Posterior Superior Iliac Spine (PSIS): Posterior termination of the iliac crest; attachment for posterior sacroiliac ligaments.
• Posterior Inferior Iliac Spine (PIIS): Located below the PSIS; forms the superior boundary of the greater sciatic notch.
• Greater sciatic notch: A large indentation on the posterior margin; converted into the greater sciatic foramen by pelvic ligaments.
• Iliac fossa: The large, smooth concavity on the internal surface; origin of the iliacus muscle.

B. Ischium

The posterior-inferior component of the hip bone, forming the posterior aspect of the acetabulum and the inferior body of the pelvis.

• Ischial spine: A pointed projection from the posterior margin; separates the greater and lesser sciatic notches.
• Lesser sciatic notch: Located below the ischial spine; converted to the lesser sciatic foramen by the sacrospinous and sacrotuberous ligaments.
• Ischial tuberosity: The rough, weight-bearing prominence; this is the primary point of contact when sitting (the "sitting bone").
• Ischial ramus: The anterior extension that fuses with the inferior pubic ramus to form the ischiopubic ramus.

C. Pubis

The anterior component of the hip bone, forming the anterior aspect of the acetabulum and the anterior body of the pelvis.

• Superior pubic ramus: Extends from the body of the pubis to the acetabulum; contains the pectineal line.
• Inferior pubic ramus: Extends from the pubic body to fuse with the ischial ramus.
• Pubic crest: The superior border of the pubic body; attachment for the rectus abdominis muscle.
• Pubic tubercle: A small prominence on the pubic crest; medial attachment point of the inguinal ligament.
• Pectineal line: A sharp ridge on the superior pubic ramus; forms part of the pelvic brim.

Major Openings & Landmarks

• Obturator Foramen: A large opening formed by the ischium and pubis, almost completely covered by the obturator membrane in life. The obturator nerve and vessels pass through a small gap called the obturator canal (superior part of the foramen).
• Acetabulum: The deep, cup-shaped socket on the lateral aspect of the hip bone.
  • The ilium forms the superior roof.
  • The ischium forms the posterior-inferior portion.
  • The pubis forms the anterior portion.
  • The acetabular fossa is the non-articular depression at the center.
  • The lunate surface is the articular (cartilage-covered) crescent-shaped rim that contacts the femoral head.

Sacrum & Coccyx Details

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2. Pelvic Divisions & Spaces

The pelvis is divided into functional spaces by the pelvic brim (pelvic inlet), creating the greater (false) pelvis above and the lesser (true) pelvis below. Understanding these divisions is critical for both anatomical study and clinical practice, especially in obstetrics.

The Pelvic Inlet (Pelvic Brim)

The pelvic inlet is the superior opening of the true pelvis, bounded continuously by the following structures:

• Posterior: Sacral promontory and alae (wings) of the sacrum.
• Lateral: Arcuate line of the ilium.
• Anterolateral: Pectineal line of the pubis.
• Anterior: Pubic crest and superior border of the pubic symphysis.

The Pelvic Outlet

The pelvic outlet is the inferior opening of the true pelvis, bounded by:

• Anterior: Pubic arch (subpubic angle).
• Lateral: Ischial tuberosities.
• Posterolateral: Sacrotuberous ligaments.
• Posterior: Tip of the coccyx.

True vs. False Pelvis

Pelvic Cavity - Dimensions & Axis

The Pelvic Axis is an imaginary curved line passing through the center of the pelvic cavity from the sacral promontory to the pubic symphysis. The fetal head must align with this axis during normal labor. The axis is not straight - it curves posteriorly at the inlet and anteriorly at the outlet.

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3. Joints & Ligamentous Support

The stability of the pelvic ring depends on a combination of strong ligaments and specialized joints designed primarily for weight transfer rather than mobility. Understanding these structures is essential for comprehending pelvic fracture mechanics and stability.

A. Pubic Symphysis

Secondary Cartilaginous Joint (Amphiarthrosis): Unites the left and right pubic bones anteriorly via an interpubic fibrocartilage disc. It is a slightly movable joint that allows for limited movement during walking and, importantly, expansion during childbirth under hormonal influence. It is reinforced by superior and inferior pubic ligaments.

B. Sacroiliac (SI) Joints

The SI joints are among the strongest joints in the body, designed to transfer weight from the upper body to the lower limbs. They are synovial joints in their anterior portion but have extensive fibrous connections posteriorly.

• Type: Synovial (anterior) + Fibrous (posterior).
• Articular Surface: Auricular (ear-shaped) surfaces of the sacrum and ilium.
• Function: Weight transfer, minimal movement.
• Stability: Extremely stable due to interlocking surfaces and massive ligaments.
• Clinical: A common source of lower back pain (SI joint dysfunction).

C. Sacrococcygeal Joint

The articulation between the apex of the sacrum and the base of the coccyx. It is a symphysis type joint with an intervertebral disc that may undergo fusion with age. The coccyx is capable of limited backward movement during defecation and childbirth.

Key Pelvic Ligaments

1. Sacroiliac Ligaments (Anterior, Posterior, Interosseous):
   • The anterior ligaments are thin.
   • The posterior sacroiliac ligaments are massive, multi-layered structures that are the primary stabilizers of the SI joint.
   • The interosseous sacroiliac ligaments fill the space between the sacrum and ilium posteriorly, forming the strongest ligamentous structure in the body.
2. Sacrotuberous Ligament: Extends from the posterior sacrum and posterior superior iliac spine to the ischial tuberosity. It is a broad, strong ligament that prevents upward tilting of the sacrum and converts the greater sciatic notch into the greater sciatic foramen.
3. Sacrospinous Ligament: Extends from the lateral sacrum and coccyx to the ischial spine. It is thinner and more triangular. Together with the sacrotuberous ligament, it converts the lesser sciatic notch into the lesser sciatic foramen.

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4. Pelvimetry & Sexual Dimorphism

The bony pelvis exhibits significant sexual dimorphism, with the female pelvis specifically adapted for childbirth. Pelvimetry is the measurement of pelvic dimensions, critical for assessing whether a vaginal delivery is feasible.

Anatomical Sex Differences

Obstetric Conjugates

These are anteroposterior (AP) measurements of the pelvic inlet.

• Anatomical (True) Conjugate: From the sacral promontory to the superior border of the pubic symphysis. This is the shortest AP diameter but cannot be measured clinically. (Normal: >= 11 cm).
• Obstetric Conjugate: The narrowest fixed distance over which the fetal head must pass. Measured from the sacral promontory to the posterior protrusion (most prominent point) of the pubic symphysis. This is the most critical measurement for assessing pelvic adequacy. (Normal: >= 10 cm).
• Diagonal Conjugate: The only conjugate that can be measured clinically via manual vaginal examination. It extends from the sacral promontory to the inferior border of the pubic symphysis. The obstetric conjugate is estimated by subtracting 1.5-2 cm from the diagonal conjugate. (Normal: >= 11.5 cm).

Caldwell-Moloy Classification

This system categorizes the female pelvis into four types based on the shape of the pelvic inlet:

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5. Clinical & Applied Anatomy

Understanding pelvic anatomy is essential for managing trauma, performing diagnostic procedures, predicting obstetric outcomes, and recognizing hormonally mediated changes during pregnancy.

Fracture Mechanics - The Pelvic Ring

Bone Marrow Aspiration

The posterior iliac crest is the preferred site for bone marrow biopsy and aspiration because:

• It is easily accessible and palpable.
• It contains abundant hematopoietic (red) marrow in adults.
• The risk of damaging major vessels or nerves is minimal.
• The bone is relatively superficial (covered only by skin, subcutaneous tissue, and gluteal muscles).

Procedure Note: The needle is inserted 2-3 cm posterior to the posterior superior iliac spine (PSIS) and advanced through the cortical bone into the marrow cavity.

Obstetric Complications: Obstructed Labor (Dystocia)

Pelvic outlet narrowing or contraction of the interspinous diameter can lead to cephalopelvic disproportion (CPD) - a mismatch between fetal head size and pelvic dimensions.

• Inlet contraction: Short obstetric conjugate (< 10 cm) leads to failure of fetal head engagement.
• Mid-pelvic contraction: Narrow interspinous diameter (< 10 cm) leads to deep transverse arrest.
• Outlet contraction: Narrow subpubic angle leads to difficulty with fetal head extension and delivery.

Hormonal Modifications During Pregnancy