HLTH1008 Chap.2 The Bony Pelvis, Sacroiliac Joint and Pelvimetry
The Bony Pelvis, Sacroiliac Joint and Pelvimetry
The bony pelvis is taught as a ring with openings that must be measured. This chapter covers the three parts of the hip bone, the anteroposterior, oblique and transverse diameters of the inlet and outlet with their average values, the features that separate a male from a female pelvis, and the ligaments that close the sciatic notches into foramina.
It then explains nutation and counternutation of the sacrum and the tilts and rotations of the whole pelvic ring with their planes and axes. Spotter stations on this topic flag a landmark or ligament and then ask what attaches to it or passes beside it. The landmarks named here are also the attachment points used in the gluteal and thigh chapters.
What this chapter covers
- 01
Ilium, ischium, pubis and the triradiate cartilage
- 02
Inlet and outlet diameters
- 03
Male and female pelvis compared
- 04
Sacrospinous, sacrotuberous and iliolumbar ligaments
- 05
Greater and lesser sciatic foramina
- 06
Nutation and counternutation
- 07
Pelvic tilt and rotation planes
Worked example · free
Sexing a pelvis on the bench
- +1The subpubic angle is the first feature to read: greater than 100° in the female, less than 90° in the male.
- +1An oval inlet and a wide, shallow, cylindrical true pelvis are female features.
- +1An oval or triangular obturator foramen is female; a round one is male.
- +1All features agree, so the pelvis is female; add the shorter, wider sacrum if it is visible.
Key terms
- Pelvic inlet
- The upper opening of the true pelvis, bounded by the pelvic brim.
- Subpubic angle
- The angle beneath the pubic symphysis between the two inferior pubic rami.
- Sacrospinous ligament
- The ligament from the lateral sacrum to the ischial spine.
- Counternutation
- Extension of the sacrum, which widens the brim and narrows the outlet.
- Lateral pelvic tilt
- A frontal-plane movement of the pelvis about an anteroposterior axis.
- Sacrotuberous ligament
- The ligament from the sacrum to the ischial tuberosity that closes the lesser sciatic foramen.
- Iliolumbar ligament
- The ligament joining the lower lumbar transverse processes to the iliac crest.
- Triradiate cartilage
- The Y-shaped cartilage in the child's acetabulum where ilium, ischium and pubis meet.
The Bony Pelvis, Sacroiliac Joint and Pelvimetry FAQ
What are the average diameters of the pelvic inlet?
The course gives about 10.5 to 11 cm anteroposteriorly from the pubic symphysis to the sacral promontory, about 12 to 12.5 cm obliquely from one sacroiliac joint to the opposite iliopubic eminence, and about 13 to 13.5 cm transversely across the widest part of the brim.
Why does nutation matter in childbirth?
Nutation flexes the sacrum so that the brim's anteroposterior diameter falls while the outlet's rises, and counternutation does the reverse. Those opposite changes alter the space available at each opening, which is why the model paper asks how the movement changes the pelvic inlet.
What passes through the greater sciatic foramen?
Piriformis passes through and divides it. Above piriformis run the superior gluteal nerve and vessels; below it run the sciatic, inferior gluteal and pudendal nerves, the posterior cutaneous nerve of the thigh and the inferior gluteal and internal pudendal vessels.
How can you tell a male pelvis from a female pelvis?
Start with the subpubic angle, under 90° in the male and over 100° in the female. Then check the inlet, heart-shaped in the male and oval in the female, the depth of the true pelvis, the shape of the obturator foramen and the length and width of the sacrum.
What do anterior and posterior pelvic tilt do to the spine?
Both happen in the sagittal plane about a coronal axis. Anterior tilt produces hip flexion and increases the lumbar lordosis, while posterior tilt produces hip extension and flattens the lumbar curve, which is why pelvic posture and low back shape are examined together.
Exam move
Draw the pelvic inlet and outlet from above and below and mark each diameter between its two landmarks before adding the average number. Learn the male and female table as six pairs of contrasts, starting with the subpubic angle. For the foramina, write the boundary ligaments first and then the contents, above and below piriformis.
Finish by explaining nutation aloud in one sentence for each opening, naming the direction of change. On a model, place a finger on each landmark of every diameter and say its name, then repeat with the ligaments and foramina, because the practical item list asks for both the bones and the soft tissue attached to them.
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