Bony pelvis and pelvic floor
Bony pelvis, pelvic floor, muscles and ligaments on imaging
The pelvis is a ring of two hip bones and the sacrum, closed below by a muscular floor. On radiographs, CT and MRI the radiologist reads its lines, joints and growth centres, and CPSP asks about the brim and diameters, the acetabular lines, pelvic ring injury patterns, normal variants in children, and the levator ani on dynamic imaging.
Bones, joints and growth centres
- Each hip bone forms from ilium, ischium and pubis, which meet in the acetabulum at the Y-shaped triradiate cartilage: a symmetrical, smooth Y-shaped lucency in a child, not a fracture. It fuses in mid-adolescence.
- Ischiopubic synchondrosis can look swollen and lucent at 4–10 years, often on one side only: a normal variant.
- Iliac crest apophysis ossifies from lateral to medial; its grade (Risser) estimates remaining spinal growth in scoliosis.
- Sacrum: the ala forms by fusion of costal elements and transverse processes. The iliolumbar ligament runs from the transverse process of L5 to the iliac crest, so it identifies L5 on MRI.
- Sacroiliac joint: lower anterior part is synovial (narrow, even space on CT); upper posterior part is a syndesmosis of the interosseous ligament, which normally looks wide and irregular.
- Pubic symphysis: secondary cartilaginous joint. Wide separation with no symphysis in a newborn with exposed bladder is bladder exstrophy (mesoderm fails to invade the cloacal membrane).
The pelvic brim and diameters
- Brim (inlet): sacral promontory, ala of sacrum, arcuate line of ilium, pectineal line (pecten pubis), pubic crest, upper border of the symphysis.
- Outlet (diamond): pubic arch in front, ischial tuberosities at the sides, sacrotuberous ligaments posterolaterally, coccyx behind. A line between the tuberosities divides the perineum into urogenital and anal triangles; the tuberosities carry body weight when sitting.
| Diameter | Measured from | Size and use |
|---|---|---|
| True (anatomical) conjugate | Promontory to upper border of symphysis | About 11 cm |
| Obstetric conjugate | Promontory to nearest point on the back of the symphysis | Shortest inlet AP diameter the head must pass; about 1.5–2 cm less than the diagonal |
| Diagonal conjugate | Promontory to lower border of symphysis | Felt on vaginal examination (about 12.5 cm) |
| Right oblique | Right sacroiliac joint to left iliopubic eminence | Sagittal suture lies here in left occipitoanterior |
| Interspinous | Between ischial spines | Narrowest plane; station 0 is level with the spines |
| Pelvis type | Inlet and features | |
| Gynaecoid (commonest, most favourable) | Round inlet, wide subpubic angle (over 90°, the best female marker) | |
| Android | Heart-shaped inlet, converging side walls, prominent spines, narrow angle (deep transverse arrest) | |
| Anthropoid | Oval, AP longer than transverse (direct occipitoposterior) | |
| Platypelloid | Flat, transverse much longer than AP |
Radiographic lines and joint disease
- Iliopectineal line marks the anterior column; ilioischial line (the quadrilateral surface seen end-on) marks the posterior column. A break in one line localises the acetabular fracture.
- Protrusio acetabuli: acetabular floor projects medial to the ilioischial line (rheumatoid arthritis, Paget disease).
| Appearance | Diagnosis |
|---|---|
| Triangular sclerosis on the iliac side of lower SI joints, joint normal, multiparous woman | Osteitis condensans ilii |
| Erosion and sclerosis both sides of the symphysis in an athlete | Osteitis pubis |
| Enlarged hemipelvis, thick iliopectineal line, high alkaline phosphatase | Paget disease |
| Dense focus with spiky margins blending with trabeculae | Bone island |
| Many small round dense foci around both acetabula | Osteopoikilosis |
| Apparent widening of SI joints and symphysis on dialysis | Subchondral resorption (hyperparathyroidism) |
| Square iliac wings, flat acetabular roofs, small sciatic notches | Achondroplasia |
| H-shaped sacral uptake in an osteoporotic woman | Sacral insufficiency fracture |
Pelvic ring injury
| Pattern | Mechanism and signs |
|---|---|
| Anteroposterior compression ("open book") | Symphysis widened, anterior sacroiliac ligaments torn, posterior ones intact |
| Lateral compression (commonest) | Horizontal overlapping rami fractures, buckle of the sacral ala |
| Vertical shear | Whole hemipelvis displaced upwards; rami and SI joint disrupted |
- Sacral fracture zones: lateral to the foramina, through the foramina, through the central canal (highest risk of bladder and bowel nerve injury).
Ligaments and foramina
- Sacrospinous and sacrotuberous ligaments convert the sciatic notches into greater and lesser sciatic foramina. The pudendal nerve crosses the ischial spine and sacrospinous ligament: block it at the ischial spine, and sacrospinous fixation sutures placed too close can trap it. It is stretched at this fixed point as the head descends in a long second stage.
- Inguinal ligament: anterior superior iliac spine to pubic tubercle.
- Obturator hernia passes through the upper lateral obturator canal, between pectineus and obturator externus; typical in thin elderly women.
- Ischial tuberosity: hamstring avulsion in sprinters; ischial bursitis from prolonged sitting on hard seats.
Pelvic floor
- Pelvic diaphragm = levator ani + coccygeus. Levator ani: pubococcygeus (with puborectalis and pubovaginal fibres) and iliococcygeus (from the tendinous arch over obturator internus fascia and the ischial spine). Coccygeus lies on the sacrospinous ligament.
- Nerve supply: nerve to levator ani (S4) from above, with pudendal branches from below.
- Urogenital hiatus transmits urethra and vagina; it is wider in women, a reason prolapse is commoner. Behind the anus the two sides meet in the anococcygeal body, forming the horizontal levator plate on which rectum and upper vagina rest.
- Imaging after childbirth: 3D ultrasound shows pubovisceral (pubococcygeus) avulsion from the pubic bone; dynamic MRI measures organ descent below the pubococcygeal line (lower symphysis to last coccygeal joint). A sagging levator plate and widened hiatus let the uterine ligaments stretch over years, giving prolapse.
- Perineal body: central point for bulbospongiosus, transverse perinei, external anal sphincter and levator fibres; the perineal membrane attaches to the ischiopubic rami.
- Retropubic space (Retzius): between pubis and bladder; a full bladder lifts the peritoneum, so a suprapubic catheter stays extraperitoneal.
Remember The obstetric conjugate, not the true conjugate, is the shortest AP space at the brim.
Exam trap Iliopectineal line = anterior column; ilioischial line = posterior column. Do not swap them.
Exam trap A symmetrical Y-shaped lucency in a child's acetabulum is the triradiate cartilage, not a fracture.
Test yourself on this topic
5 of the 63 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
A motorcyclist is crushed front-to-back between two vehicles. The pelvic radiograph shows the pubic symphysis widened to 4 cm. CT shows widening of the front of both sacroiliac joints, while the posterior parts of the joints stay closely apposed. Apart from the symphyseal ligaments, which ligaments have been torn?
Answer: Anterior sacroiliac ligaments
Anteroposterior compression opens the pelvis like a book. When the symphysis widens by more than about 2.5 cm, the anterior sacroiliac ligaments, together with the sacrospinous and sacrotuberous ligaments, must also have failed, so the front of each sacroiliac joint opens. The posterior sacroiliac and interosseous ligaments, the strongest in the pelvis, stay intact and act as the hinge, so the ring is rotationally unstable but vertically stable. If these posterior ligaments also tear, the hemipelvis becomes completely unstable.
Open-book (APC) injury
- Symphysis over 2.5 cm wide
- Anterior SI, sacrospinous, sacrotuberous torn
- Posterior SI ligaments act as the hinge
- Large pelvic volume: venous bleeding
Remember Book opens at the front; the spine of the book is the posterior SI ligaments.
Exam trap A pelvic binder closes the book and reduces bleeding; it should go on before imaging if the patient is unstable.
Why the other options are wrong
- Posterior sacroiliac ligaments Still intact here, since the back of the joints is not widened; they act as the hinge.
- Iliolumbar ligaments Join L5 to the iliac crest; not the hinge of an open-book injury.
- Inguinal ligaments Run from the ASIS to the pubic tubercle; they do not stabilise the ring.
- Lacunar ligaments Lie at the medial end of the inguinal ligament; not part of the pelvic ring.
The broad lateral mass (ala) on each side of the upper sacrum carries the auricular surface for the ilium. The ala is formed by fusion of which two parts of the sacral vertebrae?
Answer: Costal elements and transverse processes
Each vertebra has a costal element, which forms a rib in the thoracic region and the anterior part of the transverse process elsewhere. In the sacrum, the costal elements of the upper sacral vertebrae fuse with the transverse processes and with each other to form the lateral mass (ala). The auricular surface on its side articulates with the ilium at the sacroiliac joint. The bodies fuse in the midline, and the laminae and spinous processes form the posterior wall and median sacral crest.
Sacral lateral mass
- Ala = costal elements + transverse processes
- Carries the auricular surface
- Bodies fuse in the midline
- Spines form the median sacral crest
Remember Sacral ala is the sacrum's fused ribs.
Exam trap The costal element also explains cervical ribs, which come from the costal part of C7.
Why the other options are wrong
- Laminae fused with spinous processes Form the posterior wall of the canal and the median sacral crest.
- Bodies and intervertebral discs Bodies fuse in the midline, with remnants of the discs between them.
- Pedicles and superior articular processes Form the pedicles and the intermediate crest, not the ala.
- Laminae and transverse processes Laminae form the roof of the canal, not the lateral mass.
After a fall, a 10-year-old boy has a pelvic radiograph showing a Y-shaped lucent band in the floor of each acetabulum, symmetrical on both sides, with smooth margins. What is this lucency?
Answer: Triradiate cartilage
The acetabulum is formed by the ilium, ischium and pubis, which meet at a Y-shaped plate of hyaline cartilage called the triradiate cartilage. It allows growth of the acetabulum and appears as a symmetrical Y-shaped lucency on radiographs of children. It fuses in mid-adolescence, at about 14 to 16 years. Its symmetry and smooth margins distinguish it from a fracture. Early injury to it can cause a shallow acetabulum.
Triradiate cartilage
- Junction of ilium, ischium, pubis
- Y-shaped lucency in children
- Fuses at about 14-16 years
- Injury may cause a shallow acetabulum
Remember Symmetrical Y in a child's acetabulum: triradiate cartilage.
Exam trap Always compare both sides; growth cartilage is symmetrical, fractures are not.
Why the other options are wrong
- Acetabular fracture Would be unilateral, irregular and usually displaced.
- Ischiopubic synchondrosis Lies between the ischial and pubic rami, below the acetabulum.
- Os acetabuli A small ossicle at the acetabular rim, not a Y-shaped band.
- Iliac crest apophysis Lies along the iliac crest, not in the acetabulum.
An 82-year-old thin woman has small bowel obstruction. CT shows a loop of ileum passing out of the pelvis through the upper lateral part of the obturator foramen, lying between pectineus and obturator externus. Through which opening has the bowel herniated?
Answer: Obturator canal
The obturator foramen is closed by the obturator membrane except at its upper lateral corner, where the obturator groove of the pubis forms the obturator canal. The obturator nerve and vessels pass through it to the medial thigh. Bowel herniating through the canal comes to lie between pectineus and obturator externus. Obturator hernia is commonest in thin elderly women, and pressure on the obturator nerve can cause pain down the medial thigh to the knee.
Obturator hernia
- Through the obturator canal
- Upper lateral part of obturator foramen
- Lies between pectineus and obturator externus
- Thin elderly women
Remember Hernia behind pectineus: obturator canal.
Exam trap The hernia is too deep to feel, so CT is usually needed to find it.
Why the other options are wrong
- Femoral canal Lies medial to the femoral vein, below the inguinal ligament, not through the obturator foramen.
- Greater sciatic foramen Opens into the buttock, above or below piriformis.
- Lesser sciatic foramen Opens into the perineum, around the ischial spine.
- Deep inguinal ring Opens into the inguinal canal above the inguinal ligament.
The sacroiliac joint has a lower, anterior part and an upper, posterior part of different structure. What is the upper, posterior part?
Answer: It is a syndesmosis of interosseous ligament
The sacroiliac joint has two parts. The anterior and inferior part, between the auricular surfaces, is a synovial joint lined by cartilage. The posterior and superior part is a syndesmosis, where the very strong interosseous sacroiliac ligament fills a wide, irregular gap between the bones. On CT this ligamentous part looks wide and irregular and should not be mistaken for erosion. Sacroiliitis begins in the lower synovial part.
Sacroiliac joint
- Anteroinferior: synovial
- Posterosuperior: syndesmosis
- Filled by interosseous ligament
- Assess sacroiliitis in the synovial part
Remember The upper back of the SI joint is ligament, not joint space.
Exam trap Reading the ligamentous part as widening can lead to a false diagnosis of sacroiliitis.
Why the other options are wrong
- It contains a large synovial recess with fluid The synovial part is the narrow anteroinferior portion.
- It is a fibrocartilaginous symphysis A symphysis is found at the pubis, not here.
- It is the site of early erosion Erosions of sacroiliitis begin in the synovial part.
- It is a synostosis that fuses early The joint does not normally fuse in young adults.
Where this sits in the CPSP syllabus
- Diagnostic Radiology, Paper 2: Pelvis, Bony pelvis, pelvic floor, muscles and ligaments