Pelvis

Regularly overlooked injuries

▪ Detached acetabular fragment in a patient with a dislocated hip.

▪ Avulsed apophysis from the proximal femur or from the innominate bone.

The standard radiograph

AP view.

Abbreviations

AIIS, anterior inferior iliac spine; AP, anterior-posterior; ASIS, anterior superior iliac spine; RTA, road traffic accident; SI joint, sacro-iliac joint.

Normal anatomy

Normal AP view

The pelvis comprises three bone rings:

▪ The main pelvic ring

▪ Two smaller rings formed by the pubic and ischial bones

The robust sacro-iliac joints and the pubic symphysis are part of the main bone ring. The sacro-iliac joints are the strongest joints in the body and resist the normal vertical and anterior-posterior displacement forces; the pubic symphysis is the weakest link in the pelvic ring ¹^–⁴.

Arcuate lines are visible as smooth curved borders on the radiograph. They outline the roofs of the sacral formina.

Developing skeleton: Synchondroses

In children the synchondrosis (cartilaginous junction) between each ischial and pubic bone can sometimes appear confusing. In early childhood these unfused junctions may simulate fracture lines. Subsequently, between the ages of five and seven years, they may mimic healing fractures.

Developing skeleton: Pelvic bone apophyses⁵^–⁷

In adolescents and young adults the pelvis shows several small secondary centres (the apophyses). These should be radiographically identical on the two sides.

Apophyses are secondary centres that contribute to the eventual shape, size, and contour of the bone but not to its length. These centres are traction epiphyses as muscles originate from or insert into them. They are vulnerable to severe and acute muscle contraction, and also to repetitive forceful muscle pulls when jumping, hurdling, turning suddenly, or—occasionally—when dancing.

Each apophysis has a physis. The physis (growth plate) is a particularly weak part of the developing skeleton.

Age of appearance and fusion of apophyses at the hip and pelvis (years)⁵^,⁷

Apophysis	First seen on radiograph	Fuses to skeleton
Innominate bone
AIIS	13–15	16–18
ASIS	13–15	21–25
Ischial tuberosity	13–15	20–25
Iliac crest	13–15	21–25
Femur
Lesser trochanter	11–12	16–17
Greater trochanter	2–3	16–17

Note that some apophyses remain unfused and normal, but are still vulnerable into the early 20s, well after the growth of the long bones has been completed.