Procedures · Head, Spine and Pelvis

Sacrum, Coccyx and SI Joint Positioning Series

Study the sacrum, coccyx, and sacroiliac joints through their routine axial and oblique projections.

Sacrum, coccyx and SI joints

Left

StudyBucky

Loading model…

Hover a bone, name it, then click to check · drag to rotate

The routine sacrum, coccyx and si joints series

The projections this region is examined with. Set them up yourself in Practice positioning.

AP axial sacrum

Anteroposterior axial projection · central ray 15° cephalad

Demonstrates: The sacrum, both sacroiliac joints and the joint space between the fifth lumbar vertebra and the first sacral segment. Correct alignment of the central ray with the sacrum shows it at full length rather than foreshortened, and keeps the sacral foramina clear of the pubis. There is no rotation when the median sacral crest and the coccyx line up with the symphysis pubis.
Part position: Supine, with the arms at the side and the legs extended, supported under the knees. Align the midsagittal plane to the central ray and to the midline of the table, and check that the pelvis is not rotated.
Central ray: 15° cephalad, centered 2 inches (5 cm) superior to the symphysis pubis.
Why it is positioned this way: The sacrum is not upright inside the pelvis — it leans backward as it descends, so a central ray sent straight through the patient crosses it at an angle and the bone projects shorter than it is. Angling the tube up by about the same amount brings the ray back to square with the sacrum and restores its length. That is also why the direction reverses when the patient is turned over: the anatomy is unchanged, so the beam still has to arrive along the same line through it.

Also covered by this projection

Lateral sacrum and coccyxThe third projection in this routine. Taken lateral recumbent with the knees flexed and the long axis of the sacrum and coccyx aligned to the midline of the table, with the central ray 90° to the image receptor and centered 3–4 inches (8–10 cm) posterior to the ASIS. The sacrum and coccyx need separate AP projections because their angles differ, but one lateral exposure covers both — which is the reason to take it, since it spares the patient a second exposure. It is proved free of rotation by the greater sciatic notches and the femoral heads lying superimposed.

Taken prone insteadWhere the patient cannot lie supine, the same projection is obtained face down with the central ray angled 15° caudad.

A more pronounced sacral curveWhere the sacrum and pelvis are tilted further back than usual, the angle is opened up to about 20° cephalad to stay square with the bone.

AP axial coccyx

Anteroposterior axial projection · central ray 10° caudad

Demonstrates: The coccyx, projected clear of the pubis rather than superimposed on it, with the spaces between the coccygeal segments open. Segments that stay closed are either fused or still being crossed at an angle by the central ray. The coccyx sits an equal distance from each lateral wall of the pelvic opening when there is no rotation.
Part position: Supine, with the arms at the side and the legs extended, supported under the knees. Align the midsagittal plane to the midline of the table, and check that the pelvis is not rotated.
Central ray: 10° caudad, centered 2 inches (5 cm) superior to the symphysis pubis — the same point the AP axial sacrum uses.
Why it is positioned this way: This is the mirror of the sacrum projection and the pair is worth learning together: the same centring point, the angles running opposite ways. They differ because the two bones do. The sacrum leans backward as it descends, so its ray comes up from below; the coccyx hooks forward off the end of it, so its ray comes down from above. Sending the beam straight through instead leaves the coccyx lying over the pubis, where its segments cannot be assessed.

Also covered by this projection

Taken prone insteadWhere the patient cannot lie supine, the same projection is obtained face down with the central ray angled 10° cephalad and centered to the coccyx, which can be located from the greater trochanter.

A more pronounced coccygeal curveWhere the coccyx curves further forward than usual — felt on palpation, or seen on the lateral — the angle is opened up to about 15° caudad.

AP axial sacroiliac joints

Anteroposterior axial projection · central ray 30–35° cephalad

Demonstrates: Both sacroiliac joints together, with the joint space between the fifth lumbar vertebra and the first sacral segment. The two joints should look alike and sit an equal distance from the midline of the vertebrae. There is no rotation when the spinous process of the fifth lumbar vertebra sits centered in its own body and the two wings of the sacrum look symmetric.
Part position: Supine, with the arms at the side and the legs extended, supported under the knees. Align the midsagittal plane to the central ray and to the midline of the table, and check that the pelvis is not rotated.
Central ray: 30–35° cephalad — about 30° for a male patient and 35° for a female — centered to the midline 2 inches (5 cm) below the level of the ASIS.
Why it is positioned this way: This is a much steeper angle than the sacrum takes, and it is opening a different thing. The joints sit behind and below the lumbosacral junction, where the spine changes direction sharply, so a central ray sent straight through arrives across them and the two joint spaces close up. Coming steeply up from below carries the beam along the lumbosacral curve instead and separates both joints in one exposure. The larger angle suits a female patient because that curve is usually more pronounced.

Also covered by this projection

Taken prone insteadWhere the patient cannot lie supine, the same projection is obtained face down with the central ray angled 30–35° caudad and centered at the level of the fourth lumbar vertebra, slightly above the iliac crest.

Posterior oblique sacroiliac joint — RPO, demonstrating the left joint

Posterior oblique position · body rotated 25–30° · side of interest elevated

Demonstrates: One sacroiliac joint at a time, opened along its length: the joint on the ELEVATED side, which is the one farthest from the image receptor. So a right posterior oblique demonstrates the LEFT joint, and a left posterior oblique the right — the two are usually taken together so the sides can be compared. The rotation is correct when the joint stands open with no part of the wing of the ilium projected over the sacrum.
Part position: Supine to begin with, then rotate the whole body 25–30° into a posterior oblique with the side of interest raised off the table. Support the elevated hip and flex the elevated knee, and check the rotation at several points along the back — an angle-measuring device is worth using, because the two obliques must match for the sides to be comparable. Align the joint of interest to the midline of the table.
Central ray: 90° to the image receptor, centered 1 inch (2.5 cm) medial to the elevated ASIS.
Why it is positioned this way: The sacroiliac joints do not face forward. Each one runs at about 25–30° to the midsagittal plane, which is why an AP shows them foreshortened and overlapped by the ilium, and it is also where the rotation angle comes from — turning the body by the amount the joint is already turned brings the beam parallel to the joint surfaces so they separate. Only one joint can be aligned at a time, and it is the raised one: rotating a joint away from the receptor is what swings it into profile, while the joint left against the table turns further out of it.

Also covered by this projection

Angling the central ray as wellThe lower part of the joint can be opened more clearly by adding about 20° of cephalad angle, entering 1 inch (2.5 cm) medial and 1½ inches (3.8 cm) distal to the elevated ASIS.

Procedures lessonPelvis, hip & SI joints