What motion unit(s) is/are primarily affected when the right ilium moves posterior and inferior (dorsal and caudal)?

• A. Lumbar spine
• B. SI joint
• C. Hip joint
• D. Coccyx

Answer: (B)

The right sacroiliac joint is the primary motion unit affected. The sacroiliac joint is where the ilium meets the sacrum, so when the ilium moves posteriorly and inferiorly (dorsal and caudal), the articulation between the ilium and sacrum bears the immediate movement and motion at that joint. The pelvis transmits load and allows small sacroiliac motions to accommodate pelvic and spinal movements, so this ilial shift most directly alters the SI joint surfaces and ligaments.

The lumbar spine, while connected and capable of compensatory motion at the lumbosacral junction, is not the primary joint bearing this specific ilial translation. The hip joint is between the acetabulum and femur, so it isn’t the main site of motion when the ilium shifts dorsally and caudally. The coccyx is connected to the sacrum but isn’t the primary structure moved by this ilial repositioning.