What physically causes SI fixation?

• A. Ligament adhesions
• B. Myospasm
• C. Edema in joint
• D. Meniscoid entrapment

Answer: (A)

Fusing of the sacroiliac joint happens when fibrous adhesions form within the joint ligaments and capsule, effectively bridging the ilium and sacrum and restricting movement. These adhesions are a structural change that locks the joint, producing true fixation. Muscle spasm can make motion feel limited, but the joint surfaces themselves aren’t permanently bonded, and edema causes swelling and pain without creating a rigid, structural block. A meniscoid entrapment is a different intra-articular issue that can produce locking or pain, but it’s not the typical mechanism of a fixed SI joint. The key idea is that adhesions in the SI ligaments create a mechanical restraint that leads to fixation.