The intercalated segment is the proximal carpal row and Dorsal intercalated segment instability is said to occur when the lunate slips into fixed extension >10 degrees . This occurs when radial ligamentous stability is broken.
The condition is also termed as dorsal instability.
Massive ligament disruption at the time of injury, as may occur in perilunate or lunate dislocations, or gradual attrition of the secondary extrinsic stabilizers leads to abnormal extension of the lunate and carpal collapse after scapholunate dissociation.
The combined effects of an extension moment and dorsal translation of the capitate force puts the lunate into extension and exacerbate the abnormal posture of the scapholunate joint.
Capsular contracture may serve to fix the deformity.
So all this means is that in DISI, the lunate is angulated dorsally.The relative alignment of the scaphoid to the lunate forms angle of 45 degrees. When this angle is more than 70 degrees, ligaments between the scaphoid and lunate become nonfunctional. In such situations, the lunate becomes extended and maintains the position even during radial deviation.
This does not allow normal rotation and the spatial adaptability of the proximal row. A DISI deformity is seen in
- Scaphoid fractures
- Scapho-lunate dissociation
- Perilunate dislocation
- Lunate in dorsiflexed position > 10 deg.
- Lunate is palmar to capitate but faces dorsally
- Scapholunate angle >70 degrees
Note: It is important that lateralradiograph taken for disgnosis of DISI is true lateral and the wrist should not be dorsiflexed.
Nonoperative treatment does not have much role in this deformity and it would result in a slow, relentless progression to degenerative arthritis.
If the scaphoid and lunate are reducible and no degenerative changes have occurred at the radiocarpal joint, ligament repair may be attempted.
Intercarpal arthrodesis is another option.