Distal RU joint subluxation-MRI

Case Details: 49 yr old male with   twisting injury on pronated hand 4 weeks back  shows dorsal  placement of the ulnar head in relation  to radius at distal  radio ulnar joint with palmar radio ulnar ligament (black line) ill-defined  and  TFCC irregularity suggesting distal RU joint subluxation.

Teaching points by Dr MGK Murthy

1. Distal RUJ  is common site for acute and chronic wrist pain.  Joint between sigmoid notch of radius and head of ulna with ulna relatively fixed permitting translation/ rotation in pronation and supination of forearm 

2. Normal contact between two ,varies  from 60 to 80% in neutral  position to 10 to 20%  only in extreme rotation,  stabilized by TFCC (complex including TFC disk) and dorsal as well as palmar radio ulnar ligaments (deep and superficial fibres of each) with pronation and supination limited by superficial fibres attachment 

3. Line drawn form distal radial articular surface  to ulna and extent of ulna  above or below this determines ulnar variance  either positive or negative. Postive ulnar variance leads to ulnar impaction (increasing the load by ulna  and carpus)(D/D congenital). 2.5mm of +ve ulnar variance increases ulnar loading by 40%. Negative ulnar variance leads to ulnar impingement (increasing load by radius) (D/D congenital). 2.5mm - ve ulnar variance decreases ulnar load  from 18 to 4%,increasing radial load. Normal axial loading is 20%by ulna and rest by distal radio ulnar joint

4.  Dynamic evaluation in neutral / supination/ pronation by CT is ideal for stability measurement  either by congruence method or epicentre method (drawing different arcs form centre to ulnar head)

10. TFCC tears  are defined by Palmer classification type 1 (acute traumatic) and the 2 (chronic degenerative)