Ulnar variance refers to the difference between the levels of radial and ulnar articular surfaces. Normally, the articular surface lies slightly in distal position than the ulnar articular surface.
It is said to be neutral if articular surfaces of radius and ulna are at the same level.
It is positive if ulnar articular surface is distal to radial.
It is negative if ulnar articular surface is proximal to radial.
There are a lot of studies measuring ulnar variance. Most cited value of mean ulnar variance is 0.9 mm (range: -4.2 to +2.3 mm) [ref-Schind et al]
The variance determines the distribution of load in wrist joints. If it is neutral, 80% of load is born by radius and 20% by ulna.
A 2.5 mm increase in is said to increase the load on ulno-carpal joint to about 40% and decrease of 2.5 mm decrease the load to about 4%.
Negative Ulnar Variance
It is associated Kienbock’s disease [Osteonecrosis of lunate bone]
Positive Ulnar Variance
- Scapholunate instability
- Ulnar impaction syndrome
- Triangular Fibrocartilage tears
- Lunotriquetral ligament tears [Ligaments between lunate bone and triquetral bone]
- Previous excision of radial head
- Increased age
- Child gymnasts [stress related changes in the distal radial physis]
Ulnar variance is measured on posteroanterior radiographs with wrist in neutral rotaion. A transverse line is drawn perpendicular to axis of radius bone, at level of lunate fossa (touching the ulnar half of articular surface of radius) and a second transverse line at the level of the ulnar head.
The difference indicates ulnar variance.
Therefore, to determine ulnar variance on radiographs the wrist should be in neutral forearm rotation, the elbow flexed 90° and the shoulder abducted 90°.
Image Credit : http://emedicine.medscape.com/article/398406-overview
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