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.
Basically, it is the measurement of the length of the ulna compared to the radius.
It is said to be neutral if articular surfaces of radius and ulna are at the same level i.e. the difference between ulnar and radial length is <1mm.
It is positive if the ulnar articular surface is distal to radial. Positive variance can lead to wrist pain because of increased impaction and stress on the lunate and triquetrum.
It is negative if the ulnar articular surface is proximal to radial.
The variance is lower in males than females and increases with age.
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)
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 the ulnocarpal joint to about 40% and a decrease of 2.5 mm decrease the load to about 4%.
The following table shows the variation of the load with a change in Ulnar variance
|Ulnar Variance||Length Difference||Load Passing Through Radius||Load Passing Through Ulna|
Negative Ulnar Variance
- Kienbock’s disease [Osteonecrosis of lunate bone]
- Reverse Madelung disease
Positive Ulnar Variance
- Scapholunate instability
- Ulnar impaction syndrome
- Triangular Fibrocartilage tears
- Lunotriquetral ligament tears [Ligaments between lunate bone and triquetral bone]
- Previous excision of the radial head
- Increased age
- Child gymnasts [stress related changes in the distal radial physis]
- Madelung deformity
- Ulnar Abutment Syndrome
Ulnar variance is measured on posteroanterior radiographs with the wrist in neutral rotation.
It is measured in the following position
- The shoulder is abducted to 90 degrees
- The elbow is flexed to 90 degrees
- The forearm is in neutral position and hand aligned with forearm axis
A transverse line is drawn perpendicular to the axis of the radius bone, at the 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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