Ulnar variance refers to the relative lengths of the ulna and radius bones at the wrist joint. It is a measurement crucial for assessing wrist anatomy and pathology. The normal ulnar variance can vary slightly, but it is typically neutral or slightly negative in a standard posteroanterior (PA) radiographic view.
Based on the provided reference, the mean ulnar variance in the standard PA position was:
- Right side: -0.52 ± 2.1 mm
- Left side: -0.34 ± 1.8 mm
This indicates that, on average, the ulna is slightly shorter than the radius (negative variance) in the standard PA view, with a typical range encompassing neutral and slightly positive or negative values.
Understanding Ulnar Variance Measurement
Ulnar variance is measured on a standard PA radiograph of the wrist. A line is drawn perpendicular to the long axis of the radius through the distal articular surface (the end surface that meets the wrist bones). Another line is drawn parallel to this through the distal articular surface of the ulna. The distance between these two lines represents the ulnar variance.
There are three main types:
- Negative Ulnar Variance: The ulna is shorter than the radius (the ulnar line is proximal to the radial line). This is commonly seen as a normal variation.
- Neutral Ulnar Variance: The ulna and radius are level at the joint surface (the lines are at the same level).
- Positive Ulnar Variance: The ulna is longer than the radius (the ulnar line is distal to the radial line). This can sometimes be associated with certain wrist conditions.
Factors Affecting Ulnar Variance
It's important to note that ulnar variance is not a fixed number for an individual. It can change depending on several factors, including:
- Radiographic View: As noted in the reference, changes in ulnar variance occur on different radiographic views of the wrist (though specifics were in the unprovided Table 1). Wrist position (like pronation, supination, flexion, or extension) can influence the apparent relative lengths.
- Grip Position: Studies have shown that gripping can increase positive ulnar variance.
- Age and Growth: Ulnar variance can change during skeletal growth.
- Trauma or Pathology: Fractures (especially distal radius fractures) or conditions like Kienböck's disease can alter ulnar variance.
Why is Ulnar Variance Important?
Measuring ulnar variance helps clinicians:
- Assess Wrist Anatomy: Understand the baseline relationship between the ulna and radius.
- Diagnose Conditions: Positive ulnar variance is associated with increased load bearing on the ulna side of the wrist and conditions like tears of the Triangular Fibrocartilage Complex (TFCC) or ulnocarpal impaction syndrome. Negative ulnar variance is associated with conditions like Kienböck's disease (avascular necrosis of the lunate bone).
- Plan Treatment: Guide surgical decisions, such as whether to perform procedures that shorten the ulna or lengthen the radius to correct significant variance issues.
In summary, while the reference provides specific mean values for the standard PA view (-0.52 ± 2.1 mm on the right and -0.34 ± 1.8 mm on the left), ulnar variance is best understood within its context, considering the measurement technique, wrist position, and potential influence of pathology.
