Ulnar Variance: Calculation Methods, Importance, and Interpretation

How do you calculate ulnar variance?

Ulnar variance is calculated by measuring the longitudinal relationship between the distal articular surfaces of the radius and ulna on a standard posteroanterior (PA) radiograph of the wrist, providing insight into the relative lengths of these forearm bones at the wrist joint.

What is Ulnar Variance?

Ulnar variance refers to the difference in the lengths of the radius and ulna at their distal ends, where they articulate with the carpal bones to form the wrist joint. This measurement is crucial in understanding the biomechanics of the wrist, as even subtle discrepancies can significantly impact load distribution, joint congruity, and the risk of various pathologies. It's typically expressed in millimeters (mm) and can be categorized as neutral, positive, or negative.

Why is Ulnar Variance Important?

The precise relationship between the distal radius and ulna has significant clinical implications:

• Load Distribution: The radius typically bears approximately 80% of the axial load across the wrist, with the ulna bearing the remaining 20%. Ulnar variance alters this distribution, potentially increasing stress on specific structures.
• Wrist Pain and Dysfunction: Abnormal ulnar variance is frequently associated with conditions such as ulnar impaction syndrome (positive ulnar variance), Kienböck's disease (osteonecrosis of the lunate, often linked to negative ulnar variance), and tears of the Triangular Fibrocartilage Complex (TFCC).
• Surgical Planning: For wrist injuries or deformities, orthopedic surgeons often assess ulnar variance to guide treatment decisions, including osteotomies (bone cutting procedures) to correct length discrepancies.
• Post-Traumatic Assessment: After distal radius fractures, evaluating ulnar variance is essential to ensure proper healing and alignment, minimizing the risk of long-term complications.

Methods of Calculation: Radiographic Assessment

The calculation of ulnar variance is primarily performed using radiographic imaging, specifically a standard posteroanterior (PA) view of the wrist. Accuracy is paramount, as subtle variations can have clinical significance.

Standard PA Radiograph

This is the most common and reliable method. For an accurate measurement, the radiograph must be taken with the shoulder abducted to 90 degrees, elbow flexed to 90 degrees, and the forearm in neutral rotation, with the wrist in neutral flexion/extension and ulnar/radial deviation, and the fingers gently extended.

Key Measurement Points:

• Radial Reference Line: A line drawn perpendicular to the long axis of the radius, tangent to the most distal point of the articular surface of the radius (the radial styloid process or the lunate fossa, depending on the most distal point).
• Ulnar Reference Line: A parallel line drawn tangent to the most distal articular surface of the ulna (the ulnar head).

Calculation Steps:

1. Identify the Long Axis: On the PA radiograph, mentally or physically draw a line along the long axis of the radial shaft.
2. Draw Radial Tangent Line (Line A): Draw a line perpendicular to the radial long axis, touching the most distal point of the articular surface of the radius. This point is typically the lunate fossa or, in some cases, the radial styloid.
3. Draw Ulnar Tangent Line (Line B): Draw a second line parallel to Line A, touching the most distal point of the articular surface of the ulnar head.
4. Measure the Distance: Measure the perpendicular distance between Line A and Line B.

Interpretation of the Measurement:

• Neutral Ulnar Variance: If Line A and Line B are at the same level, the ulnar variance is 0 mm. This is considered the normal relationship.
• Positive Ulnar Variance: If the ulna extends distally beyond the radius (Line B is distal to Line A), the ulnar variance is positive. For example, +2 mm means the ulna is 2 mm longer than the radius at the wrist joint. This can lead to increased load on the TFCC and ulnar carpal bones.
• Negative Ulnar Variance: If the ulna is shorter than the radius (Line B is proximal to Line A), the ulnar variance is negative. For example, -2 mm means the ulna is 2 mm shorter than the radius. This can decrease load on the TFCC but may increase stress on the lunate, potentially contributing to Kienböck's disease.

Other Radiographic Views

While the standard PA view is primary, other views or techniques may be used for specific diagnostic purposes:

• PA Radiograph with Grip: Sometimes, a PA radiograph is taken while the patient is gripping a dynamometer. This can reveal dynamic changes in ulnar variance, as the radius may shorten with axial loading, potentially increasing positive ulnar variance.
• Lateral Radiograph: While not used for direct ulnar variance measurement, a lateral view helps assess wrist alignment and pathology in other planes.
• CT Scans: In complex cases or for surgical planning, CT scans can provide more detailed 3D anatomical information, allowing for precise measurements and evaluation of joint congruity.

Interpreting Ulnar Variance

The interpretation of ulnar variance is crucial for diagnosis and treatment. Normal ulnar variance is typically considered to be within a range of ±2 mm, with 0 mm being ideal. However, individual variations exist, and the clinical context is always paramount.

• Neutral Ulnar Variance (0 mm): The distal articular surfaces of the radius and ulna are at the same level. This is generally considered the most biomechanically favorable alignment.
• Positive Ulnar Variance (>0 mm): The ulna is longer than the radius. This increases axial load on the ulnar side of the wrist, predisposing to conditions like ulnar impaction syndrome (wear and tear of the TFCC and ulnar carpal bones), TFCC tears, and degenerative changes.
• Negative Ulnar Variance (<0 mm): The ulna is shorter than the radius. This shifts the load more towards the radial side. While it may reduce ulnar-sided pain, significant negative variance can increase stress on the lunate bone, making it more susceptible to osteonecrosis (Kienböck's disease).

Clinical Considerations and Limitations

It's important to recognize that while a specific measurement is taken, several factors can influence the perceived ulnar variance:

• Patient Positioning: Even slight pronation or supination of the forearm, or wrist flexion/extension/deviation, can alter the apparent relative lengths of the radius and ulna on a radiograph. Strict adherence to standardized positioning protocols is essential.
• Radiographic Technique: The angle of the X-ray beam can affect measurements.
• Dynamic Changes: Ulnar variance can change with forearm rotation and axial loading (e.g., during gripping), which static radiographs may not fully capture.
• Growth Plate Status: In growing individuals, the relative growth rates of the radius and ulna can change ulnar variance over time.
• Individual Variability: There is a normal range of variation, and a value slightly outside 0 mm may be asymptomatic in some individuals.

Therefore, while the calculation provides a quantitative measure, it must always be interpreted in conjunction with the patient's symptoms, physical examination findings, and other diagnostic imaging.

Conclusion

Calculating ulnar variance is a fundamental step in the radiographic assessment of the wrist. By precisely measuring the relationship between the distal radius and ulna, clinicians gain invaluable insight into the biomechanical forces at play within the wrist joint. This measurement serves as a critical diagnostic tool, guiding the understanding of wrist pathologies, informing treatment strategies, and contributing to optimal patient outcomes in orthopedics and sports medicine.