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Anatomy and Biomechanics

The bony anatomy of the wrist includes carpal bones which articulate with one another, distally with the metacarpals and proximally with the radius and ulna. This unique articulation allows motion of the wrist in multiple planes.

Biomechanically the flexion and extension movement of the wrist involve a motion equally through the radiocarpal and the metacarpal articulation. However, the radioulnar deviation has a major contribution from the midcarpal joints as compared to the radiocarpal articulation (60:40).

What is Ulnar variance and what causes the abnormal variance?

Ulnar variance is defined as the length of the Ulna as compared to the radius. The ulnar variance increases towards more positive value on pronation and while performing a grip. Neutral ulnar variance is defined when the difference between the length of the ulna and the radius is between 0-1 mm. Scores below and above this range are defined as negative and positive ulnar variance respectively.

Variation in loads passing through the radius and ulna depends on the relative length of the two bones. With neutral ulnar variance 80% of the load passes through the radius which varies inversely proportional to the Ulnar variance.

The abnormality in the Ulnar variance can be congenital, after fracture of the distal radius or after growth arrest involving the distal radius and ulna, injuries of the distal radioulnar joint and as complication of surgeries involving the forearm bones.

How to measure ulnar variance?

The recommended radiological X-ray view for measuring ulnar variance is a PA view of the wrist with shoulder abducted to 90 degrees, elbow flexed to 90 degrees and neutral rotation of the forearm. Two lines are drawn perpendicular to the respective shafts; one passing tangentially to the distal articular surface of the ulna and the second line crossing tendentially through their lunate fossa of the radius. The distance between these two lines is measured to give ulnar variance.

Ulnar Variance and associated pathologies

Many wrist pathologies are seen to be associated with different values of Ulnar variance.

Positive Ulnar variance is associated with Ulnar abutment syndrome, arthritis of the ulnar quadrant of the wrist, Lunotriquetral ligament tears and Triangular Fibrocartilage complex (TFCC) tears. The thickness of the TFCC varies inversely with the Ulnar variance.

Negative Ulnar variance is associated with Kienbock’s disease and Ulnar impingement syndrome where the distal ulna impinges on the radius in the proximal sigmoid notch.

Some Common Conditions Associated with Abnormal Ulnar Variance

Ulnar impaction syndrome (ulnocarpal abutment syndrome)

This is a degenerative condition of the ulnar quadrant of the wrist and is a common cause of ulnar-sided wrist pain. This results in increased load bearing across the ulnar head, TFCC and ulnar-sided carpals, and subsequent degeneration of the TFCC, chondromalacia of the osseous structures involved (ulna and carpals, especially the lunate), and disruption of the triquetrolunate ligament.

Kienbock's Disease

Kienböck’s disease is avascular necrosis of the lunate bone. It is also known as lunatomalacia. This usually affects adults between 20-40 years of age and is more common in males involved in manual trade. Generally, it is found on one side of the wrist but bilateral care is non-unknown. The patients present with dorsal wrist pain with decreased grip strength. Proggresive loss of function is associated with carpal dysfunction and this is related to loss of function more commonly than the avascular necrosis of the Lunate bone.

TFCC and Ulnar variance

The main function of TFCC is to provide cushioning to the Ulnar carpus, stabilise the Distal radio-ulnar joint and the ulnar carpal bones. TFCC thickness varies inversely with the Ulnar variance. However the TFCC angle varies directly with the Ulnar variance. With positive Ulnar variance the loads on the ulnar side are compressive but these change to be of tension with negative ulnar variance. TFCC tearing is common with positive Ulnar variance as this is associated with thin TFCC.

References

1. https://www.orthobullets.com/hand/6046/ulnar-variance
2. Katz DI, Seiler JG, Bond TC. The treatment of ulnar impaction syndrome: A systematic review of the literature. J of Surg Orth Adv. 2010; 19(4): 218-222
3. Yoshioka H, Tanaka T, Ueno T et al. Study of ulnar variance with high‐resolution MRI: Correlation with triangular fibrocartilage complex and cartilage of ulnar side of wrist. J. Magn. Reson. Imaging, 2007; 26: 714-719
4. Schuind F, Eslami S, Ledoux P. Kienböck’s disease, The Journal of Bone and Joint Surgery, 2008, 90-B: 133-139