Why does a SLAC (Scapholunate Advanced Collapse) wrist exhibit positive ulnar variance?

Why SLAC Wrist Exhibits Positive Ulnar Variance

SLAC wrist does not inherently cause positive ulnar variance; rather, a pre-existing ulnar positive variance may become symptomatic after surgical treatment of SLAC wrist, particularly following four-corner fusion or scaphoid excision procedures. 1

Understanding the Relationship

The question reflects a common clinical misconception. SLAC (Scapholunate Advanced Collapse) wrist itself does not create positive ulnar variance. Instead, the relationship works as follows:

Pre-existing Anatomic Variance

• Many patients with SLAC wrist have coincidental ulnar positive variance that remains asymptomatic while the radiocarpal pathology dominates the clinical picture 1
• The ulnar positive variance exists independently of the SLAC pattern, which characteristically affects the radioscaphoid joint first, then progresses to the midcarpal joint, while sparing the radiolunate articulation 2, 3

Biomechanical Unmasking After Surgery

• When SLAC wrist is treated with scaphoid excision and four-corner fusion (or proximal row carpectomy), the altered wrist biomechanics can unmask previously asymptomatic ulnocarpal pathology 1
• The surgical alteration of carpal mechanics shifts load distribution, potentially increasing stress across the ulnocarpal joint in patients with pre-existing ulnar positive variance 1

The SLAC Pattern Itself

The classic SLAC pattern follows a predictable sequence:

• Stage I: Arthritis between the radial styloid and scaphoid 3
• Stage II: Arthritis extending to the entire radioscaphoid joint 3, 4
• Stage III: Midcarpal (capitolunate) arthritis develops 3, 4
• Radiolunate joint preservation: This articulation remains intact throughout all stages, which forms the basis for motion-preserving salvage procedures 2, 3

Critical Clinical Pitfall

• Always assess ulnar variance preoperatively in patients undergoing SLAC wrist reconstruction 1
• Patients with ulnar positive variance may require concurrent or staged ulnar shortening osteotomy to prevent postoperative ulnar-sided wrist pain 1
• Failure to address pre-existing ulnar positive variance can lead to new-onset ulnar-sided pain after an otherwise technically successful SLAC reconstruction 1

Surgical Outcomes Without Ulnar Considerations

When ulnar variance is not a confounding factor, SLAC wrist salvage procedures demonstrate excellent results:

• Pain relief is good to excellent in most cases 2
• Extension/flexion averages 72 degrees (53% of normal contralateral wrist) 2
• Grip strength recovers to approximately 80% of the opposite side 2
• The radiolunate joint remains well-preserved in 98% of cases at average 44-month follow-up 2