What is the radiological difference between ulnar carpal abutment syndrome and ulnar styloid impaction syndrome on imaging studies such as X-rays (X-radiation) or computed tomography (CT) scans?

Radiological Differentiation Between Ulnar Carpal Abutment Syndrome and Ulnar Styloid Impaction

The key radiological difference is the anatomical structure causing impaction: ulnar carpal abutment syndrome (also called ulnar impaction syndrome) involves the ulnar head impacting against the triangular fibrocartilage complex (TFCC) and ulnar carpus, while ulnar styloid impaction involves an elongated ulnar styloid process specifically impacting the triquetrum.

Anatomical Distinctions on Imaging

Ulnar Carpal Abutment Syndrome

• Involves the ulnar head impacting against the TFCC and ulnar carpal bones (lunate and triquetrum), resulting from excessive ulnar loading 1
• Positive ulnar variance is the hallmark finding on posteroanterior (PA) wrist radiographs, indicating the ulnar head extends beyond the radial articular surface 1
• Progressive degeneration of the TFCC and ulnar carpus is visible on advanced imaging 1
• May result from malunion of distal radius fractures causing relative ulnar lengthening 1

Ulnar Styloid Impaction Syndrome

• Involves an elongated ulnar styloid process specifically contacting the triquetrum 2, 3
• Ulnar styloid length measurements are diagnostic:
  • Ulnar styloid process index (USPI) >0.21 indicates excessive length 3, 4
  • Overall styloid length >6 mm is abnormal 3, 4
  • Ulnar styloid-capitate ratio (SCR) >0.18 ± 0.05 suggests elongation 4
• Ulnar variance may be neutral or negative, distinguishing it from ulnar abutment syndrome 3, 5
• The styloid tip specifically impacts the triquetrum, not the broader ulnar carpal articulation 2, 5

Imaging Protocol for Differentiation

Initial Radiographic Assessment

• Standard PA wrist radiographs should be obtained first to assess both ulnar variance and ulnar styloid length 6, 4
• Measure ulnar variance to identify positive variance (suggesting abutment syndrome) versus neutral/negative variance (more consistent with styloid impaction) 1, 4
• Calculate USPI and measure absolute styloid length on PA views 3, 4

Dynamic Fluoroscopy

• Dynamic fluoroscopy can confirm ulnar styloid impaction by demonstrating direct contact between the styloid tip and triquetrum during wrist motion 2
• This technique is particularly valuable for confirming styloid impaction preoperatively 2

Advanced Imaging

• 3.0T MRI without contrast is the recommended next study after radiographs, with 63-100% sensitivity for TFCC tears associated with ulnar abutment syndrome 6
• CT arthrography achieves nearly 100% sensitivity and specificity for both TFCC lesions (in abutment syndrome) and can visualize chondromalacia of the triquetrum (in styloid impaction) 6, 7
• MRI can identify chondromalacia, synovitis, and soft tissue abnormalities in both conditions 3, 5

Critical Diagnostic Pitfalls

• Do not confuse the two entities: Ulnar abutment involves the ulnar head and typically presents with positive ulnar variance, while styloid impaction involves an elongated styloid process and may have neutral or negative ulnar variance 1, 3
• Measure styloid length systematically using standardized ratios (USPI or SCR) rather than subjective assessment, as general awareness of styloid impaction remains low 4
• Obtain neutral PA radiographs for accurate measurements, as different wrist positions affect measurement outcomes 4
• Consider both conditions can coexist with distal radioulnar joint abnormalities, requiring comprehensive preoperative evaluation 1