What is scaphoid non‑union advanced collapse (SNAC) wrist, its presentation, imaging findings, and management options for a patient with chronic wrist pain after a scaphoid fracture?

SNAC Wrist: Definition, Presentation, Imaging, and Management

Definition and Pathophysiology

SNAC wrist (Scaphoid Nonunion Advanced Collapse) is a predictable pattern of progressive degenerative wrist arthritis that develops following untreated or inadequately healed scaphoid fractures, characterized by carpal malalignment, progressive collapse, and osteoarthritis. 1, 2

• SNAC wrist represents one of the two most common causes of wrist arthritis, alongside SLAC (Scapholunate Advanced Collapse) wrist 1
• The condition develops from long-standing scaphoid nonunion leading to carpal bone malalignment, progressive carpal collapse, instability, and ultimately osteoarthritis 3
• Many cases remain asymptomatic for years and may go undiagnosed until advanced stages 1

Clinical Presentation

Patients typically present with progressive wrist pain, reduced grip strength, and wrist instability, often years after the initial scaphoid fracture. 1, 4

• Pain is the predominant symptom, with progressive worsening over time 4, 5
• Reduced grip strength compared to the contralateral side is common 5
• Active range of motion becomes progressively limited 4, 5
• Patients often present an average of 5 years after the initial trauma 4
• Clinical examination reveals wrist instability and tenderness over the radiocarpal and midcarpal joints 1

Staging Classification

SNAC wrist is classified into three progressive stages based on the anatomic distribution of degenerative changes: 2

• Stage I: Arthrosis limited to the radial styloid and scaphoid interface 2
• Stage II: Arthrosis extends to involve the entire radioscaphoid joint 2
• Stage III: Additional midcarpal joint arthrosis, with degenerative changes between the capitate and lunate 2

Imaging Evaluation

Initial Radiographic Assessment

Standard wrist radiographs (posteroanterior, lateral, and oblique views) are the appropriate initial imaging study, demonstrating scaphoid nonunion and degenerative changes in the radiocarpal and midcarpal joints. 6, 7

• Radiographs provide diagnostic information including degree of carpal collapse, ulnar variance, and associated osteoarthritis to adequately plan management 6
• The scaphoid nonunion is visible on standard views, distinguishing SNAC from SLAC wrist 1, 2
• A minimum 3-view series is required; relying on only 2 views is inadequate 7, 8

Advanced Imaging for Surgical Planning

CT without IV contrast is usually appropriate when radiographic findings are uncertain regarding the amount of collapse or presence of bone fragments, and when detailed surgical planning is needed. 6

• CT provides high-detail imaging of bone cortex and trabeculae, useful for assessing trabecular bridging, nonunion characteristics, sclerosis, and fragmentation 6
• Isotropic acquisition allows creation of 3-D models to assist in surgical planning 6
• CT is particularly valuable for evaluating bone architecture and deformity assessment prior to arthrodesis procedures 9

MRI Indications

MRI without IV contrast is usually appropriate when evaluating for osteonecrosis of the proximal scaphoid pole or when assessing concomitant soft tissue injuries. 6

• MRI demonstrates nonunion and can assess for proximal pole viability, though it shows only moderate sensitivity and specificity for predicting osteonecrosis without contrast 6
• For definitive assessment of scaphoid avascular necrosis, MRI with IV gadolinium contrast is the preferred modality, achieving 100% sensitivity and 94% specificity when validated against intraoperative or histologic analysis. 9
• Contrast-enhanced MRI increases sensitivity from 36% to 66%, specificity from 78% to 88%, and overall accuracy from 68% to 83% compared to non-contrast MRI 9
• MRI can detect concomitant ligamentous injuries that may change management 8

Arthrography Considerations

CT arthrography has the highest sensitivity and specificity (94-100%) for articular cartilage injury assessment if this is a specific clinical concern in post-traumatic osteoarthritis. 6

• CT arthrography is superior to MR arthrography for detecting articular cartilage defects 6
• However, arthrography is not routinely used for standard SNAC wrist evaluation 6

Management by Stage

Stage I Management

In Stage I SNAC wrist, reconstructive procedures including scaphoid bone grafting and internal fixation remain viable options. 2

• Reconstruction of the scaphoid with bone grafting is the treatment of choice when arthrosis is limited to the radial styloid 2
• Radial styloidectomy may be performed in conjunction with scaphoid reconstruction 3

Stage II and III Management

For Stage II and III SNAC wrist, reconstruction of the scaphoid is no longer a reasonable option; motion-preserving salvage procedures are preferable, with proximal row carpectomy and four-corner fusion yielding comparable results. 1, 5, 2

Motion-Preserving Procedures:

Proximal Row Carpectomy (PRC):

• Removes the proximal carpal row (scaphoid, lunate, triquetrum) 1, 5
• Preserves wrist motion while eliminating arthritic joints 5, 2
• Comparable outcomes to partial arthrodesis procedures 1

Four-Corner Fusion (Midcarpal Arthrodesis):

• Fuses the capitate, hamate, lunate, and triquetrum with scaphoid excision 1, 3, 5
• Preserves approximately 50% of active range of motion compared to the contralateral wrist 5
• Grip strength improves to 60% of the opposite side 5
• Pain reduction to 50% of preoperative levels 5
• Radial styloidectomy is typically performed concurrently 3, 5
• Nonunion at the fusion site occurs in approximately 13-20% of cases, potentially requiring conversion to total wrist arthrodesis 5

Alternative Partial Arthrodesis Options:

• Scaphocapitolunate (SCL) arthrodesis with radial styloidectomy maintains more physiologic joint surface for radiocarpal load sharing, preserves normal ulnar-sided joints, and achieves 70-degree flexion-extension arc with pain relief in most patients 3
• Three-corner fusion and capitolunate arthrodesis are viable alternatives 1
• Soft tissue procedures (capsulodesis, tenodesis) are preferred in younger patients to avoid early salvage operations 1

Surgical Outcomes:

Partial carpal arthrodesis procedures demonstrate:

• Average arthrodesis consolidation time of 67.8 days 4
• Progressive improvement in range of motion, pain scores, and patient satisfaction over 5 years 4
• Return to pre-injury daily activities averaging 12.5 months after surgery 4
• 80% of patients return to their original occupation 5

Total Wrist Arthrodesis

Total wrist fusion represents the last line of defense, reserved for failed motion-preserving procedures or severe pan-carpal arthritis. 5, 2

• Indicated when partial arthrodesis fails or in cases with extensive arthritic involvement 5, 2
• Eliminates wrist motion but provides reliable pain relief 5

Critical Management Pitfalls

Avoid attempting scaphoid or ligament reconstruction in Stage II and III disease, as degenerative changes preclude successful outcomes with these procedures. 5, 2

• Reconstruction is only appropriate in Stage I when arthrosis is limited to the radial styloid 2
• Proximal pole fractures carry higher risk of avascular necrosis due to retrograde blood supply and warrant closer monitoring 8
• Inadequate radiographic assessment (fewer than 3 views) can lead to missed diagnosis and delayed treatment 7, 8
• Concomitant ligamentous injuries require MRI for detection and may change surgical planning 8

Postoperative Rehabilitation

Immobilization in a plaster cast for 3 weeks postoperatively is standard, with active finger motion exercises performed throughout cast treatment to prevent finger stiffness. 8, 4

• Finger stiffness is one of the most functionally disabling complications and must be actively prevented 8
• Serial clinical and radiographic follow-ups at 1,3,6, and 12 months, then annually 4