Scaphoid Nonunion (SNAC Wrist)

Scaphoid nonunion (SNU) is the persistent lack of bony healing of a scaphoid fracture beyond 4-6 months from initial injury, with progressive arthritis if untreated. It is the most common nonunion in the upper extremity, occurring in 5-15% of all scaphoid fractures, and significantly higher in fractures with risk factors. The scaphoid has unique anatomy that predisposes to nonunion: (1) Tenuous retrograde blood supply: dorsal carpal branch of the radial artery enters the dorsal ridge of the scaphoid (distal scaphoid), supplying 70-80% of the bone in retrograde fashion; the proximal pole receives no direct blood supply. This makes proximal pole fractures highly susceptible to nonunion (up to 50%) and avascular necrosis (40-50%). (2) Small contact area between fracture fragments. (3) Synovial environment of the wrist (no periosteal callus formation possible). (4) Continuous motion and shear stresses through the scaphoid. (5) Frequent missed initial diagnosis: scaphoid fractures may not be visible on initial radiographs (10-25% missed initially), with the classic anatomic snuffbox tenderness being subtle. Risk factors for nonunion: (1) Delayed diagnosis or treatment more than 4 weeks (most important). (2) Proximal pole fractures (highest risk due to retrograde vascular supply). (3) Displaced fractures (more than 1 mm displacement, 15-50 degrees angulation). (4) Unstable fracture patterns (transverse comminuted vs oblique). (5) Open injuries. (6) Smoking. (7) Diabetes. (8) Inadequate immobilization. (9) Patient noncompliance. (10) Skeletally immature patients (paradoxically lower union rates with proximal pole fractures).

Pathophysiology of scaphoid nonunion advanced collapse (SNAC): scaphoid nonunion progresses to predictable arthritis pattern called SNAC wrist, similar to scapholunate advanced collapse (SLAC) but affecting the scaphoid first. Stages of SNAC (Watson and Ryu classification): Stage I (radial styloid arthritis): early radial styloid arthritis develops between the radial styloid and the distal scaphoid fragment, due to abnormal motion. Treatment: scaphoid bone grafting plus radial styloidectomy if substantial. Stage II (proximal capitate arthritis): proximal capitate articulates abnormally with non-articular surface of unstable distal scaphoid, with arthritis at proximal capitate. Treatment: scaphoid excision and four-corner fusion. Stage III (capitolunate and radioscaphoid arthritis): more advanced arthritis with capitate-lunate joint involvement. Treatment: scaphoid excision and four-corner fusion or proximal row carpectomy. Stage IV (pancarpal arthritis): involvement of all carpal joints including the radio-lunate joint. Treatment: total wrist fusion or arthroplasty. Some authors describe a separate Stage IV-V with collapse and arthritis affecting both rows. The radio-lunate joint is preserved relatively due to its conforming shape and is the cornerstone of motion-preserving salvage operations like four-corner fusion and proximal row carpectomy.

Clinical presentation: chronic dorsoradial wrist pain (months-years after often-forgotten initial injury), decreased range of motion (especially extension), weakness with grip, snuffbox tenderness, may have visible deformity (loss of palmar concavity). Workup: detailed history of injury and prior treatment. Physical exam: snuffbox tenderness, scaphoid tubercle tenderness, decreased extension, weak grip strength, may have crepitation. Examination of contralateral wrist for comparison. Imaging: (1) Plain radiographs: PA, lateral, scaphoid views (PA with ulnar deviation and supination - 'clenched fist' or 'scaphoid view'). Look for fracture line, sclerosis, cyst formation, displacement, humpback deformity (volar angulation of scaphoid in lateral view, intrascaphoid angle greater than 35 degrees suggests humpback), and arthritic changes (radial styloid, capitate, scapholunate joint). (2) MRI: best for assessing proximal pole vascularity (signal intensity on T1-weighted images — proximal pole avascular necrosis shows decreased signal, but this is not always reliable; gadolinium enhancement is more specific) and for assessing the actual fracture line. (3) CT scan: best for assessing fracture alignment, displacement, humpback deformity (intrascaphoid angle), gap, and bone quality for surgical planning. (4) SPECT-CT or bone scan: assess vascularity if MRI ambiguous. Classification: Slade and Geissler arthroscopic classification of nonunion based on fracture line and surrounding bone quality. Treatment: depends on multiple factors: time from injury, location of fracture (proximal pole versus waist versus distal), vascularity of proximal pole, presence and stage of SNAC arthritis, patient functional demands, age, comorbidities. Treatment options: (1) Bone grafting alone for early non-displaced nonunion without arthritis: autologous iliac crest bone graft, distal radius bone graft, or tricortical iliac crest wedge for humpback correction. Russe technique (volar inlay corticocancellous bone graft, palmar approach) and modifications. Matti-Russe (volar inlay) more commonly used for waist nonunion. (2) Vascularized bone grafts for proximal pole nonunion or established AVN: (a) Pedicled bone grafts: 1,2 intercompartmental supraretinacular artery (1,2 ICSRA) graft from distal radius — most commonly used. (b) Volar carpal artery pedicled graft. (c) Free vascularized bone grafts: medial femoral condyle (MFC) free flap with descending genicular artery — vascularized osteocartilaginous graft, increasingly used for proximal pole nonunion with AVN. Free vascularized iliac crest graft. (3) Internal fixation: typically Herbert headless compression screw (Acutrak, Synthes HCS) placed antegrade or retrograde through cannulated technique, with placement guided by fluoroscopy and CT assessment. Volar approach for waist fractures, dorsal approach for proximal pole. Headless screws maintain compression across the fracture site. K-wires for additional stability. (4) Arthroscopic-assisted bone grafting and fixation: minimally invasive approach with grafting and percutaneous screw fixation under arthroscopic visualization. (5) Salvage procedures for SNAC arthritis: (a) Radial styloidectomy alone for SNAC I (limited indication). (b) Scaphoid excision and four-corner fusion (lunocapitate, hamatotriquetral, capitohamate, lunotriquetral arthrodesis with retention of radio-lunate joint and proximal row mobility) for SNAC II-III, with motion preserved (50-70% of normal flexion-extension). (c) Proximal row carpectomy (PRC, removal of scaphoid, lunate, triquetrum, with capitate articulating with lunate fossa of radius) for SNAC II-III, somewhat better motion than 4-corner but limited grip strength. (d) Total wrist fusion (radio-carpal fusion with bone graft and dorsal plate, complete loss of motion but full strength and pain relief) for SNAC IV. (e) Total wrist arthroplasty (less commonly used in young active patients due to longevity concerns). Outcomes: union rates of bone grafting for early non-displaced nonunion 80-95%; vascularized bone grafts for proximal pole AVN 60-90%; salvage procedures 80-95% pain relief but with functional limitations. Complications: persistent nonunion, infection, hardware failure, donor site morbidity, residual pain, decreased range of motion, progressive arthritis.