SNAC Wrist (Scaphoid nonunion Advanced Collapse)

Scapholunate advanced collapse (SLAC), commonly known as SLAC wrist, refers to a pattern of wrist malalignment that has been attributed to post-traumatic or spontaneous osteoarthritis of the wrist. It is a complication that can occur with undiagnosed or untreated scapholunate dissociation. It is essentially the same sequela of wrist injury causing scaphoid nonunion as seen in scaphoid nonunion advanced collapse (SNAC).

Pathology

SLAC is most commonly a consequence of undiagnosed or untreated scapholunate ligament injury and rotatory subluxation of the scaphoid bone, resulting in radioscaphoid malalignment, progressive chondromalacia, and osteoarthritis.

However, its features have also been observed in patients with idiopathic calcium pyrophosphate dihydrate (CPPD) crystal deposition disease.

Radiographic features

The pattern is that of progressive osteoarthritis, initially affecting the radioscaphoid. In later stages of the disease, osteoarthritis affects the whole radioscaphoid articulation, then the articulation between the lunate and capitate. Finally, it may involve other intercarpal joints. In addition, there is scapholunate interval widening as well as proximal migration of the scaphoid and the capitate ³.

Classification

The Watson and Ballet staging system is the most commonly used system for SLAC wrist (c. 2025) 4,8,12:

stage I: osteoarthritis of the articulation between the radial styloid and the scaphoid
stage II: osteoarthritis involving the whole radioscaphoid articulation
stage III: osteoarthritis of the radioscaphoid and capitolunate articulations
stage IV: osteoarthritis of the radiocarpal and intercarpal articulations +/- distal radioulnar joint (DRUJ)

Note that the radiolunate joint is almost preserved until the very last stages of the disease. It is also worth noting that the scaphoid fossa in the radius may be deep/preserved in cases of CPPD in contrast to post-traumatic SLAC wrist.

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Plain radiograph

Key assessing parameters include ⁹:

CT

Sagittal reformatted images from multidetector CT arthrographic data are particularly useful in demonstrating abnormal angulations of the scaphoid and lunate bones (increased scapholunate angle and dorsal or volar intercalated segment instability deformity), radioscaphoid incongruity, cartilage loss, and subchondral bone degenerative changes.

Treatment and prognosis

Non-surgical treatment includes splints, analgesia, and steroid injections ¹².

Surgical treatment for SLAC wrist is dependent on the stage of disease ^2,5,6,12:

stage 1: radial styloidectomy
stage 2: proximal row carpectomy (PRC) or midcarpal arthrodesis (e.g. two-corner (capitolunate) arthrodesis, four-corner arthrodesis)
stage 3: midcarpal arthrodesis
stage 4: complete wrist arthrodesis or total wrist arthroplasty

Partial or complete neurectomy can also be performed at any stage ¹².

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SNAC Wrist (Scaphoid nonunion Advanced Collapse)