Arthroscopic partial scaphoidectomy for scaphoid nonunion

Relevant anatomy and pathomechanics

A number of factors predispose toward a nonunion. Because of the scaphoid’s narrow waist section where the trabeculae are thinnest and are more sparsely distributed, fracture site displacement decreases the bony contact area for union. Any waist fracture with displacement of greater than 1 mm or angulation of greater than 15 degrees may lead to a nonunion if left untreated. Because the scaphoid is largely covered by cartilage, any fracture heals by intramembranous ossification, so there is no fracture callus to provide any initial stability. Premature wrist loading results in bending, shearing, and translating forces, which cause progressive flexion and pronation of the distal pole. Inadequate fracture site immobilization may lead to volar bone resorption as a response to the continued loading, which may culminate in a nonunion with a secondary humpback deformity. Displacement of the fracture is a strong risk factor for delayed or failed union. Singh et al. performed a meta-analysis of 1401 scaphoids and showed that displaced fractures of the scaphoid have a four times higher risk of nonunion than undisplaced fractures when treated in a plaster cast.

Vender et al. noted that a longstanding scaphoid nonunion leads to a sequence of arthritic degeneration known as scaphoid nonunion advanced collapse (SNAC). It differs from that seen with chronic scapholunate (SL) dissociation in that the articulation between the proximal scaphoid fragment and radius is spared from arthritic changes. The distal scaphoid fragment is no longer attached to the dorsal intercarpal (DIC) ligament, which allows it to fall into flexion, which causes incongruity between the distal scaphoid fragment and the corresponding articular surface of the radius. The site of initial degenerative change is between the radius and distal scaphoid fragment (stage 1), which stops at the site of nonunion. Narrowing of the lunocapitate joint (stage 2) occurs next and with advanced midcarpal arthritis, narrowing of the capitate–distal scaphoid fragment (stage 3) occurs. The radius–proximal scaphoid fragment and radiolunate joints remain normal, even with severe arthritis. These joints are preserved because both are spherical in nature, allowing perpendicular cartilage loading in all positions and because the proximal scaphoid fragment is still attached to the lunate via an intact SL ligament. In a study of 104 scaphoid nonunions, Inoue et al. noted a prevalence of arthritis in 22% of cases if the nonunion was 1 to 5 years old, 75% of cases in nonunions that were 5 to 9 years old, and 100% of cases in nonunions that were 10 years old or more. Osteoarthritic changes occurred initially at the scaphoid–radial styloid joint, which were manifested by radial styloid pointing and/or dorsal radioscaphoid osteophyte formation, later progressing to the midcarpal joint. Osteoarthritis at the scaphoid–radial styloid joint was significantly associated with a dorsal intercalated segmental instability (DISI) deformity. The overall incidence of DISI deformity of the wrist was 56%, and the frequency of a DISI pattern increased with a longer duration of nonunion. There was no correlation between symptoms of pain and the severity of arthritis or the duration of nonunion, but there was a good correlation between the duration of nonunion and reduced grip strength or decreased wrist motion.

Nakamura et al. categorized scaphoid nonunions into 2 types based on 3-D CT scans: a volar type, in which the distal fragment overlaps the proximal fragment volarly; and a dorsal type, in which the distal fragment overlaps the proximal fragment dorsally. Moritomo et al. demonstrated that the fracture location of a scaphoid nonunion relates to the fracture displacement, development of DISI deformity, and changes in the contact area of the bones in the radiocarpal joint. Eleven patients with scaphoid nonunions were examined with 3-D CT scans. Two patterns of displacement of scaphoid nonunions were demonstrated, 1 volar and 1 dorsal. In the volar type, the distal fragment was displaced volarly relative to the proximal fragment and became close to the radial styloid with the proximal fragment extended, resulting in a humpback deformity. All patients with a volar-type pattern scaphoid nonunion had a DISI deformity. Only a few of the patients with a dorsal-type pattern scaphoid nonunion, mostly in longstanding nonunions, had a DISI deformity. The fracture line was generally distal to the dorsal apex of the ridge of the scaphoid in the volar-type fractures and proximal in the dorsal displaced fractures. The location of the dorsal apex of the ridge of the scaphoid coincides with the location of the attachment of the proximal part of the DIC ligament, which is just distal to the attachment of the dorsal component of the SLIL. These ligaments, along with the dorsal radiocarpal ligament (DRCL), probably afford indirect dorsal stability of the scaphoid. In the volar-type scaphoid nonunion, the fracture line is distal to the attachment of the DIC ligament and the dorsal component of the SLIL, which may affect the stability of the distal fragment. This could explain why the proximal pole extends and the distal pole flexes, resulting in a DISI deformity in the volar-type of scaphoid nonunion.

In the dorsal-type scaphoid nonunion, the ligamentous attachments remain on the distal fragment, which may offer some additional stability or ability for the distal fragment to resist flexion forces. In the cases with a longstanding scaphoid nonunion, however, even the dorsal-type scaphoid nonunion can develop a DISI deformity with degenerative changes at the articulation between the proximal fragment of the scaphoid and the capitate. They also looked at the proximity map, which is the visual representation of the distance from one bone to the nearest neighboring bone and gives a qualitative assessment of the inferred contact area between the bones. In the volar-type scaphoid nonunion, the proximity map of the distal fragment of the scaphoid on the radius shifted radially compared with a normal wrist, placing it closer to the radial styloid. They called this the styloid pattern ( Fig. 19.1 A–D). In the dorsal-type scaphoid nonunion, the proximity map of the distal fragment of the scaphoid on the radius shifted dorsally compared with a normal wrist, placing it closer to the dorsal lip of the scaphoid fossa of the radius. They called this the dorsal lip pattern of proximity map ( Fig. 19.2 A–D).

Oka et al. studied the wrist kinematics in 13 patients with scaphoid nonunions during wrist flexion-extension and radioulnar deviation. Two clear patterns of interfragmentary motion of the scaphoid emerged based on the fracture location. In the mobile type scaphoid nonunion (7 cases), the fracture was located distal to the apex of the scaphoid dorsal ridge and the distal scaphoid was unstable relative to the proximal scaphoid. The distal fragment showed a “book-opening” motion from wrist flexion to extension. In the stable-type scaphoid nonunion (6 cases), the fracture was located proximal to the scaphoid apex, and the interfragmentary motion was considerably less than with the distal type. In the displaced distal scaphoid fractures, the proximal fragment of the scaphoid, lunate, and triquetrum rotated into extension and supination. The distal fragment of the scaphoid and capitate translated dorsally without notable rotation. Most distal scaphoid nonunions had a DISI deformity pattern, whereas this occurred in only 1 case of a proximal fracture.

Diagnosis

The diagnosis of a scaphoid nonunion is made by history, physical examination, and wrist radiographs. The typical patient complains of radial-sided wrist pain at rest that is exacerbated by radial deviation and by wrist extension, or with rotation and torque. The wrist examination may reveal dorsal-radial wrist swelling, tenderness over the radioscaphoid joint, and a painful scaphoid shift test. Wrist motion may be decreased, depending on the stage of degeneration. The definitive diagnosis is made radiographically. Standard posteroanterior (PA), 45-degree pronation and 45-degree supination oblique views, and a true lateral view are performed. A PA view in radial deviation extends the scaphoid and allows a better assessment of the nonunion site and degree of instability. The marked changes of an advanced SNAC wrist are easily identified. An MRI and/or CT scan may be useful to evaluate any midcarpal joint changes and DISI deformity as well as to determine whether there is a styloid pattern or dorsal lip pattern of impingement.