Volar Wedge Grafting for Scaphoid Nonunions

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ANATOMY

Scaphoid is Greek for canoe or boat, but in reality the scaphoid would be a bent and twisted boat to truly explain its configuration. Surgeons are often deceived by the plane of the scaphoid, which is 45 degrees of palmar tilt to the longitudinal axis of the forearm on the lateral projection and 45 degrees angled radially from the central axis of the forearm on the anteroposterior projection. The scaphoid is almost completely covered by cartilage with five articulating surfaces. The entire proximal half of the scaphoid is an articular surface within the radial carpal joint. Dorsal vessels entering the dorsal ridge of the scaphoid directly from the radial artery supply 70% to 80% of the bone including the proximal pole ( Fig. 21-1 ).

BIOMECHANICS OF SCAPHOID FRACTURES

The scaphoid is the most frequently fractured carpal bone. Scaphoid fractures result from a fall on an extended wrist in radial deviation ( Fig. 21-2 ). The degree of wrist extension at the time of impact determines the location of the fracture. Distal radius fractures result from a fall on a less extended wrist, whereas scaphoid fractures occur with a more hyperextended wrist. The scaphoid becomes locked in the scaphoid fossa when the wrist is in a hyperextended and radially deviated position. The radioscaphocapitate ligament acts as a fulcrum on which the distal pole of the scaphoid palmarly flexes on impact, resulting in a scaphoid fracture. Most scaphoid fractures (70% to 80%) occur at the midportion or waist, whereas 10% to 20% occur at the proximal pole ( Fig. 21-3 A–C).

Scaphoid collapse and the subsequent development of a “humpback” deformity may occur in nonunions involving the scaphoid waist. Bone resorption at the fracture site leads to collapse of the scaphoid and palmar rotation of the distal pole. The lunate becomes extended through the intact lunotriquetral ligament, leading to a dorsal intercalated segmental instability (DISI) deformity. As the lunate extends, the proximal fragment of the scaphoid also extends through the intact scapholunate (SL) ligament. This extension of the proximal fragment combined with flexion of the distal fragment leads to what is referred to as a humpback deformity ( Fig. 21-4 ).

PHYSICAL EXAMINATION

Patients with established nonunions often present with pain, occasional swelling, and weakness of the hand. They may report a remote history of a fall on a hyperextended wrist resulting in a “wrist sprain,” for which they may or may not have sought medical attention. They have difficulty with any activity that requires loading the wrist in an extended position, such as push-ups. There is tenderness to palpation over the anatomic snuffbox on physical examination. There is also decreased wrist flexion and extension compared with the contralateral side. Grip strength may also be decreased compared with the contralateral side.

IMAGING

Initial imaging should start with standard radiographs to include the following four views: posteroanterior, lateral, oblique, and scaphoid view. The scaphoid view is obtained with the wrist in slight ulnar deviation; this places the longitudinal axis of the scaphoid in the plane of the radiographic cassette. In an established nonunion, it is key to evaluate certain radiographic parameters to determine the degree of scaphoid collapse and resultant wrist deformity. The normal scapholunate angle averages 46 degrees (range 30 to 60) ( Fig. 21-5 ). In an established nonunion, a DISI deformity may develop as the scaphoid continues to collapse and the lunate assumes an extended position. This results in an increased scapholunate angle. The capitolunate and radiolunate angles should measure around 0 degrees (range −15 to 15). Again, these angles increase with a DISI deformity ( Fig. 21-5 ).

Our practice is to also obtain a preoperative computed tomography (CT) scan for all scaphoid nonunions. CT scanning is an important imaging study that can help define the personality of the fracture as well as accurately determine the extent of collapse (humpback deformity). This is essential in planning for bone-grafting procedures. The anteroposterior and lateral intrascaphoid angles as well as the height-to-length ratio measure the degree of scaphoid collapse. The intrascaphoid angles are determined by the intersection of two lines drawn perpendicular to the diameters of the proximal and distal poles. An anteroposterior intrascaphoid angle of less than 40 degrees ( Fig. 21-6 ) or a lateral intrascaphoid angle of more than 30 degrees ( Fig. 21-7 ) is considered abnormal and indicates a humpback deformity. The height-to-length ratio of the scaphoid ( Fig. 21-8 ) described by Bain and colleagues can also be accurately measured on CT scan and is used to estimate the degree of deformity. A ratio greater than 0.65 indicates collapse. With any suspicion of avascular necrosis of the proximal pole, magnetic resonance imaging (MRI) should be obtained.

INDICATIONS AND CONTRAINDICATIONS FOR VOLAR WEDGE GRAFTING

There is ample evidence in the literature that untreated scaphoid nonunions lead to a progressive pattern of wrist arthritis. Longstanding scaphoid nonunion leads to a pattern of arthritis originally described by Watson and Ballet. It is described in a four-stage progression, similar to that seen with scapholunate dissociation. Stage I is characterized by arthritic changes and osteophyte formation at the radial styloid. Stage II is characterized by arthritis at the radioscaphoid joint. Stage III progresses to involve arthritis midcarpal (capitolunate and scaphocapitate joints). Stage IV includes pancarpal arthritis with sparing of the radiolunate joint.

Volar wedge grafting is indicated for patients with a scaphoid waist nonunion associated with a humpback deformity. Amadio and colleagues noted an increased incidence of arthritis with an anteroposterior intrascaphoid angle of less than 40 degrees or a lateral intrascaphoid angle of more than 30 degrees. Absolute contraindications to the procedure include degenerative wrist arthritis (stage II or III scaphoid nonunion advanced collapse, or SNAC wrists).

It remains an area of controversy whether the procedure is indicated in stage I SNAC wrists. Our practice to combine the procedure with a radial styloidectomy in young patients with stage I SNAC wrist ( Table 21-1 ). It also remains controversial whether to perform the procedure in a patient with avascular necrosis of the proximal pole and a humpback deformity. In our practice. these patients are treated with a vascularized bone graft (1, 2 intercompartmental supraretinacular artery [ICSRA] graft) inserted volarly to correct the deformity.

TABLE 21-1

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