Scapholunate advanced collapse: a pictorial review

The scapholunate interosseous ligament binds the scaphoid and lunate together, and is the primary stabilising ligament between these two bones. The scaphotrapezium and radioscaphocapitate ligaments are secondary stabilisers of the scapholunate articulation, and therefore are not as vital as the scapholunate interosseous ligament in stabilising the scapholunate articulation [5]. The scapholunate ligament is composed of three distinct parts, which include the dorsal, membranous and volar components (Fig. 1). Even though the volar and membranous components are important for scapholunate stability, the dorsal component of the scapholunate ligament is the thickest and strongest, and therefore plays the most crucial role in stabilisation [6].

There are four commonly used terms used to describe the pathology seen in the pathway to the development of SLAC wrist. These terms are scapholunate dissociation, scapholunate diastasis, rotary subluxation of the scaphoid and dorsal intercalated segment instability (DISI). These pathological entities can be identified on imaging by measuring specific angles and distances between the different bones of the wrist (Table 1).

Scapholunate dissociation is the loss of synchronous motion or normal alignment between the scaphoid and lunate bones usually from ligamentous injury [7, 8]. The mechanism of injury in scapholunate dissociation is most commonly trauma causing wrist extension, ulnar deviation and intercarpal supination [9]. Eventually scapholunate dissociation leads to misalignment of other scaphoid joints and ultimately to osteoarthritis (SLAC wrist) [8].

Scapholunate diastasis is the term used to describe an abnormal increase in the scapholunate interval. Scapholunate diastasis occurs when there is a functionally complete tear of the scapholunate ligament. Scapholunate diastasis can be seen in the setting of scapholunate dissociation. However, scapholunate dissociation and scapholunate diastasis are not truly synonyms as one may have dissociation with a preserved width of the scapholunate interval [8].

The imaging findings which can be seen with scapholunate dissociation include scapholunate diastasis, which is described as having a scapholunate distance greater than 4 mm as measured at the midpoint of the scapholunate joint on the posteroanterior view, or having double the width of the scapholunate interval compared with a normal capitolunate joint interval [8]. Scapholunate dissociation may be suspected when the scapholunate interval is 2–4 mm [10]. Widening of the scapholunate interval on imaging has been called the “Terry Thomas sign”, referring to the famous actor with a prominent gap in his front teeth [11]. MRI is useful in that in addition to diastasis, it may also directly show a scapholunate ligament tear (Fig. 2). Ultrasound can be used as a low cost alternative to MRI to identify scapholunate ligament tears. Ultrasound is particularly useful for assessing patients with prior wrist fractures with subsequent surgical fixation hardware placement because the hardware can cause metal susceptibility artefact on MRI, limiting the ability to evaluate the wrist ligaments [12].

Rotary subluxation of the scaphoid is another term commonly encountered in the discussion of the pathway to the development of SLAC wrist. Rotary subluxation of the scaphoid is a pathological displacement of the scaphoid with or without apparent scapholunate disruption secondary to ligamentous injury. Rotation most commonly occurs with the proximal pole of the scaphoid moving dorsally and rotates onto the dorsal aspect of the radius and capitate [8] (Fig. 3). This particular pattern of scaphoid rotation has been demonstrated to occur when there is interruption of both the dorsal radiocarpal ligament and the scapholunate interosseous ligament [7]. However the scaphoid can also rotate in other directions depending on different forms of ligamentous injury [8].

There are several imaging findings that can be observed with rotary subluxation of the scaphoid. On the PA radiograph, scapholunate interval widening (scapholunate diastasis) may be demonstrated and/or foreshortening of the scaphoid can be seen, causing the “signet ring” sign (produced by the scaphoid tubercle being superimposed on the waist). On the lateral radiograph, an increased radioscaphoid angle >60° and/or a scapholunate angle >60–80° with a normal radiolunate angle can be seen [8]. The normal scapholunate angle ranges from 30 to 60° [7]. Subluxation of the scaphoid onto the dorsal rim of the radius can also sometimes be demonstrated on the lateral radiograph (Fig. 4).

DISI frequently occurs concomitantly with rotary subluxation of the scaphoid, due to alteration of the normal scapholunate articulation. It is most commonly due to ligamentous injury of the scapholunate interosseous ligament or to scaphoid non-union advanced collapse. In DISI there is dorsal angulation of the lunate as it rotates about its articulation with the radius. On the lateral wrist radiograph, DISI demonstrates dorsal tilt of the lunate with a radiolunate angle >10°, capitolunate angle >30° and/or a scapholunate angle >80° (60–80° is suspected DISI) [13] (Fig. 5). On the frontal radiograph scapholunate diastasis may be present. As opposed to DISI, volar intercalated segment instability (VISI) occurs due to injury of the lunotriquetral interosseous ligament causing volar tilt of the lunate. VISI is not associated with scapholunate dissociation, rotary subluxation of the scaphoid or SLAC osteoarthropathy.

It is controversial whether scapholunate dissociation and rotary subluxation of the scaphoid (as well as often concomitantly occurring DISI) are the same terms, or if one of these is a subset of the other. However, it seems that there are subtle but definite differences between these terms as rotary subluxation of the scaphoid and DISI focus on the individual orientation of the scaphoid and lunate, respectively, while scapholunate dissociation focuses on the relationship between the scaphoid and lunate [8].

Osteoarthritis of the wrist occurs almost exclusively (95 %) as a periscaphoid problem. There are three different patterns of arthropathy seen about the wrist which include SLAC, triscaphe arthritis (between the trapezium, trapezoid and distal scaphoid) and a combination pattern. SLAC wrist is the most common type of wrist arthritis and accounts for approximately 55 % of all wrist arthritis [14].

SLAC is an osteoarthropathy of the carpus secondary to altered stress around an unstable scaphoid. In SLAC, there is ligamentous instability of the wrist including disruption of the scapholunate ligament, which causes scapholunate dissociation, scapholunate diastasis, rotary subluxation of the scaphoid and DISI. Changes in alignment of the carpus, particularly changes in position of the scaphoid, result in inordinate stress and load to be placed predominantly on the radioscaphoid and capitolunate joints.

The sequence of events and mechanical alterations seen in SLAC wrist are sequential and consistent with a predictable progressive pattern of involvement. Three stages of SLAC arthropathy have been described: where osteoarthritic changes initially occur at the most radial aspect of the radioscaphoid joint (Stage I), progressing to involve the entire radioscaphoid joint (Stage II) and eventually affect the capitolunate joint (stage III), with sparing of the radiolunate joint until very late in the arthritic disease process (Fig. 6). Eventually more pronounced separation between the proximal pole of the scaphoid and the lunate occurs. This allows the capitate to migrate proximally, displacing the lunate ulnarward culminating with the SLAC pattern of osteoarthropathy [8, 14, 15].

The most common cause of SLAC is rotary subluxation of the scaphoid, which can be attributed to the elliptical configuration of the radioscaphoid joint. Even with advanced cases of SLAC, the articulation between the lunate and radius is grossly preserved due to the spherical nature of this joint, as opposed to the elliptical nature of the radioscaphoid joint [4, 16, 17].

Due to the elliptical configuration of the radioscaphoid articulation, pathological changes in position of the scaphoid in relationship to the remaining carpus results in incongruence, excessive loading and contact at the dorsal and volar aspects of the radioscaphoid joint. The normal radioscaphoid joint can be envisioned to resemble two nested spoons which sit parallel on top of one another. With flexion and extension of the normal wrist, full articular contact occurs between the radioscaphoid joint surfaces with congruence and evenly distributed load throughout the joint. If the axis of the spoons changes and they are now perpendicular to each other as one may think happens with even minor rotation of the scaphoid (rotary subluxation), then the contact surfaces are disrupted and the load is shifted to the dorsal and volar aspects of the radioscaphoid articulation leading to destruction and degeneration [4, 16, 17] (Fig. 7).

On imaging, the three stages of SLAC arthropathy can be demonstrated. In stage I SLAC the most radial portion of the radioscaphoid joint is affected first by osteoarthritic changes. The earliest changes seen are sharp spurring at the articular/non-articular junction on the radial side of the scaphoid and at the radial styloid process tip, with loss of the normal rounded curvature of the radial styloid process (Fig. 8). Subsequently, in Stage II SLAC the rest of the radioscaphoid joint is affected with progression to narrowing of the radioscaphoid joint (Fig. 9), followed by Stage III SLAC where there is narrowing of the capitolunate joint (Fig. 10). Ultimately, proximal migration of the capitate with ulnarward displacement of the lunate occurs [8, 14, 15] (Fig. 11).

SNAC is due to a non-united fracture of the scaphoid and is a common cause of wrist arthropathy, with a pattern of osteoarthritic change which is very similar to the pattern seen with SLAC wrist. Although SNAC and SLAC are in fact two different derangements, they have a similar pathophysiology and therefore SNAC wrist can be thought of as a variant of SLAC wrist. The difference between these two entities is that as opposed to SLAC where the scapholunate ligament is interrupted with resultant scapholunate diastasis, with SNAC the scaphoid is fractured and the scapholunate ligament and joint are usually preserved. The proximal pole of the fractured scaphoid behaves similar to a small lunate because the fragment is a small spheroid shaped bone which is situated within the spheroidal portion of the scaphoid fossa. This configuration of the non-united scaphoid causes osteoarthritic changes to occur between the radius and the distal scaphoid fracture fragment, progressing proximally but only up to the non-union site [3, 8].

Imaging findings seen with SNAC include arthropathy similar to the imaging findings seen with SLAC. The articulation between the distal non-united fragment of the scaphoid and the distal radius is affected first with joint space narrowing and osteophyte formation. Avascular necrosis of the proximal scaphoid fragment may be evident, although the articulation between the proximal scaphoid fragment and the radius is typically preserved (Fig. 12). Progression to affect the scaphocapitate and capitolunate joints eventually occurs [3].

SLAC wrist is commonly (26 % of the time) seen in patients with CPPD arthropathy [2]. The pathogenesis is pyrophosphate deposition in the interosseous ligaments (scapholunate) leading to ligament laxity and disruption and thus rotational alteration of the scaphoid. Associated findings suggesting CPPD associated SLAC wrist include bilateral involvement, triangular fibrocartilage calcifications, articular and peri-articular soft tissue calcifications, prominent subchondral cysts and osteoarthritic changes involving the metacarpophalangeal joints. Although not entirely specific for CPPD, hook-like or drooping osteophytes at the radial aspects of the metacarpal heads can often be seen [2, 18] (Fig. 13).

Asymptomatic SLAC wrist does occur and requires no treatment [19]. For symptomatic SLAC and SNAC arthropathy conservative non-surgical measures are the first line therapy. Non-surgical management includes wrist splinting, oral analgesics and intra-articular steroid injections. When symptoms persist or worsen after conservative measures are attempted, it is then that surgical treatment options are often considered. Surgical options for SLAC and SNAC include arthrodesis which can be complete or partial (four corner arthrodesis or capitolunate arthrodesis), denervation, proximal row carpectomy and radial styloidectomy (Fig. 14). Distal scaphoid pole excision is an additional surgical option which can be used in the treatment of SNAC, and is typically implemented during the early stages of SNAC related arthropathy prior to the development of capitolunate osteoarthritic changes [20, 21]. Also, the presence or absence of avascular necrosis can influence SNAC treatment options. The choice of surgical procedure which is ultimately performed for SLAC and SNAC depends on multiple factors, including both personal surgical preference and the stage of arthritic progression, noting that surgical procedures are commonly combined to achieve optimal results [21].