1 Scaphoid Anatomy

10.1055/b-0034-80566

1 Scaphoid Anatomy

Watts, Adam C., McLean, James M., Fogg, Quentin, Bain, Gregory I.

The anatomy of the scaphoid and its articulation with neighboring bones are central to developing a comprehensive understanding of carpal mechanics. This chapter presents a review of scaphoid anatomy from a surgical perspective.

▪ Surface Anatomy

The scaphoid lies at ∼45 degrees to the long axis of the wrist in both the radial and palmar directions ( Fig. 1.1 ).1 The scaphoid tubercle is located at the base of the thenar eminence, in line with the radial border of the long finger. The tubercle extends volarward and is more readily palpable with the wrist dorsiflexed in radial deviation.2 The waist can be palpated between the radial styloid and the base of the trapezium in the anatomical snuffbox. The proximal pole can be palpated on the dorsum, distal to and in line with Lister’s tubercle.

▪ Osteology of the Scaphoid

The scaphoid originates from the fusion of two chondrification zones, known as the centralia.3 This process is not completed until the seventh week, when the embryo reaches 50 mm in crown-to-rump length.4 If these two elements do not fuse, then a supernumerary bone, interposed between the distal part of the scaphoid and the neck of the capitate, may be found (os centrale, present in 1.5% of the population; Fig. 1.2 ).3 The scaphoid ossification center commonly appears radiographically in the fourth year of life in girls and the fifth in boys, after the capitate and hamate, which may be seen to ossify before the end of the first year, and the triquetrum, which ossifies in the second to fourth year.5 Double ossification centers are not uncommon for the scaphoid, and if these fail to fuse, the result is a bipartite scaphoid, which can be misinterpreted as a frac-ture.6 Bipartite scaphoids are rare, usually bilateral, and they can be distinguished from a fracture by their smooth cortical edges, lack of history of trauma, and absence of displacement or degenerative changes.7

**Fig. 1.1** Surface anatomy of the scaphoid. (From Srinivas RR, Compson J. Examination of the wrist: surface anatomy of the carpal bones. Curr Orthop 2005;19:171–179. Reprinted with permission.)

**Fig. 1.2** Radiographic image of the wrist demonstrating the os centrale—a small ossicle sitting between the scaphoid and capitate.

▪ Osseous and Ligamentous Anatomy

The scaphoid has four articular facets covering 80% of its surface and forms an important link between the proximal and distal carpal rows ( Fig. 1.3 ).8 ^, 9 The kinematics of the wrist are governed by the ligamentous attachments of the scaphoid and the shape of the bones with which it articulates.

Proximal Pole

Proximally, the scaphoid articulates with the scaphoid fossa of the distal radius and the lunate. The orientation of the scaphoid fossa is 11 degrees volar and 21 degrees ulnar relative to the long axis of the radius, and therefore at ∼65 degrees to the long axis of the scaphoid in the frontal plane. This orientation prevents dorsal and radial translation of the scaphoid. Dorsal entry of a guidewire for scaphoid fixation is difficult without marked wrist flexion, as the proximal pole is largely covered by the dorsal lip of the radius. Some authors have promoted percutaneous fixation of scaphoid fractures via a dorsal approach, stating that it is easier to place the screw down the anatomical axis of the scaphoid.10 However, an anatomical study has demonstrated the hazards to superficial structures with this percutaneous technique, particularly to the extensor pollicis longus, the extensor indicis, and extensor digitorum communis to the index finger, and the terminal branch of the posterior interosseous nerve. Therefore, some authorities recommend a mini open approach just distal and ulnar to Lister’s tubercle.11

Scapholunate Interosseous Ligament

The scapholunate interosseous ligament (SLIL) divides the radiocarpal joint from the lunate facet,6 which may be either semilunar6 or ovoid.12 The strong, thick dorsal region is similar in composition to the thinner palmar region, both having a histological organization composed of transversely oriented (dorsal) or oblique (palmar) collagen fascicles, consistent with true capsular ligaments.9 In contrast, the thin proximal region (< 1 mm) is composed of fibrocartilage, with comparatively few collagen fascicles and no blood vessels, nerves, or perifascicular spaces.9 Normally, the SLIL, along with the lunotriquetral ligament, prevents communication between the radiocarpal and midcarpal joints. An arthrogram with injection of contrast into the radiocarpal joint that shows leakage into the midcarpal joint confirms the tearing of at least part of one of the ligaments but does not diagnose instability.

**Fig. 1.3** Anatomy of the scaphoid bone. (From Schmidt HM, Lanz U. Surgical Anatomy of the Hand. Stuttgart: Thieme; 2003:46.

Less than 20 degrees of motion is possible at the scapholunate (SL) joint.9 The dorsal and palmar regions are important in maintaining normal function of the SL joint and carpal kinematics.9 The dorsal region is most critical for resistance to palmar-dorsal translation, and the palmar region is critical for constraint of rotation.9 The proximal fibrocartilaginous region is well suited to accept the compression and sheer loads at the radiocarpal joint.9

Scapholunate Articulation

The shape of the lunate is thought to play an important role in the kinematics of the wrist, and the lunate may be thought of as the proximal “anchor” of the scaphoid via the strong dorsal SLIL. The distal articular surface of the lunate may have either a single facet (type 1) that articulates with the capitate or a double facet articulating with the capitate and the hamate (type 2).13 The condylar double-facet midcarpal articulation permits only flexion and extension of the proximal carpal row, restricting radial translation. It has been proposed that this affects the pattern of carpal collapse in association with scaphoid waist fracture nonunion, where the type 2 lunate morphology has been shown to be associated with a significantly decreased incidence of dorsal intercalated segment instability (DISI).14

Scaphoid Waist

The waist serves as a site for ligamentous attachments,9 ^, 15 for joint capsule attachment,8 and for the passage of nutrient vessels from the radial artery proper (or dorsal radiocarpal arch) into the scaphoid via numerous dorsal foramina.8 ^, 16 A morphological analysis of the scaphoid by Fogg suggests that there are variations in the morphology of the scaphoid that can be classified into two types.17 ^, 18 A type 1 (rotating) scaphoid is characterized by a single high crest obliquely oriented across the dorsal aspect of the waist; the type 2 (flexing) scaphoid is characterised by three low crests similarly oriented and located ( Fig. 1.4 ).17 These ridges correspond to the attachment of the dorsal joint capsule and, importantly, the dorsal intercarpal ligament and fibers of the radioscaphocapitate ligament radially.8

**Fig. 1.4** Variations in the anatomy of the wrist dorsal cortex. (From Ceri N, Korman E, Gunal I, Tetik S. The morphological and morphometric features of the scaphoid. J Hand Surg [Br.] 2004;29(4):393–398. Reprinted with permission.)

Radioscaphocapitate Ligament

The radioscaphocapitate (RSC) ligament lies in the volar concavity of the scaphoid waist, originates from the radial styloid process as far ulnarly as the middle of the scaphoid fossa,9 and acts as a fulcrum around which the scaphoid rotates. It contains a high density of mechanoreceptors, suggesting a mechanical and proprioceptive role.19 In wrists with a flexing (type 2) scaphoid, the RSC ligament is attached to the waist of the scaphoid9 and passes to the capitate, whereas in those with a rotating (type 1) scaphoid, it has no scaphoid attachments.19

Dorsal Intercarpal Ligament

The dorsal intercarpal ligament (DIC) is a weak capsular ligament that functions as a stabilizer of the wrist and a dorsal hammock restraint for the proximal pole of the capitate.20 It inserts into the proximal crest on the waist of the type 2 scaphoid but passes over the type 1 scaphoid without attachment to reach the margin of the scaphotrapezioltrapezoidal (STT) complex. The DIC ligament has been found to be richly innervated with nerve endings associated with the posterior interosseous nerve, suggesting a proprioceptive role.19 It is strengthened by the dorsal radiocarpal ligament, which extends distally in an oblique ulnar direction from the dorsal lip of the radius to attach to the dorsal aspect of the triquetrum, interdigitating with fibers of the DIC ligament9 ^, 16 ^, 21 and controlling ulnar drift of the carpus.

The type 1 and type 2 variations may play a more important role in the carpal collapse patterns observed by Haase et al 14 than the shape of the distal lunate articulation alone. It may be that the ligamentous restraint of the volar RSC ligament and the DIC ligament at the scaphoid waist in concert with the type 2 lunate morphology provides enough stability to prevent collapse of the carpus into a DISI pattern. This is supported by the findings of Moritomo et al,22 who demonstrated that the DISI deformity is less likely to be seen in fractures proximal to these ligamentous attachments, as they restrain the distal scaphoid fragment and maintain stability. A wrist with a flexing scaphoid and type 2 lunate may be less likely to develop a state of scaphoid nonunion advanced collapse (SNAC) following fracture nonunion, but this remains speculative at this point.

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