Epidemiology

The scaphoid is the most commonly fractured carpal bone, accounting for approximately 70% of carpal fractures, and the second most common fracture of the upper extremity after distal radius fractures. The majority occurs from a low-energy injury, such as a fall onto an outstretched wrist from standing height. High-energy mechanisms such as a fall from a height or motor vehicle injury account for the remainder.

The highest incidence of fractures occur in younger age groups; 1 study found the highest incidence in males between the ages of 20 and 29 years old. Similarly, a Norwegian study found an average age of male individuals with scaphoid fractures to be 25 years old. Wolf recently studied the US military population and found an incidence of scaphoid fractures to be 121 per 100,000 person-years. The higher incidence was in males in the 20- to 24-year age group is likely owing to the more active nature of the military population. Wolf’s study using a public database of acute injuries with the US general population, found that there is a male predominance of 66.4%, and thus the remaining 33.6% female representing a higher incidence than the typically reported in the previous studies. They postulated that the increased incidence in females over the years were likely owing to an increased participation in organized sports.

Anatomy

The scaphoid has an unusual shape; the name is derived from the Greek word “skaphe” for ‘boat.’ The early 20th century nomenclature used ‘navicular,’ Latin for ‘boat.’ Given its odd and complex configuration, defining the exact fracture pattern or degree of displacement can be problematic. It appears concave in both ulnar and palmar axes. Its long axis is on an oblique plane. It is the largest bone in the proximal carpal row.

Four different regions of the scaphoid have been described. They are the tubercle, waist, distal pole, and proximal pole. The scaphoid is 75% articular, especially the ulnar side. Proximally, the scaphoid articulates with the distal radius at the scaphoid fossa, and distally with the trapezoid and trapezium. Ulnarly, it articulates with the lunate proximally and the capitate distally. The volar surface is partly nonarticular. The tubercle, which points radiovolarly, serves as an attachment for several ligaments and is also almost entirely covered by the crossing flexor carpi radialis (FCR) tendon. The scaphoid is oriented in the carpus with an intrascaphoid angle averaging approximately 40° in the coronal plane and 30° in the sagittal plane. Heinzelmann and colleagues found that male scaphoids were significantly longer (by 4 mm) and wider in their proximal pole than female scaphoids. The implications for surgical screw sizing based on sex and habitus often leads to recommending smaller screw sizes for female patients when considering operative fixation.

The majority of scaphoid fractures occur at the waist, and this higher incidence may also be related to the structural properties of the bone. Bindra studied cadaveric scaphoid with computed tomography (CT) and found that the bone is most dense at the proximal pole, where the trabeculae are the thickest and are more tightly packed, whereas the trabeculae in the waist are thinnest and sparsely distributed.

The dorsoradial ridge separates the dorsal and proximal articular surfaces from the distal volar aspect. The ridge is a narrow and nonarticulating area with several vascular perforations allowing important perfusion of the scaphoid. About 70 to 80% of the intraosseous vascularity and the entire proximal pole are supplied from branches of the radial artery entering through this ridge. Having a singular dominant intraosseous vessel predisposes the scaphoid to avascular necrosis (AVN) and nonunion if fractured. With the predominantly articular nature of the scaphoid, there are few potential sites for the entrance of perforating vessels; thus, it has a tenuous vascular supply. The major palmar blood vessels arise from either the radial artery directly or the superficial palmar arch and divide into several smaller branches before coursing obliquely and distally over the palmar aspect of the scaphoid to enter through the region of the tubercle. The anterior interosseus artery provides collateral circulation to the scaphoid. In addition, Herbert and Lanzetta have hypothesized that there must be some blood supply through the scapholunate ligament complex. From their cases series, proximal pole fragments remained viable when their only remaining attachment was to the SLIL.

Given the predominantly articular surface of the scaphoid, its attached ligaments play a critical role in stability and mechanics of the wrist. The scaphoid links the proximal and distal carpal rows, and as such influences motion at each row depending its position and functional demand. The scapholunate ligament is intra-articular and connects the scaphoid and lunate at the proximal aspect of their articulation with 3 main parts. The dorsal aspect of the ligament is the strongest and thickest, and composed of transverse collagen fibers. The dorsal portion is twice as strong as the palmar portion. The dorsal region resists palmar-dorsal translation and gap. The volar portion is not as strong as the dorsal portion, and the proximal or membranous portion is made of fibrocartilage and is not truly a ligament.

The radioscaphocapitate (RSC) ligament originates from the radial styloid and lies in the volar concavity of the scaphoid waist. It attaches the capitate ulnarly. The RSC acts as fulcrum to allow scaphoid rotation. It may also have a proprioceptive role, because it contains a high density of mechanoreceptors. The scaphocapitate ligament originates from the distal scaphoid. It inserts into the volar waist of the capitate distal to the RSC ligament. This ligament, along with the scaphotrapezial ligament, functions as a primary restraint of the distal pole.

With displaced scaphoid fractures, the proximal pole extends because of its attachment to the lunate through the scapholunate ligament, whereas the distal fragment remains flexed because of its attachment to the trapezium and trapezoid via the scaphotrapezial ligament. These deforming forces lead to a humpback deformity. The anatomy of the scaphoid and its associated ligaments contribute greatly to the risk of malunion and nonunion. It is almost completely covered with articular cartilage, limiting the amount of surface area for bone contact and healing. Displacement of these articular fracture fragments can also allow synovial fluid to pass between them and delay or halt healing.