Anatomy

Within the hand, distal to the radius and ulna, there are 27 bones that are prone to injury and an open fracture. Each has its unique anatomy, blood supply, and soft tissue coverage. Divided broadly, they can be separated into 3 regions: the phalanges, the metacarpals, and the carpal bones.

The soft tissue coverage of the phalanges consists of skin, tendon, ligament, areolar connective tissue, and nail. The 14 phalanges of each hand are devoid of muscle. As a result, the digits are prone to open injury with minimal amounts of trauma or fracture displacement, especially in the dorsal surface where the fascial layers lack the robustness of the palmar side. Furthermore, these structures do not possess the bulk or vascularity of muscle, potentially limiting their ability to fight infection.

The metacarpals share some morphologic features with the phalanges. Among these are palmar layers of tough fascia and alveolar connective tissue, and a dorsal surface with a thin covering of skin, tendon, and fascia. However, the metacarpals also benefit from the presence of interosseous, thenar, and hypothenar musculature, providing bulky coverage and blood supply. As a result, the metacarpals are vulnerable to dorsal open injuries and wounds but benefit from a robust blood supply.

The carpal bones possess the most dense soft tissue coverage of the osseous regions of the hand. However, they have the most fragile blood supply because of their absence of muscular coverage and otherwise extensive articular nature. Subsequently their blood supply is derived from their ligamentous and capsular attachments, structures that can be readily compromised with trauma. However, these soft tissue attachments, combined with the deep position of the carpus and its highly congruent and strong intercarpal attachments, provide resistance to open fractures in this region.

Vascular Supply

The digits receive most of their blood supply via the palmar digital arteries, with contribution from the dorsal digital arteries. Distally, these palmar arteries anastomose to form the blood supply to the digital pulp. The palmar digital arteries run superficial to the digital nerves and lie directly deep to the skin. As a result of their position, these vessels are easily injured during digital trauma, compromising blood supply and increasing infection risk of the digit. This effect can be mitigated by the arterial anastomoses in the digit, which provide redundant blood supply in case of injury. Degloving, ring avulsion, and other circumferential injuries are a particular risk for dysvascularity, and loss of both radial and ulnar digital arteries can result in an avascular digit.

More proximally, the hand benefits from a robust and redundant vascularity. The vascular supply of the hand is provided by the palmar arches and variable dorsal arches, anastomotic networks composed of contributions from radial and ulnar circulation. These networks provide multiple perforators supplying both the metacarpal bones and the soft tissues surrounding them. The intrinsic muscles of the hand also possess multiple points of vascular supply, and provide a vascular bed that can supply the metacarpals. This region of the hand is more resistant to devascularization from trauma, although extensive soft tissue damage can still compromise its blood supply.

Proximal devascularization in hand injuries does not guarantee loss of blood supply distal to the injury. Although certain sites in the hand and wrist (eg, the proximal scaphoid) have tenuous vascular supply, the extensive network of anastomoses means that blood flow often has many alternate paths to reach distal structures. This supply is especially relevant to the carpus.

Because of the extensive articulations of the carpal bones, many have a limited, tenuous blood supply. A landmark study examining 75 cadaver limbs showed that the scaphoid received most of its blood supply via distal, dorsal nutrient vessels, with no intraosseous anastomosis to the palmar circulation. In 70% of capitates examined, most of the blood supply was dorsally based and did not anastomose with the palmar circulation. Likewise, in 8% of the lunate specimens examined, the vascular supply of the bone arose from a single vessel.

Although the other carpal bones possess more redundant blood supply, all are vulnerable to disruption from high-energy injuries. In the case of the scaphoid and the capitate, in particular, small soft tissue disruptions may result in avascular bone stock, increasing the risk of infection and nonunion.

Severe vascular injuries of the hand may require emergent revascularization, regardless of the level of contamination of the wound. Although the literature is limited with regard to thrombosis and infection rate of revascularized hands in open fractures, inadequate perfusion necessitates emergent surgical intervention. Primary repair or grafting of damaged vessels prevents ischemic injury to distal structures, and must be performed if collateral circulation is not adequate. Poor blood supply is clearly a risk factor for subsequent infection. However, the hand differs to some extent from other sites of open fracture in its extensive network of collateral circulation. Gustilo-Anderson type IIIc lower-limb fractures carry an infection risk as high as 39% according to one series. However, there are limited data on infection rates in open hand injuries with vascular compromise.

Soft Tissue Envelope

The hand has a unique soft tissue envelope, both protective and potentially injurious in the setting of open hand fractures. The palmar surface of the hand benefits from a robust skin and dense subcutaneous tissue via its glabrous skin and deep fascial connections and muscular subcompartments. Significant trauma is required to result in open fractures on the palmar side. In contrast, the dorsal hand has only a thin layer of skin with minimal alveolar subcutaneous tissue, leaving the osseous and tendinous structures prone to ready exposure even with minor trauma.

When soft tissue loss is present, the hand poses a unique challenge in coverage. Securing adequate coverage in hand trauma is necessary to protect the deep osseous and soft tissue structures such as the many nerves, vessels, and tendons. However, many soft tissue coverage options exist that are indicated based on the nature of the injury and surgeon preference, including primary closure, secondary closure, acellular dermal substitutes, local rotational or advancement flaps, pedicled flaps, and free flaps. Each option has its own unique characteristics and risks and benefits for infection that must be taken into account in the setting of an open fracture.

Contamination

Frank contamination of an open fracture intuitively increases the risk for infection. Contamination of wounds often occurs as a result of injury mechanism, because debris is deposited into the wound site. These contaminant particles provide a nidus for bacterial growth, as well as serving as a source of bacterial bioburden. Certain types of contamination warrant specific consideration. Among these are soil contamination, which carries a high risk of anaerobic infection ; fecal contamination, which carries a risk of polymicrobial and gram-negative infection ; and bite wounds, which may be contaminated by organisms including Eikenella and Pasteurella species. A 1978 study performed by Lawrence and colleagues analyzed bacterial cultures of open fractures at time of presentation, and found that infections developed in a small proportion of patients (3 of 95 fractures), and only in those with high levels of contamination, providing evidence that initial degree of contamination affects infection risk.

Tscherne and Oestern attempted to quantify this risk with their classification of open fractures. This classification takes into account the severity of associated soft tissue disruption, ranging from grade I (small puncture wound, negligible contamination, low-energy fracture) to grade III (heavy contamination, extensive soft tissue damage, associated neurovascular injury) and grade IV (traumatic amputation). A 2015 retrospective review of 122 patients by Matos and colleagues found that Tscherne II and III fractures were associated with a significantly higher rate of infection (48% and 26% respectively). Although this study examined both upper-limb and lower-limb injuries, it did not differentiate hand injuries specifically.