Intra-articular Fractures and Dislocations at the Base of Metacarpals 2 to 5

20 Intra-articular Fractures and Dislocations at the Base of Metacarpals 2 to 5

Michael Schädel-Höpfner

Abstract

Fractures and fracture dislocations of the base of the metacarpals 2 to 5 are rare but meaningful injuries. Anatomy and function of the carpometacarpal (CMC) joints differ from the second through the fifth ray, with little mobility radial and rather high mobility ulnar. Clinical signs of fractures and dislocations may be discreet. The extent of these injuries is regularly underestimated in the initial radiographical evaluation of the hand. Computed tomography (CT) is generally recommended to reveal the whole extent of the damage to bones and joints. Accurate restoration of bony and articular anatomy is required to preserve hand function and to prevent painful osteoarthritis. Best results are achieved by early surgical intervention. Depending on the extent of injury, different and individual treatment strategies should be applied. The operative concept includes closed or open reduction techniques and fixation by Kirschner’s wires (K-wires), screws, or plates. If treated properly, good results can be expected in the medium and long term.

Keywords: metacarpal, carpometacarpal joint, fracture, fracture dislocation, reduction, fixation

20.1 Introduction

Fractures of the base of the metacarpals 2 to 5 and CMC fracture dislocations are rare injuries that may affect in addition the corresponding carpal bones and the attached ligaments. CMC fracture dislocations have been described first in 1844 by Blandin.1 Dobyns et al found only three cases of these injuries in 1,621 fractures of the hand, that is, less than 0.2%.² CMC fracture dislocations can occur as well in the first ray, where they are rather common and known as Bennett’s and Winterstein’s fractures.³

For the metacarpals 2 to 5, the spectrum of injury extends from simple, nondisplaced fractures of the base of the one or several metacarpals to severe injuries with CMC fracture dislocations and considerable damage to soft tissues. Stable, isolated fractures are uncommon, whereas unstable fracture dislocations prevail. The challenges of these injuries are manifold and concern accurate diagnosis, treatment strategy, precise reduction, and stable fixation.

20.2 Trauma Mechanism and Anatomy

CMC injuries of the fingers 2 to 5 mostly result from high-energy injury trauma such as axial compression due to punch (54%), traffic injury (23%), and fall (14%).4 More than half of these injuries affect the fifth ray due to its exposed position at the ulnar side of the hand and because of its higher mobility.⁵ However, there exists no single, reproducible pathomechanism that generally causes CMC injuries.^5,6 Even under controlled laboratory conditions, different fracture patterns have been produced in this area of the hand.⁴

In contrast to the thumb CMC joint, the metacarpals 2 to 5 are fixed to the distal carpal row by stable ligamentous attachments. Stability increases from ulnar to radial, supported by the shape of the base of the metacarpals 2 and 3 that fit exactly to the corresponding surfaces of trapezoid and capitate.^7,8 The metacarpal bones are fixed to the distal carpal row by dorsal and palmar carpometacarpal ligaments. In addition, stability is increased by intermetacarpal ligaments. Most rigid is the fixation of the base of the second metacarpal that holds a curved articular surface to the trapezoid and strong ligaments to the surrounding bones. Furthermore, the second metacarpal offers a radial and ulnar condyle to articulate with the trapezium and the base of the third metacarpal, respectively.⁹ Stability is also added by the insertion of the tendon of the extensor carpi radialis longus on the base of the second metacarpal. For this reason, the second CMC joint is rarely affected by fractures, and fracture dislocations are seen only on very rare occasions ( Fig. 20.1).

Nevertheless, CMC joints exhibit some mobility, especially for the fifth and fourth finger. El-Shennawy et al⁸ examined the mobility of these joints within a three-dimensional kinematic analysis on cadaver wrist. They found a considerable range of motion for extension–flexion of the little finger with up to 44 degrees, and up to 20 degrees for the ring finger. This rather high mobility allows a powerful grip, particularly when encompassing objects, as well as forming a mold, for example, for scooping water. In contrast, mobility for extension–flexion is small for the second and third CMC joint with only 11 and 20 degrees, respectively.⁸ Thus, the second and third CMC joints have a more stabilizing function and are recognized as central column of the hand.⁷ As a consequence of these biomechanical considerations, for injuries of the fifth and fourth CMC joint, due to their high mobility and importance for grip function, best possible reconstruction of the articular anatomy should be aimed at.

20.3 Classification

The typical injury pattern of CMC fracture dislocations includes a proximal–dorsal displacement of the shaft of the involved metacarpal, with one or more fragments of the base being retained on the palmar side. In addition, there may be fractures of the distal carpal row, most frequent shear or avulsion fractures of the dorsal cortex of the hamate. The whole extent of fractures, displacement, and dislocation can be best demonstrated by CT.

Fig. 20.1 Injury of the carpometacarpal joint of the second ray with nondisplaced fracture of the trapezoid that should be treated nonoperatively. (a, b) Plain radiographs being slightly suspicious of a trapezoid fracture. (c, d) Computed tomography confirms a fracture of the trapezoid with no displacement.

There exists no generally accepted and helpful classification of fractures and fracture dislocations of the base of metacarpals 2 to 5. Although metacarpal and carpal fractures are included in the AO/OTA classification for fracture of the hand,10 this classification is not sufficient to respect all features of the specific injuries. All classifications lack an adequate consideration of possible fracture dislocations and carpal fractures.

Thus, in the clinical routine it has been established to describe the following major issues of the injury:

• Number and position of the involved metacarpals (mostly fifth and fourth)

• Simple fracture, simple dislocation, or fracture dislocation of the metacarpal (mostly fracture dislocation)

• Direction of dislocation of the metacarpal (mostly dorsally)

• Additional fracture of the distal carpal row (mostly hamate)

Only for the fractures of the base of the fifth metacarpal, there exists a specific classification. On the base of an analysis of 64 cases, Kjaer-Petersen et al¹¹ have described four typical fracture patterns.^11,12 However, this classification is not being routinely used because it has only little impact on decision making.

Nevertheless, injuries of the base of the fifth metacarpal display some particularities. Displacements and dislocations occur as a general rule due to the force exerted by the tendon of the extensor carpi ulnaris muscle. This leads to a typical proximal displacement of the distal shaft fragment, whereas basal fragments may be retained palmar and radial when their ligamentous attachments are intact.¹³ In analogy to the fractures of the base of the first metacarpal, these injuries have been named baby Bennet¹⁴ or mirrored Bennett’s fracture.¹⁵ Further, radial displacement of the shaft fragment is caused by the hypothenar muscles. The avulsion fracture of the extensor carpi radialis tendon presents another unstable situation.^16,17 Consequently, the majority of injuries of the base of the fifth metacarpal requires reduction and fixation.

20.4 Clinical Signs and Tests

Usually, the patient reports some sort of injury, particularly if there was a severe blunt force impact to the hand resulting from a fall or traffic injury. In case of a punch, the history may be disguised, and some patients appear at the doctor with a significant delay.

Fractures of the base of metacarpals 2 to 5 and even fracture dislocations can be easily overlooked during clinical examination and on plain radiographs.18–20 Clinical symptoms may be mild in injuries with no displacement, but even with distinct displacement. Local tenderness and swelling at the base of the respective metacarpal or the whole back of the hand are suggesting a fracture. A restricted and painful mobility of the CMC joints and a loss of grip power should be considered as indicative. Displacement may be palpable if swelling is not pronounced. A “dinner-fork deformity” with a step on the dorsal contour of the hand, similar to the “four-chette displacement” in distal radius fractures, can be found especially in serial fracture dislocations.⁶ Open injuries are rare but implicate an immediate operative treatment ( Fig. 20.2).