The carpometacarpal (CMC) joints of the long fingers have not received the same level of interest as their equivalent in the thumb. Yet they deserve special attention because their only similarity to the trapeziometacarpal (TMC) joint is their anatomic position in the hand. The differences in injury patterns are significant. Whereas fractures are more common for the thumb column (Bennett and Rolando fractures), dislocations predominate for the other four rays. The differences are also anatomic; the extremely compact and virtually fixed CMC block is in distinction to the mobility and detached position of the thumb.
This explains the greater vulnerability and therefore more frequent injuries of the TMC, compared with CMC dislocations. These are rare injuries, estimated at fewer than 1% of injuries in the hand.
We observe that more than a century and a half after their first descriptions, fewer than 300 studies have so far been reported, explaining to a certain degree why they have been overlooked.
However, if one considers the cases that go unnoticed or unpublished, these dislocations are far from rare. They must be routinely investigated because all authors agree not only about the difficulty of diagnosis but also on the excellent prognosis if treatment is early. Conversely the impact on hand function may be critical if the injury is not recognized. This makes treatment more difficult and outcomes uncertain.
Dislocations of the trapeziometacarpal joints must also be diagnosed and treated early, because the functional consequences for the thumb and hand function are significant.
Dislocations and Fracture-Dislocations of the Carpometacarpal Joints of the Long Fingers
The CMC joints are complex and differ according to the fingers. Their stability is secured by joints and strong palmar, dorsal and intermetacarpal (IMC) ligaments ( Figs. 6.1 and 6.2 ). Tendon insertions, acting as guy-ropes, add a dynamic compressive effect. All of this results in an extremely stable block.
Like the foot, the hand has both a transverse and a columnar structure. The second row of carpal bones, stabilized by the transverse carpal ligament, provides a stable transverse arch that serves as a foundation for the metacarpals. These form the longitudinal arches, and it is common to distinguish the fixed block of the second and third CMCs from the relatively mobile block of the fourth and fifth CMCs.
The articulation of the index finger with the trapezium and trapezoid, and the middle finger with the capitate, is roughly horizontal, notched and crenellated and therefore very congruent. Each metacarpal base receives two ligaments per surface in a triangular array, originating from the corresponding bones of the carpus. The very strong dorsal ligaments are reinforced by the insertion of the extensor carpi radialis longus on the second metacarpal bone and the extensor carpi radialis brevis on the third metacarpal bone. On the palmar surface the flexor carpi radialis, with its bifurcated insertion on both bases, counterbalances. This configuration severely restricts movement for the second and the third CMC. Thus through its central position and its rigidity, this block represents the cornerstone of the system.
The hamatometacarpal (HMC) complex is different because it is made up of two modified saddle joints and a more lax dorsal ligament assembly. This allows 10–15 degrees of flexion-extension for the fourth CMC on an axis that is collinear with the third ray. This is because the HMC joint is perpendicular to this axis. It is interesting to observe the different anatomic configurations at this level, including an articulation with the capitate, increasing the stability of the metacarpal. The fifth CMC, composed of the base of the fifth metacarpal and the ulnar aspect of the hamate, is more mobile. The flexion-extension arc varies between 20 and 30 degrees, to which we must add the possibility of a few degrees of rotation. This, combined with a more oblique joint orientation, allows an opposition movement from the fifth ray toward the tubercle of the scaphoid but also deepening of the metacarpal arch when grasping, essential for the conservation of force. The ligament assembly is identical to the neighboring block, apart from the presence of only a single ligament on the dorsal surface of the fifth CMC. On the palmar surface the flexor carpi ulnaris, via the pisometacarpal ligament, attempts to oppose the strong pull imposed by the insertion of the extensor carpi ulnaris on the dorsal surface of the base of the fifth metacarpal.
Finally, the terminal cohesion of the assembly is ensured by the strong palmar and dorsal IMC ligaments holding the metacarpal bases together.
CMC dislocations differ according to the number of rays affected and the direction of displacement of the metacarpals in relation to the carpus:
complete, when all the digits are dislocated
partial, when two or three rays are affected
isolated, when a single ray is affected
dorsal ( Fig. 6.3 )
palmar ( Fig. 6.4 )
lateral ( Fig. 6.5 )
divergent, representing the association of a dorsal and palmar dislocation ( Fig. 6.6 )
All combinations are possible and have already been reported. There is, however, a predominance of certain types of injuries that can be explained by a combination of anatomic weaknesses and mechanism of injury.
Dorsal dislocations are by far the most numerous, despite the stronger dorsal ligaments. They serve to oppose the posterior displacement of the metacarpal bases during grasp, to which the action of the extensor carpi is added. They are more numerous and more powerful than their palmar counterparts.
These are followed by palmar dislocations and lateral dislocations, particularly of the fifth ray, more so in a radiopalmar than ulnar-palmar configuration, and even rarer affecting several metacarpals (see Figs. 6.5a and b ).
Finally, there are the rare divergent dislocations (five reported cases), almost always combining a dorsal second and third metacarpal displacement with a palmar fourth and fifth metacarpal displacement, (see Fig. 6.26 ) or a case of dorsal second metacarpal and palmar third metacarpal–fourth and fifth metacarpal dislocation.
If we examine the number of rays affected, none of the types of dislocations is more common than the other. In the last two groups, however, ulnar CMC injuries are clearly predominant, mostly affecting the fourth and fifth metacarpal ( Fig. 6.7 ) but especially the isolated fifth metacarpal. The mobility and ligament laxity of the HMC joint explain this greater fragility. Border rays are also more exposed to trauma. Second metacarpal and second and third metacarpal dislocations are thus second in frequency ( Fig. 6.8 ). Third metacarpal and fourth metacarpal or third–fourth metacarpal dislocations are very rare because they are protected by their central position. As regards the very rare three-ray dislocations, the second and third metacarpal–fourth metacarpal block is paradoxically more common than the third metacarpal–fourth and fifth metacarpal block.
Finally, we should acknowledge several reported cases of dislocation of the five metacarpals, with palmar displacement a singularity.
It takes an extremely violent trauma to disrupt this very solid anatomic construct. These injuries therefore are associated with polytrauma, following a road accident (car, but mostly motorcycle) or a fall from a height.
Biomechanically, complete dislocations imply the rupture of both palmar and dorsal ligaments.
Partial or isolated dislocations involve the three ligament structures because they also require rupture of the IMC ligaments between affected and unaffected rays (see Fig. 6.7 ). These ligaments are crucial for the preservation of stability:
Experimentally their presence prevents a partial dislocation despite the section of palmar and dorsal planes.
After a trauma their presence prevents dislocations, “failed dislocations” ( Fig. 6.9 ), confirming the experimental observations.
They act as stabilizers because fixing the second and third CMCs usually proves sufficient to stabilize even complete dislocations.
Fracture-dislocations of the CMCs follow the same principles and only demonstrate the strength of the ligament structures. The most typical forms are:
the equivalent of the fifth-metacarpal Bennett fracture, where the IMC ligaments between the fourth and fifth metacarpals cause a fracture, separating the medial corner of the base of the fifth metacarpal ( Fig. 6.10 ); and
HMC fracture-dislocations, where the integrity of the posterior ligaments leads to a coronal fracture, detaching the dorsal cortex of the hamate ( Fig. 6.11 ).
There are two probable mechanisms: direct and indirect force. Direct force on the metacarpal bases causes anterior dislocation if the force is dorsal and posterior dislocation if the force is palmar ( Figs. 6.12 and 6.13 ). Indirect force is transmitted from the metacarpal heads toward their bases and carpus. The dislocation will be palmar if the wrist is in extension during the impact, dorsal if the wrist is in flexion ( Figs. 6.14 and 6.15 ). This mechanism appears to be the source of almost all cases concerning dislocations or HMC fracture-dislocations. The similarity of injuries reported in the literature is remarkable because it is almost always a punch on a hard surface, possibly explaining the frequency in dominant hands and young men. The relative breakability of the hamate compared with the posterior ligament structures suggests a weakness for this bone in the coronal plane.
Thomas Birch showed that this injury is the result of a violent blow to the head of the fourth and fifth metacarpals when the wrist is in flexion and ulnar deviation. The hamate is then trapped between the triquetrum and the two metacarpal bases ( Fig. 6.16 ), the impact causing a fracture and flexion causing dorsal displacement. Perez, in a review of hand trauma in boxers, reaffirms that the least traumatic punch occurs with the second and third metacarpal heads, with the wrist straight and in slight extension. Cain et al. specify the mechanism based on cases all presenting with a fracture of the fourth metacarpal associated with a fifth-ray HMC fracture-dislocation. It is the primary impact on the head of the fourth metacarpal that causes the fracture, and its shortening transmits the force to the fifth metacarpal. The degree of flexion of the fifth ray on impact will then define the fracture of the hamate. Minimal flexion causes a large fragment fracture with significant comminution. The size of the fragment and the comminution may be reduced progressively as flexion increases, resulting in a pure dislocation by rupture of the dorsal ligaments. In our experience a concomitant impact on the head of the fourth and fifth metacarpals causes a dorsal subluxation of the bases, usually without a fracture, because the energy is diluted in both rays. However, the deferral of force over the entire HMC joint surface detaches a dorsal fragment of the hamate, involving its two metacarpal joint facets (see Fig. 6.11 ).