Thumb Metacarpal Base Fracture and Injury
There are three types of thumb metacarpal base fractures:
There are five metacarpal bones in the hand. The first metacarpal bone is connected at the base to the trapezium at the first carpometacarpal joint; a saddle shaped joint. The articulation between the first metacarpal bone and the trapezium allows for inherit stability. Both articular surfaces are convex and concave. Movement at the base of the thumb occurs around two axis:
And rotation. Counting the fingers with the thumb is called opposition.
The Bennett fracture is a fracture dislocation that extends into the carpometacarpal (CMC) joint articulation. There is a volar-ulnar beak fracture fragment. The mechanism of injury is an axial overload along the first metacarpal with simultaneous flexion (think “fist fight”). The key is the volar-ulnar beak fracture fragment attached to the palmar oblique ligament. The distal part of the first metacarpal is adducted by the adductor pollicis muscle. The metacarpal head is displaced into the palm. The metacarpal shaft is displaced proximally by the abductor pollicis longus muscle (posterior interosseous nerve). The metacarpal is displaced dorsally, radially, and supinated. The small volar lip ulnar beak fragment remains in its position attached to the trapezium by the oblique ligament. The main deforming forces are the Abductor Pollicis Longus and the Adductor Pollicis muscles. The fracture is best seen with hyperpronated AP thumb with is the Robert view and true lateral view. Treatment consists of a closed reduction with percutaneous pinning or an open reduction internal fixation if the fragment is large enough (more than 20%). The first metacarpal must be reduced to restore the articular surface. Reduction will be done with longitudinal traction, as well as pronation of the metacarpal. Pressure on the thumb base to push it back to its place by abduction and extension of the metacarpal. Closed reduction and percutaneous pinning is used to treat the majority of cases. The pinning will go from the thumb to the trapezium or from the thumb to the second metacarpal. Pinning may also go to both. The physician should add a thumb spica splint and accept up to 2 mm articulation of incongruity. The sensory branch of the radial nerve, the radial artery, and tendons are in the area. Be aware of these structures during surgery, as well as reverse Bennett fractures.
A reverse Bennett fracture occurs at the base of the fifth metacarpal bone which extends into the carpometacarpal (CMC) joint. It is an intra-articular fracture of the base of the 5th metacarpal and the extensor carpi ulnaris pulls on the 5th metacarpal.
A Rolando fracture is a comminuted intra-articular fracture through the base of the first metacarpal bone with same deformity forces as the Bennett fracture. The base of the first metacarpal is split onto volar and dorsal fragments, making a “Y” fracture or a transverse fracture. Sometimes three or more intra-articular fragments are present. Treatment for Rolando fractures is difficult and controversial. It may be treated by ORIF, a closed reduction percutaneous pinning, or an external fixation. The prognosis is worse than a Bennett and may end in arthritis. A majority of cases do well even with residual displacement or incongruity.
An extra-articular fracture has the best outcome. There are up to 30° angulation is tolerated well. The thumb carpometacarpal joint is a saddle joint with a lot of mobility. These fractures can be treated with a thumb splint and may be splint up to 30°. Angulation of more than 30° may lead to MCP hyperextension. The physician should reduce and perform a closed pinning. In high demand athletes, an ORIF may be used.