5. THUMB METACARPAL
5.1. Thumb Metacarpal Base (Intra-articular)
These are common injuries, particularly in young active males. Most are very simple but comminuted fractures (Rolando or multifragmented) of the base of the metacarpal are difficult to treat.
(a) Bennett’s fracture dislocation
Essentially this is a dislocation with a small bony fragment attached to the very strong volar-ulnar “beak” ligament. The metacarpal needs to be reduced closed and held with two 1.1 mm K-wires passed into the index metacarpal and into the trapezium. The key is reduction of the dislocation rather than wiring of the fracture fragment. This should be protected in a Bennett’s cast for 5 weeks from surgery, the wires removed and then the thumb mobilised. When there is a larger deep fracture fragment, this can be fixed with a screw. This allows early mobilisation but requires open surgery. As closed reduction and wiring does so well this is rarely indicated. Occasionally there is no fracture fragment i.e. a pure dislocation. This should be held reduced with 1 or 2 K wires for at least 4 weeks to allow thw important local ligaments to heal. (WE NEED A PHOTO OF Bennetts and treatment)
(b) Simple peri-articular fractures of the base
Typically this is a short oblique fracture that extends proximally from the radial joint
margin distally and ulnarly. The key is maintenance of the articular surface and alignment of the thumb metacarpal. If there is little displacement the fracture should simply be held in POP. If there is significant shortening or angulation of the metacarpal shaft the fracture will need to reduced and held. This can be with 2 -3 1.1 mm K-wires passed closed across the fracture and into the trapezium, trapezoid or index metacarpal or occasionally open with screws although this is infrequent.. K-wire fixation needs POP support for 4-5 weeks. (we need a picture with K wires)
(c) Comminuted peri-articular fractures of the base of the thumb (Rolando or multifragmented)
This is a comminuted intra articular fracture at the base of the thumb metacarpal often following a falll. Conservative management is not an option unless there is minimal displacement, which is rare. The reduction cannot be held simply with wires, it requires some further fixation. Plates can be used for Rolando fractures, but are technically very difficult. To maintain longitudinal stability an external fixator can be applied between the thumb and index metacarpals. It is usually possible to hold the thumb out to reasonable length with two 1.6 mm K-wires into the index metacarpal. We believe this is the simplest and most effective approach. It allows easy imaging of the CMC joint and easy access if local reduction and fixation is required, although this is uncommon. If the reduction of the joint surface is inadequate the joint will typically be opened by a Moberg incision and fixation with wires and probably some bone graft taken from the distal radius. The external fixation elements (wires or a frame) between thumb and index finger will be required for at least 4 and probably nearer 6 weeks depending upon the rigidity of the fracture fixation.
Note that with a substantial external fixator frame it can be very difficult to achieve adequate radiographs of the base of the thumb. 2 large (1.6 mm) K-wires make peri- and post-operative assessment easier but are less rigid.which may be preferable as it may reduce the long-term stiffness.
5.2. Thumb Metacarpal shaft (Extra-articular)
There is typically an adduction/flexion angulation, usually presenting in young adults . With an angulation of less than 20°, a very good result should be achieved with 3-4 weeks of immobilisation. It is probably worth trying to hold these in plaster, but it is difficult, as the plaster tends to push on the base of the proximal phalanx and not on the distal metacarpal and thus can accentuate the angulation of the fracture, especially if there is any MP joint hyper mobility. Greater angulations need manipulation and stabilisation typically with two 1.1 mm K-wires. The fracture and wire construct needs to be supported in a POP for 4-5 weeks. In adolescence some remoulding may occur and thus a greater deformity can be accepted. Obviously this is always a careful judgement.
5.3. Thumb metacarpal – Neck
This is a very uncommon injury. Angulation of up to 10-200 can be accepted and possibly more, especially if there is significant MP joint hyperextension. If there is too much angulation this can be reduced under LA and held with two 1.1 mm K-wires across the fracture as for the distal fractures of the proximal phalanx (see section …..) As the thumb MP joint is typically so forgiving of immobilisation passing the K wires across the joint in order to achieve stability is very acceptable.
5.4. Thumb Metacarpal – Head (intra-articular)
This is an uncommon injury. It can be treated similar to finger metacarpal head fractures (section 1.5, p….). Again the joint is reasonably forgiving but any significant mid articular step (>1mm) should be reduced and held. Surgical access is easier but a perfect reduction is probably less important as although the thumb is so important to hand function a stiff thumb MP joint is not a major handicap. If a good enough reduction can be achieved closed this can be held with 2-3 (1.1 mm) K-wires with POP support for 4-5 weeks. If open reduction is required the fracture reduction is best held with 1-2 screws and mobilised early.