Metacarpal shaft fractures occurring as a result of an athletic injury are almost always closed and simple. A spiral pattern is most commonly seen and results from a combination of impact and torsion.
Fracture stability is a function of fracture configuration and periosteal disruption. A spiral configuration is unstable by nature, but actual instability occurs only if periosteal disruption is substantial enough to cause allows intrinsic muscle imbalance or external forces to cause fracture displacement. Fractures seldom displace more than when initially seen. Thus, an un-displaced or minimally displaced spiral metacarpal shaft fracture only requires protection from additional injury while healing.4, 14 Such a fracture may go directly to protective casting or splinting such as recommended above, but with the same time line for resumption of playing activities.
The amount of instability parallels the amount of displacement in spiral metacarpal shaft fractures. Shortening, angulation, and rotation may occur, often in some combination. Instability may be further compounded in cases of multiple metacarpal fractures.
Shortening is usually limited to less than four millimeters by the inter-metacarpal ligaments.8 Extensor tendon lag is not usually problematic within these parameters. 21 Thus, shortening alone is not usually troublesome in metacarpal shaft fractures.
Metacarpal shaft fractures tend to angulate dorsally due to unbalanced intrinsic muscle forces.19, 20 Each metacarpal will tolerate approximately 100-150 of fracture angulation beyond its carpal metacarpal joint mobility before digital clawing occurs. The second and third metacarpals constitute part of the rigid longitudinal arch of the hand and are immobile or relatively immobile at their carpal metacarpal joints. Thus, angulation of these two metacarpals is poorly tolerated. The 4th and 5th metacarpals have progressively more carpal metacarpal mobility. Consequently, these joints tolerate angulation better than the 2nd and 3rd metacarpals. We usually prefer to correct clinically visible metacarpal angulation, especially when it is associated with clawing or the absence of knuckle contour.2 Nevertheless, we have all seen excellent pain free functional results when one or any combination of these deformities remain after treatment (or lack of treatment), provided there is no significant rotational deformity.
Rotational deformity of metacarpal fractures is poorly tolerated. A small amount of rotation at the fracture site quickly translates into overlap or scissoring of the injured finger during digital flexion. Rotational deformity is suggested when a gap is seen between the fragments of a spiral fracture on antero-posterior x-ray. Because of the laxity of the MPJ collateral ligaments in extension, metacarpal (digital) rotation is not clinically apparent. It is therefore essential that finger position, angulation and rotation be assessed in MPJ flexion (actively or passively achieved), which tightens the collateral ligaments, and, therefore, reflects the true position of the distal bone fragment.
Successful reduction of displaced spiral metacarpal shaft fractures achieved by closed manipulation may be maintained by functional bracing4, 13, 14, 23, transcutaneously introduced Kirschner wires11, 15, 22, or mini external fixation6, 17, 18, 24. Intramedullary wires maintain alignment, but do little to control rotation. Transfixation wires and mini external fixation stabilize all aspects of the deformity, but should be inserted with the metacarpophalangeal joints flexed to prevent intrinsic muscle tightness. Kirschner wires splint, but do not compress the fracture.
When ORIF is necessary or when secure internal fixation is considered advantageous for fracture healing and rehabilitation, spiral fractures that are twice or greater than the diameter of the bone may be secured by mini screws3, 5, 9, 10. Dividing the fracture length by the bone diameter provides a guideline for the number of screws needed for fixation. Usually two mini screws are placed at positions that divide the fracture into thirds. Occasionally, for longer fractures, three mini screws are positioned to separate the fracture into quarters. At least one mini screw must be placed perpendicular to the long axis of the bone to provide maximum resistance to shear displacement. Mini screws positioned perpendicular to the fracture have maximal compressive force. Often, by following the spiral fracture line, mini screws may be inserted. These mini screws satisfy the criteria for both maximum shear resistance and compression. The mini screws also neutralize the bending and rotational forces placed on each other during rehabilitation. A variety of wiring techniques1, 7, 11, 12, 15, 16 or mini external fixation techniques6, 17, 18, 24 may also be used in the absence of mini screws or at the discretion of the surgeon.
Wires are usually removed after the fracture is sufficiently healed. Mini screws need only to be removed for cause.
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