A metacarpal fracture is often caused when a direct force is applied such as punching. Another cause is when a sudden, forceful tork is applied pulling a finger in a specific direction, or a direct blow, such as getting stepped on. (fractura de boxeador)
Fractures typically cause pain, swelling, stiffness, decreased use of the hand, and bruising (discoloration) at the site of the fracture.
Fractures or, in other words, breaks can vary in severity. They can be stable and well aligned or unstable and broken in many places. If not evaluated and treated properly the use of the hand can be significantly affected.
The best way to treat this is to seek a medical evaluation and get x-rays. Treatment will vary on what x-rays show. A splint or a cast may be used to protect the fracture while it heals. Sometimes surgery is necessary to align the bone fragments back into position to allow the body to heal correctly.
There are several non-surgical and surgical options to treat this problem.
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Source and for more information: American Academy of Orthopaedic Surgeons (AAOS)