The CMC joint of the thumb allows a wide range of motion while maintaining stability for grasp and pinch.With this in mind, failure to properly recognize and treat the Bennett fracture will not only result in an unstable, painful, arthritic CMC joint with diminished range of motion: it will also result in a hand with greatly diminished overall function.In the case of the Bennett fracture, the proximal metacarpal fragment remains attached to the anterior oblique ligament, which in turn is attached to the tubercle of the trapezium bone of the CMC joint. This ligamentous attachment ensures that the proximal fragment remains in its correct anatomical position.The distal fragment of the first metacarpal bone possesses the majority of the articular surface of the first CMC joint. Unlike the proximal fracture fragment, strong ligaments and muscle tendons of the hand tend to pull this fragment out of its correct anatomical position.Specifically:

tension from the abductor pollicis longus muscle (APL) subluxates the fragment in a dorsal, radial, and proximal direction
tension from the APL rotates the fragment into supination
tension from the adductor pollicis muscle (ADP) displaces the metacarpal head into the palmTension from the APL and ADP muscles frequently leads to displacement of the fracture fragments, even in cases where the fracture fragments are initially in their proper anatomic position. Because of the aforementioned biomechanical features, Bennett fractures nearly always require some form of intervention to ensure healing in the correct anatomical position and restoration of proper function of the thumb CMC joint.
In such cases, closed reduction followed by immobilization in a thumb spica cast and serial radiography may be all that is required for effective treatment. For Bennett fractures where there is between 1 mm and 3 mm of displacement at the trapeziometacarpal joint, closed reduction and percutaneous pin fixation (CRPP) with Kirschner wires is often sufficient to ensure a satisfactory functional outcome. The wires are not employed to connect the two fracture fragments together, but rather to secure the first or second metacarpal to the trapezium. For Bennett fractures where there is more than 3 mm of displacement at the trapeziometacarpal joint, open reduction and internal fixation (ORIF) is typically recommended.Regardless of which approach is employed (nonsurgical, CRPP, or ORIF), immobilization in a cast or thumb spica splint is required for four to six weeks. Prognosis
If intraarticular trapeziometacarpal fractures (such as the Bennett or Rolando fractures) are allowed to heal in a displaced position, significant post-traumatic osteoarthritis of the base of the thumb is virtually assured. Some form of surgical treatment (typically either a CRPP or an ORIF) is nearly always recommended to ensure a satisfactory outcome for these fractures, if there is significant displacement.The long-term outcome after surgical treatment appears to be similar, whether the CRPP or the ORIF approach is used. Specifically, the overall strength of the affected hand is typically diminished, and post-traumatic osteoarthritis tends to develop in almost all cases. The degree of weakness and the severity of osteoarthritis does however appear to correlate with the quality of reduction of the fracture.