The metacarpal bone at the base of your thumb and the trapezium bone in your wrist meet to form the carpometacarpal (CMC) joint.  Smooth cartilage covers the ends of the metacarpal and trapezium.  The cartilage allows the bones to glide easily during motion.  Strong ligaments and muscles hold the CMC joint in position and provide a combination of mobility and stability.
 
Your thumb joint is the only joint in your hand that can move in all directions.  Your thumb can move up and down, in and out, and rotate slightly to touch your fingers (opposition).  You move your thumbs hundreds of times each day. The movements of the thumb allow it to work as an anchor to help the fingers hold objects.  The thumb works as a grip when you use a hammer or hold a glass.  The thumb also acts as a stabilizer to allow your fingers to manipulate items.  You use such precision movements for such activities as counting coins and handwriting. A tendon that attaches to the base of the thumb metacarpal, called the abductor pollicis longus, is responsible for the joint dislocation that can occur with a Bennett’s fracture.

A Bennett’s fracture occurs when the metacarpal bone in the thumb breaks and dislocates at the base of the thumb.  Forces that impact the bent thumb cause this injury.  Bennett’s fractures can result from any significant forces placed on the base of the thumb, such as sports, falls and accidents.

A Bennett’s fracture causes your thumb to feel painful and stiff.  Your thumb area will swell.  Your thumb may be unstable and therefore difficult to move normally.  Over time, it is common for an old poorly treated Bennett’s fracture to develop arthritis.  This is because the joint at the base of the thumb was not realigned properly.

Your doctor can diagnose a Bennett’s fracture by examining your hand and taking X-rays.  In some cases, a computed tomography (CT) scan may be used to provide a more detailed image.

Small uncomplicated fractures, without displacement of the bone fragments, are treated non-surgically.  This means that the bones can be held in the correct alignment without surgically opening the skin.  In this case, a splint and or a cast is used.  A thumb spica cast or splint is worn to maintain positioning while the Bennett’s fracture heals.  Fractures that require realignment are reduced surgically and stabilized with pins, screws or plates.

A Bennett’s fracture can also be treated with open reduction and internal fixation (ORIF).  This means that an incision is made at the base of the thumb.  Surgical hardware, such as screws , pins or plates are used to align the fracture to allow it to heal in the correct position.  A thumb spica cast or splint is worn for several weeks following surgery.  Therapy for range of motion of the thumb is started when your doctor determines it is safe to do so.

Casts are usually worn for four to six weeks.  Once removed, you will participate in hand therapy exercises to increase the mobility, flexibility, and strength in your hand.  Generally, Bennett’s fractures caused by low impact forces have better outcomes than injuries caused by high forces.