A Bennett fracture is a serious injury involving the base of the thumb. This specific type of break is characterized by its instability and its involvement with the main joint of the thumb, the carpometacarpal (CMC) joint. This injury requires careful medical attention to ensure proper healing and restoration of full hand function. The primary concern is the high risk of long-term problems, such as arthritis, if the joint surface is not perfectly realigned during treatment. This fracture is considered the most common type of break affecting the thumb.
Defining the Fracture and Its Anatomy
A Bennett fracture is classified as an intra-articular fracture-dislocation at the base of the first metacarpal bone, which is the long bone of the thumb. An intra-articular fracture means the break extends into the joint surface of the CMC joint. The fracture creates two distinct fragments, with one remaining stable and the other being pulled out of alignment.
The mechanism of this fracture creates a small, triangular fragment located on the palmar-ulnar side of the metacarpal base, known as the Bennett fragment. This small piece remains firmly held in its correct position by the volar oblique ligament, which tethers it to the trapezium. The larger, main shaft of the metacarpal bone, which includes the majority of the joint surface, is pulled away from the joint.
This displacement and subluxation of the main metacarpal shaft is caused by the strong, unopposed pull of the abductor pollicis longus (APL) tendon. The APL muscle exerts a force that pulls the larger fragment of the thumb bone proximally, dorsally, and radially, effectively dislocating it from the joint. This anatomical feature makes the Bennett fracture inherently unstable.
Common Causes and Immediate Signs
The typical cause of a Bennett fracture is an axial load applied while the thumb is in a partially flexed position. This type of injury often occurs when a person falls onto an outstretched hand or when a forceful impact is delivered, such as punching a hard object. The compression force travels through the thumb bone and causes the oblique fracture line at the joint base.
Immediate signs of this injury include intense, acute pain concentrated at the base of the thumb and wrist. Rapid swelling and bruising develop quickly around the area, especially over the CMC joint. Individuals with this fracture will experience a significantly reduced range of motion and difficulty gripping or pinching objects.
Physical examination often reveals visible instability at the base of the thumb. Physicians rely on imaging, typically X-rays taken from multiple angles, to confirm the diagnosis and assess the degree of displacement and joint involvement.
Establishing the Treatment Plan
Treatment for a Bennett fracture is determined by the degree of bone fragment displacement and joint instability. The primary goal of any intervention is anatomical reduction—perfectly realigning the joint surface to prevent the long-term development of post-traumatic arthritis. If the articular step-off or gap after fixation is greater than two millimeters, the risk of later arthritis increases.
For many cases, the most common surgical approach is Closed Reduction and Percutaneous Pinning (CRPP). This method involves maneuvering the bone fragments back into place without making a large incision to expose the joint. Once realigned, thin metal wires (K-wires) are inserted through the skin (percutaneously) to hold the reduction stable while the bone heals. CRPP is often preferred when feasible because it involves less surgical trauma and a quicker operation time.
When the fracture is highly unstable, severely comminuted (broken into multiple pieces), or cannot be satisfactorily reduced using the closed technique, Open Reduction and Internal Fixation (ORIF) becomes necessary. This procedure requires a surgical incision to visually access the joint, allowing the surgeon to physically realign the fragments with precision. The fragments are then secured using small screws, plates, or wires, providing a rigid internal fixation.
Recovery and Rehabilitation
Regardless of whether CRPP or ORIF is performed, the first phase of recovery involves immobilizing the thumb to allow for bone healing. This is typically done with a cast or a thumb spica splint for a period that usually lasts between four to six weeks. If K-wires were used, they are removed in an outpatient setting after the initial immobilization period is complete.
Once the pins are removed and the initial healing is confirmed, the second phase of recovery begins with rehabilitation. Hand therapy or physical therapy is necessary to restore strength, range of motion, and fine motor skills that were lost due to the injury and subsequent immobilization. Rehabilitation exercises focus on gentle stretching to minimize stiffness and progressive strengthening to rebuild the thumb muscles.
The full recovery timeline often takes several months, and the patient’s commitment to the rehabilitation program plays a large part in the final outcome. A potential long-term concern is post-traumatic arthritis if the joint surface was not perfectly restored. Patients may also experience continued pain or stiffness if the bones did not heal in the correct anatomical position.