Bennett Fracture

Etiology

Bennett fractures most commonly result from an axially directed force to the thumb, often with the digit flexed. Given the mechanism of this traumatic injury, it may be associated with fractures involving the adjacent carpal bones (eg, trapezium or hamate) and/or ulnar collateral ligament injuries of the thumb metacarpophalangeal (MCP) joint.[2][3] 

First metacarpal base fractures are typically classified as 1 of 5 types based upon articular involvement and the specific fracture pattern, as follows:

• Extra-articular transverse
• Extra-articular oblique
• Intra-articular Bennett (articular involvement with a volar-ulnar fragment)
• Intra-articular Rolando (complete articular involvement, often with a "T"- or "Y"-shaped fracture morphology)
• Intra-articular comminuted [4]

Gedda further sub-classified Bennett fractures into one of three types:[5]

• Type 1: Intra-articular fracture with CMC subluxation
• Type 2: Intra-articular fracture (possibly impacted) without CMC subluxation
• Type 3: Intra-articular fracture with CMC dislocation [5]

Epidemiology

Metacarpal fractures represent approximately 33% of hand fractures.[6] Fractures at the base of the thumb have been estimated to account for 80% of all thumb fractures and 4% of all hand fractures.[7][4] While most are extra-articular, 30% are considered Bennett fractures.[8] Men more commonly sustain Bennett fractures, which tend to occur most frequently in young adults, followed by older women.[6] The underlying mechanism of injury also varies by age. Younger patients primarily sustain Bennett fractures from recreational or sports activities, middle-aged individuals often experience injuries related to work, and older patients often sustain them from falls.

Pathophysiology

As previously noted, a Bennett fracture typically results from application of an axially directed force onto the thumb, often with the digit in a flexed position, resulting in an intra-articular fracture at the base of the thumb involving the first CMC joint. The orientation of the fracture pattern creates a volar-ulnar bony fragment, which is stabilized at its articulation on the trapezium by the volar (anterior) oblique ligament. The remainder of the first metacarpal, no longer anchored by the volar oblique ligament due to the fracture, displaces proximally and dorsally with supination due to the deforming forces primarily applied by the abductor pollicis longus and the adductor pollicis, as well as the extensor pollicis longus.[9] Consequently, the thumb may adopt a shortened and adducted posture.

History and Physical

A thorough history should be performed in all patients suspected of having sustained a Bennett fracture. This includes the mechanism and chronicity of the injury, as well as any prior treatment or reduction attempts. The patient's hand dominance, although it does not directly guide the management of a Bennett fracture, could impact the recovery course and patient satisfaction if the dominant hand is injured. Additionally, a comprehensive medical history and medication reconciliation should be performed, as certain comorbidities and medications, including anticoagulants and antibody-based therapies, may influence the selection and timing of treatment interventions. 

Clinical examination should include an evaluation of the entire hand and wrist, as there may be other concurrent injuries. In the setting of a Bennett fracture, inspection may reveal swelling/ecchymosis and potentially a deformity at the base of the thumb (depending on the degree of displacement). Palpation would demonstrate tenderness in the region of the first CMC joint with limited range of active thumb motion due to pain. Flexor pollicis longus and extensor pollicis longus function should be examined with active thumb interphalangeal joint flexion and extension, respectively, to confirm tendon integrity. A neurovascular examination is also performed to evaluate thumb perfusion and digital nerve continuity.   

Evaluation

Diagnostic evaluation for a Bennett fracture is conducted primarily through imaging studies. Standard hand radiographs should be obtained, including anteroposterior (AP), lateral, and oblique views. Additional dedicated views of the thumb should be obtained to provide further detail about the injury, including a Roberts view and a true lateral of the thumb.[9] The Roberts view is a true AP view of the first CMC joint and thumb. This view is performed with full pronation of the forearm while the dorsal aspect of the thumb is placed against the radiographic plate, and the x-ray beam is directed 90 degrees to the plate. Another view is the Bett view, obtained with the palm over-pronated 20 degrees from flat against the radiographic plate, and the beam directed 15 degrees proximal-to-distal. Stress radiographs can also be performed on an AP view, which may demonstrate subluxation of the first metacarpal base radially relative to the trapezium. Alternatively, dynamic stress views utilizing fluoroscopy can be considered.[10] In the setting of equivocal radiographs or more complex patterns, a CT of the thumb can be obtained. 

Treatment / Management

The primary objective in the management of a Bennett fracture is to restore articular congruity and overall fracture alignment. There is some debate in the literature regarding the acceptable amount of articular step-off at the fracture site, particularly in non-athlete populations. Some authors have found limited correlation between the quality of articular reduction and radiographic or subjective outcomes.[11][12]. At the same time, certain biomechanical studies have demonstrated that a 2 mm persistent articular step-off does not alter contact pressures at the first CMC joint.[13] It has, therefore, been suggested that bony apposition of the fragments within 2 mm and the correction of any joint subluxation should be tolerated without increasing the risk of posttraumatic arthritis. Many clinical studies, nevertheless, suggest that anatomic reduction is preferred.[14] Consequently, most guidelines recommend articular step-off to be less than 1 mm without CMC subluxation.      

Nonoperative Management

Bennett fractures may be treated nonoperatively if there is less than 1 mm of displacement and articular step-off. If the initial displacement exceeds this, closed reduction can be attempted. Proper reduction requires traction along the thumb in an axial direction with volar abduction and pronation while applying an external force over the first metacarpal base.[9] Thumb extension (known as the "hitchhiker position") has been shown to worsen fracture displacement and should be avoided.[1] Once within acceptable parameters, a thumb spica splint or cast is applied. Serial radiographs of the hand/thumb should be obtained weekly for the first three weeks to assess for worsening displacement, which may warrant surgical intervention.

Closed reduction and splinting of Bennett fractures was the initial treatment described for these injuries, which remained the preferred method of treatment until the 1970s.[15][14] Historical reports have shown reasonable outcomes with this management, although more recent studies have shown poorer outcomes when treating certain Bennett fractures with splinting/casting alone.[16] 

Operative Management

Fractures with greater than 1 mm of displacement or articular step-off that cannot be successfully reduced and stabilized generally require operative treatment. A variety of surgical interventions exist,[17][18][19] as follows:(A1)

• Closed reduction and percutaneous pinning: Direct fixation of the volar-ulnar bony fragment itself is not typically required to achieve adequate overall reduction and stability. A single Kirschner (K)-wire can be placed trans-articularly through the first metacarpal shaft and into the trapezium to abut the shaft against the volar-ulnar fragment. Alternatively, the Iselin technique involves dual K-wire fixation between the first and second metacarpal shafts without trans-articular fixation.[20]  
(B3)

• Open reduction and internal fixation: A dorsal incision centered over the first CMC joint and the thumb metacarpal base is made. Under direct visualization, the fracture is reduced and fixed with a plate/screw construct, lag screw, or K-wire(s). 

• Arthroscopic-assisted reduction and screw fixation: To aid in reduction efforts, first perform CMC joint arthroscopy to directly visualize the articular surface, facilitate reduction, and confirm an acceptable reduction.[21] Then, a cannulated screw (or K-wire) can be inserted into the volar-ulnar fragment for fixation. 

• Augmentation with a suture-button: Cortical suture-buttons may be utilized to augment screw fixation.[17]

In cases of a Bennett fracture malunion or nonunion, potential surgical treatment options include intra-articular or extra-articular osteotomies, trapezium resection with or without interposition arthroplasty, implant arthroplasty, first CMC arthrodesis, and small fragment excision with suture-button suspension.[22] Following surgical intervention, the patient is placed into a thumb spica splint.  

Differential Diagnosis

The differential diagnosis includes the following: 

• Rolando fracture (a complete intra-articular fracture at the base of the first metacarpal that may be comminuted)
• Trapezium fracture
• Metacarpal shaft fracture
• First CMC dislocation

Prognosis

Due to their potential instability and articular involvement, the prognosis of patients with Bennett fractures is variable.[23][24] Based on a study by Kjaer-Petersen et al that reviewed 41 Bennett fractures, long-term outcomes are often determined by the quality of fracture reduction. In their analysis, 86% of patients with an anatomic reduction (less than 1-mm step-off) had no residual symptoms, whereas only 46% of patients with good or poor reduction (greater than 1-mm step-off) remained asymptomatic.[15] The type of intervention has also been shown to impact ultimate outcomes. Duche et al evaluated closed reduction percutaneous pinning and showed that patients experienced satisfactory medium-term functional outcomes, but with moderate loss of range of motion and pinch strength.[20] In a systematic review of patient outcomes following surgical treatment for Bennett fractures, Velasquez-Giron et al demonstrated that patients treated with closed reduction percutaneous pinning had less postoperative pain than those undergoing open reduction and internal fixation or arthroscopy-assisted reduction and screw fixation.[25] However, they also concluded that all three techniques ultimately yielded similar functional outcomes and similar rates of posttraumatic CMC arthritis.    

Complications

Following a Bennett fracture, patients may experience thumb stiffness. To mitigate this, range-of-motion exercises should begin as soon as clinically appropriate, which may be several weeks after the injury in patients treated nonoperatively with splinting/casting, 5 to 10 days after screw or plate fixation, or 4 weeks after closed reduction percutaneous pinning (after the pins are removed).[26] Malreduction of the fracture may result in a permanent thumb deformity with more significantly diminished pinch strength and/or instability at the CMC joint. Further, residual articular incongruity can increase the risk of posttraumatic CMC arthritis.