The carpal bones are the eight small bones that form the wrist, acting as a flexible bridge between the forearm bones (radius and ulna) and the hand’s metacarpals. These bones are arranged in two rows, working together to allow the wrist its wide range of motion and stability. The scaphoid bone is the most commonly fractured carpal bone, accounting for approximately 60 to 70% of all fractures within the wrist joint.
The Scaphoid Defining the Most Commonly Fractured Carpal Bone
The scaphoid is an elongated, boat-shaped bone situated on the thumb side of the wrist, within the proximal row of carpal bones. Its unique position makes it especially vulnerable to injury. Anatomically, the scaphoid acts as a structural link, bridging the proximal and distal carpal rows.
This bridging function means the scaphoid is involved in nearly every wrist movement, particularly flexion. Unlike other carpal bones, the scaphoid rotates and translates as the wrist flexes and extends. This dynamic movement exposes the bone to high mechanical stress during traumatic events. The scaphoid is divided into three regions: the distal pole, the waist, and the proximal pole. The narrow waist is the most common fracture site, making up about 70% of all scaphoid breaks.
Mechanism of Injury How Scaphoid Fractures Occur
Most scaphoid fractures result from a specific accident known as a Fall Onto an Outstretched Hand (FOOSH) injury. This instinctive reaction involves landing on the palm with the wrist forcibly bent backward, or hyperextended, often with a slight deviation toward the thumb side.
When the wrist is forced into extreme hyperextension, the scaphoid is compressed violently between the radius and the capitate bone. This impact drives the scaphoid directly against the dorsal rim of the radius, causing the bone to fracture in tension. The resulting force typically creates a fracture across the narrowest section, the waist. This biomechanical event is responsible for the high incidence of scaphoid fractures in younger, active individuals involved in sports or high-impact activities.
Unique Healing Challenges of the Scaphoid
Scaphoid fractures are problematic because of the bone’s unusual and tenuous blood supply. Blood vessels primarily enter the scaphoid at its distal pole and then flow in a retrograde (backward) direction to supply the rest of the bone.
A fracture, particularly one at the waist or proximal pole, can completely sever the blood flow to the proximal fragment (the piece closest to the forearm). When the blood supply is cut off, the bone fragment is deprived of the oxygen and nutrients needed for repair, which significantly slows or prevents healing. This leads to a higher risk of nonunion (failure of the bone to heal fully) or avascular necrosis (AVN), the death of bone tissue due to lack of blood supply.
The risk of AVN is directly related to the fracture location; breaks in the proximal pole carry the highest probability of bone death. Nonunion rates can be as high as 10% even with modern treatment. If a displaced fracture is left untreated, the risk of nonunion climbs much higher, potentially leading to Scaphoid Non-union Advanced Collapse, a severe form of wrist arthritis.
Diagnosis and Treatment Pathways
Diagnosing a scaphoid fracture can be challenging because initial symptoms are often mild and mistaken for a simple wrist sprain. Standard X-rays taken immediately after the injury may not clearly show the fracture, especially if it is non-displaced. Due to this difficulty, specialized scaphoid view X-rays are often taken. If clinical suspicion remains high despite negative initial images, the wrist is typically immobilized.
Advanced imaging, such as a Computed Tomography (CT) scan or Magnetic Resonance Imaging (MRI), is frequently used to confirm an occult (hidden) fracture or to evaluate the extent of displacement. Early diagnosis is important to prevent the severe complications associated with nonunion and AVN.
For non-displaced scaphoid fractures, the standard treatment is long-term immobilization in a cast, typically a thumb spica cast that includes the thumb. Healing can require a prolonged period, commonly ranging from 6 to 12 weeks or even longer, due to the bone’s tenuous blood supply. Displaced fractures or those that fail to heal after a period of casting often require surgical intervention. Surgery usually involves internal fixation, where a surgeon aligns the bone fragments and secures them with a small screw or pins to promote stable healing.