The scaphoid bone is one of the eight carpal bones, located on the thumb side of the wrist, just above the forearm bones. It is the largest bone in the wrist’s proximal row. The scaphoid links the proximal and distal rows of carpal bones, performing a mechanical function essential for overall wrist stability and coordinated motion. A fracture to this bone is the most common carpal bone injury. When left untreated, it presents a high risk of severe, long-term complications due to its distinct anatomical features and challenging healing process.
Unique Healing Challenges of the Scaphoid
The scaphoid bone has a limited blood supply that makes fracture healing difficult. Unlike most bones, the scaphoid receives approximately 80% of its blood from vessels entering primarily at its distal pole (closer to the fingers) and flowing backward toward the proximal pole. This pattern, known as retrograde blood flow, leaves the proximal pole in a precarious position.
A fracture, especially one occurring through the bone’s “waist,” can easily sever the limited blood vessels supplying the proximal fragment. When this blood supply is disrupted, the detached proximal fragment is left with little to no source of oxygen and nutrients. This compromised circulation significantly slows the bone’s repair mechanisms, making the scaphoid prone to healing failure even with timely treatment.
Scaphoid Nonunion and Chronic Instability
An untreated scaphoid fracture carries a high risk of nonunion, the failure of the two broken fragments to fuse. Nonunion can occur in up to 12% of missed fractures, with higher rates for displaced or unstable breaks. When the fragments do not unite, the wrist loses the structural integrity provided by the scaphoid, which stabilizes the carpal bones.
This mechanical failure leads to chronic instability, causing the carpal bones to move abnormally. The resulting malalignment places abnormal stress on surrounding ligaments and bones. Patients experience chronic pain, significant loss of grip strength, and limited range of motion. The wrist may collapse into a “humpback” deformity, an angular bend in the scaphoid that disrupts wrist mechanics.
Bone Death and Wrist Collapse
A severe consequence resulting from the initial vascular disruption is Avascular Necrosis (AVN), or osteonecrosis, where bone tissue dies due to a lack of blood supply. Since the proximal fragment relies on precarious retrograde blood flow, it is the most susceptible part of the scaphoid to this complication. AVN occurs in 15% to 50% of scaphoid fractures, with incidence increasing the closer the fracture is to the proximal pole.
Once the bone cells die, the fragment loses structural strength and eventually collapses, changes shape, and may fragment. This collapse leads to a significant shortening of the scaphoid, drastically altering the alignment of the entire carpal structure. The resulting permanent deformation causes a substantial loss of motion, severely limiting hand use. AVN complicates future surgical repair, as the dead bone tissue is unable to heal, often necessitating complex procedures like vascularized bone grafts.
Long-Term Degenerative Arthritis
Instability and collapse stemming from a chronic nonunion inevitably lead to degenerative arthritis. The abnormal motion of the wrist bones causes uneven wear on the cartilage surfaces, a process that can begin within a year of the injury. This condition follows a specific pattern of joint destruction known as Scaphoid Nonunion Advanced Collapse (SNAC wrist).
SNAC wrist arthritis typically begins at the joint surface between the radius (forearm bone) and the fractured scaphoid. As the disease progresses, arthritis spreads to the joint between the capitate and lunate bones, eventually affecting most wrist joints. This destruction of the joint cartilage results in lifelong pain, stiffness, and weakness that severely restricts wrist function. Advanced arthritis often limits treatment options to major salvage procedures, such as partial or total wrist fusion, which permanently restrict motion to alleviate pain.