The hamate bone is one of the eight small, irregularly shaped carpal bones that form the wrist, linking the forearm and the hand. Situated in the distal row of the carpus, its unique structure and prominent bony projection make it susceptible to specific types of injury, often associated with sports or repetitive motion.
Location and Articulation
The hamate bone is located on the ulnar side of the wrist, at the base of the palm. It has a distinctive wedge shape, with its broader base pointing toward the fingers and its narrow top pointing toward the forearm. This positioning places it in the distal carpal row alongside the capitate, trapezoid, and trapezium bones.
The hamate bone connects with several neighboring bones to form the complex wrist joint. Proximally, it forms a joint with the triquetrum bone, while laterally, it articulates with the capitate bone. Distally, the hamate connects directly to the fourth and fifth metacarpal bones, which lead into the ring and little fingers.
The Distinctive Hook of the Hamate
The most notable feature of the hamate bone is a curved, hook-like projection extending toward the palm, known as the hamulus. This prominent bony extension projects forward from the main body of the hamate, measuring approximately 7 to 10 millimeters in length. The hook itself is covered by the soft tissues of the palm and is not involved in any bony articulations.
The hamulus serves as an attachment site for several soft tissue structures, giving it considerable functional importance. It anchors the transverse carpal ligament, which forms the roof of the carpal tunnel, helping to define the pathway for the median nerve and finger flexor tendons. The hook also provides attachment for the pisohamate ligament and several small muscles that control the little finger, known as the hypothenar muscles. This anchoring function helps maintain the structural integrity of the palm.
Biomechanical Role in Wrist Stability
The hamate bone supports the hand’s natural arch. Its trapezoidal shape contributes to the structural integrity of the carpal arch, which is crucial for transferring forces effectively from the hand to the forearm during gripping and loading activities.
The hamate bone also functions as a pulley for the flexor tendons that control the ring and little fingers. By providing a stable, curved surface, the hamulus ensures that these tendons can glide smoothly and efficiently as the fingers flex. Furthermore, the hamate forms the lateral boundary of Guyon’s canal, a narrow passageway in the wrist that protects the ulnar nerve and ulnar artery as they travel into the hand.
Causes and Mechanisms of Hamate Bone Fractures
Fractures involving the hamate bone are uncommon carpal injuries, but they occur frequently in athletes and individuals who use tools involving repetitive impact. The hook of the hamate is the part most often fractured, rather than the body of the bone.
The injury typically results from direct, blunt force or repetitive microtrauma to the hypothenar eminence. A common mechanism involves the butt of a sports club or bat repeatedly striking the hamulus during a forceful swing, such as in golf, baseball, or racquet sports. The force of impact, or even the friction from the equipment handle, can cause a stress fracture or an acute break in the hook. A fracture can also occur from a fall onto an outstretched hand, though this is less typical for the hook itself.
Symptoms of a hook of the hamate fracture include pain and tenderness on the ulnar side of the palm, often aggravated by gripping or pushing movements. Because of the hamate’s proximity to Guyon’s canal, a fracture fragment can sometimes irritate or compress the nearby ulnar nerve. This compression can lead to symptoms like numbness, tingling, or weakness in the little finger and the adjacent side of the ring finger.