The Anatomy of the Hamate

A Carpal Bone at the Base of the Hand

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The hamate is a triangular-shaped bone that is made up of a flatter portion (known as the body) and a curved prominence (called the hook or hamulus) that juts off of the bone towards the palm of your hand.

The structure is a part of the distal (top) row of carpal bones on the medial (pinkie side) border of the hand and its hook makes up a portion of anatomical tunnels through which several nerves in your arm and hand travel.

Fractures to both the hook and the body portion of the hamate bone are possible. While this injury is uncommon, it is more likely in certain individuals and can cause significant wrist pain and loss of hand function.

Anatomy

The hand is a complex junction of many different bones that must work in synchrony to carry out your daily tasks. The radius and ulna bones in the lower arm connect to eight carpal bones that are arranged into two rows of four bones each.

The lower (proximal) row consists of the scaphoid, lunate, triquetrum, and pisiform. On top of these structures is the distal row of bones which includes the trapezium, the trapezoid, the capitate, and the hamate. Just above these two carpal rows lie the five long metacarpal bones situated beneath your fingers.

The triangular hamate bone and its hook-shaped prominence is found beneath the fourth and fifth metacarpals on the ulnar side of the hand. The structure also connects to the capitate bone as you move laterally towards the thumb and to the triquetrum as you move proximally down the hand toward the wrist.

The hook of the hamate also helps to form two important structures in the wrist. The first is the inner (medial) border of the carpal tunnel, through which the median nerve runs. The curved portion of the hamate bone also makes up the outside (lateral) border of Guyon’s canal, a tunnel containing the ulnar nerve.

Function

One of the primary functions of the hamate bone, and specifically its hook, is to serve as an attachment point for several muscles and ligaments. These include the flexor carpi ulnaris, the opponens digiti minimi, and the flexor digiti minimi muscles.

The pisiform-hamate ligament and the transverse carpal ligament (sometimes referred to as the flexor retinaculum) also connect to the bone’s curved prominence.

Together, these structures use the hook of the hamate as an anatomical pulley which enables them to flex or curl the fourth and fifth fingers. This important action allows the hand to form a strong grip when holding or manipulating objects. It also assists with many different fine motor activities performed by the hand.

Associated Conditions

Injury to the hamate bone is a relatively rare occurrence, with a fracture being the most common condition involving this structure. In spite of this, a report in the American Journal of Roentgenology estimates that just 2% to 4% of carpal fractures affect the hamate.

When the bone is injured, the hook portion is most commonly fractured because it protrudes outward and is more prominent. This type of fracture is most likely to occur in individuals who participate in sports like golf, baseball, or cycling that require frequent and repetitive gripping.

These activities can lead to an acute injury or a stress fracture to develop slowly over time. Other blunt traumas like a car accident, a fall on an outstretched hand, or a high-velocity impact to the hand (like from a baseball) can also cause an injury to the hamate’s hook.

Less commonly, the body of the hamate can also be fractured. This type of injury is most frequently caused by a high-energy injury like a punch, a fall, or a car accident and may be seen in combination with other carpal fractures.

Regardless of location, individuals with hamate fractures often present with vague and non-specific symptoms which can make diagnosis challenging and delay proper treatment.

Typically, a fracture to either the body or the hook of the hamate causes symptoms like pain and tenderness over an area called the hypothenar eminence (the padded section at the bottom of the hand under the fourth and fifth fingers).

Swelling and bruising may also occur in the hand or fingers depending on the acuity of the injury. In addition, weak and painful gripping may also be present. In more chronic situations, numbness, tingling, and weakness in the fourth and fifth fingers may also occur if the hook of the hamate is involved due to its proximity to the ulnar nerve in Guyon’s canal.

Treatment

Diagnosis of hamate fractures can be challenging with standard X-ray imaging and is often missed initially. If hand symptoms cause a doctor to be suspicious of this type of injury, more specific X-ray views or even magnetic resonance imaging (MRI) or a computed tomography (CT) scan may be necessary to properly identify the fracture.

Once a hamate injury is located and diagnosed, treatment can occur. Initially, fractures to the body or the hook of the hamate that are non-displaced (ones that don’t cause the bone to change alignment) are typically treated conservatively with immobilization in a cast. This is typically worn for a period of six weeks.

There is some debate about whether immobilization with a cast is always beneficial or whether early surgical intervention is sometimes warranted. This is especially true for individuals looking to quickly return to a sport.

If casting is unsuccessful at reducing the symptoms or if a portion of the hamate bone breaks free or is misaligned, surgery may be necessary. In this case, free-floating boney fragments are excised and hardware may be placed in the wrist to stabilize the injured bone (called an open reduction internal fixation or ORIF).

Following either immobilization in a cast or a surgical procedure, physical or occupational therapy is typically initiated to regain range of motion in the hand and restore your function. Be sure to speak to your physician if you have sustained an injury to your hand or wrist so they can properly diagnose it and determine which type of treatment is most suitable for you.

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