Humerus

The humerus is the single bone of the arm region (Figure 8.2.1). At its proximal end is the head of the humerus. This is the large, round, smooth region that faces medially. The head articulates with the glenoid cavity of the scapula to form the glenohumeral (shoulder) joint (see Chapter 9).

The distal end of the humerus has two articulation areas, which join the ulna and radius bones of the forearm to form the elbow joint.

Ulna

The ulna is the medial bone of the forearm. It runs parallel to the radius, which is the lateral bone of the forearm (Figure 8.2.2).

In the anatomical position, with the elbow fully extended and the palms facing forward, the arm and forearm do not form a straight line. Instead, the forearm deviates laterally by 5–15 degrees from the line of the arm. This deviation allows the forearm and hand to swing freely or to carry an object without hitting the hip.

Radius

The radius runs parallel to the ulna, on the lateral (thumb) side of the forearm (see Figure 8.2.2). The head of the radius is a disc-shaped structure that forms the proximal end.

Carpal Bones

The wrist and base of the hand are formed by a series of eight small carpal bones (see Figure 8.2.3). The carpal bones are arranged in two rows, forming a proximal row of four carpal bones and a distal row of four carpal bones.

The carpal bones form the base of the hand. This can be seen in the radiograph (X-ray image) of the hand that shows the relationships of the hand bones to the skin creases of the hand (see Figure 8.2.4). Within the carpal bones, the four proximal bones are united to each other by ligaments to form a unit.

The four distal carpal bones are also held together as a group by ligaments. The proximal and distal rows of carpal bones articulate with each other to form the midcarpal joint (see Figure 8.2.4). Together, the radiocarpal and midcarpal joints are responsible for all movements of the hand at the wrist.

In the hand, the carpal bones form a U-shaped grouping. A strong ligament called the flexor retinaculum spans the top of this U-shaped area to maintain this grouping of the carpal bones (Figure 8.2.5).  Together, the carpal bones and the flexor retinaculum form a passageway called the carpal tunnel. The tendons of nine muscles of the anterior forearm and an important nerve (the median nerve) pass through this narrow tunnel to enter the hand. Overuse of the muscle tendons or wrist injury can produce inflammation and swelling within this space. This produces compression of the nerve, resulting in carpal tunnel syndrome, which is characterized by pain or numbness, and muscle weakness in those areas of the hand supplied by this nerve.

Metacarpal Bones

The palm of the hand contains five elongated metacarpal bones. These bones lie between the carpal bones of the wrist and the bones of the fingers and thumb (see Figure 8.2.3). The metacarpal bones are numbered 1–5, beginning at the thumb.

The first metacarpal bone, at the base of the thumb, is separated from the other metacarpal bones. This allows it a freedom of motion that is independent of the other metacarpal bones, which is very important for thumb mobility. The remaining metacarpal bones are united together to form the palm of the hand.  The anterior movement of these bones, particularly the fifth metacarpal bone, increases the strength of contact for the medial hand during gripping actions.