The humerus is the long bone of the upper arm or forelimb in humans and other vertebrates. It is the largest bone in the arm, extending from the shoulder to the elbow. The humerus is a key part of the skeletal system, contributing to the structure and movement of the arm.[6] It serves as the primary support for muscles involved in arm movement, providing attachment points for various tendons and ligaments.
Location
The humerus is located in the upper arm, extending from the shoulder joint (where it articulates with the scapula) to the elbow joint (where it articulates with the radius and ulna). It lies between the shoulder and the elbow, forming the major bone of the upper limb.
Structure and Anatomy
The humerus is a long, tubular bone that forms the upper part of the arm, extending from the shoulder to the elbow. It has a complex structure with various regions that allow for movement, muscle attachment, and articulation with other bones.
Proximal End of the Humerus
The proximal end of the humerus is closer to the shoulder and contains several important anatomical features:
Head of the Humerus
The rounded, ball-like structure that fits into the glenoid cavity of the scapula to form the shoulder joint (glenohumeral joint). This ball-and-socket joint allows for a wide range of arm movements.
Anatomical Neck
The narrow region just below the head, where the humerus tapers. It separates the head from the tubercles and is a common site for fractures.[4]
Greater Tuberosity
A large, bony prominence located laterally (on the outer side) of the humerus, just below the anatomical neck. It serves as an attachment point for the rotator cuff muscles, including the supraspinatus, infraspinatus, and teres minor.
Lesser Tuberosity
A smaller bony prominence located medially (on the inner side) of the humerus, also below the anatomical neck. It serves as the attachment site for the subscapularis muscle, another rotator cuff muscle.
Intertubercular (Bicipital) Groove
A groove that runs between the greater and lesser tuberosities. It allows the tendon of the long head of the biceps brachii muscle to pass through and attach to the scapula.
Shaft of the Humerus
The shaft of the humerus is the elongated portion of the bone between the proximal and distal ends, forming the main body of the humerus.[1]
Medial and Lateral Borders
The medial and lateral borders of the shaft are the sides of the bone, where muscles and ligaments attach. These borders are critical for muscle movement and stabilization.
Deltoid Tuberosity
A roughened area located on the lateral side of the shaft, about halfway down the bone. This serves as the attachment site for the deltoid muscle, which is responsible for shoulder abduction.
Radial Groove
A spiral groove running along the posterior surface of the humerus. This groove accommodates the radial nerve and deep brachial artery, providing neurovascular support to the arm.
Distal End of the Humerus
The distal end of the humerus forms the lower portion of the arm and connects to the forearm at the elbow joint. It includes several key features that articulate with the radius and ulna.
Trochlea
A spool-shaped structure located on the medial side of the distal humerus. It articulates with the ulna, forming part of the elbow joint, allowing for flexion and extension of the forearm.
Capitulum
A rounded, ball-like structure located laterally on the distal humerus. It articulates with the head of the radius, allowing for rotation of the forearm (pronation and supination).
Medial and Lateral Epicondyles
The prominent bony knobs on the distal humerus, located just above the trochlea and capitulum.[8] These epicondyles serve as attachment points for various muscles, including the flexors and extensors of the forearm.
- Medial Epicondyle: Larger and more prominent, provides attachment for the flexor muscles of the forearm.
- Lateral Epicondyle: Smaller, provides attachment for the extensor muscles of the forearm.
Coronoid Fossa
A shallow depression located on the anterior surface of the humerus, just above the trochlea. It accommodates the coronoid process of the ulna during elbow flexion.
Olecranon Fossa
A larger depression located on the posterior surface of the humerus, just above the trochlea. It accommodates the olecranon process of the ulna during elbow extension.
Articulations
The humerus articulates with several bones at its proximal and distal ends:
- Proximal End: The head of the humerus articulates with the scapula at the glenoid cavity to form the shoulder joint (glenohumeral joint).
- Distal End: The trochlea articulates with the ulna and the capitulum articulates with the radius, forming the elbow joint.
Muscle Attachments
The humerus serves as a key attachment site for numerous muscles involved in arm and shoulder movement, including:
- Rotator Cuff Muscles: The greater and lesser tuberosities are sites for the attachment of muscles like the supraspinatus, infraspinatus, subscapularis, and teres minor.[7]
- Deltoid Muscle: Attaches to the deltoid tuberosity for shoulder abduction.
- Biceps Brachii: The long head of the biceps brachii attaches to the intertubercular groove.
- Forearm Muscles: The medial and lateral epicondyles serve as attachment points for the muscles of the forearm, responsible for flexion, extension, and rotation of the wrist and fingers.
Nerve and Vascular Structures
- Radial Nerve: The radial nerve passes through the radial groove on the posterior surface of the humerus. It provides motor innervation to the posterior muscles of the arm and forearm.
- Brachial Artery: The brachial artery runs along the medial side of the humerus, supplying blood to the arm and hand.
Bone Composition
The humerus, like other long bones, consists of:
- Compact Bone: Forms the dense outer layer of the bone, providing strength and protection.
- Spongy Bone (Cancellous Bone): Found at the proximal and distal ends of the humerus, providing structural support and housing the bone marrow.[5]
Function
The humerus serves several key functions related to movement, support, and stability in the upper limb:
- Supports Upper Limb Movement
- The humerus is crucial for enabling the movement of the arm. Its articulation with the scapula at the shoulder joint and with the radius and ulna at the elbow joint facilitates a wide range of motions, including lifting, rotating, and reaching.
- Facilitates Shoulder and Elbow Articulation
- At the shoulder, the humeral head forms a ball-and-socket joint with the scapula (glenohumeral joint), enabling multi-directional movement of the arm. At the elbow, the humerus articulates with the radius and ulna, allowing flexion, extension, and rotation of the forearm.
- Provides Muscle Attachment Points
- The humerus serves as a central attachment site for numerous muscles that control the movement of the shoulder, elbow, wrist, and hand. These muscles include the rotator cuff, deltoid, biceps brachii, and forearm flexors and extensors, among others.
- Transmits Forces
- As the largest bone in the upper limb, the humerus plays a role in transmitting mechanical forces from the shoulder to the forearm. It helps to distribute the weight and stress from the hand to the shoulder during physical activities, such as lifting or pushing.
- Protects Nerves and Blood Vessels
- The humerus houses important structures such as the radial nerve and brachial artery within the radial groove, which are essential for motor and sensory function in the arm and hand. These structures are protected by the bone, ensuring proper function.
Clinical Significance
The humerus is clinically significant due to its role in movement, weight-bearing, and its vulnerability to various injuries.[3] Some of the key clinical issues associated with the humerus include:
- Fractures of the Humerus
- Humeral fractures can occur due to trauma, falls, or accidents. Common types include fractures of the proximal humerus (near the shoulder), shaft fractures, and distal fractures (near the elbow). These fractures may result in pain, limited range of motion, and require surgical intervention or immobilization.
- Proximal Humerus Fractures
- These are particularly common in elderly individuals due to osteoporosis. Such fractures can affect the shoulder’s range of motion and may lead to long-term disability if not properly treated.
- Radial Nerve Injury
- The radial nerve runs along the humerus in the radial groove. Fractures of the humeral shaft can damage the radial nerve, leading to “wrist drop” (inability to extend the wrist and fingers) and sensory loss along the back of the hand and forearm.
- Osteoarthritis of the Shoulder
- The humeral head forms the ball of the ball-and-socket joint at the shoulder. Osteoarthritis in the shoulder joint can lead to pain, stiffness, and a decreased range of motion, affecting daily activities.
- Biceps Tendon Rupture
- The long head of the biceps tendon attaches to the intertubercular groove of the humerus. Tears or ruptures of this tendon, often due to overuse or trauma, can lead to weakness in the arm and a visible bulge in the biceps muscle.
- Infection and Bone Diseases
- The humerus can be affected by conditions like osteomyelitis (bone infection) or tumors, such as osteosarcoma. Infections can cause pain, swelling, and fever, while tumors may require surgery and chemotherapy.[2]
- Frozen Shoulder (Adhesive Capsulitis)
- Though primarily affecting the shoulder joint, frozen shoulder can lead to limited movement of the humerus due to inflammation and fibrosis of the joint capsule. This condition results in pain and stiffness around the shoulder, often making it difficult to lift the arm.