The shoulder is a complex structure present on the upper body that connects the arms to the torso. It consists of many components that work together to provide stability and a wide range of movement.
The shoulder is a large and complex ball-and-socket or spheroidal joint that contains many bones, muscles, tendons, and ligaments. These structures combine to form and stabilize this highly flexible joint, providing the arms with a wide range of movement. However, this mobility means the shoulder is susceptible to damage from overuse, instability, or injury.
In this article, we will discuss the anatomy, function, and structures of the shoulder.
At the shoulder, three major bones meet and create a 90-degree angle. Two of these bones — called the clavicle and scapula — form the
The junctions of these bones and the sternum, or chest bone, create three joints: the glenohumeral, the acromioclavicular, and the sternoclavicular joints. These joints are responsible for the shoulder’s wide range of motion.
Other structures, such as cartilage, ligaments, muscles, and tendons then surround the joints to connect the bones and provide stability. Muscles also allow for motion.
Click on the BodyMap above to interact with a 3D model of the shoulder.
The shoulder consists of three main bones. These are:
- The clavicle: Also known as the collar bone, this bone connects the arm to the chest and is present in front of the scapula. It attaches to many muscles that connect it to the arm, the neck, and the chest.
- The scapula: The scapula is commonly known as the shoulder blade. It has a triangular shape. It attaches to the body primarily through muscle and effectively “floats” off the back of the chest. The scapula has a bony component, known as the acromion, which attaches to the clavicle. The glenoid is a shallow socket on the scapula.
- The humerus: The humerus is the upper arm bone. It is a long bone between the elbow and the shoulder. The humerus has a spherical portion at the top of the arm, which acts as the “ball” to the “socket” of scapula.
There are three joints present in the shoulder. These
The glenohumeral joint: This joint is a ball-and-socket joint. It connects the humerus to the scapula. It is the most mobile joint in the body. The joint allows for movement of the shoulder in multiple planes, including arm rotation and moving the arm up and away from the body. The acromioclavicular joint: This connects the clavicle to the scapula. The scapula has a bony component known as the acromion, where the clavicle attaches. The joint stabilizes the shoulder and helps with movement such as raising the shoulders. The sternoclavicular joint: This connects the clavicle to the sternum. The sternum, also known as the breastbone, is the flat bone in the center of the chest. The joint allows the clavicles to move.
The intrinsic muscles of the shoulder
- The deltoid: Located on the outside of the shoulder, the deltoid has a triangular shape. It attaches to the clavicle and scapula. The deltoid allows the arm to move forwards, backwards, sideways, and away from the body. It also stabilizes the shoulder during carrying and lifting.
- Teres major: This muscle originates from the scapula and attaches to the humerus. Teres major allows for the internal rotation of the arm.
- Supraspinatus: This muscle originates from the scapula and attaches to the humerus. It stabilizes the glenohumeral joint and allows for the movement of the arm away from the body.
- Infraspinatus: This muscle originates from the scapula and attaches to the humerus. It externally rotates the arm and stabilizes the glenohumeral joint.
- Teres minor: Teres minor originates from the scapula and attaches to the humerus. It laterally rotates the arm and stabilizes the glenohumeral joint.
- Subscapularis: This muscle originates from the scapula and attaches to the humerus. It allows for the rotation of the arm towards the body and stabilizes the glenohumeral joint.
The supraspinatus, infraspinatus, teres minor, and subscapularis muscles are known as the rotator cuff muscles. They all originate from the scapula and attach to the humerus. These muscles collectively help raise and rotate the arm.
The extrinsic muscles of the shoulder
- The trapezius: The trapezius originates from the skull and attaches to the clavicle and scapula. It raises the scapula.
- Latissimus dorsi: This muscle originates from the lower spine and attaches to the humerus bone. It brings the arm backwards and towards the body and provides assistance for pull-up and rowing movements.
- Levator scapulae: This is a long and slender muscle that helps to elevate the scapula. It also helps to rotate the glenoid cavity, stabilize the spine, and extend and laterally flex the neck.
- Rhomboid major and rhomboid minor: The rhomboid muscles primarily stabilize the scapula, keeping it in position, and help rotate the glenoid cavity.
While the shoulder is very mobile, this flexibility also makes it prone to instability and damage. These injuries can impact the range of motion and cause pain in the shoulder. Some of the most common conditions that affect the shoulders include:
Rotator cuff injuries
- Tears: This is a common injury where a tendon is torn, meaning it no longer fully attaches to the bone. The main causes of a rotator cuff tear are injury and age-related degeneration. While treatment can vary due to the severity, it often involves rest, pain relief medication, and rehabilitation exercises. In more severe cases, it may require surgery.
- Bursitis: This refers to inflammation of the
bursa, which are fluid-filled sacs that cushion and protect tissues by reducing friction. Infections or too much friction from repetitive movements or trauma can irritate and inflame the bursa. Treatment options can involve antibiotics, rest, corticosteroid injections, and surgery in more severe cases.
- Tendinitis: This describes inflammation of the tendons and may cause difficulty lifting the arms. It typically occurs following irritation or damage to the tendons from sporting or repetitive overhead movements. Some people may also refer to it as a shoulder impingement. Treatment often involves rest, physical therapy, and injections, or surgery if these methods do not work.
Arthritis describes inflammation of the joints. In people with arthritis, the cartilage is damaged and the joint no longer has a smooth surface.
Arthritis can affect the joints of the shoulders, including the acromioclavicular and glenohumeral joints, and may cause pain, limited range of motion, and a clicking noise.
Types of arthritis that may affect the shoulder include:
- post-traumatic arthritis
- rheumatoid arthritis
- rotator cuff tear arthropathy
- avascular necrosis
Treatment will depend on the type and severity of arthritis, but will typically begin with nonsurgical options such as rest, physical therapy, and pain relief medications. In cases where these options are unsuccessful or the pain causes disability, a doctor may consider surgery.
Fractures are also known as broken bones. As such, a shoulder fracture describes when a break occurs to the scapula, clavicle, or top of the humerus. Fractures can occur due to:
- car accidents
In many cases, a doctor can treat shoulder fractures without surgery. This usually involves immobilizing the shoulder using a sling and allowing it to heal. However, if the bone is completely out of position, it may require surgery. This typically involves fixing the fracture with plates and screws or rods inside the bone.
A shoulder dislocation describes when the humerus comes partially or fully out of the glenoid. Due to its mobility, the shoulder is the most commonly dislocated joint, with forward, or anterior, dislocations accounting for nearly
To treat a shoulder dislocation, a doctor will likely perform a closed reduction. This is a procedure where they put the humerus back into the joint socket. A doctor may then advise resting or immobilizing the shoulder, before performing some rehabilitation exercises when pain and swelling resolve.
Sometimes a dislocation is so severe that it requires surgery. A person may also require surgery if the shoulder becomes unstable after a dislocation, to prevent recurrent dislocations.
A person can try to ensure their shoulder remains healthy and has a good range of motion by doing the following:
- stretching the shoulder muscles adequately before exercise
trying not to overusethe shoulder muscles when working or playing sport
- strengthening the shoulder muscles through an exercise program
- not ignoring shoulder pain and consulting a doctor if the pain continues
- trying to avoid activities that may result in a fall or accident
The shoulder is a complex structure that consists of many bones, muscles, and tendons. These components combine to form a mobile joint that provides the arms with a wide range of movement.
However, as a mobile joint that many people frequently use, the shoulder is prone to instability and injury. Conditions such as rotator cuff injuries, shoulder arthritis, fractures, and dislocations can cause pain and impact the function of the shoulder. Prompt treatment, rest, stretching, and exercise can help to strengthen the shoulder and aid recovery.