A visible indentation in the shoulder area when the arm is lifted or moved can be alarming, as the shoulder usually maintains a rounded contour during movement. This hollowing suggests that the underlying structures responsible for the shoulder’s shape are not functioning or are no longer present as expected. The causes range from issues with the nerves that control the shoulder muscles to damage within the joint’s mechanics.
The Mechanics of the Shoulder Dent
The characteristic rounded shape of the shoulder is primarily formed by the deltoid muscle, a large, thick, triangular muscle that covers the shoulder joint. This muscle is composed of three heads—anterior, lateral, and posterior—each contributing to the overall contour and range of motion. The lateral head is particularly responsible for lifting the arm out to the side, a movement known as abduction.
When the arm is raised, the deltoid muscle must contract and bulge outward to execute the movement and maintain the shoulder’s convex appearance. A noticeable “dent” or flattening means the expected muscular bulk is absent or significantly reduced. This absence of contour is often a direct result of the muscle not contracting with sufficient force or having undergone shrinkage, known as atrophy.
The rotator cuff muscles, particularly the supraspinatus, initiate the first few degrees of arm lift. However, the deltoid is the main driver and bulk provider for the majority of the movement. If this visual mass is compromised, the skin may sink into the hollow left behind, creating the indentation when the arm is activated.
Causes Related to Muscle Atrophy and Nerve Function
The most direct cause of a dent in the shoulder is atrophy of the deltoid muscle, which is almost always linked to a problem with its nerve supply. The axillary nerve provides the motor function to the entire deltoid muscle; therefore, damage to this nerve quickly leads to weakness and muscle wasting.
Axillary nerve damage, or palsy, commonly occurs following a traumatic event such as a shoulder dislocation or a fracture of the humerus. The nerve runs close to the shoulder joint, making it vulnerable to stretching or compression. Within weeks, the deltoid can shrink visibly, resulting in a distinct flattening of the shoulder contour and difficulty lifting the arm away from the body.
Parsonage-Turner Syndrome (PTS), also known as brachial neuritis, is a less common cause. This neurological disorder involves the sudden onset of severe shoulder pain, followed by rapid weakness and wasting of the muscles in the shoulder girdle. PTS involves inflammation and damage to the brachial plexus, the network of nerves that supplies the arm and shoulder, and can specifically affect the axillary nerve.
Another potential cause is cervical radiculopathy, where a nerve root is compressed or irritated as it exits the spine in the neck. If the affected nerve root is the fifth cervical nerve (C5), it interferes with the motor signals traveling to the deltoid muscle. This interruption prevents the muscle from receiving the necessary input to contract and maintain its size, eventually leading to atrophy and the visible dent.
Causes Related to Structural Damage and Instability
In some cases, the dent involves structural issues within the shoulder joint that alter movement mechanics, independent of deltoid atrophy. Massive or chronic rotator cuff tears, especially involving the supraspinatus and infraspinatus tendons, can lead to muscle atrophy in the shoulder blade area. The resulting shrinkage changes the overall shape, sometimes making the deltoid region appear more sunken.
A severe, long-standing tear can also cause the head of the humerus to move upward, further disrupting the normal muscle mechanics and contour. This mechanical failure contributes to an abnormal depression or hollowing, particularly when attempting to raise the arm. The atrophy associated with these tears often reflects a long duration of the injury, typically a year or more.
Shoulder instability following a dislocation can also contribute to an altered contour. A severe dislocation can cause a Hill-Sachs lesion, a compression fracture resulting in a dent on the back of the humeral head. This bony indentation interferes with the smooth motion of the joint, and the resulting instability can visually distort the shoulder profile when the arm is lifted.
Prior shoulder surgery, such as extensive rotator cuff repair or stabilization procedures, can sometimes leave scar tissue or necessitate muscle adjustments that permanently change the visible contour. While functionally stable, the resulting external appearance may feature a slight depression not present before the operation.
When to Consult a Healthcare Professional
A noticeable dent in the shoulder, especially one accompanied by other symptoms, warrants a professional medical evaluation. You should consult a doctor if the dent appeared suddenly, particularly following a fall or injury. This rapid onset suggests acute nerve damage or a severe structural tear that requires prompt attention.
Additional warning signs include a rapidly worsening inability to lift the arm, severe or persistent pain, or any accompanying numbness, tingling, or a burning sensation. These neurological symptoms are suggestive of nerve involvement, such as axillary nerve palsy or Parsonage-Turner Syndrome.
To determine the exact cause, a healthcare professional will conduct a thorough physical examination, testing the strength and sensation around the shoulder. Diagnostic tools like electromyography (EMG) and nerve conduction studies assess nerve function. Magnetic resonance imaging (MRI) or ultrasound can visualize the muscles, tendons, and joint structures to confirm atrophy or tears. Early diagnosis allows for timely intervention, such as physical therapy or surgical repair, which can significantly improve the long-term outcome.