A visible lump or protrusion on the shoulder can be alarming, often leading to concern that a bone has moved out of place. The shoulder girdle is a complex structure, and what appears as an abnormal bony prominence can range from a normal anatomical landmark to a sign of serious injury or a progressive condition. This article provides general information, but it is not a substitute for professional medical advice. Any new or painful lump should always be evaluated by a healthcare provider for an accurate diagnosis and appropriate treatment plan.
Identifying Normal Shoulder Anatomy
The shoulder includes several bones that lie close to the skin, which can become more noticeable in individuals with less overlying muscle or fat tissue. The clavicle, or collarbone, is easily felt from the sternum across the chest to the tip of the shoulder. Its outer end forms a joint with the acromion, the highest point of the shoulder blade.
This junction, known as the acromioclavicular (AC) joint, often presents a slight, normal prominence. The spine of the scapula, a ridge of bone running across the back of the shoulder blade, can also be quite visible. A normal bony prominence will feel firm, fixed, and present on both shoulders, though one side might be slightly more noticeable.
An anatomical landmark will not usually cause acute pain or instability unless aggravated by movement or position. Distinguishing a fixed, symmetrical structure from a newly appeared or mobile lump is the first step in self-assessment.
Acute Injuries Causing Bony Displacement
A sudden, visible bump appearing immediately after trauma is often caused by a mechanical disruption of the joint. The most common cause is an acromioclavicular (AC) joint separation, frequently called a “shoulder separation.” This injury happens when the ligaments connecting the collarbone and shoulder blade are stretched or torn, typically from a fall directly onto the point of the shoulder.
In more severe grades of an AC separation (Grade III), the supporting coracoclavicular ligaments tear completely. This allows the weight of the arm to pull the shoulder blade down while the clavicle is pushed upward. This results in a distinct step-off where the end of the collarbone sits higher than the acromion. Symptoms include immediate, severe pain, swelling, and difficulty lifting the arm.
A fractured clavicle can also create a prominent bony deformity, especially when the broken ends are displaced. Fractures in the middle third of the collarbone are the most common, and the pull of neck muscles can lift the inner bone fragment upward. This displacement can cause the broken bone to press against the skin, a sign known as “tenting,” which requires urgent medical attention.
Chronic Conditions and Postural Changes
When a shoulder prominence develops slowly without a specific injury, the cause may relate to degenerative changes or muscle control issues. Acromioclavicular joint osteoarthritis is a common chronic condition where the cartilage in the AC joint wears down, leading to the formation of bone spurs (osteophytes). These bony growths cause a gradual, hard enlargement or bump at the joint site.
This type of prominence is accompanied by localized pain and tenderness, particularly when the arm is moved across the chest or lifted overhead. The degenerative process often results from years of wear and tear, repetitive stress, or may follow an old AC joint injury. The pain is generally chronic and slow to worsen, distinguishing it from an acute injury.
Another non-traumatic cause is scapular winging, where the shoulder blade protrudes noticeably from the back. This occurs due to weakness or paralysis of the muscles that hold the scapula flat against the rib cage, most commonly the serratus anterior muscle. Nerve damage can cause this muscle dysfunction, leading to the medial border of the scapula sticking out when pushing against a wall or raising the arm.
When to Seek Medical Attention
While many shoulder lumps are benign, certain signs indicate the need for prompt medical evaluation. Seek immediate care if the protrusion appeared after significant trauma, such as a fall, and is accompanied by severe, debilitating pain or the inability to move the arm. Other red flags include any visible open wound, numbness or tingling in the arm or hand, or the bone pressing tightly against the skin.
For a lump that developed gradually, a medical assessment is warranted if it is growing rapidly, feels hot or tender to the touch, or is larger than five centimeters. A physician will perform a thorough physical examination, assessing the shoulder’s range of motion and stability. Diagnostic imaging, such as X-rays, is the standard next step to visualize bony structures, confirm the location of any fracture or displacement, and check for degenerative changes like bone spurs.