Bone spurs, medically known as osteophytes, are bony projections that can form along the edges of bones, particularly in joints. In the shoulder, these growths often appear where bones meet, such as near the acromion (the top part of the shoulder blade) or within the glenohumeral joint (the main ball-and-socket joint of the shoulder). They develop when the body produces extra bone in response to ongoing stress, degeneration, or injury. While some bone spurs remain small and cause no issues, others can become problematic if they press on nearby nerves, tendons, or soft tissues. This pressure can lead to symptoms like pain, stiffness, and a reduced range of motion in the shoulder.
The Body’s Response to Joint Degeneration
One of the primary reasons bone spurs develop in the shoulder is joint degeneration, often associated with aging and osteoarthritis. Osteoarthritis involves the gradual breakdown of cartilage, the smooth connective tissue that cushions the ends of bones and allows them to glide easily against each other. As this protective cartilage wears away, bones may begin to rub directly against one another.
In an attempt to repair the damaged cartilage and stabilize the joint, the body initiates a healing response by laying down new bone. This compensatory mechanism results in the formation of bone spurs along the joint margins. Approximately 70% of individuals over 60 show signs of bone spurs, often in major joints like the shoulder, highlighting the common, age-related nature of this process. These bony growths can form on the humeral head or glenoid, potentially affecting the smooth movement of the ball-and-socket joint and leading to mechanical issues.
Mechanical Stress and Overuse Injuries
Physical stress, whether from acute injuries or chronic overuse, contributes to bone spur development in the shoulder. Repetitive overhead activities, common in sports like tennis, baseball, or swimming, or in occupations requiring frequent arm elevation, place immense stress on the shoulder joint. This ongoing mechanical stress can destabilize the joint and damage surrounding tendons and ligaments.
Bone spurs often develop beneath the acromion, the bony arch at the top of the shoulder, narrowing the space for the rotator cuff tendons and leading to shoulder impingement, where tendons get pinched during arm movement. Shoulder injuries like rotator cuff tears can also directly contribute to spur formation. A portion of the rotator cuff tendons passes through a narrow area between bones, and if a bone develops a sharp edge or spur, it can rub against and even tear the tendons over time.
The Role of Inflammation and Instability
Chronic inflammation and joint instability are additional factors that can lead to bone spur formation in the shoulder. Conditions like chronic tendinitis, which is inflammation of tendons, or bursitis, inflammation of the fluid-filled sacs that cushion joints, can trigger bone spur development. Persistent inflammation can lead to calcification and eventually new bone growth as the body tries to repair the affected tissues.
For instance, the subacromial bursa in the shoulder, which helps the rotator cuff tendons glide smoothly, can become inflamed due to injury or overuse. This inflammation can irritate surrounding structures and contribute to the formation of bone spurs on the underside of the acromion. These spurs, in turn, can exacerbate the inflammation and lead to a cycle of pain and further bony growth. A chronically unstable shoulder joint can also promote bone spur development. When the shoulder is repeatedly dislocated or experiences chronic looseness, it leads to abnormal joint mechanics and increased stress, prompting the body to form bone spurs to limit excessive movement.