Bone spurs, medically termed osteophytes, are projections of extra bone that grow along the edges of bones, often near joints. In the shoulder, this bony overgrowth is the body’s reaction to instability or damage within the joint structure. Spurs frequently develop on the underside of the acromion, around the glenohumeral joint (the main ball-and-socket joint), or in the acromioclavicular (AC) joint. The underlying cause is typically chronic wear or irritation that the body attempts to stabilize with new bone tissue.
Degenerative Joint Disease
Osteoarthritis (OA) is a primary driver of bone spur formation in the shoulder, affecting both the glenohumeral and AC joints. This condition involves the progressive breakdown of articular cartilage, the smooth tissue that cushions the ends of bones. Cartilage erosion leads to bone-on-bone friction and an irregular joint surface, which the body registers as structural failure.
In response to this instability, the body attempts to widen the joint surface area to better distribute forces and limit movement. This results in the formation of osteophytes along the joint margins, particularly in the AC joint. Although intended for stabilization, this new bone growth often causes stiffness and further mechanical issues within the joint.
Response to Tendon and Ligament Damage
Bone spurs also form in the shoulder as a reaction to chronic irritation and damage to soft tissues, separate from general joint surface wear. This mechanism is commonly seen in shoulder impingement syndrome, where the rotator cuff tendons become pinched. Impingement occurs when the space beneath the acromion, through which the tendons pass, is narrowed.
Chronic mechanical rubbing or small tears in the rotator cuff tendons, especially the supraspinatus, trigger inflammation in the surrounding bone. The bone reacts by growing an acromial spur on the underside of the acromion, which worsens impingement by further reducing space for the tendons. The body may also attempt to reinforce ligament or tendon attachment sites, leading to calcification and ossification at that junction.
Underlying Risk Factors
Several factors increase susceptibility to the degenerative and mechanical causes of shoulder bone spurs. Advancing age is a non-modifiable risk factor, as the likelihood of developing osteoarthritis and joint wear increases over time. Individuals with a family history of arthritis may also have a genetic predisposition to forming osteophytes.
Repetitive occupational or athletic activities are also major contributors, particularly those involving frequent overhead motions. Sports like baseball, tennis, and swimming, or jobs requiring constant lifting, place chronic stress on the shoulder joint’s soft tissues and cartilage. Previous acute shoulder trauma, such as fractures or dislocations, can also disrupt normal joint mechanics and accelerate the degenerative processes leading to spur formation.
The Biological Process of Bone Growth
The formation of a bone spur is fundamentally a process of misguided bone repair that begins at a cellular level. When chronic stress or injury damages the joint surface or tendon attachment points, a signal is sent to the body’s bone-building cells. These specialized cells, known as osteoblasts, are recruited to the site of injury or instability.
The osteoblasts begin to secrete a new bone matrix, primarily consisting of type I collagen. This matrix then undergoes calcification, where mineral crystals, mainly hydroxyapatite, are deposited into the organic framework. This sequence of events, known as ossification, results in the hard, bony projection of the osteophyte. The spur is an adaptive response where the body attempts to stabilize a damaged area by laying down extra bone.