Frozen Shoulder (Adhesive Capsulitis): Causes, Stages, and Risk Factors

How Does Frozen Shoulder Occur?

Frozen shoulder, medically known as adhesive capsulitis, is a perplexing condition characterized by progressive pain and stiffness in the shoulder joint, resulting from inflammation, thickening, and contraction of the joint capsule, often without a clear initiating cause.

What is Frozen Shoulder (Adhesive Capsulitis)?

Frozen shoulder is a debilitating condition affecting the glenohumeral joint, leading to significant pain and a severe restriction of both active and passive range of motion. The term "frozen" accurately describes the sensation of the shoulder becoming progressively stiff and immobile, as if locked in place. While the exact trigger often remains elusive, the underlying pathology involves changes within the joint capsule itself.

The Anatomy of the Shoulder Joint

To understand how frozen shoulder occurs, it's essential to briefly review the shoulder's anatomy. The shoulder is a ball-and-socket joint, where the head of the humerus (upper arm bone) fits into the glenoid cavity of the scapula (shoulder blade). Encasing this joint is a fibrous tissue envelope called the joint capsule. This capsule, along with ligaments and tendons, provides stability and contains synovial fluid, which lubricates the joint and allows for smooth, frictionless movement. In a healthy shoulder, the capsule is loose and pliable, permitting the wide range of motion characteristic of the shoulder.

The Pathophysiology: How It Develops

Frozen shoulder develops through a complex pathological process that primarily affects the glenohumeral joint capsule. While the precise initiating event is often unknown (idiopathic), the changes observed within the capsule are consistent:

• Inflammation: The initial phase typically involves inflammation of the joint capsule and the synovial lining. This inflammation leads to pain, particularly at rest and with movement, and is often the first symptom experienced.
• Fibrosis and Thickening: As inflammation persists, the normally thin and flexible joint capsule begins to undergo a process called fibrosis. Collagen fibers, which make up the capsule, become disorganized and excessively deposited, causing the capsule to thicken significantly. This thickening is most pronounced in the inferior (bottom) part of the capsule and around the rotator interval (the space between the supraspinatus and subscapularis tendons).
• Contraction and Adhesions: Concurrently, the fibrotic tissue contracts, making the capsule tighter and reducing its internal volume. This contraction pulls the joint surfaces closer together. Furthermore, adhesions (bands of scar-like tissue) form between the folds of the capsule, and between the capsule and the humeral head, effectively "gluing" the structures together. These adhesions physically restrict the movement of the humeral head within the glenoid cavity.
• Loss of Synovial Fluid: While not always a primary cause, some research suggests a reduction in the volume or quality of synovial fluid within the joint, further contributing to stiffness and reduced lubrication.

Together, these changes transform the once pliable and spacious joint capsule into a stiff, contracted, and adherent structure, severely limiting the shoulder's range of motion.

Stages of Frozen Shoulder

Frozen shoulder typically progresses through three distinct, albeit sometimes overlapping, stages, each with characteristic symptoms and duration:

• Freezing (Painful) Stage: This is the initial stage, lasting from 2 to 9 months. It is characterized by the gradual onset of pain, which often worsens at night and with movement. As pain increases, the shoulder begins to lose range of motion, though stiffness may not yet be the dominant symptom.
• Frozen (Stiffening) Stage: Lasting from 4 to 12 months, this stage is defined by significant stiffness and a marked reduction in range of motion. Pain may begin to subside during this phase, but the severe limitation of movement makes daily activities challenging. The shoulder feels "frozen."
• Thawing (Recovery) Stage: This final stage, which can last from 5 months to 2 years or even longer, involves a gradual return of range of motion. Pain typically continues to decrease, and the shoulder slowly regains its mobility. Complete recovery is common, though some individuals may experience residual stiffness.

What Causes Frozen Shoulder?

While the pathophysiology describes how the capsule changes, the etiology refers to what triggers these changes. Frozen shoulder is broadly categorized into two types:

• Primary (Idiopathic) Frozen Shoulder: In the majority of cases (approximately 70%), there is no identifiable cause or preceding event. This is why it's termed "idiopathic," meaning of unknown origin. Research continues to explore genetic predispositions, inflammatory responses, and microtrauma as potential factors.
• Secondary Frozen Shoulder: This type occurs following a specific event or in association with other medical conditions. The event or condition is thought to initiate the inflammatory and fibrotic process in the shoulder capsule.

Risk Factors for Frozen Shoulder

While primary frozen shoulder remains idiopathic, several risk factors are strongly associated with its development, particularly the secondary type:

• Age and Sex: Frozen shoulder most commonly affects individuals between 40 and 60 years old, and it is more prevalent in women than men.
• Diabetes Mellitus: This is the most significant systemic risk factor, with diabetic individuals having up to a five-fold increased risk. High blood glucose levels are thought to contribute to glycation of collagen, making it stiffer and more prone to fibrosis.
• Thyroid Conditions: Both hyperthyroidism (overactive thyroid) and hypothyroidism (underactive thyroid) are associated with an increased risk.
• Cardiovascular Disease: Conditions like heart disease, high cholesterol, and stroke have been linked to a higher incidence of frozen shoulder.
• Parkinson's Disease: Individuals with Parkinson's disease are also at a higher risk.
• Immobility or Trauma: Any prolonged immobilization of the shoulder, such as after a fracture, surgery, or stroke, can predispose an individual to frozen shoulder. Even minor trauma that leads to avoidance of movement can be a trigger.
• Surgery: Procedures like mastectomy or open-heart surgery, which can lead to arm immobility or altered shoulder mechanics, can sometimes precede the development of frozen shoulder.

Why Does It Happen to Some People and Not Others?

Despite extensive research, the precise reasons why some individuals develop frozen shoulder while others with similar risk factors do not remain largely unknown. It is likely a multifactorial condition, involving a complex interplay of genetic predisposition, metabolic factors, inflammatory responses, and potentially subtle biomechanical stressors. The body's individual response to inflammation and tissue repair mechanisms likely plays a critical role in determining who progresses to the fibrotic and contractile changes characteristic of adhesive capsulitis.

Conclusion

Frozen shoulder is a complex and often perplexing condition marked by progressive pain and stiffness due to inflammation, thickening, and contraction of the shoulder joint capsule. While often idiopathic, it is strongly associated with certain medical conditions, most notably diabetes, and periods of shoulder immobilization. Understanding the anatomical changes and the stages of its progression is crucial for both diagnosis and the development of effective management strategies aimed at restoring shoulder function and alleviating pain.