Shoulder pain is a common complaint, and shoulder impingement and frozen shoulder are two conditions frequently confused. While both cause pain and limit arm movement, they are fundamentally different in their underlying causes and how they affect the shoulder’s anatomy. The shoulder is a complex ball-and-socket joint, and understanding whether the problem lies with the surrounding soft tissues or the joint capsule is key to proper diagnosis and treatment.
Understanding Shoulder Impingement
Shoulder impingement, often called subacromial impingement, is a mechanical issue involving the compression of soft tissues within the shoulder joint. This occurs when the rotator cuff tendons and the subacromial bursa—a fluid-filled sac that reduces friction—are pinched beneath the acromion, the bony projection forming the top edge of the shoulder blade.
The compression leads to inflammation, causing tendinopathy of the rotator cuff tendons or bursitis. This pinching is often caused by repetitive overhead activities common in certain sports or occupations, leading to overuse injury. Anatomical factors, such as a hooked acromion shape or bone spurs, can also reduce the available space and contribute to the problem.
The resulting friction and irritation cause pain, particularly when the arm is lifted overhead, and may lead to weakness. Impingement focuses on a structural crowding issue where tissues are physically squeezed during movement.
Understanding Frozen Shoulder
Frozen shoulder, medically known as adhesive capsulitis, involves the shoulder joint’s protective lining rather than external compression. The condition is characterized by the thickening and tightening of the joint capsule, the strong connective tissue surrounding the glenohumeral joint. This process involves inflammation followed by the deposition of excessive collagen, causing scar tissue, or adhesions, to form within the capsule.
This scar tissue formation causes the capsule to contract and shrink, reducing the volume of the joint space. The cause is often unknown, though it is sometimes linked to immobility following an injury or surgery. It is also more common in individuals with diabetes or thyroid disorders. Frozen shoulder typically progresses through three recognizable stages.
The first stage, the “freezing” phase, involves a gradual onset of increasing pain, and movement begins to decrease. The subsequent “frozen” stage sees the pain stabilize, but stiffness and loss of motion become most pronounced. Finally, the “thawing” phase is the long period where the range of motion slowly begins to return.
Key Differences in Symptoms and Movement
The presentation of pain and restriction is the most telling difference, directly reflecting their distinct underlying pathologies. With shoulder impingement, pain is localized and occurs during specific movements, often forming a “painful arc” when the arm is raised between 60 and 120 degrees. The functional limitation is primarily due to pain, and the passive range of motion—how far the arm can be moved by someone else—is usually preserved.
Frozen shoulder results in a global and severe restriction of movement in all directions, affecting both active and passive motion. The tightening of the joint capsule physically prevents the joint from moving, meaning a healthcare provider cannot move the arm further than the patient can. A hallmark sign is the significant loss of external rotation, the inability to rotate the arm outward. Daily activities like reaching behind the back or into a back pocket become severely limited due to this overall stiffness.
Distinct Paths to Recovery
The contrasting nature of these conditions necessitates entirely different approaches to treatment. For shoulder impingement, the primary goal is to reduce inflammation of the tendons and bursa, and then improve shoulder mechanics. Treatment involves rest, anti-inflammatory medications, and physical therapy focused on strengthening the rotator cuff muscles to create more space under the acromion. Corticosteroid injections are often used to reduce acute inflammation and pain, facilitating participation in a strengthening program.
Recovery from frozen shoulder is usually a much longer process focused on restoring joint mobility. Physical therapy involves mobilization techniques and stretching exercises designed to break up scar tissue and stretch the contracted joint capsule. For cases that fail to respond to conservative treatment, procedures like hydrodilatation, which injects sterile fluid to stretch the capsule, or surgical manipulation under anesthesia may be required to restore movement. The full recovery timeline can span months to years, significantly longer than recovery for shoulder impingement.