The shoulder joint, which functions like a ball and socket, can be severely damaged by arthritis or injury, often requiring joint replacement surgery. Traditional Total Shoulder Arthroplasty (TSA) mimics natural anatomy and requires a healthy, functional rotator cuff for stability. When the rotator cuff is severely damaged or irreparable, a specialized procedure called Reverse Total Shoulder Arthroplasty (RTSA) is used instead. RTSA is designed to restore function in a shoulder that has lost the dynamic support of its rotator cuff.
The Rotator Cuff Condition Necessitating Reverse Replacement
The primary condition necessitating reverse shoulder replacement is Rotator Cuff Arthropathy, which results from a massive, long-standing, and irreparable tear of the rotator cuff tendons. Normally, the rotator cuff keeps the ball of the upper arm bone (humerus) centered within the shoulder socket (glenoid) during movement. When these tendons are torn beyond repair, this centering function is lost.
This loss of dynamic stability allows the powerful deltoid muscle to pull the humeral head upward, a phenomenon known as superior migration. This causes the joint ball to rub against the underside of the acromion, the bony arch above the shoulder. This repeated friction leads to progressive wear of the articular cartilage, causing severe arthritis and chronic pain.
The resulting joint instability and inability to lift the arm, known as pseudoparalysis, signals that a traditional replacement is not possible. The reverse replacement is specifically indicated to bypass this irreparable rotator cuff failure and restore the patient’s ability to elevate the arm.
The Mechanical Shift: Deltoid Compensation for Rotator Cuff Failure
Reverse Total Shoulder Arthroplasty fundamentally changes the shoulder’s biomechanics to compensate for the failed rotator cuff by physically reversing the ball and socket components. In RTSA, a metal ball is fixed to the glenoid (socket), and a plastic socket is placed on the upper end of the humerus.
This anatomical reversal lowers and medializes the joint’s center of rotation, moving it closer to the body. This shift dramatically increases the lever arm and tension of the deltoid muscle. With this improved mechanical advantage, the deltoid acts as the primary motor for raising the arm, replacing the function previously performed by the rotator cuff.
The spherical metal ball acts as a fixed fulcrum, allowing the deltoid to compress the joint and efficiently lift the arm. This mechanism allows the patient to regain functional arm elevation, even though the cuff muscles are non-functional.
Surgical Handling and Post-Operative Status of Rotator Cuff Tissue
During reverse shoulder replacement, the damaged and often retracted rotator cuff tendons are encountered, but most tissue is not repaired for function. The superior and posterior cuff tendons (supraspinatus, infraspinatus, and teres minor) are typically scarred and atrophied from disuse and are left alone. The new joint mechanics bypass the need for these tendons to perform rotation and stability.
The one exception is the subscapularis tendon, located on the front of the shoulder. This tendon is often detached or cut by the surgeon to gain access to the joint space during the procedure. The subscapularis is the only rotator cuff tendon routinely repaired in RTSA.
It is repaired primarily to provide crucial anterior stability to the prosthetic joint and prevent forward dislocation. Overall rotational function, especially external rotation, is often permanently limited because the posterior tendons remain non-functional. The remaining rotator cuff tissue is typically scarred, atrophic, and contributes very little to active movement.