How Reverse Shoulder Replacement Works: Understanding the Mechanics and Your New Shoulder

By Eric J. Sanders, MD – Shoulder and Elbow Surgeon at Texas Orthopaedic Associates

If you've been told you need a reverse total shoulder arthroplasty (reverse shoulder replacement), you might be wondering how this procedure helps restore function when the rotator cuff is torn or worn out.

Reverse shoulder replacement is not just a different implant—it’s a complete shift in shoulder biomechanics designed to let your deltoid muscle do the job your rotator cuff can no longer perform.

In this post, I’ll explain—in clear, patient-friendly terms—how reverse shoulder replacement works, what changes it makes to your shoulder structure, and what you can expect in terms of motion, strength, and limitations.

What Is a Reverse Shoulder Replacement?

A reverse total shoulder arthroplasty (rTSA) is a type of shoulder replacement where the normal anatomy is reversed:

• The ball (glenosphere) is placed on the shoulder blade (scapula)

• The socket (humeral cup) is placed on the top of the arm bone (humerus)

This "reversal" is intentional—it changes the mechanics of the shoulder so the deltoid muscle becomes the primary mover of the arm when the rotator cuff is torn or irreparable.

Key Biomechanical Changes That Make It Work

✅ Medialization of the Center of Rotation

In a normal shoulder, the center of rotation lies near the anatomic joint line. In reverse shoulder replacement, the center of rotation is moved medially and inferiorly—closer to the chest wall.

Why this matters:
This medial shift increases the deltoid’s mechanical advantage by improving the length-tension relationship, allowing it to lift the arm even without a functioning rotator cuff.

📘 As discussed in PMC6942071, medialization enhances deltoid tension and stability while reducing stress on the implant interface.

✅ Deltoid Lengthening and Function

The design of the implant also lengthens the deltoid muscle, improving its strength and effectiveness.

Why this matters:
A longer, properly tensioned deltoid muscle can now lift and rotate the arm, assuming the duties once performed by the torn rotator cuff.

This is especially important for activities like:

• Reaching for a cabinet

• Lifting light objects

• Performing personal care tasks

✅ Compensation for Rotator Cuff Tears

The reverse shoulder replacement is particularly suited for rotator cuff tear arthropathy—a condition where the rotator cuff is torn and the shoulder becomes arthritic and unstable.

The new biomechanics allow the deltoid to compensate for cuff deficiency, providing stability and motion that wouldn't be possible with a traditional implant.

📘 According to PMC8883988, reverse arthroplasty is now the gold standard in patients with cuff-deficient shoulders due to its ability to restore function and reduce pain.

✅ Arm Lengthening and Change in Shoulder Contour

Reverse shoulder replacement often results in subtle lengthening of the arm—usually by 2–3 centimeters.

Why this matters:

• It helps maintain joint tension and implant stability lowering the risk of dislocation

• The shoulder contour often appears leaner and less rounded

• Patients may notice their arm feels “longer” or “tighter” during early recovery, but this sensation usually normalizes

This change also contributes to the improved biomechanics of the deltoid, especially in forward elevation.

The Role of the Scapula After Reverse Shoulder Replacement

✅ Increased Scapular Contribution

After rTSA, the shoulder blade (scapula) plays a much larger role in arm motion. You may notice your shoulder blade moves more during arm elevation.

Why this matters:

• The deltoid originates from the scapula

• Improved deltoid leverage depends on how well the scapula moves

• Patients with stiff or dyskinetic scapulae may experience limitations in range of motion

📘 A study in PMC8559568 notes that scapulothoracic rhythm significantly increases after reverse shoulder replacement, supporting the new mechanics.

Expected Limitations After Reverse Shoulder Replacement

While rTSA restores impressive function, there are some natural limitations due to implant design.

❌ External Rotation and Behind-the-Back Movements

• External rotation (turning your hand outward) may be limited, especially without a functioning teres minor or posterior deltoid

• Movements like tucking in a shirt or reaching behind your back can be difficult or limited

❌ Implant Impingement and Range of Motion

All implants have a built-in limit to prevent:

• Impingement (where implant components contact one another or bone)

• Instability or dislocation

Surgeons carefully choose implant angles and positioning to maximize safe range of motion, but extremes of movement—especially internal rotation or combined motions like abduction with rotation—may feel restricted.

🧠 Your shoulder is more stable, but it’s designed to move within a safe arc. Therapy focuses on functional motion, not extreme flexibility.

Summary: What Patients Should Know

A reverse shoulder replacement changes how your shoulder works—and that’s exactly the point. By reversing the anatomy and improving the deltoid’s ability to move your arm, this procedure restores function in shoulders that have lost their rotator cuff.

What You Can Expect:

• Excellent pain relief

• Strong forward elevation

• Reliable function for daily activities

• Some limitations in rotation

• A different—but often stronger and more stable—shoulder than before

Final Thoughts from Dr. Sanders

As a shoulder specialist, I often tell my patients: Reverse shoulder replacement isn’t just a fix—it’s a redesign. Understanding how it works empowers you to get the most from your recovery and therapy.

If you're considering surgery or recovering from one, know that this procedure is backed by decades of innovation and real-world success. With commitment to rehab and care, most patients regain confidence in their shoulder—and in themselves.

Dr. Eric J. Sanders is a shoulder and elbow surgeon at Texas Orthopaedic Associates in Dallas and Plano. He specializes in complex shoulder conditions, including rotator cuff tear arthropathy and revision shoulder surgery. Learn more or request a consultation here.