Introduction
As a shoulder replacement surgeon, the question that I am asked most frequently prior to surgery is the difference between an anatomic and reverse total shoulder arthroplasty. I think that is
Shoulder arthroplasty has become an essential procedure for managing a wide variety of shoulder joint conditions, particularly those involving severe arthritis or rotator cuff deficiency. Two principal types of shoulder arthroplasty are utilized: Anatomic Total Shoulder Arthroplasty (TSA) and Reverse Total Shoulder Arthroplasty (RTSA). Although they both serve to restore function and reduce pain, they operate on distinctly different biomechanical principles and are suited to different patient populations. In this article, we’ll delve into the biomechanical underpinnings of RTSA, contrasting them with those of TSA, and examine how these approaches address differing anatomical and functional challenges in the shoulder.
Understanding Shoulder Replacement Biomechanics
In short, the difference between the two implants on a surface level can be summarized as such:
Anatomic Total Shoulder Arthroplasty (TSA) attempts to mimic the natural joint anatomy by replacing the humeral head with a metal implant and the glenoid with a polyethylene socket.
Reverse Total Shoulder Arthroplasty (RTSA) reverses the natural anatomy of the shoulder by placing a "ball" component on the glenoid and a "socket" component on the humerus. This configuration shifts the shoulder's center of rotation and relies on the deltoid muscle for primary movement, bypassing the need for a functioning rotator cuff.
For those of you out there who are more mechanically-minded, I put together a brief highlights summary of TSA and RTSA biomechanics. This may help you understand why we utilize one implant versus the other.
Biomechanics of Anatomic Shoulder Arthroplasty (TSA)
Glenohumeral Joint Mechanics: The convex humeral head and concave glenoid provide a fulcrum for movement, closely resembling the native joint mechanics.
Role of the Rotator Cuff: In anatomic TSA, a functional rotator cuff is crucial. The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) compress the humeral head against the glenoid, providing both stability and strength in shoulder elevation and rotation.
Center of Rotation: Anatomic TSA maintains the shoulder’s natural center of rotation, which is close to the center of the humeral head and aligned with the glenoid fossa.
Limitations: Anatomic TSA is suitable primarily for patients with preserved rotator cuff function. When the rotator cuff is deficient, TSA may lead to joint instability, migration of the humeral head (proximal humeral escape), and poor functional outcomes.
Biomechanics of Reverse Total Shoulder Arthroplasty (RTSA)
RTSA was developed to address the limitations of TSA, particularly in patients with rotator cuff deficiency. Its biomechanical design shifts the functional dynamics significantly:
Reversed Joint Geometry: The glenoid component becomes a convex ball (glenosphere), while the humeral component is concave. This "reversed" setup effectively medializes and lowers the center of rotation, providing inherent joint stability by altering the forces applied to the joint.
Altered Center of Rotation: The medialization and inferiorization of the center of rotation are core biomechanical changes. This shift reduces shear forces on the glenoid component and leverages the deltoid muscle more effectively, compensating for the deficient rotator cuff. By moving the center of rotation closer to the scapula, RTSA reduces the torque on the glenoid implant, thus enhancing implant stability.
Deltoid Muscle as Primary Mover: In RTSA, the deltoid muscle becomes the primary driver of arm elevation and abduction, compensating for the lack of rotator cuff function. The repositioned center of rotation creates a mechanical advantage for the deltoid by increasing its lever arm length. This adaptation allows the deltoid to initiate and maintain arm elevation with less strain.
Reduction of Glenohumeral Joint Forces: The design of RTSA decreases joint reaction forces by improving stability and reducing the demand on the remaining soft tissues. The change in orientation helps prevent the superior migration of the humeral component, addressing a common problem seen in patients with rotator cuff insufficiency undergoing TSA.
Clinical Indications and Outcomes
Anatomic TSA:
Best Candidates: Patients with intact rotator cuffs and primarily degenerative osteoarthritis.
Outcomes: Typically restores a near-normal range of motion and provides high levels of patient satisfaction. However, failure rates are higher in patients with cuff deficiency.
RTSA:
Best Candidates: Patients with irreparable rotator cuff tears, rotator cuff arthropathy, or severe proximal humerus fractures.
Outcomes: While RTSA often results in a limited range of motion compared to TSA, it provides functional outcomes superior to TSA in rotator cuff-deficient patients. Patients experience reliable pain relief, improved shoulder stability, and adequate function for daily activities.
Key Differences and Biomechanical Considerations
Parameter | Anatomic TSA | Reverse TSA |
Implant Positioning | Humeral head and glenoid socket mimic natural anatomy | Reversed position of ball and socket |
Center of Rotation | Near the center of humeral head | Medialized and inferiorized near the glenoid |
Primary Mover | Rotator cuff muscles with deltoid assistance | Deltoid muscle as primary mover |
Indications | Patients with intact rotator cuff | Patients with rotator cuff deficiency |
Range of Motion | Typically high, closer to natural shoulder | Limited, but functional for daily tasks |
Stability Considerations | Dependent on rotator cuff for joint compression | Inherent stability due to medialized center of rotation |
Complications and Long-Term Considerations
RTSA carries specific risks, including scapular notching, which occurs as the humeral component contacts the scapula during certain movements. This contact can lead to bone loss over time. This complication has largely been eliminated by more recent lateralized glenosphere designs and more knowledge about proper implant positioning.
In addition, the increased reliance on the deltoid in RTSA can lead to fatigue or overuse-related issues, particularly in active individuals. Both procedures demand precise surgical technique to optimize implant positioning, ensure longevity, and reduce complication rates.
Conversely, TSA’s primary complications involve glenoid component loosening, especially in the setting of cuff deficiency.
Conclusion
From a biomechanical perspective, RTSA is a revolutionary option for patients with rotator cuff deficiency, providing a stable shoulder joint by shifting the center of rotation and using the deltoid as the primary muscle for arm elevation. TSA, on the other hand, is ideal for patients with intact rotator cuffs, closely replicating natural shoulder biomechanics. The choice between TSA and RTSA should be tailored to each patient’s anatomy, functional demands, and underlying pathology, as each procedure offers unique advantages and considerations.
As research advances, new designs and techniques will continue to evolve, potentially blending elements of both approaches to further improve outcomes and extend the functional lifespan of shoulder implants.