Reverse Total Shoulder Replacement: Muscle Biomechanics, Key Muscles Affected, and Rehabilitation

What muscles are affected in reverse total shoulder replacement?

Reverse total shoulder replacement (RTSR) fundamentally alters the biomechanics of the shoulder joint, shifting the primary responsibility for arm elevation from the often-damaged rotator cuff to the deltoid muscle, while also impacting the function and role of other surrounding musculature.

Understanding Reverse Total Shoulder Replacement (RTSR)

Traditional total shoulder replacement (TSR) mimics the native anatomy, with a ball (humeral head) articulating with a socket (glenoid). However, TSR is often contraindicated or less effective when the rotator cuff muscles are severely damaged or dysfunctional, as these muscles are crucial for stabilizing the traditional joint and initiating movement.

Reverse total shoulder replacement (RTSR) is a specialized surgical procedure designed for individuals with irreparable rotator cuff tears, severe glenohumeral arthritis, or complex fractures. Its "reverse" nature refers to the prosthetic components: the ball (glenosphere) is fixed to the shoulder blade (scapula), and the socket (humeral cup) is attached to the upper arm bone (humerus). This inversion of the ball-and-socket configuration profoundly changes the joint's center of rotation and, consequently, the biomechanics of the surrounding muscles.

The Native Shoulder: A Muscular Overview

To appreciate the changes brought about by RTSR, it's essential to understand the healthy shoulder's muscular landscape. The glenohumeral joint, a highly mobile ball-and-socket joint, relies heavily on a complex interplay of muscles for stability and movement.

• Rotator Cuff: Comprising four muscles—supraspinatus, infraspinatus, teres minor, and subscapularis—the rotator cuff is paramount for dynamically stabilizing the humeral head within the shallow glenoid fossa and initiating/controlling various movements, particularly elevation and rotation.
• Deltoid: This large, multi-pennate muscle (anterior, middle, and posterior heads) is the primary mover for gross arm elevation (abduction and flexion).
• Scapular Stabilizers: Muscles like the trapezius, rhomboids, levator scapulae, and serratus anterior anchor and control the scapula, providing a stable base for glenohumeral joint movement and ensuring proper scapulohumeral rhythm.
• Other Shoulder Girdle Muscles: Muscles such as the pectoralis major, latissimus dorsi, biceps brachii, and triceps brachii also contribute to shoulder movement and stability.

The Biomechanical Shift in RTSR

The key innovation of RTSR is the medialization and inferiorization of the center of rotation. By moving the "ball" medially and inferiorly, the deltoid muscle fibers become significantly lengthened at rest and achieve a more advantageous lever arm. This allows the deltoid to effectively elevate the arm, even in the absence of a functional rotator cuff. The rotator cuff, which is often irreparable in RTSR candidates, becomes largely ineffective or bypassed as a primary mover for elevation.

Key Muscles Affected by RTSR

The impact of RTSR on the shoulder musculature is extensive, affecting their function, recruitment patterns, and importance in rehabilitation.

• Deltoid Muscle:
  • Primary Mover for Elevation: Post-RTSR, the deltoid muscle assumes the critical role of the primary elevator of the arm. Its new line of pull, due to the altered center of rotation, significantly improves its mechanical advantage for abduction and flexion.
  • Increased Importance: The strength and endurance of the deltoid become paramount for functional arm movement. Rehabilitation heavily focuses on strengthening all three heads of the deltoid.
• Rotator Cuff Muscles:
  • Diminished or Eliminated Role in Elevation: The very reason for RTSR is often the irreparable damage to the rotator cuff. Post-surgery, these muscles are no longer effective or necessary for arm elevation due to the altered joint mechanics.
  • Altered Function: While their role in elevation is minimized, the remaining integrity of certain rotator cuff muscles (e.g., subscapularis for internal rotation, infraspinatus/teres minor for external rotation) may still contribute to rotational movements or provide some residual stability, depending on their pre-operative condition and the surgical approach. However, their primary stabilizing and dynamic centering role is largely bypassed by the design of the prosthesis.
• Scapular Stabilizers:
  • Enhanced Importance: Although not directly operated on, the muscles that stabilize the scapula (trapezius, rhomboids, levator scapulae, serratus anterior) become even more critical. A stable and properly moving scapula provides the necessary foundation for the deltoid to exert its force effectively.
  • Compensation and Dyssynergia: Without a functioning rotator cuff, there's a greater reliance on these muscles for compensatory movements. Poor scapular control can lead to inefficient movement patterns and potential pain.
• Other Shoulder Girdle Muscles:
  • Pectoralis Major and Latissimus Dorsi: These large muscles can become relatively more dominant in internal rotation and adduction. Their strength may need to be balanced with the deltoid and scapular stabilizers to prevent imbalances.
  • Biceps Brachii and Triceps Brachii: The long heads of both the biceps and triceps cross the glenohumeral joint and contribute to stability. In RTSR, the long head of the biceps may be tenotomized (cut) or tenodesed (reattached) to address pre-existing pathology or to prevent impingement on the prosthetic components. This can alter its contribution to shoulder stability and flexion.

Functional Implications and Rehabilitation Focus

The profound muscular changes in RTSR necessitate a highly specific rehabilitation approach. The focus shifts from strengthening the rotator cuff (as in traditional shoulder rehab) to:

• Maximizing Deltoid Strength: This is the cornerstone of post-RTSR function. Exercises target all parts of the deltoid.
• Optimizing Scapular Control: Ensuring the scapula moves efficiently and provides a stable base for the deltoid's action is crucial for range of motion and preventing compensatory patterns.
• Managing Rotational Limitations: Patients often experience limitations in external and internal rotation due to the altered joint mechanics and the often-dysfunctional rotator cuff. Rehabilitation aims to optimize functional rotation within the new biomechanical constraints.
• Avoiding Impingement: Certain movements, particularly combined adduction and internal rotation, may be restricted to prevent dislocation or impingement of the prosthesis.

Conclusion

Reverse total shoulder replacement profoundly re-engineers the shoulder's muscular mechanics. It fundamentally shifts the burden of arm elevation from a damaged rotator cuff to a newly empowered deltoid muscle. While the rotator cuff's role in elevation is largely bypassed, the function of scapular stabilizers becomes even more critical for providing a stable platform. Understanding these intricate muscular adaptations is key for healthcare professionals and patients alike, guiding effective rehabilitation strategies to optimize functional outcomes after this transformative surgery.