Reverse Shoulder Arthroplasty: Dislocation Risks, Symptoms, and Prevention

Can you dislocate a reverse shoulder?

Yes, while less common than in a native shoulder, a reverse total shoulder arthroplasty (RTSA) can indeed dislocate. Understanding the unique biomechanics of this prosthetic joint is crucial to comprehending its dislocation risk.

Understanding the "Reverse Shoulder"

The term "reverse shoulder" refers to a reverse total shoulder arthroplasty (RTSA), a specialized surgical procedure designed to restore function and alleviate pain in individuals with severe shoulder conditions, particularly those involving irreparable rotator cuff tears accompanied by significant arthritis (rotator cuff tear arthropathy). Unlike a conventional shoulder replacement where a ball is placed on the humerus and a socket on the scapula, the RTSA literally reverses this anatomy:

• A hemispherical socket (glenosphere) is fixed to the scapula (shoulder blade).
• A concave cup (humeral cup) is attached to the upper end of the humerus (arm bone).

This ingenious reversal changes the center of rotation of the shoulder joint, moving it medially and inferiorly. This shift significantly improves the mechanical advantage of the deltoid muscle, allowing it to compensate for a non-functional or deficient rotator cuff and elevate the arm.

The Biomechanics of a Reverse Shoulder

The primary biomechanical advantage of an RTSA lies in its ability to tension the deltoid muscle. By moving the center of rotation, the deltoid's lever arm is lengthened, enabling it to become the primary elevator of the arm. This is critical because the rotator cuff, which normally initiates and assists with shoulder elevation and rotation, is often compromised in patients requiring an RTSA.

While this design effectively restores overhead arm movement, it also alters the joint's stability. The large glenosphere and the relatively shallow humeral cup create a unique articulation. Stability relies heavily on:

• Deltoid tension: A properly tensioned deltoid provides dynamic compression across the joint.
• Soft tissue balancing: The surrounding muscles and remaining capsule play a role.
• Implant design and positioning: The size and orientation of the components are critical.

Dislocation Risk in Reverse Shoulder Arthroplasty

Despite the design's success in restoring function, dislocation remains a potential, albeit less frequent, complication compared to a native shoulder. The incidence of dislocation post-RTSA ranges from 1% to 10%, varying based on patient factors, surgical technique, and implant design.

Factors contributing to dislocation include:

• Implant Design and Positioning:
  • Component Malposition: Incorrect angle or placement of the glenosphere or humeral cup can lead to impingement or reduced stability.
  • Glenosphere Size and Lateralization: Smaller glenospheres or insufficient lateralization may offer less inherent stability.
  • Humeral Component Neck-Shaft Angle: An inappropriate angle can lead to impingement with surrounding bone or soft tissue.
• Soft Tissue Imbalance:
  • Insufficient Deltoid Tension: If the deltoid is not adequately tensioned, the joint lacks crucial dynamic stability.
  • Scar Tissue and Adhesions: Formation of excessive scar tissue can restrict movement and alter biomechanics.
  • Lack of Rotator Cuff: While the RTSA compensates, the absence of rotator cuff integrity means there are fewer secondary stabilizers.
• Patient Factors:
  • Non-compliance with Post-operative Precautions: Failing to adhere to activity restrictions is a leading cause.
  • Trauma: Falls or direct impacts can force the joint out of place.
  • Prior Failed Surgery: Revision surgeries often carry a higher dislocation risk due to altered anatomy and tissue quality.
  • Neurological Conditions: Conditions affecting muscle control (e.g., stroke) can impair stability.
• Surgical Approach: Some surgical approaches (e.g., superior approach) may be associated with a slightly higher dislocation risk than others (e.g., deltopectoral approach) due to differences in soft tissue dissection.

The most common positions that lead to dislocation of a reverse shoulder are typically those involving adduction, internal rotation, and extension of the arm. This combination can cause the humeral cup to lever off the glenosphere, particularly if the deltoid is relaxed or insufficient.

Signs and Symptoms of a Dislocated Reverse Shoulder

A dislocated reverse shoulder is a medical emergency requiring immediate attention. Signs and symptoms are usually unmistakable:

• Sudden, severe pain: Often sharp and incapacitating.
• Obvious deformity of the shoulder: The shoulder may appear "squared off" or abnormal.
• Inability to move the arm: Any attempt at movement is excruciating and futile.
• A "popping" or "clunking" sensation at the time of injury.

Management and Prevention

Immediate Action: If a dislocation is suspected, it is critical to seek emergency medical attention immediately. Do not attempt to reduce the dislocation yourself, as this can cause further damage to the implant or surrounding tissues.

Treatment:

• Closed Reduction: In some cases, a physician may be able to manipulate the arm to put the joint back into place without surgery. This is typically done under sedation.
• Open Reduction: If closed reduction is unsuccessful or contraindicated, surgical intervention may be required to reposition the components.
• Revision Surgery: For recurrent dislocations or issues related to implant malposition, revision surgery may be necessary to adjust or replace components.

Prevention: Prevention is paramount and largely relies on strict adherence to post-operative guidelines:

• Post-operative Precautions: Patients are typically given specific instructions on movements to avoid, especially during the initial healing phase. These commonly include:
  • Avoiding reaching behind the back (excessive internal rotation and adduction).
  • Avoiding reaching across the body (excessive adduction).
  • Avoiding hyperextension of the arm.
  • Limiting lifting heavy objects.
• Rehabilitation: A structured physical therapy program is crucial for strengthening the deltoid and other stabilizing muscles, improving range of motion within safe limits, and ensuring proper joint mechanics.
• Patient Education: Understanding the mechanics of the reverse shoulder and the importance of adhering to precautions empowers patients to protect their new joint.
• Surgical Technique: Optimal implant placement, appropriate soft tissue tensioning, and meticulous surgical technique by an experienced surgeon are critical for minimizing dislocation risk.

Conclusion

While the reverse total shoulder arthroplasty is a highly effective procedure for restoring function in complex shoulder conditions, it is not immune to complications. A "reverse shoulder" can indeed dislocate, although less frequently than a native shoulder. Understanding the unique biomechanics of this prosthetic joint, adhering to post-operative precautions, and engaging in a dedicated rehabilitation program are essential steps in minimizing the risk of dislocation and ensuring the long-term success of the surgery. For individuals with an RTSA, vigilance and adherence to medical advice are key to maintaining shoulder stability and function.