Continuing Education Activity

This activity outlines the biomechanics, indications, contraindications, procedure steps, and complications of reverse shoulder replacement.

Objectives:

• Assess the biomechanics behind the reverse shoulder arthroplasty.

• Identify when reverse shoulder arthroplasty is a valid treatment option.

• Differentiate when reverse shoulder arthroplasty is contraindicated.

Introduction

Reverse total shoulder arthroplasty (RTSA) is increasingly gaining popularity worldwide in treating various traumatic and degenerative glenohumeral diseases and irreparable rotator cuff arthropathies (see Image. Reverse Total Shoulder Arthroplasty). The number of performed RTSA in the United States of America has increased from 22,835 in 2011 to 62,705 in 2017.[1] RTSA was first described by Beddow and Alloy in 1970. However, they did not publish their surgical outcomes.[2] In 1987, Grammont et al reported the first 8 cases of rotator cuff arthropathy treated by an RTSA prototype called the “Trompette” (Medinov).[3] RTSA was introduced in the United States of America in 1998 after being used in Europe for several years.[4] The RTSA is a semi-constrained prosthesis with components different from the anatomical total shoulder arthroplasty prosthesis. The anatomical shoulder arthroplasty consists of a concave glenoid socket and proximal humeral ball prosthesis (see Image. Anatomical Shoulder Arthroplasty). In the reverse shoulder arthroplasty, the anatomy is reversed to a glenoid ball (glenosphere) and concave proximal humerus component.[5]  

Anatomy and Physiology

Understanding the shoulder joint anatomy and biomechanics is essential for selecting the suitable prosthesis for shoulder arthroplasty and successful outcomes. The Shoulder joint complex includes 4 combined articulations: the acromioclavicular joint, the glenohumeral joint (GH), the sternoclavicular joint, and the scapulothoracic joint.

 The Glenohumeral (GH) Joint

GH joint is a synovial multiaxial ball and socket articulation between the humeral head and the bony glenoid that permits a wide range of motion.[6] Dynamic and static stabilizers achieve GH stability. Dynamic GH stabilizers include the long head of the biceps and the rotator cuff muscles. The rotor cuff muscles are subscapularis, supraspinatus, infraspinatus, and teres minor. During shoulder movements, the dynamic GH stabilizers keep the humeral head in a central position in the glenoid fossa. Static GH stabilizers include the glenoid bony configuration, glenoid labrum, shoulder joint capsule, and GH ligaments (superior, middle, and inferior GH ligaments).[7] The humeral head is retroverted about 20 degrees to the distal humerus inter epicondylar axis. The humeral neck-shaft angle is about 130-140 degrees.[8] The glenoid is titled upward about 5 degrees and retroverted about 5 degrees from the scapular body axis.

Reverse Total Arthroplasty Biomechanics

In the RTSA, the shoulder center of rotation is displaced inferiorly and medially. This modification in the center of rotation allows the deltoid muscle to work on a longer lever arm. This mechanical advantage allows more deltoid muscle fibers to act on shoulder abduction. Also, the downward humerus displacement increases the deltoid muscle tension.[9] The RTSA transforms the shear forces around the shoulder into compressive forces, creating a rotational moment that allows the deltoid muscle to start arm abduction.[4] The larger glenohumeral surface area in RTSA increases the shoulder stability and the potential range of movement. Also, the deltoid neutralization in RTSA minimizes the upward humeral translation associated with rotator cuff arthropathy, which enhances shoulder stability.[10]

Indications

RTSA has been used to manage and treat the following conditions:

• Rotator cuff arthropathy is a degenerative shoulder disease caused by a massive rotator cuff tear causing arthritic glenohumeral joint and proximal humerus migration (see Image. Advanced Glenohumeral Arthritic Changes).[11][12][13]
• Pseudo-shoulder paralysis due to an irreparable massive rotator cuff tear[14]
• Acute 3 or 4 parts proximal humerus fractures[15]
• Post-traumatic glenohumeral arthritis 
• Chronic irreducible shoulder dislocation[16]
• Revision surgery for failed anatomical total shoulder arthroplasty[17]
• Revision surgery for failed proximal humerus hemiarthroplasty[18]
• Inflammatory joint conditions, eg, rheumatoid arthritis[19] 

Contraindications

RTSA is contraindicated in patients with the following:

• Axillary nerve palsy
• Deficient deltoid muscle[20]
• Active infection
• Significant glenoid deficient bony stock
• Skeletal immaturity
• A neuromuscular disorder that may increase the risk of prosthesis dislocation

Equipment

The constrained RTSA prosthesis's initial designs were unsuccessful and had to be withdrawn because of the high failure rate.[21] In 1981, Grammont et al introduced the concept of medialization of the humeral head center of rotation to enhance the deltoid muscle lever arm. In 1991, a second-generation RTSA prosthesis was developed, and it has been used without change for about 15 years.[22] The newer prosthesis involved 2 main modifications: a large hemispherical glenoid component and increased humeral component inclination to the shaft to 155 degrees.[23] 

The large size of the glenoid hemisphere causes medialization of the center of rotation. The bigger humeral neck-shaft ankle lowers the center of rotation and the proximal humerus, which increases the deltoid tension. The common problems associated with the newer prosthesis design were scapular notching, bony impingement, and polyethylene wear.[23] Over the last 20 years, more modifications have been introduced to overcome second-generation prosthesis problems. Prothesis designs with smaller neck-shaft angles (135 or 145 degrees) have shown a lower incidence of bony impingement and scapular notching.[24] Currently, various models of RTSA are available on the market, and their designs vary.[25] The RTSA usually involves the following components:

• A cementless glenoid base plate; is seated on the prepared glenoid and fixed by multidirectional screws.
• The modular geosphere is usually half a sphere and is secured at the glenoid base plate.
• The humeral stem is inserted into the prepared humerus. Two types of stems are available: cemented and cementless press-fit stems. 
• The humeral cup is fitted on top of the humeral stem. It is designed to secure the concave insert on top of it.
• A concave polyethylene insert is fitted on top of the humeral cup. It is concave and articulates with the geosphere.

Personnel

To perform a reversed shoulder arthroplasty safely, the minimum operative team required to include:

• Anesthetist
• Anesthetist assistant
• Scrub nurse
• Operative room runner
• Shoulder surgeon
• Surgeon assistant
• Recovery nurse
• Prothesis company representative (optional): If the team is unfamiliar with the surgical instrument kit and the various prosthesis parts, sometimes the company representative's presence is required to identify them.

Preparation

The RTSA preoperative preparation includes obtaining a complete history, performing a full clinical assessment, arranging the appropriate imaging modalities, and medically optimizing the surgery.

Patient History

Detailed history, including personal, medical, and social history, is mandatory to identify all medical comorbidities, social circumstances, and the patient’s functional baseline. For example, patients who live on their own or usually use a mobility aid need extra support postoperatively.

Clinical Assessment

Full preoperative clinical assessment should be performed, including:

• The current shoulder's active and passive range of movement
• Rotator cuff examination,
• Axillary nerve function,
• Deltoid muscle function,
• Skin condition over the involved shoulder,
• The entire upper limb neurovascular status.

IMAGING MODALITIES

Shoulder X-rays

Anteroposterior, Y-scapular, and axillary views are usually obtained before RTSA. X-rays are useful for preparative templating, identifying bony lesions, and assessing bone quality.

Computed Tomography (CT) Scan

CT scan is beneficial in assessing the humerus and glenoid bony stock and glenoid version for preoperative planning. A preoperative 3-dimensional (3D) CT scan can determine the proximal humeral retroversion.[26] The 3-dimensional glenoid assessment using 3D-CT scan as a part of preoperative planning has been shown to guide surgeons in achieving more accurate glenoid component positioning.[27][28]

Magnetic Resonance Imaging (MRI) Scan

MRI could be used to assess the integrity of the rotator cuff before the surgery.

Medical Optimization

Patients with medical comorbidities, eg, diabetes, anemia, cardiac or renal diseases, require medical assessment, medication review, and optimization before the surgery.

Technique or Treatment

Anesthesia

General anesthesia associated with a regional nerve block, eg, interscalene brachial plexus block, is usually used for RTSA.[29] Prophylactic intravenous antibiotics and tranexamic acid are given during the induction.

Patient Position

For RTSA, the patient is usually positioned in a beach chair with the chest tilted to 60 degrees. The shoulder should be on the edge to allow full arm extension and adequate exposure of the humerus during the procedure. The anesthetic team should be on the other side of the operative table to give the surgeons enough access to the surgical field.

Examination Under Anesthesia

The shoulder's passive range of motion could be masked by pain, and it is challenging to determine the exact range while the patient is awake. After patient positioning, examining the shoulder for passive range movement and soft tissue tension is useful to allow proper planning for soft tissue contracture release as required.

Surgical Approach

RTSA is commonly performed through the deltopectoral approach or the anterior superior approach.[30] The deltopectoral approach allows better visualization of the lower glenoid, which is important for correct inferior sitting of the base plate. Other advantages of the deltopectoral approach include axillary nerve identification and protection, a lower incidence of axillary nerve palsy, and better access to the humerus shaft, mainly in revision procedures.[31] On the other hand, postoperative instability could be higher after the deltopectoral approach compared to the anterior superior approach.[32] The selection of the operative approach should be based on the surgeon's experience and patient suitability.

Deltopectoral Approach

The bony landmarks for the deltopectoral approach are the coracoid process, the acromion, and the proximal humerus. The inter-nervous plane for the deltopectoral approach is between the axillary nerve, which innervates the deltoid muscle, and medial and lateral pectoral nerves, which innervate the pectoralis major muscle. A 10 to 15-cm-long skin incision is required along the line between the coracoid process and the proximal humeral shaft over the deltopectoral groove. Careful dissection is required to identify and protect the cephalic vein. The cephalic vein is usually mobilized laterally toward the deltoid muscle. The conjoint tendon (the short head of the biceps and coracobrachialis) is then identified and retracted medially without much traction to protect the musculocutaneous nerve. The clavipectoral fascia should be incised lateral to the conjoint tendon to expose the subscapularis muscle. The subscapularis muscle can be divided lateral to the musculotendinous junction to reveal the shoulder joint capsule.

Anterior Superior Approach

 Bony landmarks are the acromion, acromioclavicular joint, and anterior border of the clavicle. Skin incision starts 1 cm medial to and on the anterior half of the acromioclavicular joint. The incision extends in the line of the anterior clavicle border to the point 3 cm lateral to the acromion. The anterior deltoid muscle fibers are identified and detached from the acromion. The subacromial bursa is excised, and the rotator cuff muscles are explored.[30]

PROCEDURE STEPS

Humeral Head Dislocation

Regardless of the surgical approach, the humeral head is dislocated by adducting and externally rotating the arm while pushing the elbow upward and forward.[29] Anterior and inferior osteophytes are removed from the proximal humerus to identify the level of humeral resection and the anatomical neck.

Humeral Preparation

The humeral head is resected slightly below the greater tuberosity tip. Intramedullary or extramedullary guides can guide the humeral head resection depending on the surgeon's preference. Resection guides are specific for each prosthesis type and have different resection angles. The humeral head is usually resected at about 30 degrees of retroversion. The forearm axis with the elbow flexed at 90 degrees is used as a reference axis for retroversion.[29] The humeral canal is reamed using different reamer sizes and then prepared by humeral stem broaches. A version rod could be attached to the broach handle to monitor the version during humerus preparation. Two types of humeral stems are available, whether press-fit cementless or cemented. The size of the press-fit cementless stem is equal to the size of the final humeral broach size. The cemented stem size is smaller by 2 mm compared to the final broach size, allowing space for the cement around the stem. 

Glenoid Exposure

The subscapularis muscle is released and retracted medially by a glenoid retractor to expose the glenoid. The labarum is excised from all around the glenoid for full glenoid exposure. The Diathermy tip is used to identify the anterior bony margin of the glenoid (position at 5 o’clock on the right shoulder and 7 o’clock on the left shoulder). The glenoid's bony margins are adequately exposed and examined to address any bony deficit in the glenoid before seating the base plate. Eccentric reaming and bone graft can manage glenoid bony defects under 25 degrees. For example, suppose there is bone loss from the superior glenoid. In that case, the upward tilt of the glenoid component is avoided by reaming inferiorly and superior bone graft insertion to maintain the desired inferior tilt of 10 degrees. Glenoid bony defect larger than 25 degrees requires a patient-specific graft prepared using preoperative 3D planning software. 

Glenoid Preparation 

Identifying the lower glenoid rim for proper base plate seating is important. Inferiorly placed base plate with 10 degrees inferior tilt was shown to reduce the base plate failure.[33] A central pilot wire is inserted to guide the glenoid reamer. The glenoid reamer is applied along the guidewire to prepare the glenoid surface and expose the subchondral bone for adequate seating of the base plate. Glenoid reaming could be performed at 0 or 10 degrees of inferior tilt. An inferiorly tilted position for the glenoid component increases the deltoid muscle tension due to inferior displacement of the humerus and reduces early glenoid failure. Still, it does not reduce the incidence of scapular notching.[33][34] The pilot wire is then removed from the glenoid. The base plate holder is used to apply for the glenoid plate, and it is flushed to the bone. The base is compressed to the exposed glenoid subchondral bone by the central screw. Superior and inferior screws aiming for dense bone are inserted to secure the base plate and reduce the micromotion at the plate-bone interface.

Glenosphere Trial and Offset Selection

Lateralized and eccentric glenosphere in modern prosthesis designs reduces the scapular notching and allows a better range of motion without bony impingement.[35] Depending on the prosthesis type, different geosphere sizes (32 to 42 mm) are available. A glenosphere trial determines the suitable diameter size, offset, and eccentricity direction. The suitable glenosphere should allow enough separation from the scapular pillar anteriorly, posteriorly, and inferiorly and free humeral component movement without superior impingement. The 36 mm glenosphere size is commonly used for female and 40 mm for male patients. Trials with different lateral offsets and eccentricities can be carried out to find adequate soft tissue tension. Proper soft tissue tension is necessary for prosthesis stability and deltoid muscle function after RTSA.[36]

Humeral Tray and Polyethylene Insert Trial

The humeral tray and polyethylene insert trials are mounted onto the stem trial, and the prosthesis trial is reduced and checked. Different tray and insert heights can be tried to achieve the optimal range of motion,  soft tissue tension, and stability. The deltoid muscle, conjoint tendon, and triceps muscle tension are tested to ensure proper soft tissue tension. Joint stability in all directions without impingement is checked at this stage. The proximal humerus can be resected into a lower level if excessive soft tissue tension or the prosthesis trial cannot be reduced. 

Definitive Prosthesis

Once all components are confirmed and checked, trial components are removed, and the definitive components are prepared for use. The glenosphere is implanted first while there is more space, followed by implanting the humeral stem (cemented or cementless). If a cemented stem is to be used, antibiotic-loaded cement is recommended. A cement restrictor is inserted, and the humeral canal is washed and dried before cementing. Once the stem is secured, the humeral cup is mounted onto the humeral stem, followed by the polyethylene insert on top. The prosthesis is reduced, and a final check is carried out for stability and soft tissue tension. The wound is then washed out with normal saline.

Subscapularis Muscles Repair and Wound Closure

It is still controversial whether to repair the subscapularis muscle during RTSA. Some published evidence indicates increased shoulder internal rotation after subscapularis muscle repair.[16][37] However, several studies showed no difference in shoulder instability in subscapularis muscle repair.[38][39][40] If the subscapularis muscle is of good soft tissue quality, the surgeon may decide whether to repair it.

Post-operative Care

The operated shoulder is immobilized in a board arm sling after the surgery for 2 weeks. The external rotation beyond the neutral level of the arm is avoided for the first 4 weeks. Pendulum shoulder exercises and elbow movement can start early. Post-operative X-rays are obtained to check the prosthesis components, anchoring screws, and the bone of the humerus and glenoid. Regular follow-up should be organized, starting 2 weeks after the surgery, to check the surgical wound and exclude any complications.

ADVANCES of RTSA

Stemless Humeral Component

The stemless humeral component is also known as the canal-sparing humeral component. The canal-sparing component is especially useful for patients who have a humeral deformity. The stemless humeral component preserves the bony stock of the proximal humerus if it is needed for future secondary revision procedures.[41] Humeral stemless components have been reported to reduce operative time and blood loss in anatomical total shoulder arthroplasty.[42] Midterm follow-up results for RTSA with the stemless humeral components are promising, with no reported component loosening.[41][43]

Preoperative 3D Planning Software

Digital software is a new development to allow virtual RTSA preoperative planning. This software creates a 3D virtual shoulder model from the shoulder 3D CT scan. It enables the surgeon to navigate the shoulder, identify bony loss, and plan for different component sizes, glenoid component tilt, and humeral component versions.[44] 3D preoperative planning is particularly beneficial for preparing patient-specific grafts for complex glenoid bone defects.

Complications

Prothesis Infection

The infection rate post-RTSA ranges from 1 % to 10 %.[16][45][46] Rheumatoid arthritis was found to increase the risk of postoperative infection in patients having RTSA.[19] Also, the infection rate is higher in revision RTSA compared to primary RTSA.[4] Acute infection (within 3 weeks) is manageable by debridement, antibiotics, polyethylene insert exchange, and retention of the prosthesis components. The presentation of infection after 3 weeks usually requires 1 or 2 stages of revision surgery.

Prothesis Instability and Dislocation

The reported prosthesis dislocation rate after RTSA is about 3.6%.[32] Factors that cause prosthesis instability after RTSA include inadequate soft tissue tension, mechanical impingement, deltoid dysfunction, axillary nerve palsy, deficient bony glenoid, malpositioned components, small glenosphere, and too medialised glenoid base plate.[47][48] Instability after RTSA is usually anterior, with the arm extended, adducted, and internally rotated.[49] The deltopectoral approach has been associated with a higher dislocation rate than the anterosuperior trans deltoid approach.[30]

Axillary Nerve Palsy

Transient axillary nerve neuropraxia may result from nerve traction caused by the arm position during glenoid exposure.[50] Humeral lengthening in some RTSA designs can cause brachial plexus traction and axillary nerve palsy.[4]

Scapular Notching

Mechanical impingement of the humeral prosthesis against the scapular neck may result in scapular notching.[4] There is a high incidence of scapular notching associated with the Grommet-style RTSA (51 to 96%).[51][52] The risk of inferior scapula notching could be reduced by placing the glenosphere inferiorly in the Grammot-style RTSA.

Sirveaux classified the scapular RTSA notching based on the extent of scapular bony defects in X-rays into 4 grades:

• Grade 1: scapular bony defects are confined to the pillar
• Grade 2; scapular bony defects extend to the glenoid baseplate lower screw
• Grade 3; scapular bony defects extend beyond the glenoid baseplate lower screw.
• Grade 4; loose glenoid base plate[53]

Base Plate Failure

The glenoid baseplate failure can result from increased motion at the baseplate–bone interface and inadequate bone growth into the plate. Several advancements in prosthesis design have been introduced to overcome this mode of prosthesis failure. The use of variable angle locking screws to fix the base plate has been suggested to engage the dense bone in the coracoid base scapular spine.[54] A central compressive locking screw has been introduced to compress the base plate into the glenoid and minimize plate micromotion.[55] The combination of base plate inferior tilt and locking screws has been proven to minimize the shearing forces at the baseplate-bone interface and minimize the failure rate.[56] To treat this mode of failure, a revision arthroplasty surgery is required to change the base plate. Bone grafts and locking screws are usually needed.[57]

Other Less Common Complications After RTSA

• Iatrogenic intraoperative Periprosthetic fracture
• Vascular injury
• Heterotopic ossification
• Surgical scar complications

Clinical Significance

RTSA is gaining popularity around the world to treat different shoulder pathological conditions in adult populations. More than 60,000 RTSA are performed yearly in the USA with satisfactory results. RTSA is mainly used to treat patients who suffer from deficient rotator cuff function with or without glenohumeral articulation arthritis. The alteration of the shoulder's rotation allows the deltoid muscle more mechanical advantage to mobilize the arm.

Enhancing Healthcare Team Outcomes

The healthcare practitioners managing patients with shoulder disease need to be familiar with the RTSA procedures. Understanding the indications and biomechanics for RTSA is necessary to select suitable patients for this type of treatment. Arthroplasty nurses and physicians should provide clear postoperative counseling about arm usage after the surgery. Patient education about safe arm use after RTSA reduces the risk of prosthesis dislocation. Post-operative proper pain management and physiotherapy lead rehabilitation are essential to achieve enhanced recovery and reduce the length of hospital stay. Regular follow-up consultations after the surgery with serial postoperative X-rays and clinical assessment are required to identify any postoperative complications.