4.1 Total Shoulder Arthroplasty (TSA): Anatomic vs. Reverse TSA Indications & Precautions

Surgical Anatomy and Indications: Anatomic vs. Reverse Total Shoulder Arthroplasty

Total shoulder arthroplasty encompasses two fundamentally distinct biomechanical reconstructions: Anatomic Total Shoulder Arthroplasty (aTSA) and Reverse Total Shoulder Arthroplasty (rTSA). The selection between these procedures is governed by the structural integrity and functional competence of the dynamic rotator cuff musculature (supraspinatus, infraspinatus, teres minor, and subscapularis) and the morphology of the glenoid vault.

               SHOULDER ARTHROPLASTY BIOMECHANICAL COMPARISON

       [Anatomic TSA (aTSA)]                      [Reverse TSA (rTSA)]
                 |                                         |
   - Intact, functional rotator cuff         - Massive/irreparable rotator cuff tear
   - Humeral head on glenoid socket          - Rotator cuff tear arthropathy (CTA)
   - Normal anatomical center of rotation    - Glenosphere on glenoid, cup on humerus
   - Requires subscapularis detachment       - Medialized & distalized center of rotation
     and 6-week repair protection            - Deltoid acts as primary abductor/elevator
   - Precautions: NO active IR, limit        - Precautions: NO combined extension,
     passive ER to 30°-40°                     adduction, and internal rotation

Anatomic Total Shoulder Arthroplasty (aTSA)

Anatomic TSA replaces diseased articular surfaces with a prosthetic humeral head and a polyethylene glenoid component, replicating normal native glenohumeral anatomy.

• Primary Indications: Primary glenohumeral osteoarthritis (OA), avascular necrosis (osteonecrosis) of the humeral head, and post-traumatic arthritis with an intact, functioning rotator cuff.
• Surgical Interval: The procedure is executed almost universally via the deltopectoral approach, exploiting the internervous plane between the deltoid (axillary nerve) and pectoralis major (medial and lateral pectoral nerves). The cephalic vein is identified in the deltopectoral groove and typically retracted laterally with the deltoid to preserve its tributary drainage.
• Subscapularis Management: To gain entry to the glenohumeral joint, the anterior dynamic stabilizer—the subscapularis tendon—must be mobilized. This is accomplished via a subscapularis tenotomy (incising the tendon medial to the lesser tuberosity), a subscapularis peel (elevating the tendon directly off the bone), or a lesser tuberosity osteotomy (LTO) (taking a bone wafer for bone-to-bone healing). The divided subscapularis is repaired with heavy non-absorbable sutures at closure.
• Healing Biology & Biomechanical Constraints: Tendon-to-bone or tendon-to-tendon healing requires 6 to 12 weeks for collagen cross-linking and tensile strength maturation. Disruption of this repair results in anterior prosthetic instability, chronic weakness, and catastrophic failure.

Reverse Total Shoulder Arthroplasty (rTSA)

In 1985, French surgeon Paul Grammont revolutionized shoulder reconstruction by reversing the normal ball-and-socket geometry. A hemispherical prosthetic ball (glenosphere) is secured to the scapular glenoid baseplate (metaglene), and a concave polyethylene socket (humerosocket) is mounted on the humeral stem.

• Primary Indications:
  1. Rotator Cuff Tear Arthropathy (CTA): End-stage degenerative arthritis characterized by massive, irreparable rotator cuff tears, superior migration of the humeral head (acetabularization of the acromion and femoralization of the humeral head), and pseudoparalysis of elevation.
  2. Massive Irreparable Rotator Cuff Tears: Patients with profound functional deficits who have failed conservative and arthroscopic interventions.
  3. Complex Proximal Humerus Fractures: Comminuted 3-part or 4-part fractures in elderly, osteoporotic patients where tuberosity healing around a hemiarthroplasty is unpredictable.
  4. Revision Shoulder Arthroplasty: Conversion of failed aTSA or hemiarthroplasty complicated by secondary rotator cuff insufficiency.
  5. Tumor Resections: Reconstruction following proximal humerus oncologic resections.

Comparison of Clinical Indications and Structural Prerequisites

Biomechanics of Reverse TSA: Grammont Principles and Scapular Notching

To restore functional elevation in a shoulder devoid of rotator cuff dynamic stabilization, the Reverse TSA relies on two fundamental biomechanical alterations described by Paul Grammont:

1. Medialization of the Center of Rotation: In native anatomy, the center of rotation (COR) lies within the spherical humeral head. In rTSA, placing the glenosphere directly on the glenoid fossa shifts the COR medially and onto the bone-glenosphere interface. This medial shift dramatically reduces the shear forces (torque) experienced at the glenoid fixation baseplate, minimizing early aseptic loosening.
2. Distalization (Lowering) of the Humerus: Seating the humerosocket inferior to the COR distalizes the humeral shaft relative to the acromion. This tensions the anterior and middle deltoid fibers and lengthens the deltoid moment arm (lever arm). As a result, when the deltoid contracts, its vector is converted from an upward shearing force into a powerful rotary torque that elevates the arm.

                      GRAMMONT BIOMECHANICAL ALTERATIONS

            Native Shoulder Anatomy               Reverse TSA (Grammont)
            -----------------------               ----------------------
              [ Acromion ]                              [ Acromion ]
                   |                                         |  (Deltoid pretensioned)
             (Rotator Cuff)                                  v
                   |                                 [ Glenosphere (Medial COR) ]
           (Humeral Head COR)                                |
                   |                                 [ Humerosocket (Distal) ]
              [ Humerus ]                                    |
                                                        [ Humerus ]

    * Loss of rotator cuff in native anatomy causes humeral head to migrate upward.
    * rTSA medializes and distalizes the COR, allowing the deltoid to elevate the arm.

Scapular Notching and Sirveaux Classification

A recognized mechanical phenomenon unique to rTSA is scapular notching, caused by repetitive mechanical contact of the medial polyethylene cup rim against the inferior pillar of the scapular neck during adduction and internal rotation.

• Sirveaux Classification of Scapular Notching:
  • Grade 1: Defect limited to the inferior border of the scapular pillar.
  • Grade 2: Bone defect extending to the inferior baseplate fixation screw.
  • Grade 3: Bone defect extending beyond the inferior fixation screw.
  • Grade 4: Severe bone defect extending underneath the central post/baseplate, risking catastrophic component loosening.
• Surgical Prevention: Utilizing eccentric (inferiorly offset) glenospheres, lateralized glenoid baseplates, inferior tilt of the glenosphere (10°), or bone graft augmentation behind the metaglene (BIO-RSA) reduces notching incidence.

Instability Vectors and Positional Precautions: aTSA vs. rTSA

Postoperative movement restrictions differ fundamentally between aTSA and rTSA based on the direction of mechanical vulnerability:

• Anatomic TSA Instability Mechanics: Anterior dislocation occurs primarily in combined external rotation, extension, and abduction (stressing the healing subscapularis and anterior capsule).
• Reverse TSA Instability Mechanics: The rTSA prosthesis is remarkably stable in forward flexion and external rotation. However, it is most vulnerable to posterior-inferior dislocation when placed in combined shoulder extension, adduction, and internal rotation (e.g., reaching behind the back to tuck in a shirt, placing a wallet into a rear pocket, or pushing up from a seated position with the arm behind the torso). In this position, the medial humeral cup levers against the inferior scapular pillar, popping the humerosocket off the glenosphere.

Postoperative Nursing Management, Rehabilitation Protocols & Complications Surveillance

Evidence-based orthopaedic nursing care following shoulder arthroplasty requires vigilant neurovascular assessment, precise immobilization positioning, pain control, and early recognition of mechanical or infectious complications.

Sling Immobilization and Positioning Protocols

• Sling Wear Duration: Patients are maintained in an abduction sling (often featuring an anterior foam abduction bolster or pillow) for 4 to 6 weeks postoperatively. The sling must be worn continuously, including during sleep, except for hygiene and prescribed home exercises.
• Supportive Bed Positioning: When supine, a folded towel or pillow must be placed behind the operative elbow to prevent shoulder hyperextension (which places anterior tension on the healing capsule and subscapularis repair).
• Early Distal Motion: Active range of motion of the operative hand, wrist, and elbow, along with gentle pendulum (Codman) exercises, is initiated on postoperative day 1 to prevent dependent edema and joint stiffness.

Comprehensive Neurovascular Surveillance

• Axillary Nerve (C5–C6): The axillary nerve wraps tightly around the surgical neck of the humerus and travels adjacent to the inferior glenoid capsule. During rTSA, distalization of the humerus and aggressive soft-tissue retraction can produce traction neurapraxia. Nurses assess axillary nerve integrity by:
  1. Motor: Testing isometric contraction of the anterior/middle deltoid (without moving the shoulder joint).
  2. Sensory: Evaluating light touch sensation over the lateral aspect of the deltoid muscle (the classic "regimental badge" area).
• Musculocutaneous Nerve (C5–C7): Vulnerable to retractor pressure over the coracobrachialis/short head of biceps. Assessed via sensation over the lateral forearm and biceps muscle tone.
• Radial, Median, and Ulnar Nerves: Complete distal sensory and motor assessment (wrist/finger extension, finger opposition, interosseous abduction) to rule out brachial plexus stretch injuries.

Specific Complications in Shoulder Arthroplasty

1. Acromial and Scapular Spine Stress Fractures: A complication unique to rTSA, occurring in 1% to 4% of patients. Distalization of the humerus places continuous, unaccustomed tensile strain on the deltoid origin at the acromion and scapular spine. In osteopenic or osteoporotic bone, this leads to insufficiency stress fractures, presenting as sudden localized pain and abrupt loss of active elevation weeks or months postoperatively.
2. Indolent Periprosthetic Joint Infection (Cutibacterium acnes): Cutibacterium acnes (formerly Propionibacterium acnes) is an anaerobic, slow-growing Gram-positive bacillus that resides deep within the sebaceous glands and hair follicles of the axilla and shoulder girdle. Unlike acute staphylococcal infections that cause high fevers and gross erythema, C. acnes infections present indolently as vague, persistent, deep aching pain and progressive stiffness or loosening without systemic fever or markedly elevated ESR/CRP. Diagnosis requires holding anaerobic synovial fluid and tissue cultures for a minimum of 14 days.