Anatomy of the Shoulder

The shoulder is a ball-and-socket joint formed by the head of the upper arm bone (humeral head) sitting in a shallow socket on the shoulder blade (the glenoid). The joint surfaces are covered with articular cartilage. The joint is stabilised by the capsule, the glenohumeral ligaments and the labrum (the fibrocartilaginous ring around the socket).

Most of the movement is generated by the rotator cuff (supraspinatus, infraspinatus, teres minor and subscapularis) working together with the deltoid muscle.

Anatomical Total Shoulder Arthroplasty (TSA)

In TSA a metal ball replaces the humeral head and a polyethylene (high-density plastic) socket replaces the glenoid, reproducing the natural anatomy of the shoulder. Because most of the movement is generated by an intact, functioning rotator cuff, TSA requires the rotator cuff to be intact and functional in order to give a good result.

• Typical candidates: glenohumeral osteoarthritis, avascular necrosis, and advanced joint damage with an intact rotator cuff
• Advantages: preserves anatomical biomechanics, offers a greater potential range of motion, and provides long-lasting pain relief and durability
• Disadvantages: if the rotator cuff is insufficient or tears over time, the prosthesis may migrate superiorly and pain may develop

Reverse Shoulder Arthroplasty (RSA)

In RSA the anatomy is reversed: a metal ball (glenosphere) is placed on the glenoid side and a polyethylene socket on the humeral side. This mechanical change moves the centre of rotation of the shoulder medially and inferiorly, increasing the efficiency of the deltoid muscle and allowing the arm to be raised even when the rotator cuff is weak or absent.

• Typical candidates: rotator cuff arthropathy (arthritis secondary to cuff tearing), massive irreparable rotator cuff tears, failed previous shoulder replacement (revision), complex proximal humerus fractures (particularly in older patients) and inflammatory arthritis
• Advantages: excellent pain control; active elevation of the arm without a rotator cuff; high patient satisfaction
• Disadvantages: range of motion (particularly internal and external rotation) is more limited than after TSA, and there are complications specific to RSA such as scapular notching, dislocation and acromial fracture

Which Patient, Which Prosthesis?

The decision is individualised according to the patient's age, activity level, shoulder pathology, rotator cuff status, bone quality and comorbidities.

• Intact rotator cuff + glenohumeral arthritis → TSA
• Rotator cuff arthropathy or massive irreparable cuff tear → RSA
• Complex proximal humerus fracture in an older patient → RSA
• Severe glenoid bone loss → RSA is generally safer
• Shoulder arthritis in a young, active patient → TSA is preferred, although the indications for RSA have broadened in recent years
• Failed previous TSA → RSA is frequently used at revision

Symptoms and Indications

• Persistent shoulder pain: pain at rest and particularly worse at night
• Loss of movement: difficulty raising the arm, reaching overhead and combing the hair
• Stiffness: marked restriction, particularly of external rotation and abduction
• Crepitus: a grating sensation and sound with movement
• Loss of strength: inability to raise or pull with the arm
• Failure of non-operative treatment: persistent symptoms despite medication, physiotherapy and injections

Diagnosis

Diagnosis is based on a detailed history, physical examination and imaging. The patient's age, daily activities, duration of pain, previous operations and comorbidities (diabetes, rheumatological disease, smoking) are reviewed. Examination assesses active and passive range of motion, rotator cuff strength (Jobe, lift-off and external rotation strength tests), joint line tenderness and neurovascular status.

Plain radiographs show joint space narrowing, osteophytes, calcific deposits and superior migration of the humeral head; they are the first and decisive investigation. CT is required for surgical planning to assess glenoid bone loss, glenoid version and subluxation; three-dimensional CT now allows patient-specific surgical planning. MRI assesses the rotator cuff muscles, fatty infiltration and the labrum and cartilage, and provides information that is critical to the choice between TSA and RSA.

Before Deciding on Surgery

Shoulder replacement is major surgery and is not reversible. Non-operative treatments should be tried before the decision is made:

• Activity modification and avoiding lifting weights
• Analgesics / NSAIDs (under medical supervision)
• Physiotherapy: range of motion, stretching and balanced strengthening
• Intra-articular injections (corticosteroid, hyaluronic acid)
• Stopping smoking (this significantly affects surgical success)
• Weight control and blood glucose control (in patients with diabetes)

Expected Outcomes

With appropriate patient selection, shoulder replacement is a highly successful operation for relieving pain and improving function. After TSA the great majority of patients are largely free of pain and can use the shoulder comfortably in daily life; most can comb their hair, wash and reach overhead without pain.

After RSA, pain control and patient satisfaction are high; patients can usually raise the arm above shoulder height and reach the top of the head or a cupboard. Both prostheses are expected to be long lasting; modern prosthetic systems have 10–15 year survival rates of 85–95%.

Risks Common to Both Prostheses — Common (Under 5%)

• Superficial wound infection (usually treated with antibiotics)
• Postoperative pain, swelling and temporary bruising
• Temporary restriction of movement or stiffness — resolves with regular physiotherapy
• Transient symptoms related to anaesthesia such as nausea and dizziness
• Temporary altered sensation after an interscalene block

Risks Common to Both Prostheses — Less Common

• Deep infection (periprosthetic infection): 0.5–2%; frequently requires revision and prolonged antibiotics; the most common organism is Cutibacterium acnes
• Nerve injury (particularly the axillary or musculocutaneous nerve; very rare, <1%)
• Periprosthetic fracture: a fracture of the humerus or glenoid during or after surgery
• Loosening of the components: may occur over the years, particularly on the glenoid side
• Wear: wear of the polyethylene socket over the years
• Heterotopic ossification (bone formation in soft tissue)
• Deep vein thrombosis / pulmonary embolism (very rare in the upper limb)
• Serious anaesthetic complications (very rare)

Risks Specific to Anatomical Total Shoulder Arthroplasty (TSA)

• Subscapularis insufficiency: failure of the tendon released during surgery to heal, causing loss of internal rotation strength and shoulder instability
• Rotator cuff insufficiency or tearing over time: superior migration of the humeral head, with pain and loss of function
• Loosening of the glenoid component (the most common cause of failure)
• Posterior subluxation / instability (particularly if glenoid retroversion has not been adequately corrected)
• More frequent need for revision to RSA: if the rotator cuff becomes insufficient

Risks Specific to Reverse Shoulder Arthroplasty (RSA)

• Scapular notching: erosion of bone where the glenosphere contacts the scapular neck; reduced by lateralised designs
• Dislocation: 2–5%; particularly in the early period with extreme rotation and movements taking the arm behind the body
• Acromial or scapular spine fracture (stress fracture): 1–2%; related to the increased tension on the deltoid
• Restriction of external and internal rotation (reaching into a pocket, reaching behind the back); this usually resolves with physiotherapy and rehabilitation
• Baseplate loosening (in the long term, on the glenoid side)

Factors that Increase Risk

• Smoking (increases the risk of wound healing problems and infection)
• Poorly controlled diabetes
• Immunosuppressive therapy and rheumatological disease
• Previous shoulder surgery and revision cases
• Advanced age and osteoporosis; obesity
• Active or recently treated infection (dental, urinary, skin)

Contact Your Doctor Immediately If You Have

• Sudden increase in pain or swelling
• Fever above 38 °C
• Discharge, malodour or separation of the wound
• Numbness, tingling or loss of strength in the arm or hand
• Sudden deformity of the shoulder, a feeling of dislocation or excessive mobility
• Firm, one-sided swelling or redness of the leg, or shortness of breath
• Chest pain

Long-term Outcomes and Survival

Modern shoulder prostheses have 10-year survival rates of 85–95% — that is, the likelihood that the prosthesis is still functioning without needing revision at 10 years.

Because RSA has a shorter history than TSA, very long-term (15+ year) data are limited; however, 10-year survival rates are comparable with TSA. Appropriate patient selection, modern prosthetic designs, experienced surgery and compliant rehabilitation are the fundamental determinants of success.