Total Shoulder Arthroplasty (Anatomic)

Definition Anatomic Total Shoulder Arthroplasty (TSA) is a surgical procedure that involves replacing the damaged humeral head with a metal sphere and the glenoid with a polyethylene dish, strictly replicating the native anatomy. The success of the procedure relies entirely on a functioning rotator cuff to compress the head into the glenoid ("concavity compression") and generate rotation.

Epidemiology

• Trends: While Anatomic TSA volume is increasing globally, Reverse TSA (rTSA) volume has grown exponentially and now surpasses aTSA in many registries (including Australia and USA). This is due to expanded indications for Reverse TSA (such as cuff tear arthropathy, fractures, and revision), as well as the desire to avoid late glenoid loosening associated with anatomic implants.
• Demographics: Typically active patients aged 60-75 years with primary osteoarthritis. Patients under 50 present a significant challenge due to concerns about implant longevity—specifically the "polyethylene problem."
• Risk Factors:
  • Primary: Osteoarthritis (genetic, age-related).
  • Secondary: Previous trauma (Post-traumatic OA), instability surgery (Capsulorraphy Arthropathy—over-tightening leads to posterior wear), Avascular Necrosis (Steroids, Alcohol, Sickle Cell), Inflammatory Arthritis (Rheumatoid), and Hemochromatosis ("Iron fist, iron shoulder").

Pathophysiology of Osteoarthritis Primary OA is characterized by a predictable sequence of joint destruction:

1. Cartilage Loss: Early loss of articular cartilage, often starting centrally or posteriorly.
2. Posterior Wear: The hallmark of shoulder OA is posterior glenoid wear (retroversion) and posterior humeral subluxation. This eccentric loading creates a "Rocking Horse" phenomenon. The humeral head acts as a fulcrum, levering the glenoid component out of the bone if not corrected.
3. Soft Tissue Contracture: The subscapularis and anterior capsule become contracted and scarred, drastically limiting external rotation. The posterior capsule stretches due to chronic subluxation but is rarely the primary problem.
4. Osteophyte Formation: Large osteophytes form on the inferior humeral head ("Goat's Beard") which can encroach on the axillary nerve space.

Normal Anatomy

• Glenoid Version: Typically retroverted 2-8° relative to the axis of the scapula body. However, the scapula itself is anteverted 30° on the thorax. The net result is a glenoid face that is slightly retroverted relative to the body axis.
• Inclination: Superior inclination is typically 0-5°.
• Critical Shoulder Angle (CSA): The angle between the glenoid inclination and the lateral acromion edge.
  • CSA under 30°: Associated with Osteoarthritis. The deltoid force vector compresses the joint, leading to cartilage wear.
  • CSA over 35°: Associated with Cuff Tears. The deltoid force vector shears superiorly, pulling the head into the supraspinatus.

Biomechanics of Concavity Compression The shoulder is inherently an unstable joint (golf ball on a tee). Stability in aTSA is dynamic, provided by the rotator cuff.

• Concavity Compression: The rotator cuff muscles pull the humeral head into the glenoid concavity, creating a stable fulcrum.
• Force Couples:
  • Coronal Plane: Deltoid (upward force) vs Supraspinatus/Infraspinatus (compressive/downward force).
  • Axial Plane: Subscapularis (anterior) vs Infraspinatus/Teres Minor (posterior).
• Implication: Any deficiency in the cuff leads to eccentric loading (edge loading) of the glenoid component, causing the "Rocking Horse" phenomenon and early loosening. This is why an intact cuff is non-negotiable for aTSA.

Biomechanical Goals of aTSA

1. Restore Version: Correct retroversion to neutral or slight retroversion (within 10°) to prevent eccentric loading. This centers the humeral head on the glenoid, distributing forces evenly across the cement interface.
2. Restore Head Height: Reproduce the relationship between tuberosities and articular surface height (typically 8mm above the Greater Tuberosity). Restoring the native Center of Rotation (COR) is vital for cuff mechanics. If the head is placed too high, the cuff is over-tensioned; if too low, the cuff is lax, leading to instability.
3. Restore Offset: Lateralization is critical for proper deltoid tensioning and the length-tension relationship of the rotator cuff. Loss of global offset leads to weakness and impingement.

Navigation and PSI In complex B2/B3 glenoids, standard instrumentation is inaccurate (often under-correcting retroversion).

• PSI (Patient Specific Instrumentation): Pre-operative CT planning creates a 3D mold that sits on the glenoid face/coracoid to guide the central pin. Evidence suggests improved accuracy of version correction compared to standard guides. The guides are typically printed in sterile nylon and allow for precise pin placement within 2-3 degrees of the plan. This is particularly useful in "B2" glenoids where the native landmarks are eroded.
• Navigation/Robotics: Real-time feedback on version and inclination. Useful for trainees and difficult morphology. Optical or accelerometer-based systems are available, though cost remains a barrier to widespread adoption in the public system.

Diagnosis of Infection Diagnosing shoulder periprosthetic joint infection (PJI) is notoriously difficult due to the low-virulence nature of C. acnes.

• Serum Markers: CRP and ESR are often NORMAL.
• Aspiration: High rate of dry tap or false negatives.
• Intra-op: Fluid/Tissue cultures are gold standard. Hold cultures for 14 days as C. acnes is slow growing.
• Sonication: Explant sonication increases sensitivity by disrupting the biofilm.

Nerve Injury Nerve injury is a feared complication.

• Axillary Nerve: 1-2% incidence. Traction injury or direct laceration during inferior release. Monitor deltoid function.
• Musculocutaneous Nerve: Retractor injury. Biceps weakness. Do not place retractors deep to conjoined tendon.
• Suprascapular Nerve: Injury during posterior release or retractor placement.

Rotator Cuff Failure Secondary cuff failure leads to superior migration and "Rocking Horse" loosening of the glenoid. This is the main reason for avoiding aTSA in patients with questionable cuff status. If the cuff fails, the mechanics of the joint are destroyed, and conversion to Reverse TSA is required.

• Pain: 90-95% achieve excellent pain relief. Anatomic TSA is the gold standard for pain relief in OA.
• Function: Range of motion typically superior to Reverse TSA (better rotation). Patients often forget they have a replacement.
• Satisfaction: High patient satisfaction scores (Subjective Shoulder Value ~90%).
• Durability: Young age (under 55) is strongest predictor of revision. Glenoid loosening remains the primary mode of failure.

Epidemiology According to the AOANJRR, the use of Anatomic TSA is declining relative to Reverse TSA, even for osteoarthritis, especially in patients over 70 years. However, Anatomic TSA remains the preferred option for younger patients to maximize range of motion.

Prosthesis Selection Most glenoids used in Australia are cemented All-Polyethylene. Metal-backed/Hybrid glenoids have a higher revision rate for loosening but are sometimes used for augmented fixation in B2/B3 glenoids. The registry continues to show superior long-term survival for cemented all-poly glenoids.

Cost and Access The prostheses are covered under the TGA and private health insurance. Public hospital access is limited by waiting lists. New technologies like Patient Specific Instrumentation (PSI) and Navigation are increasingly utilized to improve glenoid component positioning, especially in challenging B2/B3 deformities.