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.

The word "anatomic" is the operative principle of aTSA: the reconstruction must reproduce the native centre of rotation (COR), head height, radius of curvature and global offset. The topic repeatedly warns against "over-stuffing" the joint, but the concept deserves its own treatment because it is one of the most common technical errors and a frequent cause of a stiff, painful, or failed anatomic replacement.

What overstuffing means Overstuffing is an increase in the effective soft-tissue tension and joint volume beyond native anatomy. It occurs when the composite humeral construct is too big or sits too proud, most often because:

• The humeral head component is too thick or too large in diameter (over-sized), over-tensioning the cuff and capsule.
• Peripheral (inferior "goat's beard") osteophytes are NOT removed before templating, so the anatomic neck is misjudged and an oversized head is selected.
• The neck cut is too low/conservative or the head is seated too proud, lateralising and elevating the COR.
• Global (medial-to-lateral) offset is over-restored, lengthening the deltoid and cuff moment arms excessively.

Consequences

• Loss of motion, particularly external rotation and elevation, because an over-tensioned subscapularis and posterior cuff cannot excurse.
• Increased subscapularis repair tension, raising the risk of early repair failure and anterior escape.
• Eccentric glenoid rim loading and edge loading, accelerating polyethylene wear and the rocking-horse mechanism that drives glenoid loosening.
• Persistent pain and stiffness, and a shoulder that never feels "natural" despite a technically seated implant.

Conversely, under-stuffing (a head that is too small or a neck cut too high) leaves the cuff lax and the joint unstable. The goal is a balanced, tension-neutral reconstruction.

How to avoid it

• Remove osteophytes first, then define the true anatomic neck and template head diameter and thickness to native geometry.
• Match native head size (coverage and radius of curvature); resist the temptation to up-size for "stability".
• Restore, not exceed, offset and head height so the COR is reproduced rather than lateralised.
• Check passive external rotation on the table — the reconstructed shoulder should reach roughly neutral to greater than 30 degrees of ER with the repaired subscapularis under acceptable tension; a shoulder that will not externally rotate past neutral suggests overstuffing.
• Assess the "shuck" / translation — an appropriately balanced head allows about 50 percent posterior translation that reduces spontaneously, indicating neither over- nor under-stuffing.

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.

Because aseptic glenoid loosening is the leading long-term failure mode of anatomic TSA, an examiner will expect you to interpret peri-glenoid radiolucent lines and to grade them. The topic's viva on the asymptomatic 2 mm peg lucency invokes the "Lazarus lines" scale but never defines it — this section closes that loop.

The scale Franklin originally described a system for radiolucency around keeled glenoid components; Lazarus and colleagues modified it for pegged components (J Bone Joint Surg Am, 2002). Both use an ordinal 0-to-5 scale in which higher grades indicate a greater extent and completeness of lucency around the pegs (or keel):

• Grade 0 — no radiolucency.
• Grades 1-2 — incomplete lucency around one or more pegs (thin, partial).
• Grades 3-4 — complete lucency around one, then progressively all, of the pegs/keel.
• Grade 5 — gross loosening: component tilt, shift, subsidence or migration.

How to use it

• Radiolucent lines are extremely common and often benign. In the Lazarus series, only 20 of 328 glenoids showed NO radiolucency on the immediate postoperative film — so an isolated thin, incomplete line is expected, not alarming.
• Progression matters more than a single film. Serial radiographs (matched AP and axillary) looking for widening lucency, new complete peri-peg lucency, or a change in component position are what distinguish stable fixation from evolving loosening.
• Correlate with symptoms. A low-grade, non-progressive lucency in a pain-free patient is surveillance, not surgery ("treat the patient, not the X-ray"); a rising grade, component shift (grade 5), or new start-up/rest pain triggers an infection work-up (including Cutibacterium acnes) and CT assessment of bone stock before considering revision — typically to a reverse prosthesis.

• 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.

Global Epidemiology Primary glenohumeral OA is the dominant indication for anatomic TSA worldwide, typically in patients aged 60-75. Across large national registries (AOANJRR, UK NJR, US AJRR), reverse TSA volume has grown sharply and now exceeds anatomic TSA for many indications, driven by expanded reverse indications and concern over late glenoid loosening. Anatomic TSA remains preferred in younger, higher-demand patients with an intact cuff to maximise rotation.

Society Guidance (Side by Side)

Registry Evidence Registries consistently show cemented all-polyethylene glenoids out-survive metal-backed designs, and that younger age is the strongest predictor of revision (aseptic glenoid loosening). Augmented all-poly components are increasingly used for B2/B3 bone loss.

High- vs Limited-Resource Practice In high-resource settings, preoperative CT, 3D planning, patient-specific instrumentation (PSI), and navigation are used to improve glenoid version correction, especially in B2/B3 deformity. In limited-resource settings, plain radiographs with an axillary view guide planning, standard instrumentation and cemented all-poly glenoids predominate, and hemiarthroplasty or non-operative care may be selected where implant cost or revision capacity is constrained.