Shoulder biomechanics
Humerus
- Neck-Shaft Angle: 130 degrees, with 30 degrees retroversion from the epicondylar axis.
- Head Eccentricity: 9mm posterior to the central axis of the shaft.
Glenoid
- Tilt: 5 degrees superior tilt and 7 degrees retroverted from the plane of the scapula.
- Scapula Alignment: Scapula is anteverted by 30-40 degrees to the coronal plane, making the overall glenoid position 25-35 degrees anteverted, corresponding to humeral retroversion.
Clavicle
- Firmly fixed to the scapula by coracoclavicular ligaments.
- Circumducts around the sternoclavicular joint (SCJ).
- Disruption of the acromioclavicular (AC) and coracoclavicular (CC) ligaments can lead to scapulothoracic instability.
Shoulder Motion
- Glenohumeral Motion:
- Responsible for the first 30 degrees of abduction (ABD) and 60 degrees of forward flexion (FF).
- The rotator cuff stabilizes the humeral head, providing a fulcrum for the deltoid to act upon.
- Scapulothoracic Motion:
- As shoulder movement increases, the humeral head translates inferiorly.
- The scapula moves to maintain articulation with the glenoid, preventing impingement and maintaining deltoid length.
- GH vs SC Motion Ratio: 2:1 during most arm elevation, transitioning to 1:1 at the end range.
Glenohumeral Stability
Stability is achieved through both static and dynamic stabilizers, creating a concavity compression force.
Static Stabilisers
| Component | Function |
|---|---|
| Labrum | Provides 20% of stability, deepens glenoid concavity, attaches biceps and GH ligaments. |
| Articular Geometry | Slight mismatch in the radius of glenoid and humeral head helps stability. |
| GH Ligaments & Capsule | SGHL, MGHL, and IGHL provide key support in various movements. |
| Negative Pressure | Creates suction effect for stability, lost with capsular tears. |
| Surface Area Mismatch | The larger humeral head increases joint reaction force, compressing it into the glenoid for stability. |
Labrum: Triangular cross-section like the meniscus, deepening the glenoid by 9mm supero-inferiorly and 5mm antero-posteriorly.
GH Ligaments:
- Superior GH Ligament (SGHL): Arises from the supraglenoid tubercle and attaches to the lesser tuberosity, preventing internal rotation (IR) of the adducted arm.
- Middle GH Ligament (MGHL): Contributes to anterior stability below 90 degrees abduction.
- Inferior GH Ligament (IGHL): Primary stabilizer in abduction, forming a “hammock” around the humeral head.
Coracohumeral Ligament: Limits external rotation (ER) and internal rotation (IR), stopping humeral head translation.
Dynamic Stabilisers
| Component | Function |
|---|---|
| Rotator Cuff | Compresses the humeral head into the glenoid, providing a fulcrum for power muscles. |
| Biceps Tendon | Depresses the humeral head, stabilizing against the glenoid. |
| Scapular Rotators | Levator scapulae, rhomboids, trapezius, serratus anterior - position the scapula. |
| Deltoid | Provides vertical shear force in the adducted position, contributing to concavity compression. |
| Proprioception | Vital for stability, especially in multi-directional instability (MDI). |
Shoulder Replacement
Keys to a successful outcome in shoulder replacement include careful patient selection, restoration of anatomical alignment, and durable fixation.
Glenoid Component Fixation
| Design | Details |
|---|---|
| Flat-backed | Prone to loosening in the long term. |
| Spherical-backed | More popular due to less loosening. |
| Pegged or Keeled | Pegged designs are currently preferred. |
Types of Shoulder Replacement
| Option | Details |
|---|---|
| Hemiarthroplasty | Suitable for lower-demand patients; avoids glenoid issues, but function and pain outcomes not as good as total shoulder replacement (TSR). |
| Total Shoulder Replacement (TSR) | Full replacement of both humeral head and glenoid. |
| Resurfacing | Requires adequate bone stock, lower morbidity if successful. |
| Reverse Shoulder Arthroplasty (RSA) | Ideal for patients with cuff deficiency; utilizes the deltoid for abduction. |
Principles of Reverse Shoulder Arthroplasty (RSA)
- RSA optimizes the position of the deltoid as an abductor by adjusting the glenoid and humeral component positions:
- Humeral Shaft Lowering: Increases deltoid tension.
- Glenoid Medialization: Increases the deltoid lever arm and reduces torque on the metaglene, minimizing the risk of loosening.
- Humeral Component Orientation: Placed almost horizontally, improving stability throughout the range of motion around the metaglene.