Forces acting on shoulder during anterior dislocation

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anterior shoulder dislocation forces mechanism

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Forces Acting on the Shoulder During Anterior Dislocation

Mechanism Overview

Anterior dislocation accounts for ~96–98% of all glenohumeral dislocations. The humeral head is forced anteriorly past the glenoid rim, most commonly to a subcoracoid position (>99% of cases).

Primary Force Vectors

1. Abduction + External Rotation (Most Common)

The classic mechanism involves the arm in abduction (~90°) and external rotation — common in throwing, tackling, or falling on an outstretched arm. The forces at play:

External rotation torque winds up the anterior capsule and inferior glenohumeral ligament (IGHL), which act as a cam.
Once the IGHL is maximally stretched, continued external rotation levers the humeral head anteriorly over the glenoid rim.
The subscapularis (primary anterior dynamic stabilizer) is overcome by the external rotation force.
The humeral head translates anteroinferiorly, avulsing the anteroinferior labrum (Bankart lesion).

"This is the subcoracoid, anterior shoulder dislocation, usually occurring when an abducted, extended, and externally rotated upper extremity takes a major jolt. The resulting lever forces the proximal humerus anteriorly." — Pfenninger & Fowler's Procedures for Primary Care

2. Posterolateral Direct Force

A direct blow applied posterolaterally to the humeral head (e.g., tackling in collision sports with the arm extended):

Drives the humeral head anteriorly across the glenoid rim.
The glenoid rim acts as a fulcrum, and the posterior force vector translates to anterior displacement.
Creates a Hill-Sachs lesion: impaction fracture of the posterolateral humeral head against the anterior glenoid rim.

3. Fall on Outstretched Hand (FOOSH) — Older Patients

Ground reaction force travels up the long axis of the humerus.
With the arm forward-flexed and externally rotated, this axial load is converted into an anterior shear force at the glenohumeral joint.
More common in patients >40 years and associated with rotator cuff tears (up to 80% incidence in patients >60 years).

The Mechanism Illustrated

Shoulder dislocation mechanisms — A: volleyball player falling with arm abducted/externally rotated causing anterior dislocation; B: rugby tackle with posteriorly-directed force; C: fall causing posterior dislocation

Figure: A — Fall onto forward-flexed, externally rotated arm → anterior dislocation. B — Tackling with arm extended + posteriorly directed force → anterior dislocation. C — Fall onto forward-flexed, internally rotated arm → posterior dislocation. (Rockwood & Green's Fractures in Adults, 10th ed.)

Force Summary Table

Force Component	Direction	Effect
External rotation torque	Lateral → anterior spin	Overcomes IGHL, levers humeral head anteriorly
Abduction	Superolateral	Tightens inferior capsule; positions humeral head at glenoid rim edge
Direct posterolateral blow	Posterior → anterior	Directly translates head forward over rim
Axial load (FOOSH)	Distal → proximal	Converted to anterior shear at glenohumeral joint
Extension force	Posterior	Stretches anterior capsule, reduces anterior restraint

Structures Resisting These Forces (and Failing)

Stabilizer	Type	Failure Pattern
Inferior glenohumeral ligament complex (IGHLC)	Static	Avulsion from glenoid rim → Bankart lesion
Anterior labrum	Static	Tear/detachment (Bankart lesion)
Subscapularis	Dynamic	Overpowered by external rotation force
Anterior capsule	Static	Stretching, rupture, or HAGL lesion (humeral avulsion)
Glenoid rim bone	Static	Bony Bankart fracture (high-energy mechanisms)

Associated Injuries by Force Magnitude

Low-energy: Capsulolabral avulsion only (Bankart lesion)
Moderate: Hill-Sachs lesion (posterolateral humeral head impaction fracture)
High-energy: Bony Bankart, greater tuberosity fracture, axillary nerve injury (≤13.5%), axillary artery injury (rare), rotator cuff tear
Subclavicular/intrathoracic dislocation: Requires additional strong lateral-to-medial force vector — extremely rare

"Subclavicular and intrathoracic dislocations are extremely rare and involve the addition of strong lateral to medial forces." — Rosen's Emergency Medicine, 10th ed.

Sources:

Rockwood & Green's Fractures in Adults, 10th ed., p. 1248–1249
Rosen's Emergency Medicine, 10th ed., p. 654
Pfenninger & Fowler's Procedures for Primary Care, 3rd ed.

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