Shoulder Instability
By Dr. Albertson.B.Mukhim
Junior Resident
Department of Orthopaedics
Introduction
The shoulder , by virtue of its anatomy and biomechanics, is one of the most unstable and frequently
dislocated joints in the body, accounting for nearly 50% of all dislocations, with
a 2% incidence in the general population.
 Factors that influencethe probability of recurrent dislocations are age, return to contact or collision
sports, hyperlaxity, and the presence of a significant bony defect in the glenoid or humeral head.
 In a study of 101 acute dislocations, recurrence developed in 90% of the patients younger than 20 years
old, in 60% of patients 20 to 40 years old, and in only 10% of patients older than 40
 years old.
 Contact and collision sports increase the recurrence rate to near 100% in skeletally immature athletes.
 Burkhart and DeBeer, Sugaya et al., and Itoi et al. have shown that glenoid bone loss of more
 than 20% results in bony instability and increased recurrence rates.
 This is because the “safe arc” that the glenoid provides for humeral rotation is diminished, resulting
in instability when the deficient edge is loaded at extremes of motion.
NORMAL FUNCTIONAL ANATOMY
 An understanding of the normal functional anatomy of the shoulder is necessary to understand the
factors influencing the stability of the joint.
 The bony anatomy of the shoulder joint does not provide inherent stability.
 The glenoid fossa is a flattened, dish-like structure. Only one fourth of the large humeral head articulates
with the glenoid at any given time.
 The glenoid is deepened by 50% by the presence of the glenoid labrum.
 The labrum increases the humeral contact to 75%.
 Integral to the glenoid labrum is the insertion of the tendon of the long head of the biceps, which inserts
on the superior aspect of the joint and blends to become indistinguishable from the posterior glenoid
labrum.
 Matsen et al. suggested that the labrum may serve as a “chock block” to prevent excessive humeral
head rollback.
 The shoulder joint capsule is lax and thin and, by itself, offers little resistance or
 stability.
 Anteriorly, the capsule is reinforced by three capsular thickenings or ligaments that are intimately fused
with the labral attachment to the glenoid rim.

 The superior glenohumeral ligament attaches to the glenoid rim near the apex of the labrum
conjoined with the long head of the biceps. On the humerus, it is attached to the anterior aspect of
the anatomic neck of the humerus .

 The middle glenohumeral ligament has a wide attachment extending from the superior glenohumer
alligament along the anterior margin of the glenoid down as far as the junction of the middle and inferior
thirds of the glenoid rim. On the humerus, it also is attached to the
 anterior aspect of the anatomic neck.
 The inferior glenohumera ligament attaches to the glenoid margin from the 2- to 3-o’clock positions
anteriorly and to the 8- to 9-o’clock positionsposteriorly. The humeral attachment is below the level of
the horizontally oriented physis into the inferior aspect of the anatomic and surgical neck of the humerus.
The anterosuperior edge of this ligament usually is quite thickened.
 O'brien dicussed this ligament as a hammock type of model.
 Anterior band restraints external rotation and inferior translation in 90⁰ abduction.
 Posterior band restraints posterior translation in all positions of abduction.
 Restraints at extremes of motion and hence assist in the roll back of humeral head.
 Main stabilizer in anterior and posterior stress when shoulder is abducted 45⁰ or even more.
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The muscles around the shoulder also contribute significantlyto its stability.
The action of the deltoid (the principal extrinsic muscle) produces primarily vertical shear forces,
tending to displace the humeral head superiorly.
The intrinsic muscle forces from the rotator cuff provide compressive or stabilizing forces. Concavity
compression is produced by dynamic rotator cuff muscular stabilization of the humeral head when the
concavity of the glenoid and labral complex is intact.
 Loss of the labrum can reduce this stabilizing effect by 20%. In the concavity of the glenoid-labral
complex, synchronous eccentric deceleration, and concentric contraction of the rotator cuff and biceps
tendon are necessary for humeral stability during mid ranges of humeral motion.
 Asynchronous fatigue of the rotatorcuff from overuse or incompetent ligamentous support can
 result in further damage to the static and dynamic supports.

 MRI studies have shown fatty infiltration and thinning of the subscapularis tendon in recurrent anterior
instability.
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 Importance of synchronous mobility of the scapula and glenoid to shoulder stability and emphasized the
importance of this dynamic balance to appropriate positioning of the glenoid articular surface so that the
joint reaction force produced is a compressive rather than a shear force.
 With normal synchronous function of the scapular stabilizers, the scapula and the glenoid articular
structures are maintained in the most stable functional position.
 Strengthening rehabilitation of the scapular stabilizers (serratus anterior, trapezius, latissimus dorsi,
rhomboids, and levator scapulae) is especially important in patients who participate
 in upper extremity-dominant sports.
 Although the glenoid is small, it has the mobility to remain in the most stableposition in relation to the
humeral head with movement. Rowe compared this with a seal balancing a ball on its nose.
 The glenoid also has the ability to “recoil” when a sudden force is applied to the shoulder joint, such as in
a fall on the outstretched hand. This ability to “recoil” lessens the impact on the shoulder as the scapula
slides along the chest wall.
Scapular dyskinesis is an alteration of the normal position or motion of the scapula during coupled
scapulohumeral movements and can occur after overuse of and repeated injuries
to the shoulder joint.
A particular overuse muscle fatigue syndrome has been designated the SICK scapula: scapular
malposition, inferior medial border prominence, coracoid pain and malposition, and dyskinesis of
scapular movement.
Pathoanatomy and Developmentof Instability
 The anterior band of the inferior glenohumeral ligament (IGHL) is the primary stabilizer that limits
anterior translation in 90° of abduction. Perthes and Bankart described the detachment of this
intraarticular ligament from the anterior glenoid rim as the typical lesion found in
 recurrent anterior dislocation. The Bankart’s lesion is seen in 87–100% of initial dislocation.

• The Bony Bankart lesion (a piece of glenoid fractures along with the ligamento-labrocapsular
complex; called the glenolabral articular disruption
lesion.
• Anterior labroligamentous periosteal sleeve avulsion (ALPSA): ALPSA results from healing of
Bankart lesion to medial glenoid and is more often a feature of recurrent
instability.
• Bankart lesion associated with superior labrum anterior and posterior (SLAP) (Taylor and
Arciero): They result from a more extreme trauma and can be seen both in acute and recurrent
instability.
• Superior labrum anterior cuff (SLAC) lesion: This is superior labrum anterior cuff lesion in
which the anterior supraspinatus exhibits a partial or complete tear resulting from instability.
• Humeral avulsion of glenohumeral ligament (HAGL):Capsuloligamentous stretching and
detachment of capsule/ligament from humerus
• Massive rotator cuff tear seen by Robinson et al. is cited by him as an important cause of
recurrent dislocation of shoulder.
• Bony pathology seen with anterior dislocation shoulder: Greater tuberosity fracture, glenoid
rim fracture and Hill-Sachs lesion (posterosuperior impaction fracture
of humeral head; are found frequently.
Loss of 20–30% glenoid rim is associated with instability as is presence of large Hill-Sachs
lesions.
Biomechanics of shoulder joint
 The glenoid and its labrum do not offer a deep stabilizing socket like the acetabulum. Only 25-30% of
the humeral head is in contact with the glenoid surface at any given anatomical position.
 The glenohumeral joint does not offer isometric articular ligaments that provide stability during the
mid-arc of motion.
 The glenohumeral ligaments play important stabilizing roles in extremes of motion.
 The mechanics of glenohumeral stability is dependent on the interplay between the net joint reaction
force acting on the humeral head during movement and the shape of the glenoid cavity.
 The glenohumeral joint will not dislocate as long as the net humeral reaction force is directed within
the effective glenoid arc.
 The effective glenoid arc is the arc of the glenoid that is able to support the net humeral joint reaction
force.
 The effective glenoid arc in a particular direction is also referred to as the balance stability angle. It is the
maximal angle that the net humeral joint reaction force makes with the glenoid center line in a particular
direction.
The concept of " stability ratio" was floated to further characterize the bony stability.
 It is defined as the force necessary to dislocate the head of the humerus from the glenoid
divided by the compressive load.
 The stability ratio depends upon the concavity of the glenoid fossa when the friction
effects are assumed to be minimal. It increases with higher glenoid depth.
 The stability ratio is preferred in the laboratory being relatively easy to measure—usually a fixed
compressive load is applied, and the displacing force is progressively increased until dislocation occurs.
 Clinically, the stability ratio is judged by the load and shift test, wherein the examiner applies a
compressive load pressing the humeral head into the glenoid while noting the amount of translating
force necessary to move the humeral head from stable position.
 It gives a fair idea of the adequacy of the glenoid concavity and is one of the most practical ways to
detect deficiencies of the glenoid rim.
THE GLENOIDOGRAM
It is a better idea to understand how glenoid “works” rather than to determine how it
“looks”.
Glenoidogram is an ingenious tool to explain this. The glenoidogram is the path taken
by the center of the humeral head as it translates across the face of the glenoid in a
specified direction away from the glenoid center line.
The height of the glenoidogram reflects the amount of work (and hence the stability)
needed to dislocate the humeral head for a given compressive load .
The shape of the glenoidogram indicates the extent of the effectiveness of glenoid arc
in that direction.
The glenoidogram is oriented with respect to the “glenoid center line”, a reference line
perpendicular to the center of the glenoid fossa.
A good example of use of glenoidogram is from the poor glenoid version that can arise
from loss of part of the glenoid rim.
SCAPULOHUMERAL RHYTHM
 The angle between the glenoid and the moving humeral head has to be maintained within a safe zone of
30° of angulation during activities to decrease shear and translatory forces. So for all the complementary
movements of humerus the scapula must be actively positioned muscularly.
 This also entails stabilizing the scapula to act as a stable base during movements. Losing scapular controls
imbalances the length-tension relationships.
 The combined and synchronized movements between the scapula and the humerus during abduction is
termed “scapulohumeral rhythm”.
 The “rhythm” involves a scapular rotation during abduction, which decreases the shearing effect
between the humeral head and the glenoid and allows the glenoid to stay centered under the humeral
head.
 There is a ratio of 2:1 for humeral rotation to scapular rotation that varies with the stage of abduction
and also with loading of the arm.
Force Couple Mechanism onHumeral Head
 Rotator cuff and long head of biceps (considered the fifth tendon of rotator cuff ) generate force couples
in coronal and transverse planes. It is essential to stabilize shoulder for varied and large rotational
movements.
 There are two force couples coronal and transverse
 acting across the glenohumeral articulation.
 Deltoid and supraspinatus give abduction moment equally. So one would assume that the coronal
abduction moment is unbalanced; however, the resultant of force vectors of these both muscles are so
oriented that actually a compressive force is produced that improves the stability of joint.
 The supraspinatus force (coronal force couple) is directed medially and inferiorly that nullifies upward
force of deltoid in a moment advantage .
 The supraspinatus is responsible for approximately 50% of the torque occurring with shoulder abduction
and flexion, rest contributed by deltoid. As the weight of the arm pulls downward, the force of the
supraspinatus pulls slightly above horizontal, helping to steer the humeral head and producing abduction
of the arm.
 Initially it was thought that the only function of the supraspinatus was to help in the initiation of
abduction.
Classification Of shoulder instability
1. According to degree of dislocation/instability
a) Subluxation
b) Dislocation
2. According to number of dislocation
a) Single dislocation
b) Recurrent dislocation
3. According to duration of symptoms/dislocation
a) Acute dislocation
b) Chronic dislocation
4. According to direction of instability/dislocation
a) Anterior instability
b) Posterior instability
c) Inferior instability
d) Multi-directional instability(MDI)
5. According to etiology of instability
a) Traumatic
b) Atraumatic
6. Matsen's Classification
a) TUBS- Traumatic, Unidirectional, Bankart's Lesion, usually require Surgery.
b) AMBRII- Atraumatic, Multidirectional, Bilateral, usually require Rehabilitation,
( if don't respond to rehab then surgery involving) Inferior Capsular Shift
and Rotator Interval Closure.
7. Stanmore instability Classification system
It recognizes that there are two broad reasons why shoulders become unstable.
a) Structural changes due to major trauma such as acute dislocation or recurrent
micro trauma; and
b) Unbalanced muscle recruitment (as opposed to muscle weakness) resulting in
The humeral head being displaced upon the glenoid .
From a clinical and therapeutic point of view to you three polar types of disorder
can be identified :
Type 1- Traumatic structural instability.
Type 2- Atraumatic (or minimally traumatic) structural instability
Type 3- Atraumatic non- structural instability (muscular dyskinesia)
The Stanmore triangle depicting the three polar types of instability.
Polar types 1 and 2 usually require surgery while polar 3 usually require
rehabilitation
Thank you