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Shoulder Instability:

Shoulder instability in a common problem seen by upper limb surgeons. The shoulder girdle is a very complex anatomical structure comprising of 30 muscles and 4 joints. In order to be able to place the hand in as large a sphere as possible, the biomechanical design of the shoulder is inherently unstable.

A finely tuned balance exists between enhanced mobility and instability. The various anatomical components i.e. bones and joints, muscles, ligaments etc, must all work together in harmony to maintain this equilibrium. Disruption to any of these parts can tip the balance and leave the shoulder joint unstable. Treatment of instability is thus complex requiring both input of surgeon and
specialist physiotherapy.

Shoulder Stabilisers
The shoulder is stabilised by both static and dynamic restraints

Static Restraints
1. Glenohumeral Capsule
2. Joint version - 30º retro version 5º superior version
3. Labrum - may deepen socket by 50%
-Increase surface area contact
-Buttress / Chock block
-Attachment of ligaments
-Plunger / Suction effect

4. Shallow ball and socket - gives a little stability
5. Negative Intracapsular Pressure
6. Wet surface cohesion
Dynamic Restraint
The Rotator Cuff functions to achieve:
1. Active compression
2. Pretensioniong of capsule
3. Proprioreception

Shoulder instability is commonly classified into:
1. The direction of instability i.e. anterior, posterior, multidirectional, and
2. The cause of instability (Stanmore Classification / Bayley’s Triangle)
(a) Structural damage
(b) Pre-existing structural weakness and therefore predisposition to injury
(c) Abnormal muscle pattern activation / imbalance

Frequently following a structural injury, a secondary abnormal muscle patterning problem can develop. A dedicated physiotherapy rehabilitation program, both pre-and post-operatively can address these muscle patterning issues and increase the likelihood of a success following surgical repair.

Mr Ronan McKeown,
The Newry Clinic, Windsor Avenue, Newry, Co.Down, BT34 1EG.

Tel: (028) 3025 7708
Email: [email protected]


MB. BCh. BAO. Dip. Sports Med. MD.
MFSEM (Sports & Exercise Medicine).
FRCSI (Trauma & Orthopaedics).

GMC No. 4128195
IMC No. 255842


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