Shoulder Instability: An Introduction

I love the shoulder joint, its a marvellous unique structure within the human body, it is the most mobile joint we have, no other joint can match it in the degrees of freedom it has. But, this mobility comes at a price, a reduction in its stability. The shoulder joint has to constantly maintain a balancing act between mobility and stability and sometimes this balancing act can go wrong, in both directions, with shoulders either losing their mobility and becoming too stiff and restricted, but what we are going to look at is when they go the other way and lose their stability.

Before I begin, lets talk about the terminology and classifications used when describing shoulder instability.

When a shoulder is unstable it can dislocate, this is the complete separation of the ball of the shoulder, called the head of the Humerus (HOH) from the socket, called the Glenoid. An unstable shoulder can also sublux, this is slightly different from a dislocation in that its a partial and temporary 'slip' of the head of the Humerus from the centre the Glenoid, finally an unstable shoulder can give sensations of instability without any actual physical separation of the joint, and this is termed apprehension.

Within the orthopaedic surgical world the terms TUBS and AMBRIs have been widely used, and still are, when diagnosing unstable shoulders.

TUBS stands for

  • Traumatic
  • Unilateral (one shoulder)
  • Bankart and Hill-Sachs Lesions (bony injuries that I will discuss later)
  • Surgery

AMBRI stands for

  • Atraumatic (no trauma)
  • Multidirectional (loose in various planes)
  • Bilateral (both shoulder affected)
  • Rehabilitation
  • Inferior capsular shift (if rehab fails)

However it is now recognised that these classifications are way too simplistic for the diagnosis and management of shoulder instability and they don't account for all types of instability. When these classifications are followed it has, and does often, lead to failures in surgery and management and can make matters worse for patients with unstable shoulders.

So I, and all those who I work with, use the Stanmore Triangle Classification System when describing shoulder instability, and I highly recommend you do too, this is an excellent way of diagnosing unstable shoulders and planning their management. It was designed by Mr Ian Bayley FRCS and my mentor in all things shoulders Mr Simon Lambert FRCS, two of the countries top specialist shoulder surgeons at the Royal National Orthopaedic Hospital Stanmore, Herts.

Stanmore Classification of Shoulder Instability

The Stanmore triangle uses three classifications of shoulder instability based around if a shoulder has some structural traumatic or non traumatic defects that are contributing to instability, with a third classification for the non structural but adverse muscle patterning shoulders, they are as follows

TYPE I Traumatic structural • significant trauma • often a Bankart’s defect • usually unilateral • no abnormal muscle patterning, (ie: your classic sporting injury)

TYPE II Atraumatic structural • no trauma • structural damage to the articular surfaces • capsular dysfunction • no abnormal muscle patterning • not uncommonly bilateral, (ie the classic recurrent dislocators when doing normal or sporting activities)

TYPE III Habitual non-structural (muscle patterning) • no trauma • no structural damage to the articular surfaces • capsular dysfunction • abnormal muscle patterning • often bilateral, (ie the rarer more complex shoulder with multiple factors)

Now depending on where within the triangle a shoulder lies in this classification system the management can be planned, there is also scope and flexibility for the merging of the classifications for those shoulders that don't neatly fit into one of these boxes, which in my experience most humans don't, for example it can incorporate shoulders that have had a trauma (ie Type I) but who had a pre deposition to instabilty due to the structure of their shoulder (ie Type II) so this can be called a Type I/II shoulder. Or perhaps a Type III/II shoulder which are those with muscle patterning issues, but have had over time multiple episodes of dislocations leading to structural changes over time or visa verse those with a structural defect but have developed adverse muscle patterning as a consequence, these are called Type II/III.

For more information on the Stanmore Classification of shoulder instability please read this paper and watch this excellent video via of Mr Ian Bayley himself explaining the Stanmore classification system, but do come back afterwards.

Now as shoulder instability is a massive area for one article I am breaking it down into more manageable pieces, this is just a brief introduction into the anatomy of the shoulder with some truths that aren't found in many anatomy textbooks. Each follow up post (there will be three) will look at each 'Polar Type' on the Stanmore classification system in further detail and its subsequent management.

As I mentioned at the beginning the structure of the shoulder has a balancing act to maintain between the roles of mobility and stability, first lets talk about the bony shapes and alignment of the shoulder joint and how these help or don't in shoulder stability.

The ball of the shoulder (HoH) is massive when compared to the shoulder socket (Glenoid), with only about 30% of the HoH in contact with the Glenoid at any one time, for a visual representation think of a golf ball on its tee, and then imagine the golf ball needing to stay on that tee when held horizontal!

To increase
the size and depth of the glenoid and help hold the HoH better onto it there is a ring of cartilage called the Labrum that encircles joint. This is triangular in cross session and can increase the HoH contact area by an extra 20%.

There is however vast anatomical difference in individuals shape size and positions of both the gleniod, the humeral head and the labrum. The gleniod can be a variety of shapes and sizes and can be angled in varying tilts, read here for more info. The HoH can also be varied in shape and angled differently from person to person read here for more info, and finally the labrum can be thicker or thinner in various areas, again read this if you really want to bore yourself silly. These variations in anatomy all make some shoulders more akin to patholgy and also instability than others read this for more info.

Variations in Gleniod shapes

Next is the joint capsule, the fibrous 'bag' that surrounds the whole joint, its primary role is to contain the synovial fluid which nourishes the joint surfaces, but it also helps a bit with the joints stability as it produces a negative pressure within the joint, which through hydrostatic pressure helps suck the joint surfaces together, think of when you try and get a glass slide off a wet table. Yet again the capsule has vast anatomical differences with some capsules being much more baggy or capacious than others, some thinner and some that have holes in. Are you beginning to see a pattern yet? The text books don't tell you this do they, we aren't all built the same, in fact no two or us are, variation in anatomy is just human!

Blended within the capsule are a group of thickened sections which are classed as the glenohumeral ligaments, these are called anteriorly (on the front) the superior, middle and inferior glenohumeral ligaments and posteriorly (on the back) the inferior glenohumeral ligament, but yet again there is vast anatomical differences with the ligaments with some having larger and thicker ones than others, some even have ligaments completely absent, again read this if you want more detailed info.

Finally we have the muscles and tendons of shoulder, first the rotator cuff group, made up of the supraspinatus superiorly, the subscapularis anteriorly and the infraspinatus and teres minor posteriorly, these are responsible for holding the HoH snug against the Glenoid when at rest, this is termed passive stability and helps contribute with all the other passive structures just mentioned ie the labrum, capsule and ligaments. When the other muscles of shoulder ie the Deltoid, Pecs and Latissmus Dorsi etc move the arm around, the rotator cuff work to compress and hold the HoH centred on the Glenoid, this is termed active or dynamic stability, again some vast variations can be found in the rotator cuffs strength, timing and synchronicity leading to shoulder instability issues for some, and even adverse muscle patterning with any of the other muscles that act on the shoulder also creating shoulder instability, but more on this in later posts.

So in conclusion for this brief introduction into shoulder instability we can see that there is wide variation in shoulder structural anatomy. We can see that using the Stanmore Classification of shoulder instability we can cover all types of shoulder instabilities and it gives us flexibility when co concurrent pathologies and causes of instability exist, which they often do, and helps us plan our management much more effectively.

In my following posts I will be talking about the different classes of shoulder instability as well as the treatment and management of them.

Type 1: Trauma / structural can be found here

Type 2: Non trauma / structural can be found here

Type 3: Muscle patterning / non structural can be found here

As always thanks for reading






  1. Reblogged this on sporting therapy blog and commented:
    This is the first (introductory) article, in a four part series, on shoulder joint instability by renowned, UK based, physiotherapist/lecturer/coach/trainer/author/blogger/tweeter Adam Meakins. We highly recommend reading, absorbing and utilising the information and advice he presents – an invaluable resource for all of us involved in injury rehabilitation.
    Adam is Clinical Lead Physio for Perform London and a highly respected authority on musculo-skeletal injury and rehabilitation – his passion for raising standards across the industry, together with his strong belief in scientific/evidenced based approach to injury assessment/treatment is something that we respect and champion at Sporting Therapy. If you aren’t doing so already, you can follow Adam on twitter (the Sports Physio @AdamMeakins).
    Enjoy your Monday….
    Team @sportingtherapy

Related news