Scapular Diskinesis

THE SCAPULO-THORACIC JOINT AND DISKINESIS  

The scapula is a flat blade lying along the thoracic wall.

It has 4 main functions:

  1. It provides a stable socket for the articulation of the humeral head. The Scapula moves in a coordinated way with the moving humerus to keep the centre of rotation (this is the mathematical point within the humeral head that correlates to the axis of rotation) within a physiologically tolerable range. This “safe zone” of glenohumeral activity is approximately within 30 degrees of extension or flexion from the neutral position in the scapula plane. This safe zone allows for maximum congruence due to the concavity / compression rule. Proper alignment of the glenoid allows for the most efficient position of the rotator cuff muscles to function. The coordination of scapulothoracic and glenolhumeral movement is termed Scapulohumeral Rhythm.
  2. The Scapula provides a kinematic link between the body and the arm, transferring velocity and energy proximally to distally. For example, during the tennis serve most of the energy and force needed are created by the legs, pelvis and trunk – but this force is translated to the upper limb.
  3. The Scapula provides a base for muscle attachment. These muscles can be divided into the categories:

A.Scapula Pivoters

I Serratus anterior
II Trapezius,
III Rhomboids,
IV Levator Scapulae

They are large muscles whose role is to position the scapula and therefore the glenoid optimally for the humeral head. Different parts of each muscle function independently depending on the task involved.

B.   Gleno Humeral protectors – the rotator cuff muscles. These function to stabilize the glenohumeral joint and to provide rotation (link to rotator cuff).

C.    Humeral positioners

Deltoid – this has anterior, posterior and middle fibres. The anterior fibres attach to the clavicle, the posterior attach to the spine of the scapula and the middle fibres attach to the acromium, performing abduction, flexion,extension, and rotation, depending on the poition of the glenohumeral joint.

Biceps- Mainly a flexor and humeral head depressor (also thought to have an important proprioceptive role in high level activity

Triceps- Extensor and adductor

Pectoralis Major- From a wide sterno-clavicular orogin the muscle converges on the upper humerus , as it does so, it folds on itself and forms the anterior border of the axilla or arm pit. Its main role is to adduct the arm and medially rotate the humerus.

Pectoralis Minor- This arises from the third, forth and fifth ribs and attaches to the coracoid process of the scapula. The importance of this muscle is to assist serratus anterior in the protraction of the scapula, keeping the anterior (ie glenoid) angle in apposition with the chest wall as the vertebral border is drawn forwards bt serratus anterior. The muscle is elongated when the scapula rotates in full abduction of the arm; its subsequent contraction assists gravity in restoring the scapula to the rest position.

Latissimus Dorsi-This muscle has a very long origin and a narrow insertion. The origin commences at the spine of the seventh thoracic vertebra, and extends downwards along the spinous processes and supraspinous ligaments of all the sacral and lumbar vertebrae, across from the central ridge on the posterior part of the crest of the ilium and the posterior third of the outer lip of the iliac crest. It flows over the inferior angle of the scapula, from which a few fibres arise to join the muscle.The lateral border of the muscle forms the boundary of the lumbar triangle.
The muscle passes upwards and laterally, forming thelower border of the posterior axillary fold. It inserts into the floor of the biciptal groove (approx a 2 cm insertion point). It extends the shoulder and medially rotates the humerus and working in conjunction with pectoralis is a powerful adductor .

4. The scapula allows adequate space for the clearance of the rotator cuff during elevation of the upper extremity. Retraction and depression of the scapula cause posterior tilting of the acromium increasing the sub acromial space, making room for the greater tuberosity as the arm is raised. Conversely, protraction is associated with impingement.

Bio-Mechanics and Physiology

In normal abduction, the scapula moves laterally in the first 30 – 50 degrees of glenohumeral abduction. After this the scapula rotates upwardly about a fixed axis to an arc of approximately 65 degrees as the shoulder reaches full elevation. This total movement occurs at approximately a 2:1 ratio glenohumeral/scapulo thoracic movement.

Translation of the scapula in protraction and retraction occurs around the curvature of the thoracic wall. Depending on a person’s size and vigorousness of the activity, this range can be from 15 to 18 cm. The muscles act mainly in “forcce couples”.

The muscles involved in stabilizing the scapula are the upper and lower fibres of trapezius and rhomboids coupled with the serratus anterior. The muscles causing acromial elevation are the lower fibres of the trapezius and serratus anterior muscles coupled with the upper fibres of trapezius and rhomboids.

Scapula Orientation on the chest wall 30 Degrees anterior to the sagital plane.

 

 

 

 

 

MUSCLES PROVIDING UPWARDS ROTATION OF THE SCAPULA

It is important to note that both glenohumeral and scapulo thoracic pathology will almost certainly coexist and it can be difficult to determine whether the scapula dyskinesis was instrumental in causing the gleno humeral pathology or vice versa. There will be a greater overlap with time due to the interdependency of both systems.

Orientation of the scapular on the chest wall – 3 Degrees upward rotation.