What is the “zero position?”
Remember, the dislocated shoulder WANTS to go back in! It just needs to reach a position where the humeral head is ready to slide back into the glenoid fossa – i.e. to relocate. In abduction this position is known as the “zero position”.
Technically it is the critical angle between the glenoid fossa and the humeral head at point of relocation in abduction and external rotation. In this position all the humero-scapular muscle group axes line up and lose all rotatory and transverse pull. Additionally, the articular surfaces of the humeral head and the glenoid are opposed. This allows the humeral head to slide back into place.
The importance of this relationship was first documented by Henry Milch in 1938 when he separated the muscles traversing the glenohumeral joint into 3 “cone groups.” The ‘zero position’ term and a more detailed analysis of the anatomy was made by Saha in 1983.
As the scapula has a large range of movement around the chest wall, and the key relationship is between the humeral head and the glenoid fossa of the scapula, the effective “zero position” in relation to your patient’s torso can be variable – you need to know where the scapula is before you start.
How do I know when the patient is in this position?
This is when the scapula is at full rotation and anteversion, as seen below.
In this position the humerus is 165° overhead and 45° in front. Achieving “zero position” in a patient with a dislocation is not always easy.
Fixing the Scapula
When attempting achieve “zero position” by abduction, the scapula position can be controlled. “Fixing the scapula” (by preventing rotation and anteversion – seen in red here) limits the scapular movement that normally occurs with glenohumeral movement during abduction past 30°.
Here we see the arm being abducted, whilst the operator “fixes” the scapula with his left hand. This prevents the rotation, and the “zero position” is reached more easily at about 100° of abduction.
If your patient can “fix” their own scapula then all the better. Explain to your patient that you want them to “put your chest out and your shoulders back. Try and bring your shoulder blades in towards your spine.”
Fixing the scapula means that you move the affected limb less distance towards the point at which relocation will occur.