HEADPIECE PLACEMENT.

Lee G. Yardley, D.C.

In correcting or reducing the misalignment factors of the ASC you have only 2 tools at your disposal:- The vector. Head piece placement.

The determination of how to best use these tools is developed as a result of a thorough understanding of the biomechanics of the case .

Today we are going to address the practical aspects of the headpiece placement component .

The headpiece controls 2 things:-1 The direction of head movement.

2 The speed of head movement.

The direction of head movement can occur in one of 3 ways:

1 Movement away from the side of contact/laterality

2 Movement towards the side of contact/laterality.

3 No movement at all.

OUR TOOL – THE MASTOID OUR TOOL – THE MASTOID SUPPORT HEADPIECESUPPORT HEADPIECE

Lets take a look at the makeup of the headpiece. It has two different ends.

The round end

A B C

The square end

These are labeled A,B,C & D headpiece positions.

D

WHERE IS THE CENTER OF GRAVITY WHERE IS THE CENTER OF GRAVITY

OF THE SKULL ( COG)OF THE SKULL ( COG)

CENTER OF GRAVITY

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The center of gravity of the skull is about ½ inch or 1 cm above the external auditory meatus in the saggital plane.

This translate to the center of the orbits of the eyes in the frontal plane.

If you want no head movement at all then you would place the COG of the skull on either the center of D or squarely on B.

If you want the head to move away from the side of laterality then you would want the COG to be above D on the round headpiece, or above B towards C on the square headpiece.

If you want the head to move towards the side of laterality then you would want the COG of the skull to be below D on the round headpiece, or B towards A on the square headpiece.

The speed of head movement can be modified two different ways:-

1 By modifying the length of the lever arm created between the COG of the head & the fulcrum established by the mastoid’s contact with the headpiece.

2 Increasing the resistance to the head moving by increasing the surface area contact between the head & the support.

So the further that we move the center of gravity above the fulcrum the faster the head will move away from us during the adjustment.

The further we move the head below the fulcrum the faster the head will move towards us during the adjustment.

This is true in a very pure sense on the D headpiece but on the square headpiece we have the additional factor of the resistance created by the surface area contacting the greater area offered by the square headpiece.

The resistance offered by the square headpiece increases the number of possible options offered dramatically.

While the head will still move in the direction based on how far above the fulcrum the COG is, it will encounter some level of resistance that slows down the head movement. This can provide time during the adjustment for other things to occur.

Whilst it is true that if you altered the lever arm shortening it by moving the COG of the head towards the fulcrum you might have the same net effect, the resistance offered by the square headpiece provides more room for error.

FINER POINTS OF ACCURATE FINER POINTS OF ACCURATE HEADPIECE PLACEMENT.HEADPIECE PLACEMENT.

1) The episternal notch & the glabella must be level with each other.

2) The chin should not drop down. The superior occlusal plate should be parallel with the bottom of the headpiece.

3) As you are adjusting the case, if you sense ( based on feedback) that it is not moving then gently lift the patients head & place it back. This can ever so slightly change the interface of the articulations & then pull through a few more times. You may well find that it begins to move again.

4) If it is still not moving the you might try moving the crank a quarter or half of a turn & then pull through a few more times. Simple things like these can make all the difference to reducing the subluxation.

If you make the distance of the COG from the fulcrum too large then you may well lock up the articulations & move nothing.

Another factor that may well lock up the articulations is too much angulation of the headpiece.

Angulation of the headpiece is defined as A angulation or B angulation:-

A angulation B angulation

Achieving the Correct Achieving the Correct AngulationAngulation

You do need to be careful not to have too great of angulation.

The rule of thumb for both A & B is the you must be able to slide your hand & touch the pinna of the ear without lifting the patients head. This will protect you against Parietal support which will negate all of your lever systems.

For A angulation you should have to push your hand deep into the upholstery covering whereas for the B angulation you should be able to reach under without pushing down.

DEALING WITH DEALING WITH ROTATIONSROTATIONS

Rotations can be very difficult to reduce.

If dealing with a large rotation rotate the head ever so slightly so as to rest the malar on the headpiece ( for posterior rotations). This will provide a resistance to stop the head rotating & allow more of the forces to effectively move the atlas.

Another idea is to take an x-ray film eraser ( still in the plastic) & use it as a wedge to create the same effect. This works better than the above for anterior rotations.

Also remember that with large lower angle in Basic Type Ones ( greater than 5 degrees) to pre stress. The shoulder should be moved down no more than an inch.

The more skilled you become, the more you appreciate the

importance of skillful headpiece placement.

& you increasingly appreciate the sheer brilliance of the mastoid

headpiece & what it can do for you