DYNAMIC HIP SCREW THEATRE RADIOGRAPHY
Imaged with the intensifier between the legs the objective is to reliably and repeatably produce images of the AP and lateral hip. If using a small field intensifier views of the femoral shaft below the hip will be needed for all cases.
Get the angle right
A common mistake with hip imaging in theatre radiography is to not abduct the good leg sufficiently. The surgeon will often ask the Radiographer if there is enough room for adequate imaging. If the view angle is too small (30 degrees or less) then considerable foreshortening of the femoral shaft and additional magnification will occur. So if little space is available renegotiate, then if still restricted position the C-arm as close as possible to the good leg (even if this causes some difficulty in moving to the lateral position).
When you rotate the AP image to show the femoral shaft at vertical, the rotation display should show 35 degrees or more.
Get the height right
Most orthopaedic surgeons like to work with the table height at maximum. You should find that this makes the height of the lateral hip view similar for each case so note it down. For our tables and Siemens Siremobil Compact this is 10cm. With the Philips Endura it is 16cm. If the anaesthetist tilts the table this will differ so check the clearance above the top of the traction post; for both machines four fingers or around ten centimetres is the correct height for the lateral view. Check this by placing your fist in the gap between the top of the post and the Intensifier faceplate during pre-screening.
Philips: No flip required for 'Operator Viewpoint' C-arms
Siemens: Apply both flip buttons to rotate the viewpoint to look from the foot end.
Get the cross arm right
For the Philips Intensifier the cross arm should be in the centre of its travel at the start of all cases. For the Siemens Siremobil Compact it should always be all the way back in its travel range to allow for maximum forward travel to reduce lateral magnification. (When moving to the lateral from AP view the Siemens Siremobil Compact rotates 15cms further away than other machines).
Get the centring right
For the AP view the hip will invariably be just past the traction post at a 45 degree angle. So for a 30cm intensifier the post should be just under the edge of the Intensifier heading in at 35 to 45 degrees. Now stand at the foot of the affected leg and look along the leg at the Intensifier, it should be centred in line with the middle of the leg. If not use Pan to swing left or right until it is.
Don’t use the cross arm to centre
This may seem controversial but actually can improve consistency greatly. Instead of using the cross arm to centre the AP leave it where it is and turn the track handle parallel to the femur. Move the whole machine to make adjustments proximally and distally. To adjust from side to side use the ‘Pan’ rotation lock instead. If you already do this give yourself a pat on the back!
Use your foot to check corrections
Moving the whole machine proximally and distally can result in a loss of accuracy. This is because the machine takes a good push to get moving, once it is on the move its weight means it can overshoot your desired centre. To avoid this problem stand close to the wheel and place your foot against it. Push the machine proximally with your body and observe the gap between your foot and the wheel. Get used to observing how many centimetres this is on the floor, use a ruler a couple of times if needs be to get the hang of it. Now you will easily spot if you have overshot your desired positioning and can pull the machine back again.
For movements distally first make the desired gap and place your foot in front of the wheel. Then pull the machine until it touches your foot. Don’t move your foot until the machine has stopped and then quickly apply the brake.
If you were thinking of ignoring the above advice
Don’t. Once you get the hang of it your foot will position better than your hands ever did! A note of safety here: only look at the floor when you are sure you are not running into Haygroves clamps or other instruments. If there is any danger of this watch the motion of the Intensifier then check with the floor once you have moved.
Tape the floor
Electrical tape is best for floor marking a local 99p shop does it in yellow which has the advantage of showing felt pen marks. Black electrical tape is even cheaper if you have no budget. Always fold over the end of the tape to make it easy to retrieve from the floor, it can even be re-positioned. The best way to use it is in conjunction with a simple laser cross projector. One I have used with great success is the Kamasa TD9B available for under £10 and with several advantages including the all-important AAA batteries. Models which take button cells are to be avoided due to short battery life. The laser should be attached to the machine low down on the rear corner pointing straight down at the floor. I have used Ultratape double sided foam for this but you can use adhesive magnets if you prefer. The Kamasa unit is robust but not indestructible, at the price replacement is inexpensive!
Tilt the C-arm to show the femoral head
Sometimes when space is limited it is better to tilt the C-arm towards the femoral head on the lateral view instead of panning. Undo the 'flip over' lock and tilt the C-arm slightly left or right. This will almost always be enough to show the head clearly. If contrast or quality is insufficient first increase contrast manually then collimate to half the diameter and increase dose to e.g. DR Standard for Siemens Siremobil. This may not help if your angle is badly wrong, change your angle towards forty five degrees if you can.
Know how far to move the C-arm
For a 12 inch Intensifier (this is the standard size) at the height correct for the lateral view on the AP image it is seven centimetres from image centre to any point on the outer circle of the image. Dividing this up in to three lots of two centimetres you can estimate how far to move the machine to position the head of femur towards the top of the image.
In the image above it would be better to centre more distally to show more of the femoral shaft (the centring shown here is perfect for an IMHS however). Looking at the grid you can see the number three is about right for where you need the top of the image to be, counting up to the top of the screen shows four squares which is four centimetres to move the C-arm. Place your foot four centimetres from the wheel and track the whole C-arm distally until it touches your foot. When the next image is taken the acetabulum will be shown at the top of the image as required.