Better Outcome
Distal Locking
Introduction
The surgeon needs to drill through holes in the distal (ankle) end of the tibia. It’s too far from the proximal end for a mechanical guide to help. So, the Radiographer needs to set up the C-arm exactly perpendicular, exactly on centre and at the right angle of tilt to show a view superimposing the lateral hole over the medial hole. See the video below of this on camera, in principle the appearance is the same as when screening.
Tibial Nailing
Introduction
The surgeon needs to drill through holes in the nail, at the ankle end of the tibia. It’s too far from the knee for a mechanical guide to help. So, the Radiographer needs to set up the C-arm exactly perpendicular, exactly on centre and at the correct angle of tilt to show a view superimposing the lateral hole over the medial hole. See the video below of this on camera, in principle the appearance here is the same as when screening.
Short version:
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Position tibia parallel to table edge.
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Position C-arm face parallel to table edge (So central ray is
perpendicular to leg).
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Centre just proximal to medial malleolus.
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Centre carefully, height as well.
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Do not use pan/wag. Instead track laterally.
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In case of vertical ellipse: move foot end of leg, towards you 5cm. If this makes the image worse (narrower ellipse) then instead move the leg away 5cm (from the starting position.
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In case of horizontal ellipse: angle over 5° at a time. (Note 25° angulation, goes from a full circle to a total eclipse!)
Long version:
So, distal locking shouldn't be a problem, and yet it is! Ever tried to shoot a bullet through a tiny tube not much bigger than the bullet? And this tiny target moves in such a way that it’s not even pointing your way, and the hole becomes obscured? Then, when you can see it as a circle, as you move to centre your image on it, it no longer looks round. What black magic hocus pocus is this?
Fortunately, it is all completely explicable. Here is that explanation.
The nail is straight; it runs straight down the middle of the tibia. The holes are set straight, across the nail, so the screw holes are perpendicular to the nail.
To guide screw insertion, you need to position the C-arm to look directly through the holes. The resulting image will show a ‘perfect circle’, not a vertical, horizontal or diagonal ellipse.
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Perfect circles?
In reality, because of rays being becoming further oblique away from centre you will never have two perfect circles. One can be perfect, but the other will be a couple of degrees off. In this diagram the left hole is perfectly superimposed but the right is not for this reason.
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What to do, in each case of imperfect superimposition
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Case of Vertical ellipse: The leg is skew on the table. Your C-arm is not perpendicular to the nail. Image shown on right is almost 10° from perpendicular.
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Case of horizontal ellipse: The leg is rotated (usually externally). You must rotate your C-arm over from horizontal, until you match this rotation. Image shown is about 10° rotated externally.
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Case of diagonal ellipse: The leg is both skew and rotated. Angle over 5° and reposition the leg about 7cm towards you. If this makes the ellipse narrower from side to side, reposition the leg 7cm away from you instead. Image shown is about 10° abducted and 10° rotated.
Detailed Procedure to follow
This means all you need to do, is set your C-arm perpendicular to the nail and track the whole machine sideways until the distal nail is centred on screen. There are several ways of ensuring you are at 90 degrees by measurement. But these may not be available to you. In my research, I had a very bright green laser cross line, which we used in the AP position to check that the leg was perpendicular. That's one way, if you have a laser cross.
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Assuming you haven't such a tool (I made my own!), set the C-arm as close to perpendicular to the table as you can, and the leg also perpendicular, not abducted or adducted. Centre just above the ankle and take your first image, horizontal. The hole might look like a horizontal ellipse. For each 5° of tilt of the C-arm, expect to gain around one fifth of the superimposition. So, raise your tilt 5° and take another image. It should be closer to a circle, from top to bottom. Continue 5° at a time until it is full height.
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At this point, it's a good idea to realise, that the leg should be immobilised. There is not much point, achieving perfect circles (superimposition of the holes) if the leg is allowed to rotate outwards during the process. So, the foot should be held securely somehow. Preferably not by a med student, at the wrong end of a 48-hour shift! A common favourite at my old hospital, was to balance the foot on an overturned plastic kidney bowl. A slipperier and more unstable support is hard to imagine.
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For sure, surgeons can simply rotate the leg, to help you get the perfect view. But bear in mind, they cannot readily achieve this by twisting the foot, as this will rotate an unstable fracture, or disrupt the position of a fibula plate!
What to do if the holes are not superimposed in the horizontal plane.
This is what happens if you are not, as you planned, perpendicular to the tibia/nail. In this case the surgeon can simply adduct the leg 5cm (move the foot end of the leg medially). If this improves the circle, but not perfectly, then another 5cm adduction. If it makes it worse, then abduct the leg 5cm (laterally). It shouldn't be much further than 10cm either way, otherwise your attempt at setting up both leg and machine perpendicular were quite poor.
Tips:
Once you have the perfect circle/s, a magnified view is usually appreciated.
Start with the intensifier as close as possible and mag later. Otherwise, there will not be room between X-ray tube and leg for the surgeon to drill.
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Surgeon will need to position the rear end of his driver in line with the centre of the Xray tube.
Once you are looking for perfect circles, it is not possible to tell from the image whether the leg is skew or rotated. Left and right pan-error look the same. Insufficient rotation over, looks the same as under. This is why you should make incremental corrections. 10cm movement of tube to one side = 5°, which is one fifth of a circle of correction in width. 5° of angulation over is one fifth of a circle of correction in height.
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On rare occasions, the nail may be internally rotated, such that it is not possible to angle perfectly through the holes. Your C-arm cannot angle below horizontal, unless you take the machine around to the operating side, which is not allowable! In this case, the leg and nail should be externally rotated 5° or so. To identify this problem, you will see the holes are not quite round, appearing as slightly elliptical, wider in width than height. However, when you angle over 5°, the holes will appear more elliptical (getting worse). This means that the holes are tilted below the horizontal plane and external rotation of the leg is required.
If the nail is rotated, 25° or more than your central ray, the holes will be entirely occluded! The same is possible for off perpendicular by 25° or more.
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One fifth of a hole (height or width), is approximately 5° of rotation or pan (leg abduction or adduction).
Do not move more than 5 degrees of tilt or pan at a time. This is because greater corrections can lead to confusion, where the correct angle is missed. Commonly this will appear as if the image has not changed much, if at all. For instance, if you are 5° away from spot on, and you change by 10° you will now be 5° off, the other way, which is the same shape of ellipse in appearance!
Rotation
The leg, and nail, will commonly be rotated externally. To follow this rotation, angle over the table in 5° steps, taking an image, then another 5 until the holes are superimposed (a perfect circle is seen).
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NB Sorry, I didn't specialize in Angiography!