Why do we angle the tube on a knee?

MissE · Moderator Joined: 29 Jun 2006 Posts: 156 Location: Ohio

What is the reasoning for angling the tube on a knee radiograph?

Adriana · Joined: 24 Aug 2004 Posts: 10 Location: San Antonio, Texas

On a lateral projection of the knee the tube is angled 5-10 degrees cephalic. This is done to superimpose the distal borders of the femoral condyles.
On an AP projection of the knee the tube is angled 3-5 degrees cephalic. This is done to open up the joint space.
Am I close? 1st year student here. Very Happy

xrayquilter · Power User Joined: 04 Jul 2006 Posts: 120 Location: Ohio

I'm a 1st year student, also Smile

After looking in my book, I think you hit the nail on the head with your answer! Without looking it up, I would have just said that it's done to open up the joint space!
My book says to angle 7-10* on a short patient with wide pelvis, but only angle 5* on a tall (male) patient with a narrow pelvis~~for the lateral.

For the AP, the degree of angulation depends on the size of the patient. For a patient who measures less than 19 cm. (from ASIS to tabletop), you angle 3-5* caudad. This is a person with thin thighs & buttocks.
For the patient who measures 19-24 cm. (ASIS to TT), you don't angle at all.
And for the patient who measures more than 24 cm. (ASIS to TT), you angle 3-5*cephalad. This is a person who has thick thighs & buttocks. This is the most commonly used.

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xrayquilter · Power User Joined: 04 Jul 2006 Posts: 120 Location: Ohio

Is it because the condyles aren't on the same level straight across? I may be way off base here, but isn't the lateral condyle higher than the medial? Confused

RadMaster · Site Admin Joined: 09 Aug 2004 Posts: 180

MissE · Moderator Joined: 29 Jun 2006 Posts: 156 Location: Ohio

Yes

That was the answer I was looking for. Great job everyone!!