Recurrent Dislocation
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Now some of the patients we see have recurrent
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dislocations, and I just wanna say a few words about some
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of the things that we look at.
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There are hundreds of measurements
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for looking at the patella, looking at patella our height,
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looking at trochlear morphology,
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which we normally do about three centimeters
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above the joint line.
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And, you know, I don't really, uh, find any of them
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very worthwhile because the patella motion is
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so complicated, the patella translates
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and rotates as it's coming down.
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And most of the problems that we see are in the early phases
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of flexion and then become normalized
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as the patella gets lower down and more deep into the group.
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So in your handouts for completion, I've listed some
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of the measurements that are employed.
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These are generally done on radiography or ct,
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but they can be employed also on MR imaging.
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They have to do with the transverse patello femoral
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congruence, the vertical patello femoral congruence.
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And again, a number of different measurements are used.
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I like the two European measurements,
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either the English Blackburn Peel
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or the French Cat Amp better than INS cell body.
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Uh, but they all have their problems
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with significant overlap between normal patients
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and patients who have symptomatic patello femoral tracking,
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uh, dysfunction.
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We also will do a measurement known
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as the TTTG distance, which is the horizontal distance
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between the center of the trochlea
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and the tibial tuberosity.
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Uh, anything 15
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to 20 millimeters is considered borderline abnormal
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and more than two centimeters abnormal.
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But you have to be cautious with this
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because the measurement will change with knee positioning.
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And, uh, just sort of put in a, a model over here
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that you can see that if the knee is in varis compared
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to a neutral or valgus position,
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that your TTG distance changes quite a bit.
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Now, unfortunately, most of the coils that we use
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for MR will constrain the knee position
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to relatively straight,
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but be aware of this, uh, that, that,
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that there are problems.
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And similarly, the degree of knee flexion
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and extension can also change the measurement.
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So even though we provide this measurement, it's not
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as perfect as you might think.
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Uh, if you're interested in these measurements,
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I'll refer you to an article by Avni Chara
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and Skeletal Radiology from about 10 years ago,
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where he has a nice little, uh, diagram, a little chart
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of the different structures that should be evaluated, uh,
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in, uh, patients who have mal
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Tracking. And
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in the current thinking,
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the focus is really at the upper knee,
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at looking at the trochlear morphology,
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the S trochlear morphology,
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and, uh, the surgeons really like that, uh, TTG distance,
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uh, for that reason.
Donald Resnick, MD
Professor Emeritus, Department of Radiology
University of California, San Diego
Mini N. Pathria, MD, FRCP(C)
Division Chief, Musculoskeletal Imaging
University of California San Diego
Eric Y. Chang, MD
Adjunct Professor, Radiology
University of California, San Diego
Brady K. Huang, MD
Clinical Professor of Radiology
UC San Diego Medical Center
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