Are you considering a hip injection to test for femoroacetabular impingement (FAI)? Has a hip surgeon told you he can use a hip injection to diagnose FAI or a labral tear with certainty? In this article, you'll discover the truth about hip injections for FAI (and related hip joint pathology like labral tears).
The current medical approach to the diagnosis of FAI relies on several tests. One test is anesthetic injection into your hip joint.
Hip surgeons believe that if the injection relieves pain, you have a problem inside your hip joint (like a labral tear and/or FAI). Surgeons suggest that a successful injection indicates surgery is likely to be helpful. (source).
In other words, if an injection fixes your hip pain, you need surgery to fix the problem inside your hip.
We've had many conversations with hip pain sufferers and heard and read the opinions of many surgeons.
This trust in hip injections for FAI is very strong.
But should it be? As with many parts of the femoroacetabular impingement story, there's a lot of misinformation beneath the surface.
To figure out whether a hip injection for FAI (or any other hip joint pathology) gives us accurate information about hip joint pain, we need to see what the medical literature really says.
This study (source) is one of the top 50 most-cited articles on femoroacetabular impingement. It examines the accuracy of hip injections for hip joint pathology diagnosis when compared against MRI, MRA, physical examination, and surgery.
Over two hundred forty medical papers cite this study as evidence that hip injections are accurate tools for identifying hip joint problems.
The surgeons in this study claim that hip injections are 90% accurate in diagnosing hip joint problems.
They compared the results of hip injection against their surgical findings. If a hip injection relieved pain AND surgery uncovered joint pathology (e.g. a labral tear, cartilage damage, etc.), then the injection was correct.
But there was a huge problem with the study that makes it impossible to gauge how accurate these injections really are.
Based on the study design, you cannot calculate the accuracy of hip injections with confidence. You really can't tell the usefulness of hip injections for FAI.
When you look at the full text of this article, you discover something curious. Of 40 patients, 39 had joint problems during surgical inspection.
That means 97.5% of the patients had a hip joint problem during surgical investigation.
That means that ANY "test" could have gotten a high accuracy score. Any test biased to return a positive finding would have looked "accurate" in this setup.
Think about it this way.
You have a bowl of 40 marbles. 39 are red. 1 is blue.
You take one marble out of the bowl at random and place it under a napkin so that I cannot see the color.
"Look," I say. "I have a test to determine whether the marble is red. I drop this magic quarter on table next to the napkin. If it makes a sound, it's a positive result. The marble will be red. If it is silent, it's not red."
We run the test 40 times on all 40 marbles.
My magic quarter makes a sound every single time. Because quarters hitting tables make sound every single time.
What is the accuracy of my magic quarter test?
In this case, my magic quarter was 97.5% accurate!
What if we changed the number of red and blue marbles? Let's say 37 red marbles and 3 blue marbles.
In that case, my test would be 92.5% accurate!
But would you trust my magic quarter?
Of course not. Why?
My magic quarter is only good at giving a "positive" test result. In a test sample where the correct answer is almost always positive, my test does great.
But in a test sample where there are more negative results, there's a major problem. If there were 39 blue marbles and 1 red marble, my test would fail in spectacular fashion.
As you can see, we can't really calculate the accuracy of the hip injection given the patient population. But the surgeons in this study did anyway.
37 out of 40 of the patients had a positive response to the hip injections (meaning the injections reduced their pain).
Upon surgical inspection, one of those was a false positive.
The three negative responses (meaning no relief and therefore no hip joint problems) were all false negatives.
4 incorrect answers out of 40 leads to a calculated score of 90% accuracy.
But, again, that's not an accurate interpretation.
What they demonstrated was that injections are good at giving "positive" results. In cases where it doesn't give a positive result, you still can't trust the result.
Suppose they had used a fictional belly button skin pinch test.
The skin pinch test would be simple and cheap: if pinching the skin below the belly button hurts, it's a sign of intraarticular hip joint pathology.
Running this test on the same group of 40 people, let’s assume that it hurts on all 40 people.
This belly button skin pinch test would have a higher accuracy rate than the hip injection! 97.5%! And only one false positive!
Suppose they had used a "flinch test."
If a patient flinches when you throw a fake punch at them, they have intraarticular hip pathology. All 40 patients flinch. 97.5%! And only one false positive! Same result.
This study is convincing on the face. When you read the abstract, you assume the authors did good science.
But they didn't.
And the conclusions they draw are not supported by the data.
The real accuracy of hip injections for identifying joint problems is unknown, despite what this study claims.
When you see papers on FAI like this, it should make you think carefully about what surgeons tell you.
If you're considering a hip injection to test for femoroacetabular impingement (or a labral tear) think about this first. If you're looking at a hip injection to see if surgery for femoroacetabular impingement may help you, think about the magic quarter test. And also read this article on hip injections and surgery for FAI.
And if you're interested in an depth look at the sordid history of bad science behind femoroacetabular impingement, check out this article.
For videos on femoroacetabular impingement research, check out this playlist.