All righty, so thank you for inviting me to speak about hip micro-instability. What is it and how to diagnose it? Again, my disclosures that don't relate to this study, or this presentation. So five minutes is not enough time to talk as much as we want to about hip micro-instability. I was once taught the more you know about something, the less that needs to be said. And we don't know very much here, so I could go on for quite some time. But if you want more detailed information, we've written a lot on this topic as it's evolved over the last 10 years with a pretty good review. Not a lot has changed since 2019 in our Yellow Journal article. So what is hip micro-instability? Well, we define it as extra-physiologic hip motion that causes pain with or without the symptom of hip joint unsteadiness. In all reality, rarely do individuals actually complain of instability of the hip. And for those who are shoulder surgeons, it's kind of the same thing. It's symptomatic laxity. But again, the symptom is not necessarily instability symptoms. This concept has been in evolution for a while, partly because we've shown that the femoral head moves relative to the acetabulum. When we presented this research at the ORS, most people said that I was crazy and that it didn't work like that. And as I said, while I may be crazy, the data is the data. But it took us four years to get that published because the concept was hard for a lot of people to understand when you have normal bony anatomy. But if you look at the anatomy, it is not a highly conforming fit. There is space between the femoral head and the acetabulum that does allow for the center of the femoral head to move relative to the acetabulum. Fortunately for me, there are smarter people that figured out other ways to look at this. Jacques Ménetre in Switzerland looked at ballet dancers doing the splits, as you see here, versus laying supine, and found that the femoral head subluxated on average two millimeters relative to the acetabulum in the two different positions. Josh Harris at Houston Methodist also looked at ballet dancers having them do splits radiographs. And the subluxation was on average of 1.4 millimeters, with 89 percent of their dancers having lateral subluxation, even though they were asymptomatic. Andrew Amos's lab has shown that the capsular ligaments around the hip do help control motion. Each of them is a primary restraint to hip rotation. They play an important role in femoral head motion, especially rotation, and especially the iliofemoral ligament is the main driver here. Shane Noe's group has done several studies on what happens when you cut the iliofemoral ligament, and basically it increases femoral head motion, particularly translation, as well as rotation, and range of motion and joint laxity all are affected by cutting the iliofemoral ligament. But that's looking at normal hips. So there's been a couple models that have come out with hip capsular laxity. Phil Noble in Houston did a pie cresting of the ligaments to see if you can induce capsular laxity, and we developed a model where we stretched the ligaments about the hip to produce instability. And these models have shown increased femoral head translation and rotation in the capsular laxity models. And so the concept just to kind of consider is that we know the femoral head moves relative to the acetabulum. That stresses the capsule and the labrum. With continued stresses and repeated stresses, this can lead to breakdown of the labrum and the capsule. That leads to increased reliance on muscle stabilization, and with muscle fatigue trying to maintain the femoral head within the acetabulum, you get increased femoral head motion and then more stresses on the labrum and capsule. So it's a spiral situation. The key here is how do you make the diagnosis, and I think that's still an evolution. First of all, with regard to history, they may have no injury or just a history of mild injury to the hip. They don't generally complain of hip instability. They oftentimes are diagnosed with Ehlers-Danlos syndrome, benign hypermobility syndrome, or potentially dysplasia. And the key that got me started on this was actually those individuals that have normal bony anatomy but are still having symptoms and labral tears. The things we look for, we look for bait and signs, even though it doesn't seem to correlate necessarily directly with instability versus not having instability. Those with generalized laxity, I think, are at increased risk of having micro-instability. The so-called dial test and log roll test with the patient laying supine, if you just look at the way this patient is laying, they had surgery on the right hip. They cut the iliofemoral ligament. You can see they sit in more external rotation, indicative of iliofemoral ligament insufficiency. Mark Philippon described the technique of actually rotating the foot inwards and watching it fall out and seeing the bounce back with it and how far it externally rotates. We actually just completed a study that we submitted looking at the range of motion. If you take a patient laying supine, add their flexion with their internal rotation and their external rotation, if they're female and they have an arc of greater than 200 degrees, they had a 90% likelihood of having ease of distractibility at the time of surgery and signs of micro-instability. If you're a male and had that greater than 200 degree arc, that was 75% likelihood of having instability. This is in review at this time. The tests that I like to look at for hip micro-instability, the hip abduction extension external rotation test. We abduct the patient's leg about 30 degrees. We extend the hip. We externally rotate and put an anteriorly directed force in the greater trochanter. If that causes pain anteriorly, that's consistent with hip instability. When we looked at this, it had an 81% sensitivity and an 89% specificity for the diagnosis of hip micro-instability. When we looked at the prone external rotation test described by Ben Dome, with the patient laying prone, you flex the knee, you externally rotate, put an anteriorly directed force on the greater trochanter. If it hurts anteriorly, that's consistent with instability. We found this to have a low sensitivity, only 33%, but a very high specificity at 98%. The classic test that I learned for hip micro-instability is the hyperextension external rotation test where the patient's at the end of the table. They hold on to their contralateral leg to lock in their pelvis. While the hip is hyperextended, you externally rotate. If that causes pain anteriorly, again, it was 71% sensitivity and 85% specificity for hip micro-instability. If you had two of those tests positive, you had a greater than 90% likelihood of instability. Just for completeness sake, Steve Aoki's go-to instability test, as he's described to me, is this axial distraction test. Patients supine, your knee at their issue of tuberosity, hip flexed and slightly abducted, and you pull an axial distraction force and feel the hip kind of shuck out. That to me is more of a test of the zona obicularis, but again, would be correlative with ease of distractibility. Looking radiographically, you obviously want to rule out lateral and anterior center edge angles. You want to look at the tonus angle, or roof angle, and you want to look at your fear index. Other signs, we described the so-called cliff sign. When you look at the femoral head, as it rotates out laterally, it just kind of drops off like a cliff. These are people that didn't have prior surgery. Here's another patient with bilateral cliff sign, and here on a lateral radiograph, you can actually even see a cliff sign. As regarding MRIs, John Sakian, the group at Michigan, reported having a wide anterior joint recess or thinning of the joint capsule lateral to the zona obicularis, so in this area here on an axial oblique, the recess and the capsule thickness. When we looked at this, capsule thickness of less than three millimeters in women was also consistent with micro-instability, or a sign that we saw with people that had instability. You want to look on your MR orthograms for capsular defects. Other signs, ligamentary tears, I think, also would be a clue. We recently just published a study on the criteria in the operating room for confirming the diagnosis of hip micro-instability, things that we see at the time of surgery, either before we actually make the portals, even just the ease of distractibility, and I would recommend that you look at that if you are interested in more information. So in summary, hip micro-instability is real. There's an increasing ability to diagnose it preoperatively based on examination radiographs and MRI, but more research is being done to help make the diagnosis of hip micro-instability easier. Thank you.

hip micro-instability

presentation

extra-physiologic hip motion

capsular ligaments

diagnosis