My case presentation today is Ultrasound of the Posterior Knee. I do not have any financial disclosures. I do need to disclose that I'm passionate about musculoskeletal ultrasound, and I do this every day and all day in my practice at work at Duke Medical Center. As part of this case series that we have put together as part of the AMSSM virtual case or ultrasound cases, I'm going to keep with the format. This will begin with a brief case presentation. We've been asked to do a common and a simple case presentation, not focus on the obscure cases. Then go through the ultrasound protocol for the posterior knee, and then review a pathology, and then discuss the ultrasound report. My case today is a 49-year-old has-been elite runner that ran back in the day in college. He has had intermittent knee swelling for the last three months, and also associated posterior medial knee mass. There is no trauma. It started with running a turkey trot, 5K, pushing a stroller with his son in the stroller. And then the swelling would go down a few days later, but it would occasionally just return without any incident. Pretty soon it came more frequently and occurred even in clinic when this former elite runner was doing musculoskeletal ultrasound procedures, bending and twisting the knee, and the knee would swell up. This, of course, is an example from me. My posterior knee, I put the ultrasound on and looked in my knee, and there was a knee effusion in the front. But on posterior knee exam, I saw this anechoic area of fluid between the medial head of the gastroc and semimembranosus, and you can see that there's a stalk that comes out into part of this cyst. And here's a video showing that the cyst wall is a little bit thickened, and there is septate. So this looks like it's been there for a little while. So here is a posterior medial knee mass commonly seen in patients. So as we study the posterior knee, it's important to not only review this video, but there's also the AMS system videos put together by Minderik Hall and John Finoth that are great to review. There's the European Society of Musculoskeletal Radiology Guidelines and the AMSSM AIUM checklist that follows our fellowship curriculum as part of the AMSSM. It's expected that you should be able to identify a Baker's cyst, also look at popliteal vessels. I'm going to go over some more difficult structures to identify. The peroneal nerve and the sural nerve are more of the all-star or advanced structures, but we'll go through those in a systematic approach today as well. So here is the recommended ultrasound curriculum from the AMSSM. It uses the AIUM posterior knee checklist. So we will first review the popliteal fossa, then popliteal artery and vein, semimembranosus tendon and muscle, the medial and lateral gastric muscles, the sciatic, tibial and common fibular nerves, the posterior horns of both menisci and tibiofemoral joint, and then the PCL that is sometimes indicated and can be seen in the sagittal oblique plane. So first of all, let's look at the popliteal fossa. This is bound by the semimembranosus, semitendinosus, as well as the biceps femoris and the medial and lateral gastric. You can see as we look through this that there's arteries and veins and then the nerves as we go along here in the popliteal fossa. As we look at the popliteal fossa, you'll see that there's pearls and pitfalls. Patient is usually in prone position, knee extended 5 to 10 degrees of flexion. So the high frequency linear probe, you can see the superficial tendons and nerves. And then the low frequency probe is used for body habitus that are fairly larger. And then we'll look for the deeper structures, which we can see from evaluating the PCL, as well as ganglion associated with meniscal tears. It's important as we review the posterior knee that you do not forget your physical exam. The way I was taught was that hip rot was the mnemonic, history, inspection, palpation, or in range of motion and other tests. So part of evaluating the popliteal fossa is to inspect or look for masses or varicose veins, which could also be seen. Here's an example of the popliteal artery in the vein. As you go through, you can see some valves of the popliteal artery. And as we look through this popliteal fossa and then look at the vessels, there's some pearls and pitfalls that we need to discuss. The popliteal artery is the deepest, it's lateral to the medial head of the gastro, and the popliteal vein is more superficial and often lateral to the artery. It collapses when it's prone and slight knee flexion increases distension or patient standing compression to evaluate for thrombosis. So the tibial nerve is the most superficial and lateral structure or nerve seen. So let's look at, here's the popliteal artery, here's the vein, you can see it collapse with sonopalpation or the transducer putting some pressure down, collapsing the vein. And you can see also when the knee is flexed up, this will dilate. You can see also the nerves here, and we'll get to that in just a little bit. So evaluating, part of evaluating the semimembranosus tendon and muscle is to look for where the muscle becomes a tendon and then attaches in that posterior tibia. So here's medial head of the gastroc was over here, looking at the, and now here I switch to a short axis view, I'll go back to a long axis view. So muscle tendon attachment, it's important, there could be some anisotropy right there. You would think that there would maybe a tear or some issues associated with that, but that is anisotropy. You notice on this one, there was a little bit of a small baker's cyst associated with that. And when you're looking in the posterior knee, you can see there's some articular cartilage. You could evaluate, this is the articular cartilage of the medial femoral condyle. Even though we're looking at semimembranosus muscle, you could evaluate and see other structures as well. So part of this semimembranosus, just as any tendon, there could be tears or tendinopathy, sometimes a diagnostic injection, because it's hard to find out exactly what is the cause of the associated pain, putting a little lidocaine there and if that resolves the symptoms, that could be helpful. I had a similar situation with an elite diver that every time she extended her legs out on the entrance into the water, she had pain right in the posterior medial knee that wasn't associated with baker's cyst or any other issues and we did a diagnostic lidocaine challenge test. So semimembranosus tendon and muscle can be identified saggily, just deep to the semitendinosus and proximal to the knee joint. So just as you're scanning from proximal to distal, you'll see the semimembranosus and semitendinosus together. It lies medial to the medial head of the gastroc in the posterior knee and it inserts in the posterior medial tibial epiphysis. That's the direct tendon and the medial tibial epiphysis indirect tendon. You can see here where the muscle tendons insert there. It also blends with the posterior capsule. It's prone to anisotropy and don't confuse this for a cyst or a tendinosis or a tear. So if we look at this medial head of the gastroc, you can see where the muscle tendon attachment there and you'll look at these gastroc muscles and insertion of the distal femur. So medial head of the gastroc anemias, saggily look at it. It's just lateral to the semimembranosus tendon and at the posterior knee joint, it's part of the Baker's cyst evaluation and long axis scan from the origin at the medial femur condyle to the distal myotendinous junction. The lateral gastroc is evaluated saggily. It's medial to the biceps femoris tendon at posterior knee joint and a long axis scan from the origin at the lateral femur condyle to the distal myotendinous junction. And so it's important to always be able to find where the medial head of the gastroc and the semimembranosus cross together and that's where you'll find a Baker's cyst or enlargement of the gastroc anemias, semimembranosus bursa. It's an anechoic, hypochoic fluid. As you can see here, there's reverberation of the needle when it's aspirated and you can look for this stalk of the associated Baker's cyst. So let's look at the nerves as we follow them through the posterior knee. If you look at, here's the short and then now long axis fuse, it's usually better to be seen, find a nerve in the short axis fuse. So here's the sciatic, tibial split off and here's the perineal nerve. So tibial, perineal, then that's lateral. So you can see here's the fibula as we go follow this distal. So the perineal nerve just wraps around the fibular head there. So let's just take a look at the transducer as you scan from proximal to distal. Just following this on this netter diagram, so you have the sciatic nerve, then the perineal nerve will split off and it continues with the tibial nerve. You have a medial serocontinuous nerve will split off of the tibial nerve. And as you scan proximally to distal, you'll see that there's another small nerve that splits off and that's the lateral serocontinuous nerve. So when you're scanned on the skin, this is, you're going proximal, distal pretty much staying midline on the leg. But if you're looking for your perineal nerves, you're going to start midline and you're going to wrap around that fibular head. So watch on this where you'll scan, take your little time. As soon as you hit the fibular head, you're going to wrap around more towards and to your knee. And then here is the posterior knee looking at the posterior horn meniscus. Here's the meniscus will lie in between the femoral condyle and the tibia. And sometimes you could see a meniscal cyst that's not a bigger cyst, but these are usually associated with a meniscal tear, sometimes asked to drain these or aspirate these meniscal cysts to see if this is the cause of their posterior knee pain. I find that these are reoccurring, but not as much as a bigger cyst. And then also evaluating the tibial femoral joint, looking in for any posterior spurs, cortical irregularities of the femoral condyle. So you could see, as we'll scan here in just a sec, there could be associated spurring of the posterior tibia. Also there could be, you know, see how this femoral condyle is nice and smooth and round. There could be irregularities associated with that thinning of that cartilage. And that's also to be evaluated when you're scanning this posterior knee. On the AIUM-AMSSM checklist, it says that we should evaluate PCL as indicated. So if you look at the PCL, it's just a sagittal, slightly oblique plane. So here's where you would place the probe. You're going to actually just kind of toe in just a little bit more, push more anterior on the proximal part than posterior, because you see that's a little bit deeper and this is a little bit more superficial, and you'll follow that attachment along there. So it's an oblique sagittal plane, proximal end of the transducer is rotated slightly medially, long axis to the tendon. I find that short axis is difficult to evaluate, but can be done. It's able to, you're able to evaluate the distal portion, usually it's the only one you would see, and it's usually less than one centimeter. So here's the, we're going to just take a little bit of time to go through the perineal and sural nerves. This is good practice to follow these around, see if there's any different entrapments. This will help, I find, to identify some causes of post-surgical pain. And so let's, the sural nerve is actually, if we follow the perineal nerve, it splits off of the sciatic nerve, and then it splits to the lateral sural cutaneous nerve, divides off of that, and then the perineal nerve divides in half into the superficial and deep. If you look at the sural nerve, it's actually a combination of the lateral sural cutaneous nerve and medial sural cutaneous nerve that comes from the tibial nerve joins up to cause the sural nerve. So this one, these are the branches, and these branches join to form this one, just a little bit of comparing and contrasting these two nerves. So we follow this sural cutaneous nerve, consists of the fusion of the medial, as I discussed, and the lateral sural cutaneous nerve, and there's a communicating branch here as they join together to form that sural cutaneous nerve that travels down. Posture and then lateral ankle and foot. So the sural nerve is a sensory input for the posterior lateral corner of the leg and lateral foot and fifth toe. Removal is only trivial. This is often used for biopsy or for donor graphs. So the perineal nerve is half the size of the tibial nerve, obliquely is along the lateral side of the popliteal fossa, wraps around the fibular head. And if you're trying to find it, you just always find that biceps femoris. Think of the biceps femoris as protecting the perineal nerve. So it protects it, and so you see it just medial to the biceps femoris, so you always find it that way. So find your biceps, inserting on that fibular head, and just look medially and you'll find the perineal nerve. So prior to its division, it gives off articular and lateral sural cutaneous nerves. There are three articular branches, two of these accompany the superior and inferior lateral genicular arteries to the knee. So if you're trying to find these, look for the arteries. Sometimes it helps you identify that. And the third recurrent articular nerve is given off at the point of the division of the common perineal nerve as it ascends with the anterior recurrent tibial artery through the tibialis anterior to the front of the knee. So there's motor branches of this, and those are the superficial and deep. So here's just some more netter images showing. So here's that common fibular or perineal nerve, and that's superficial, that goes lateral, and then the deep travels midline anterior and falls all the way down to the great toe. you can see here, it follows along the interosseous membrane. Sorry, that's the reason I included this image. So superficial peroneal nerve goes along, remember it's the lateral aspect. It innervates the peroneus longus and brevis. And these muscles assist with eversion and plantar flexion of the foot. So we follow this along, you'll see, here's the peroneal nerve, and then here's the deep peroneal nerve as it travels along the anterior midline of the leg and then foot ending at the great toe. So the deep peroneal nerve innervates the muscles of the anterior compartment of the leg, which include tibialis anterior extensor hallicis longus and extensor digitorum longus, and as well as the peroneus tertius. These muscles are responsible for dorsiflexion of the foot, extension of the toes, innervates the muscles of the foot, extensor digitorum brevis, extensor hallicis brevis as well. So let's follow these nerves along. Here's the posterior knee as we go along, sciatic nerve. So once there's a split there, so that becomes the tibial nerve, and this is the peroneal nerve. So let's keep an eye, let's follow this peroneal nerve. Now you notice there's a little nerve that just split off, and that's the lateral serocontinuous nerve, and that will travel down, join up with the medial serocontinuous nerve to form the sero nerve. And we can see, here's an example of the sero nerve that resides in between the medial and lateral gastrointestinal but here's, it's on a loop, a CNA loop, and you can see that here's the bifurcation once again, tibial nerve, peroneal nerve, and lateral serocontinuous nerve that travels medially. Here's another video, lateral serocontinuous nerve, that'll split off. Here's the medial serocontinuous nerve, and they will join up, and you can find, follow the artery along. Here's a lateral gastroc, medial gastroc, and as they travel down, now this becomes the Achilles, and it's a midline Achilles, and then it'll just slide lateral. So here's Achilles still, here's a sero nerve, after those, the lateral and the medial serocontinuous nerves join up, then that becomes the sero nerve. So here's on that, or so lateral gastroc, or so lateral serocontinuous nerve, or communicating branch, they join up, and they follow that gastroc, or the Achilles down. So you can see here's the medial serocontinuous nerve, and the lateral serocontinuous nerve join up, and here's the Achilles in the short axis, and then also when you get down distally, it slides laterally, so that was Achilles, now here's serocontinuous nerve following the artery down the lateral knee. So here, just here's a diagram, so Achilles, and the nerve just distally slides off laterally. So about 10 centimeters from the Achilles insertion, you will find the serocontinuous nerve right there. So it's midline, and then it just slides laterally as you glide down. So just remember as you're scanning, tibial, sciatic with tibial, let's keep midline, perineal nerve, you're gonna have to just work to keep the nerve in view as you wrap around the fibular head. So here's a superficial perineal nerve. So it splits off, let's keep an eye on the perineal nerve, going proximal to distal, here's the nerve, and I'm gonna go closer to, here's the fibular head, so wrap around the fibular head, it's gonna take me a little while to just keep that in view, so practice keeping that, you just have to toggle the transducer, see how I'm just slowing and wrapping that, and all of a sudden, here's the nerve, deep goes deep, as the name implies, superficial with perineus longest, here's the nerve as we go, continue distally, it stays in the perineus longest, and then all of a sudden, rise superficially, pops through the fascia, at this point right now, there's the superficial perineal nerve, just popped through the fascia distally. Okay, so here's the deep perineal nerve, we're gonna follow that from proximal to distal, wrap it around, wrap it around the fibular head, it stays deep, it goes along, follow the artery, it'll stay along, here's the interosseous membrane, so here's the artery and nerve, and you will follow that down, interosseous membrane between the tibia and the fibula, and it'll travel all the way down across the tibiotalar joint and to the great toe. So let's get back to our case, this former runner, that is now a sports med ultrasonographer, having posterior medial knee mass, associated with some swelling, and that is obviously a Baker's cyst, as you can see between the medial head of the gastroc and the semimembranosus, there's the Baker's cyst. Now I'm gonna teach some pathology associated with some cases, so these are some quick cases within our case, just for a teaching point, this was a 78-year-old that was sent to me to drain a Baker's cyst, and as I scanned posteriorly, didn't really see any Baker's cyst, no fluid between the medial head of the gastroc and the semimembranosus, there was just this vessel with associated acoustic shadowing, so here's a circular vessel, there's some calcifications within this, and it's causing some acoustic shadowing, so hey, what is this? I looked further, hadn't gotten any plain films on this individual yet, because they came with me, or came to me, and they had some vessel, or some plain films of a hip, they had prior trauma as an elderly gentleman, and I looked, and here's the calcified vessel, so those popliteal arteries had some calcification, and his pain was intermittent claudication, and not associated with a Baker's cyst, and you can see what this vessel looks like on a plain film, this is what it looks like on ultrasound. Here's some examples, some loose bodies in a Baker's cyst, this is what it looks on a lateral plain film, this is what we would see on ultrasound. So here's a 60-year-old OR nurse with ongoing perineal tendinopathy, I'm gonna show this, she had curbsided some of our orthopedic colleagues while she was in the OR, and they'd been treating her for some perineal tendinopathy, and asked me to see her because she wasn't getting better, and so as I was doing my physical exam, I felt a little mass on the lateral knee, which is surprising, because our Baker's cyst should be on the medial knee, and so as I scan this nerve here, you'll see this hypochoic area, so this perineal nerve also has this increases in size about six to seven times, and so this was not perineal tendinopathy, this perineal nerve had a nerve sheath tumor, so she ended up getting an MRI and having this removed. Here's a perineal, this patient had perineal pain in the nerve, in the distribution, that oversized total knee replacement, so here's the replace component, but if you could look at the nerve, here's the perineal nerve snapping back and forth, almost like you would see when the ulnar nerve snaps out of the cubital tunnel, so this was snapping back and forth, causing pain, dysthesias, and so you could see in the posterior knee, not all posterior knee pain is Baker's cyst, especially this is the lateral knee, and here is an example of someone who had a replacement, still had ongoing knee pain, and associated perineal tendinopathy, I scanned their knee, since my success of the previous one, where it was a perineal nerve, I thought, oh, gee, it's just a perineal tendinopathy, did my fancy hydrodissection along the perineal nerve, but that did not resolve any of the symptoms associated with her posterior knee pain, and so I had to look further, so here's the nerve, they're not really snapping like on our previous case, here's needle going in and injecting like we would do with a simple hydrodissection, did not resolve whatsoever, but actually, let me just put this, I'll go back to original, it was a Fabella that was associated on this first, if we look here, here's a Fabella will show up, lateral knee, here's the Fabella, you can see the acoustic shattering from that, so that would be the lateral femoral condyle, I did a lidochine challenge just on the Fabella, released or eliminated the pain, you can see this Fabella on the associated plain film, right here, they actually removed it, and the pain resolved, this is a very uncommon case, unless you think that all posterior lateral knee pain is Fabella, I hope that's not the case, but here's another case where here's a D1 track athlete, saw the orthopedist for posterior lateral knee pain with ridiculous symptoms radiating down the lateral leg, saw the team orthopedist, was referred to physical men rehab for rule-out radiculopathy, had a normal MRI, then was referred to neuro for a nerve conduction test, had a normal nerve conduction test, the neurologist then referred to a primary care sports doc that did a, they wanted a compartment testing, the compartment testing was normal, then ended up in my clinic, and with my experience of the Fabella in the past, through the process of elimination, they did a lidochine challenge test along that perineal, or that Fabella, we injected it, had him do this provocative running and whatever caused his pain, and usually before that, it resolved, and so this patient actually had this Fabella removed as well. Here's the acoustic shadow, here's caused by the Fabella right there, femoral condyle in the perineal nerve, and a two-year follow-up patient has no pain. Last case, superficial perineal neuropathy, here's someone who's dead post trauma, ankle replacement ongoing. Of course, the surgeon said that everything looks great, but the patient asking, why do I still have pain? And it was sent to me just to evaluate dynamic imaging, and I noticed some pain along that, that the pain was associated with a superficial perineal neuropathy with sonopalpation, in other words, the scanning using the transducer along the lateral, mid-leg, they reproduced the pain, and so as we follow, you could follow the superficial perineal nerve as it pops through the fascia, you kind of saw that, and then here's the nerve right here, do a little hydrodissection, test that, see if that eliminates the pain, and that did, of course. Here's the fibula causing that acoustic shadowing, and so that was a diagnostic way of seeing how you can see is that nerve pops through the fascia, that's where the pain was caused. Here's just, we didn't spend much time on evaluating the PCL. I think there are some associated PCL cysts that were sent to us to aspirate, and they could get associated or decrease pain from decompressing these pain. Remember, it's a sagittal oblique evaluation. When you do this, go ahead and put the dopplers on and evaluate the vessels as you go after, and it's usually just lying in that notch of the medial and lateral femoral or condyles, so just come medial or lateral. I prefer medial just because the perineal nerve seems to come in the way if you do a lateral approach. Here's some, there's a number of posterior knee procedures that can be done, so to help people, here's one of the earlier cases that I did when I started doing ultrasound, and it was the spouse of one of our orthopedists. He said, hey, my wife's having posterior knee pain, and I don't want anyone going there with a scope to go after this. Can you just drain it? And I said, yes, posterior knee pain. I thought it was simple Baker's cyst, and he already had an MRI looking at this. He was able to decompress this, but you could see, you could help people out with having, hopefully not having going to a surgery. Also, you could find out with these type of procedures if that's really the cause of their pain. If you decompress it and they're still having pain, there's other options. So let's go back to the original case of Baker's cyst. How would you describe this? Where would you do to put in your report? You notice that there's thickening of the wall, there's septate, there's cellular debris in there as well. So here is one way of doing the report. Explain what you're doing, indication of what transducer position of the patient, and then correlate it with if there's other imaging that you reviewed. I think it's important to document that. And then on this one, I used the words irregular hypocoeceptic cyst measuring. So it's good to put the size of the cyst. Use your instrumentation or your machine. Many of these have ability to measure and take a size measurements. And so that's important to keep down right down there. And then I don't do a lot. I don't think medically, legally, I'm going to comment on doing if there's DVTs or whatnot, but I think it is important to document if there's flow. And then obviously write down your impression there. Many of the times I am sent patients to just, they've had surgery, they've seen a bunch of different people, and they're sent to further evaluate dynamically imaging and to also just find out why they're having pain. And so if you document an ultrasound, sometimes you're not reimbursing. You spend all this time doing a physical exam and a diagnostic injection. And so on these cases, I will, here's my epic note. And I will, instead of build just a diagnostic exam that sometimes I don't get reimbursed for, I'll do a consult and a procedure with a code of ultrasound guidance because I am spending considerable amount of time reviewing the chart for what surgeries have been done. And then I'm doing a physical exam as well as an ultrasound exam and using ultrasound guidance. So in conclusion, ultrasound of the posterior knee could be, it's very simple, not a lot of structures, but just remember not all posterior knee pain is a Baker's cyst. There's lifelong learning, following the nerves is important and you could help patients help figure out why they're having ongoing pain even after they've had surgery. It's important to remember the anatomy and then you could apply this knowledge to help your patients. It's been a pleasure to be with you here today. Thank you for your time and thank you.

3rd Edition, CASE 31

3rd Edition

Knee

posterior knee

ultrasound

Baker's cyst

popliteal fossa

nerve structures

diagnostic injections

musculoskeletal ultrasound