Marlena Jbara, MD discusses normal ultrasound features of the plantar fascia and the Achilles tendon, imaging features of common foot and ankle biomechanical pathology, as well as sonographic imaging of masses.
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TAPE STARTS – [00:00]
Marlena Jbara: Hi, my name is Marlena Jbara and I am an MSK radiologist at Northwell Health at Staten Island University Hospital. In this segment, we will be discussing Part 2 of diagnostic musculoskeletal ultrasound. Disclosures: I or related party have no financial relationship to disclose. The objectives of this lecture to review the normal ultrasound features of the plantar fascia and the Achilles tendon. We will also review common imaging features in the foot and ankle of biomechanical pathologies. And lastly, we will review sonographic imaging of masses. It's very important to have a landmark. You want to know the anatomy of the part being examined. And lastly, you want to use a combination of motion artifacts and still images in the evaluation. General considerations: Some people are easier to scan than others. The water content, the patient's BMI, fat content and variations in normal anatomy can affect the scan. By making adjustments in the gain, the depth and probe frequency, we can help offset these variables. So using ultrasound to evaluate common foot and ankle pathology include evaluation of the plantar fascia, the Achilles tendon, looking at Morton's neuromas, ganglia, looking at ankle ligaments, assessing first MTP joint pathology and lastly performing ultrasound guided injections.
These are all ways that we can use ultrasound to evaluate common foot and ankle pathology. The benefits of ultrasound are numerous. The biggest benefit is that there is no radiation and it can be reproducible in trained hands. It's excellent for superficial soft tissue elements including tendons and muscle and it's patient friendly. Patient comes in, there is no preparation, the probe can be in the area of interest and we can modulate it based on the patient experience. And of course, it's a small-to-moderate expense when compared to cross-sectional imaging such as MRI and PET scanning. In the ultrasound suite, the routine exam room is equipped with adequate imaging devices. There is a transducing gel, which is a superficial gel that can be either standard or sterile and this is applied to the transducer and to the area of interest. We can perform a static exam of the architecture and we can also perform power and color Doppler vascular assessments. Dynamic imaging is possible and we can look at the peroneal evaluation and plantar plate imaging by this technique.
And so we can begin by placing the patient in the prone position for a long axis view of the plantar fascia. So the patient lays down, we make him comfortable and then we stand the patient's foot up on their toes and help to support the back of the heel and our transducer as seen here. So we have a hockey probe small foot type transducer. We are going to be looking at the plantar fascia on the order of about 14000 mHz of penetration and we are going to begin our inspection of the plantar fascia in the long axis. We then can take the patient and turn the transducer 90 degrees and also assess it in the short axis. And here you can see that the patient has now been repositioned but that's not necessary. You can stay in the prone position taking the short axis of the plantar fascia. And this represents an image, which basically is taken at the midline of the central cord of the plantar fascia. The calibers are denoting the distance, D3 denoting 3.23 mm, which would be within normal limits as the plantar fascia is generally up to a normal of 4.5 mm. Not only can we see the plantar fascia but I like to probe into the heel to assess the degree of pain and that people who tend to have small stress fractures or stress response will be very sensitive to the touch of the transducer where those cases of more chronic pain or not stress factors may have tears and pathology but don't have as much exquisite tenderness when probing the plantar fascia.
Note on this image what we can see is that we see the superficial plantar skin interface with the epidermis and the dermis and the subcutaneous soft tissues of the plantar heel. Here we are coming into the echogenic layer of the superficial plantar fascia and this is the central cord that really represents a confluence of structures but mostly the flexor digitorum brevis. We see the plantar calcaneus with its cortical echogenic surface and the anechoic blockage of the sound as most of the sound would be transmitted back from the cortical reflection. Moving on to this picture along the central cord, now we see that the calibers are denoting D2, which is 6.7 mm and that is thickened. And basically we noticed this hypoechoic plantar fascia. Again, we can find our landmarks at the epidermis and the dermis and the subcutaneous soft tissues we see the outline of the superficial portion of the plantar fascia and it's irregular sort of lobular thickening of the fascia. Let's notice the cortical reflection and even this echogenic area deep to the plantar fascia, which probably represents the earliest form of enthesophyte. The normal plantar fascial attachment as I mentioned previously would be less than 4.5 mm and what we can see here again finding our landmarks at the epidermis and dermis and subcutaneous soft tissues. We see the plantar fascia denoted in calibers by D2 and we see that this is the normal plantar fascial attachment. Again, we would probe into the heel to denote whether or not there is pain related to stress fractures in this location.
And of course, here is an example of an abnormal plantar fascial attachment. Again finding your landmarks, you are going to look for the epidermis and dermal layers, the subcutaneous soft tissues with its echogenic reflectors and then by noting the plantar fascia is thickened to this level here on D1, the calibers are denoting thickness of 5.5 mm, which is consistent with plantar fasciitis and plantar fascial thickening. These areas of hypoechoic echogenicity are going to be on the order of plantar fascial thickening and even a small partial tear present at these locations. Moving on to the long axis view of the Achilles tendon, we will position this patient in the prone position, get them comfortable. We will get ourselves comfortable and we will use the long axis of the probe and place out along the long axis of the Achilles tendon. Generally, I let the foot hang off the bed so that the ankle is supported by the bed and also it leaves the Achilles tendon at rest. And here we have a beautiful demonstration of a normal Achilles tendon insertion. Noting the echogenic fibrils within the Achilles tendon without any area of focal discontinuity, we can see the superficial subcutaneous soft tissues, the epidermis and dermis, the Achilles tendon and we can see the potential space that exist for the Achilles retrocalcaneal bursa.
So behind this echogenic reflection here this represents the posterior cortex of the calcaneus. We see the small area of fat diving in that's part of Kager's triangle fat and in this space if it would be hypoechoic and fluid filled, we would invoke the diagnosis of retrocalcaneal bursitis. In this example, we again get our landmarks, get our bearing, epidermis and dermis and then we are going to move into the Achilles tendon, which we can see one of the echogenic fibrils here. And note that we can't even really make out the dorsal extent of the tendon but it's quite thickened and from here to about here represents the echogenic thickened Achilles tendon. And what you are seeing is small echogenic areas within the Achilles tendon, which are demonstrating for us the retracted enthesophytes and dystrophic calcifications that can be seen in chronic Achilles tendon injuries. Further imaging in the long axis, we can get a little bit increased depth and we can see this echogenic foci within the substance of this very thickened, abnormal enlarged Achilles tendon consistent with the sequelae of prior interstitial tears or may even be retracted enthesophytes. In this case, we can see the Achilles tendon again and in the case of a real life subject what we can do is have the patient move up and down and move their foot dorsiflex and plantarflex and see the Achilles tendon in motion particularly if you thought that there were any interstitial tears or tears that have re-approximated. By moving the patient you get to unapproximate those tests and sort of eventuate or realize a gap within the tendon and then of course provide a cinematic clip to record the dynamic pathology.
Moving onto the short axis view of the Achilles tendon, again we have this patient's foot hanging off our table. The ankle joint is supported by the table and we have now place the transducer at 90 degrees to the long axis and perpendicular to the length of the Achilles tendon. Notice that sometimes people have more prominent grooves of wrinkles of their skin and you want to fill all these grooves with the transducer jelly so that you have good skin apposition throughout the exam. You will notice there will be big black blotchy margins along your images, if you are pressing too deeply into a structure and moving the transducer gel away from the area of interest. In the short axis of the Achilles tendon, we can clearly see the echogenic fibrils cutting cross-sections so getting your bearings, getting your landmarks again seeing the dermis and epidermis and the subcutaneous soft tissues and that we get a sense of the entire width of the Achilles tendon from medial to lateral and of course the anterior and posterior dimension that we were measuring on the sagittal image. Ultrasound, furthermore you can see what we used this small hockey probe puck. We can do along the long axis or even the short axis as seen here medially and looking at a portion of the posterior tibial tendon.
You can clearly see that this ultrasound probe being in the small footprint is well designed to achieve high image quality and being able to sort of get into these small areas and grooves along the abnormal contours of the foot and ankle. And of course, when imaging and ultrasound the Achilles tendon here are some more optimized images for seeing the Achilles and of course you get your bearing, you are seeing Achilles tendon, the fibrils within the Achilles tendon and it's nice, smooth transition to insert on the posterior calcaneus. This would again be the area of retro-calcaneal bursa. In this image, which is the same, you can see the long axis. Really, this area here is representative of what is called the watershed zone and the watershed zone is considered a relatively risky area for the development of tears. There are some thoughts on the fact that it's a hypovascular area and repairs aren't as easily obtained in this section of the Achilles tendon. And here, I have re-included that normal Achilles tendon with its normal fibrils and normal thickness usually felt to be less than 7 mm. And on the right, you can notice that it's hypoechoic and very thick. The Achilles tendon in this case would be larger than 7. These are 0.5 mm sections so this is about 1 cm of thickness for the Achilles tendon, abnormal. And to just give you another example of utility of the ultrasound in superficial body parts, here we see an x-ray, where we see some mild inflammation and here you can see an exquisite demonstration of the enlarged proximal patellar tendon or some people call it the patellar ligament in this patient who has jumper's knee also known as Osgood Schlatter's disease.
Moving on to assessment of forefoot issues, we can look at things of metatarsalgia including the interdigital nerve looking for perineural fibrosis also known as Morton's neuroma. This is just as noted as most common in the third interspace but also can be seen in the second interspace. In fact, all the interspaces are named as you well know but these are the most common locations for pathology. We can perform a dynamic ultrasound exam in our institution typically a few request and examination of a Morton's neuroma. Your patient will undergo the Mulder's maneuver and we will have that documented on a cinematic clip so that we can further demonstrate its size, the full size of the neuroma. Sometimes the neuroma is underestimated because you are only seeing the superficial part. So the Mulder's maneuver can be obtained to see the full extent of the neuroma. And of course soft tissue superficial masses, ultrasound is usually indicated as one of the initial primary investigations and in a ganglion what we are looking for, of course, is this hypoechoic or anechoic structure. And so you want to get your bearings again. You are basically going to focus on the area of interest. Usually, the patient will point to where they feel a lump or bump. You are going to place the transducer gel on the probe and also on the patient. Looking through the subcutaneous soft tissues, you will come across this hypoechoic lesion with an imperceptible posterior wall.
And notice this echogenic reflection. That's very important. This is called posterior acoustic enhancement or through transmission and it denotes the reverberation of a very homogenous fluid content structure. And therefore the more anechoic something is the higher the through transmission and the more likely this will be reflected back to the transducer sound beam. Of course, in evaluating these ganglia, we can also always put on a color or power Doppler box and in this case we see that there is a hypoechoic lesion; however, if you thought that there happens to be increased through transmission but there is internal vascularity and that's highly abnormal denoting some type of tumor. This is going to go on to further imaging either MRI or CAT scan and further denote where the boundaries are to give a preoperative planning for the surgeon who may eventually remove this lesion. Further using color Doppler, we can see the effects of this pulsatile mass and the mass actually dissipated with pressure and the pulsation dissipated but when we put on in subsequent imaging power Doppler, we could clearly see that there is a pulsatile mass in the area and this can go on to further imaging and now that we are clear that this is not just a simple ganglion. Moving on to articular diseases, find an example of demonstration that can be seen on ultrasound would be the tophaceous nodules identified in gout particularly along the first MTP joint.
In this case, the IP joint. And here we can see at the IP joint, you will notice this very echogenic heterogeneous irregular nodular tophus that's located at the interface of the joint space and when correlated with the x-ray, this is consistent with tophaceous gout. And here we can see the proximal extent of that inflammation noting dorsal to the IP joint, we still have these hypoechoic nodules seen adjacent to the bone in this patient with tophaceous gout. Further tophaceous gout overlying the IP joint as seen here in this echogenic heterogeneous collection of joint fluid and notice the cortical break identified here along the distal phalanx where you can see an adjacent gouty nodule. Of course, we can perform dynamic testing of the first MTP joint. We can look at the sesamoid complex and of course the plantar plate. This is one of the most common indications we have. Plantar plate ruptures are becoming more and more common and more easily detected using musculoskeletal ultrasound. And what we can see in this example was that there was an intracapsular lesion identified at the first MCP joint. We see this echogenic region. We could put color Doppler on this and we can also delineate the patient for further advanced imaging so that we could see what this lesion actually is.
And of course again we notice which joint motion we can see this persistent hypoechoic tissue within a joint and this may represent some kind of joint body. Of course, intraarticular masses are on the rear side. It can be a detached plantar plate and these are things that we can assess with dynamic motion of moving the great toe up and down and assessing its location, but this lesion remained the same within the joint despite the motion. Moving on to the assessment of plantar masses, one of the most common indications for imaging is to assess whether or not someone has a plantar fibroma also known as Ledderhose's disease. This represents a benign fibroblastic proliferation of the plantar fascia and the male-to-female ratio represents two to one. It's bilateral and about 20% to 50%. An ultrasound can often be seen as a hypo to mixed echogenic lesion. They can be discrete fusiform multi-nodular thickenings of the plantar fascia. Of course, we can go on to MRI and see the typical signal characteristics of being iso-to-low-intense on T1 and T2 weighted imaging with variable contrast enhancement. But I have included these examples of plantar fibromas here where you can see this fusiform hypoechoic relatively homogenous lesion that is along the long axis of the plantar fascia as denoted by the calibers. Here, another lesion along the flexor tendon seen is a plantar fibroma. Here this echogenic structure is seen usually with no Doppler hyperemia or vascularity as seen as a hypo or mixed echogenic lesion inseparable from the planar fascia.
More examples of plantar fibromatosis with this hypoechoic lesion inseparable and superficial to the plantar fascia. And we can move on to peripheral nerve sheath tumors. They represent about 10% of all foot and ankle tumors, very common in the tarsal tunnel to see Schwannoma posterior tibial nerve. It's probably the most common place that I see it, but the range of Schwannoma is about 57% neurofibroma, 29% a malignant peripheral nerve sheath tumor accounting for 14% of these types of tumors. Of course, Schwannoma and neurofibromas are benign with a slow growth rate. In the lower limb, of course, as I mentioned the posterior tibial nerve, the PTN is the most commonly involved. We can see beautiful ultrasound demonstrations of the split fat pad sign as seen in MRI and here I have included this MR image at your right. This T1 weighted image with this fusiform mass inseparable from the nerve splitting the fat as you can see. And here, the typical ultrasound appearance of Schwannoma relatively homogenous lesion that has tails that are seen to be edges of the posterior tibial nerve with its normal fibril and fibrillar ultrasound anatomy. Furthermore, we can have synovial cyst that localize with neuromas and these can be seen. They may cause pain. We may want to decompress them, of course do an excisional biopsy. We can have all those features outlined with ultrasound.
In terms of whether there is synovial or ganglion cyst, these are mostly asymptomatic and incidental in the extremities. It is of note that simple fluid, which is more anechoic or hypoechoic, that's going to be easier to aspirate and I just include this image to the right just again to remind us of the anechoic or hypoechoic lesion. The thin posterior wall and of course the increased posterior acoustic enhancement or the increased through transmission in this homogenous lesion that's collecting the sound beam and then booming it all back to the transducer. And it is of note that anechoic simple fluid is more amenable to percutaneous intervention and that makes sense. The more echogenic or heterogeneous something is the more possibility for it to contain desiccated protein material and therefore be more difficult to aspirate with routine standard sized needles. In the evaluation of ganglion, we can see its proximity to other structures and of course we can invoke Doppler imaging and assess whether or not there is actually vascularity to the lesion. We also may be able to denote a small neck that will let us know or alert the patient that may recur because there is an intraarticular extension and therefore it may fill again. And of course, being that you can see the lesion you can place a needle into the lesion under guidance usually along the long axis of the transducer. You want to see your needle tip throughout the puncture and of course you can readily use Doppler to assess any structures in the region that you think are vessels so that you can avoid them. And of course, this is our ganglion that is lateral to the base of fifth metatarsal.
We begin with this lesion that's about 3.8 cm. It's anechoic, increased through transmission and perceptible back wall and here you can see the reflections and reverberations of the needle. As it enters the lesion, you always want to be aware of where the tip is, always keeping that in the field of view and of course this aspiration is beginning and we are going to continue to see this fluid filled collection disappear. Moving on to foreign body granuloma, this is a recent case. In these cases, most foreign bodies have peripheral vascularity from the inflammation but there will be no internal vascularity and most of them are hyperechoic and that there is certain amount of granulation tissue and cellular debris within the lesion. In summary, we have reviewed the normal ultrasound features of the plantar fascia in the Achilles tendon. We have taken a look at relatively common features of foot and ankle biomechanical pathology. Certainly, this isn't the exhaustive list but this represents a representative sample of what can be seen in ultrasound and we have touched on the review of sonographic imaging of masses. Again, just a few notes that you want to have a landmark, know the anatomy of the part being examined so that you can identify when there is abnormal anatomy in the field of view. It can be very hard when you are dealing with the patient speaking, moving the machine and dealing with the pathology to keep all these features straight. So it's very helpful to have an assistant in the room if it all possible.
Of course, you want to use motion and dynamic imaging in the evaluation. In general, some people will be easier to scan than others. Those factors such as the water content, the patient's BMI, fat content of tissues and variations and normal anatomy can affect that scan but by making adjustments in the gain, the depth and the probe frequency, we can offset these variables. And again, these references are available for your review. I thank you for your generous time and attention and hope that this lecture has been of value to you.
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