Marlena Jbara, MD discusses a checklist approach to forefoot anatomy on an MRI, presents imaging sequences used to assess pathologies of the forefoot, as well as the osseous and ligamentous anatomy essential in the stabilization of the plantar plate.
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TAPE STARTS – [00:00]
Marlena Jbara: My name is Marlena Jbara and I'm an MSK radiologist from Northwell Health. This section is entitled MRI of the forefoot, a review of normal anatomy. Disclosures, I, or related party, have no financial relationship to disclose. Objectives, review MRI evaluation of the forefoot anatomy using a checklist approach in the coronal, sagittal, and axial imaging planes. We'll provide an overview of the imaging sequences used to assess pathologies of the forefoot. And lastly, we'll review the osseous and ligamentous anatomy essential in the stabilization of plantar plates.
To review the osseous anatomy of the forefoot, the forefoot is comprised of those boney structures distal to the tarsometatarsal articulations, and includes the metatarsals one through five, the hallux proximal and distal phalanges, and the proximal middle and distal phalanges, or the lesser metatarsals of second through fifth. The anatomy of the forefoot is essentially fixed on two columns, the medial and lateral column. The first metatarsal with the middle cuneiform is one of the functional units. It has about six degrees of mobility. The second metatarsal with the middle cuneiform is permanently fixed. The third metatarsal with the lateral cuneiform, also firmly fixed. And the fourth and fifth metatarsals with cuboid is relatively mobile.
Beginning with review of anatomy along these superficial dissections of the plantar forefoot, the distal plantar fascia plantar aponeurosis can be seen branching at about the level of the midfoot, corresponding to this line that's obliquely going across the lateral plantar fascia. And we can see the fence of the distal plantar fascia race as they course to the forefoot, supporting the flexor tendons at the level of the MTP joints. And you can see these are digital slips of the plantar aponeurosis going to each of the digits.
And we can see some of the transverse fasciculi here. Notice the fascial coverings still surrounding the abductor digiti minimi and abductor hallucis. And also you can get a sense of the interdigital nerve and the interdigital arteries as they course along their respective digits. The first layer removes that plantar aponeurosis and we get down deeper to the plantar fascia, and the plantar fascicles of the aponeurosis serving the digits.
And here we can see the different bands of the flexor tendons as the flexor hallucis and the extensor -- I'm sorry, the flexor tendon, serving the digits, are here. We can get a better sense of the branching portions of the interdigital nerve branches coming from branches of the medial and lateral plantar nerve, forming the interdigital network that will give rise to issues such as Joplin's neuroma. We have the superficial branch of the medial plantar artery and then again the major muscles that you're seeing here. You're seeing portions of the abductor hallucis, portions of the flexor digit brevis and abductor digiti minimi.
You can see the fiber sheets of the flexor tendons as they come to encircle each of the flexor tendons along the digits. And of course the proper plantar digital branches of the medial and lateral plantar -- I'm sorry, the medial plantar nerves can be seen here. Moving down to the second and third layers, we can further appreciate flexor tendons with the flexor hallucis and the flexor digitorum tendons. We can see cut reflections here of the abductor hallucis and the abductor digiti minimi. We can also see the lateral plantar nerve, artery and vein coursing, and becoming branching structures to join the branches of the medial plantar nerve, which will form the interdigital network.
Here we can see the sesamoids are exposed with the intersesamoidal ligament overlying the superficial flexor hallucis longus tendon. In the third layer, we can appreciate the transverse and oblique heads of the abductor hallucis, as well as the confluence of the medial and lateral head of the abductor hallucis. Notice these sesamoids become portions of the medial and lateral heads of the flexor hallucis brevis. Other things that we can appreciate here, we can see the flexor digiti minimi brevis.
And moving on to perhaps the most important transaxial anatomy, we can see at the level of the MTP joints that of the plantar plate anatomy, and that's the all-important plantar plate that really supports the metatarsal heads as they land on the ground, and part of the toe phase of gait. Essentially, what we have if we can take the M3 segment, what we can see here is that we have the extensor digitorum longus and brevis enclosed within the capsule.
We can see the proper collateral ligaments here and the accessory collateral ligaments that join to the plantar plate, seen at the plantar aspect. We can see portions of the accessory and the proper on the side as well. The plantar plate has an annular pulley that is supporting the flexor digitorum brevis and superficialis tendons. And we can further appreciate the intermetatarsal bursa seen here with B. Of course, the nerve artery and vein bundle that are implicated for Morton's neuromas are right here underneath the deep transverse intermetatarsal ligament, which really provides the inherent stability along the plantar forefoot.
Furthermore, plantar plate imaging can be seen here in this rendition where you see the lateral capsule, the accessory collateral ligaments here. We're talking about the first sesamoid hallucal complex and you could see that the flexor tendon, the flexor digitorum brevis invest the sesamoid, and then continues on to implant at the base of the hallux distal phalanx.
We have sesamoidal phalange ligaments, which can be routinely seen as seen here in this representative image demonstrating a subacute tear of the sesamoidal phalangeal ligament. Here, we can further see images of the plantar plate, the supporting structure that's deep to the metatarsal head with its fibrous ridge, and of course the superficial flexor tendon, and the extensor tendon superficial to these deeper structures. And now we can develop a checklist approach using the anatomy considerations in MRI.
We use the workhorse of imaging of the boney structures, is in the coronal plane or the long axis plane as shown here. And what we have on the left is a T1 weighted image, and therefore, the waiting is for fat signal, and we can see the subcutaneous fat layers. Other things that can be seen on this image here, we see the isointense signal intensity of muscles. So this is the flexor digitorum brevis and the portion of the abductor digiti minimi, and the portion of the abductor hallucis.
T1 weighted imaging demonstrating the fat, which is the bright signal felt with the intervening connective tissue stroma and supporting collagen network. What we have on the right is a coronal STIR image and we can see the suppressed fat, which will make everything that's fluid signal more conspicuous. So here we can see the shape of blood vessels and small little veins, and arteries that are within the sole of our foot. And of course looking at the muscle signal intensity that has not changed from T1 to T2, there is no elevated signal to suggest edema or neuritis pattern.
And we're going to begin in the coronal sequence looking at the bones, the joints, and soft tissues and that's really our checklist. In each of the areas, we'll have some special soft tissue considerations, whether be the plantar plate or the interdigital nerves. But essentially on every image, we're going to evaluate the bones, the joints and the soft tissues.
Moving from the plantar aspect to the more dorsal aspect on this image, on T1 weighted imaging, you can appreciate the sesamoids that are on the medial and lateral aspects of the flexor hallucis tendon. And here we can see some slight elevated T2 signal within the lateral sesamoid, which may indicate sesamoiditis. What we're seeing are the flexor tendons along the plantar aspect of the foot and the plantar fascial connections to the flexor tendons cannot be separated from these areas.
Now, moving through the coronal plane, we can more fully appreciate the sesamoidal complex, the tibial and fibular sesamoids. We can see the flexor digitorum brevis muscles investing the medial and lateral aspects of the sesamoids. We're getting to see a bit of the plantar head of the metatarsals and you can see here on T2 weighted image some of similar pathology. You're also getting a sense of some of the intrinsic muscles and noting there is no larger intermuscular fluid collections or signal abnormalities.
One thing we can also begin to appreciate are the interspaces coming from the plantar aspect. We're seeing some slight fullness and obliteration of the usual intermetatarsal fat. And we're going to keep our eye on these areas to look for the presence of a Morton's neuroma or intermetatarsal bursitis.
Here, we can see on a coronal T1 weighted image, we have fullness in the interspace, but we're not going to diagnose it Morton's neuroma until it's three millimeters across at least in cross-sectional dimension, which we'll see on axial images, and I'll return to that point when we get to that area. We can appreciate the beginning of the MTP joints and the fifth MTP joint, and you can see here with slight bunion head deformity, the slight varus angulation that's occurring.
In this coronal STIR image, we can see the presence of slight intermetatarsal bursitis. In this interspace at the second and the fourth interspace being physiologic and slight increase here at the first interspace, we can again appreciate the muscles and the bones to vast extent on these coronal images. And further assessment of course, we're seeing some enlargement of the interdigital nerve that does not fulfill criteria from Morton's neuroma.
Notice the normal fatty marrow of the MTP joint and all of the MTP joints for getting a really good look at what the intramedullary bone looks like and the cortical bone. And you'd be comparing and contrasting that to your coronal T2 weighted image on this side or coronal STIR, where you'd be able to pick up any fractures, bone marrow edema, or elevated T2 signal in these regions. Notice we have some elevated signal within the medial belly of the abductor hallucis, also demonstrating slight fat signal intensity. And this may be the result of some kind of neuropraxia in the past where there was a neuritis pattern that caused muscular atrophy, a slight degree of muscular atrophy there.
Further ascending to the dorsum of the foot, we can appreciate the MTP joint. Notice the exquisite detail of the cortical bone, the subchondral degenerative change, the increased sclerosis here. Small osteophytes can be seen at the median lateral aspects of the metatarsal head. We're getting a great look at the second MTP joint and appreciating though there is no bone marrow edema seen on the coronal STIR. There is relative flattening of the second metatarsal head and maybe even the third metatarsal head, and we'll see that again when we look at sagittal imagining.
We're going to be looking at the IP joint. We're beginning to see some degenerative changes and appreciate the joint space osteophytes. Notice, we're getting a nice look and we'll come back to this MRI imaging of the midfoot so I won't spend time on this section, but you can really get a sense of the Lisfranc joint by looking here on the coronal image.
Again, we're moving through our interspaces again. We have trace intermetatarsal bursitis, which is likely physiologic. So in the coronal sequences, the workhorse is really going to be about things that are occurring in the interspaces, seeing the sesamoid complex and the boney structures. That's the workhorse of the coronal long axis sequences of the foot. Things that you can't see as well, and this plane are going to be things that are in plane, so sagittal -- I'm sorry, the plantar fascia and tendon anatomy, very difficult to assess in just the coronal plane.
Again, more degenerative changes occurring at the hallux MTP joint. Notice the small osteophytes occurring and slight erosions occurring from degenerative changes. We're seeing the ends of the intermetatarsal interosseous musculature, and we can appreciate on T2 weighted images, slight elevated signal in this patient who has a neuritis pattern.
Moving closer to the dorsum of the foot, we're still appreciating the bones with its elevated T1 fatty signal and its cortical bone, having less water content, and less fat cells in it because the marrow is contained at these areas. We're appreciating some of the distal phalanges. Certainly, we're not getting a great look at them. This is not the right sequence to really sort of assess it fully so we're going to have to use our sagittal and axial images to further asses that.
And so in that way, MRI is really a three-dimensional assessment taking into account though we're looking at two-dimensional images. We're trying to see all of the pathology and reconstruct the image in three planes, sagittal, coronal and axial. And further ascending to the dorsum of the foot, we're getting a good look at the medial Lisfranc joint and some of the metatarsals. Of course, we'd be able to pick up stress fracture.
So any elevated T2 signal that we would see within the metatarsals would be indicative of the stress-related change. And furthermore, moving through the coronal sequence, we're continuing through all of the toes through the intraosseous muscles, looking at the subcutaneous soft tissues. Now, let's turn our attention to sagittal anatomy of the forefoot. The sagittal plane is going to be very useful to assess the flexor tendons, the boney structures, and the joint articulations, particularly the congruency and the superior to inferior plane is going to be well seen in this region on sagittal imaging.
To review again, of course we have the lesser metatarsal seen when the foot is viewed from a lateral aspect. We have the base of the foot of the metatarsal and all of the bases of the metatarsals. We have the MTP joints and of course the phalanges and digits. Just remember, the importance of the imaging of the plantar plate and the sagittal plate, we can appreciate the first MTP joint with its capsule. We have the accessory collateral ligament which really slings the sesamoidal complex. The sesamoids are further invested or attached to the proximal phalangeal base via the sesamoidal phalangeal ligament.
The sesamoids themselves of course are bones that are invested within a tendon and we can see the flexor hallucis brevis where probably off to one side, this is likely the medial or the lateral. We can appreciate in the non-hallux metatarsals, similar anatomy, although we do not have the sesamoid complex. We have the, in this case, the M3 segment. We can see the base of the proximal phalanx, the fibrous ridge, and the attachment of the plantar plate, the plantar plate proper and we can see portions of the intersesamoidal ligament cut in here.
We can see the deep flexor tendon and the extensor tendon is superficial to that, and the fatty path sort of supporting all of these structures. I've included some representative images of what a plantar plate tear might look like just to give you a sense of that. And what we're appreciating is the plantar plate has elevated T2 signal as it touches and attaches to the proximal phalangeal base and this is not as bright as I would expect for an acute injury. This is subacute injury.
In here, we'll look later on at this transaxial slice, and when we turn to axial imaging and we can appreciate a flexor tendon, and we can see portions of the plantar plate, and the accessory and proper collateral ligaments. And we could see that a portion of the plantar plate along this border with the accessory collateral ligament is torn corresponding to this image. And moving into the boney structures, we begin to appreciate a portion of the sesamoidal phalangeal complex, and we can see the hallux metatarsal head, proximal distal phalanges, and we can really appreciate the sesamoidal phalangeal ligament here intact.
And further, we can see the congruency of the joint. Of course, the osteophytes that exist can be seen here, no bone marrow edema, small joint effusion, maybe physiologic or slightly increased. Also, we can see the tendon, the myotendinous junction of the abductor hallucis, which is going to invest the sesamoidal complex. And further seeing here, since we're out of the boney structure, we're actually at the presence of the intersesamoidal ligaments. Notice the joint congruency, lack of bone marrow edema, osteophyte formation and the tendon muscle unit.
Further moving into sagittal imaging of the forefoot, we can see here that we actually have thinning of the phalangeal attachment of the sesamoidal complex. And though there is not a frank tear, this stint here, there's obviously chronic strain and a small joint effusion, and there's no bone marrow edema. The sesamoid has not been detached and the joint remains congruent. You're getting a little bit of the extensor hallucis tendon also on this image.
Now, we move across to the sesamoidal complex and the sesamoidal phalangeal attachment, seeing more of the extensor tendon here. Noting the boney structures and congruent and seeing the first ray in its entirety. Further looking to the sesamoid complex and its congruency with its hallucal articulation, cartilage exist between this. We're not seeing a really great demonstration but this is proportion of the sesamoidal phalangeal ligament. Again, partially seeing the bone, no bone marrow edema.
And now, we're beginning to come in to this second MTP joint. We're beginning to see part of the plantar plate complex. And in this picture, we can really get an appreciation of the plantar plate and its sesamoidal attachment -- I'm sorry, it's proximal phalangeal attachment. Notice the close proximity of the flexor tendon, seeing a portion of the extensor tendon here. Notice on T2 weighted images, no bone marrow edema.
We're beginning to see what seems to be a static mild hammer toe deformity, still the plantar plate is intact, and there's some mild plantar pericapsular elevated T2 signal, which may indicate overuse. But we'll see more about that as we move into the joint. And moving further into the joint, we can see the sesamoidal -- I'm sorry, the phalangeal plantar plate attachment of the MTP joint. Note the joint congruencies, small joint effusion, the boney structures, low in T2 signal, no fracture, no edema, no periosteal reaction.
Of course, you're seeing muscular tendons anatomy here, the subcutaneous soft tissues here and the skin surface in here. So you can see exquisite detail if there will be a formed body.
And of course moving in further to the plantar aspects of the foot, we can get a volume averaging of a portion of the plantar plate complex in here, where again, we're seeing the boney structures. More about the same boney structures, the plantar complex seen as we slice the side of this articulation. And we repeat the process going through each articulation looking at the plantar plate, the flexor tendon, its relationship, the fact that there's no bone marrow edema and the joint is congruent on every image. That's the goal of this checklist to sagittal imaging. Really, a nice example again of the congruency of the articulation, noticing the exquisite detail of the marrow cavity, the signal intensity, the apposition of the plantar plate to the MTP joint and the intrinsic muscles of the foot.
Again, moving through each articulation, seeing the boney structures, the joint congruency, the extensor and flexor tendons and the intact plantar plate is really the goals of this particular area, the imaging and sagittal imaging of the forefoot. And of course, more of the same seeing the plantar plate boney structures, extensor and flexor tendons.
Now, we can turn our attention to forefoot anatomy and plantar plate imaging in the transaxial plane. And again, just to review what we're looking for here, if we consider the M3 metatarsal head segment, when we look at this entire articulation, we see the proper collateral ligament most dorsally, the accessory collateral ligament more plantarly and the plantar plate providing an investment plantarly.
The plantar plate is joined to the remainder with the accessory collateral ligaments, and of course the capsule. The capsule retains its investment from the intraosseous muscles that came through the interspaces. Notice, we can have an intermetatarsal bursa and that superficial to the deep transverse intermetatarsal ligament seen here. Of course, deep to this, we can have a nerve artery vein bundle, and you can see here why neuromas would occur in this position.
Of course, you have your flexor digitorum longus and brevis tendons to match your extensor digitorum longus in brevis tendons here. You have some of the superficial transmetatarsal ligaments and the plantar fascia, serving the investments of the flexor tendons and we'll take a look at this now in the axial imaging. So again, the T1 weighted images on the left and the T2 weighted images on the right, and what we can appreciate here is that there's some subcutaneous soft tissue swelling here.
We are just past Lisfranc ligament. We're actually at about the level of Lisfranc ligament. So we're just beginning our perusal of the forefoot boney structures and we're going to be taking a look in this case at the boney structures looking at the marrow and seeing whether or not there's edema. In addition, we're going to have an excellent look at the muscles of the foot and be able to assess whether or not there's elevated signals seen in neuritis and trauma, and tumors.
Of course, we can see the extensor and the flexor tendons, and really the axial images are the workhorse of this exam. And I'm going to move through the next few slides and what we're looking at really comes down to abductor digiti minimi, flexor digitorum brevis and longus, and abductor hallucis musculature. Of course, the boney structure is seen here and this is considered portions of the Roman arch, the keystone configuration, giving optimal stability to the midfoot.
Again, looking at the intrinsic muscles, the elevated signal and normal fatty marrow. Of course you're appreciating the subcutaneous soft tissue edemas in here. We can also appreciate the -- you can't see all that well, but the extensor tendon sort of quickly, quite opposed to the bone and the flexor tendons of course have not yet diverged into their respective superficial and deep branches.
And again, we're appreciating the boney structures with its normal fatty marrow and no elevated T2 signal. We're seeing the intrinsic musculature, appreciating some of the nerve artery vein bundles. The distal plantar fascia can be seen here with the subcutaneous soft tissues. A little bit of edema remains along the dorsal lateral midfoot to forefoot.
And moving through these images again, we can appreciate the lack of bone marrow edema, noticing the cancellous bone with the cortical bone.
We can appreciate the extensor tendons here coming through. We're beginning to see portions of the flexor tendons, both superficial and deep. And of course the abductor hallucis, we're looking at the distal plantar fascias in here and here. Obviously, if there was a neuroma -- I'm sorry, a plantar fibroma, we could see as a hypoechoic structure variable on T2 weighted, and variable in its contrast enhancement.
In here in these transaxial slices, we can further appreciate is the extensor digitorum tendons in here. We're beginning to see the flexor tendons, both deep and superficial branches here, the intrinsic muscles of the foot, and the interosseous musculatures in here, which will provide the investment for the collateral ligament complexes at the level of the metatarsal phalangeal heads.
In here we can see the hallucal sesamoid complex along with its collateral ligament, its accessory collateral ligament and intersesamoidal ligament. We can appreciate the congruency and the sesamoidal hallucal articulation. We're beginning to see the second through fifth metatarsal heads as they're going to come to their respective positions. Here, we're beginning to see a bit of the fourth plantar plate complex. We're starting to see portions of the interosseous tendon come up to form what will be the proper and accessory collateral ligaments.
In here, we can appreciate the different intermetatarsal bursitis at the first, second and third interspace. These are physiologic amounts of these interspaces but this is slightly elevated. Notice the sesamoidal complex with its flexor tendon, the collateral ligament complex here.
We're coming into the second MTP and third MTP plantar plate complexes. Noticing we're seeing the extent of tendons, the sagittal bands, the proper and accessory collateral ligaments and portions of the plantar plate. Note there is no pericapsular soft tissue thickening seen here and we're seeing that superficial transverse metatarsal ligaments in here. You can appreciate better the deep intermetatarsal transverse ligament here. And further perusal, you can see the flexor tendons closely opposed to their respective digits, the extensor tendons, the extensor hoods and the subcutaneous soft tissue.
We can appreciate mild enlargement of the interdigital nerves seen here at the second interspace and the trace intermetatarsal bursitis in here. This is physiologic and this does not fulfill criteria for Morton's neuroma, being less than three millimeters in transverse dimension. And we continue our imaging to the tips of the toes to preserve the ability, to see any kind of glomus tumor or things at the tip of the toes that can be present.
So in summary, in this past segment, we've reviewed the MR evaluation of the forefoot anatomy using a checklist approach in the coronal, sagittal and axial imaging planes. I provided an overview of the imaging sequences used with such pathologies of the forefoot, and we reviewed the osseous and ligamentous anatomy essential in the stabilization of plantar plates.
I thank you for your generous time and attention, and hope that this lecture has been a value for you.
TAPE ENDS - [32:09]