Friday, April 25, 2008

Anterior Process Calcaneus Fracture

The calcaneus, or heel bone, is the largest tarsal bone. Most fractures of the calcaneus are typically easily seen on MRI, but if you are not careful, it is easy to miss a fracture of the anterior process of the calcaneus.

The fracture can be difficult to see on x-rays, especially the anteroposterior and lateral views. 16 year-old teenage girl with pain after trauma, with negative x-rays:

Sagittal STIR and T1 images reveal a subtle fracture (red arrows) of the anterior process of the calcaneus, with mild edema. Correlative CT scan confirms the fracture:

These fractures are typically treated with protected weight bearing. Displaced fractures involving more than 25% of the calcaneocuboid articular surface are usually treated operatively.

Note that an accessory ossicle (os calcaneus secundarius) can be difficult to distinguish from an anterior process fracture, especially if the suspected fracture is subacute or chronic in nature. History and correlation with the clinical examination will be needed in these cases.

Thus, it's a good idea to look specifically at anterior process of the calcaneus in any patient with a history of trauma, whether you are looking an x-ray, CT, or MRI. Many of these patients have the fracture missed initially, and present with chronic hindfoot pain.

Vic David MD

Friday, April 18, 2008

Euler and the Deltoid Ligament

I haven't dabbled in calculus in quite a while, but remember it fondly. I was a bit of a math dunce, but real math dudes are fiercely intelligent. One of the greatest math dudes of all time is Leonhard Euler. If you took calculus, you will remember Euler's identity, perhaps fondly, and perhaps with a shudder:

This little gem was called "the most remarkable formula in mathematics" by the physicist Richard Feynman, no math slouch himself.

OK, by now you should be asking, "What in the world has this got to do with musculoskeletal MRI". Ah, but there is an odd but delightful connection.

Euler conceived of the mathematical entity, the deltoid, which can be defined as the trace of a point on a circle, rolling inside another circle 3 or 3/2 times as large in radius:

Note the triangular shape of the deltoid, which comes from the Greek letters delta (Greek letter D) and eidos ("resemblance").

Two well known structures of the body are the deltoid muscle of the shoulder and the deltoid ligament of the medial ankle, whose shape resemble the mathematical deltoid, triangular in configuration.

Menfiardi et al. wrote an excellent article (Radiology 242:817-824, 2007) describing the anatomy of the deltoid ligament. Here is a drawing from that article:

© RSNA, 2007

Medial aspect of the ankle displays the deep and superficial layer of the MCL complex. The superficial layer is semitransparently displayed to visualize the covered portions of the deep layer. Deep layer consists of anterior and posterior tibiotalar (aTTL and pTTL) ligaments. Superficial layer consists of tibionavicular ligament (TNL), tibiospring ligament (TSL), and the tibiocalcaneal ligament (TCL). Three portions of the spring ligament complex are displayed: superomedial calcaneonavicular ligament (smCNL), medioplantar oblique calcaneonavicular ligament (1), and inferoplantar longitudinal calcaneonavicular ligament (2). aColl = anterior colliculus, pColl = posterior colliculus.

Twenty-eight year old male with history of eversion injury three months ago:
Coronal T2-weighted image with fat saturation shows a grade 2-3 sprain of the deltoid, particularly the deep fibers (red arrow). There are multiple bone bruises as well (green asterisks).

Repeat scan at 5 months, this time without fat saturation, shows improvement in edema, but persistent abnormal thickening and abnormal signal associated with the deep (red arrow) and superficial fibers (yellow arrow) of the deltoid.

Sagittal images at 5 months:

(click on image to enlarge)

(A) T1-weighted and (B) STIR images show persistent bone marrow edema subjacent to the anterior colliculus, intercollicular groove, and the posterior colliculus, the origin of the deep fibers of the deltoid ligament. Persistent edema 5 months after injury suggests that the deltoid has not healed properly, and is the cause of the patient's medial ankle pain.

Vic David MD

Friday, April 11, 2008

Tight Spaces and Tunnels

Ever been to a rock concert where everyone is packed in tight?

I have, and the feeling can be disconcerting. The feeling of having your exterior slowly squeezed like a tube of cheap toothpaste, kneaded like a loaf of dough by a series of sharp elbows... not the highlight of the concert experience.

Nerves in the body sometimes pass through tight passages, generally fascial, fibrous, or fibroosseous tunnels such as the cubital tunnel of the elbow and the carpal tunnel of the wrist. Nerves can be compressed in these tunnels, and they don't enjoy the compression, either. The patient can experience symptoms such as pain, tingling, and motor weakness.

The tarsal tunnel is a well-known structure in the medial aspect of the ankle. The tarsus is the skeletal region between the distal tibia and fibula, and the metatarsals. Some imaginative Greek thought that this region resembled an inverted wicker basket ("tarsus" in Greek). A Dionysian romp with a bottle or two of wine might put you in the frame of mind to see an inverted wicker basket here:

Tarsal tunnel syndrome is due to compression of the tibial nerve, as it swings through the tarsal tunnel, meandering its way into the foot. Tarsal tunnel syndrome can manifest as foot pain, paresthesias, and numbness. In some cases, there can be atrophy of the foot intrinsic muscles.

There are various causes, including bony prominences, varicose veins in the tunnel, and soft tissue masses. Patients with suspected tarsal tunnel syndrome are often referred for MRI scans. Here is a 54 year-old female with bilateral tarsal tunnel syndrome.

Axial image of the right ankle reveals an accessory muscle (red arrow) in the tarsal tunnel. Note the normal posterior tibial neurovascular bundle (green arrow).

For comparison, here is a normal ankle:

The posterior tibial tendon (yellow arrow), FDL (black arrow), and FHL (orange arrow) are well-seen, as is the posterior tibial neurovascular bundle (green arrow). There is no accessory muscle tissue.

Sagittal images from our symptomatic patient:

Note the accessory muscle tissue (red arrows) blending in with the quadratus plantae muscle (yellow arrow). This accessory muscle tissue is the flexor digitorum accessorius longus (FDAL). There are several accessory muscles that can occur in the region of the tarsal tunnel, including the FDAL, peroneocalcaneus internus and tibiocalcaneus internus (Sammarco and Stephens JBJS 1990; 72:453-454). Only the FDAL is reported to be associated with tarsal tunnel syndrome.

The FDAL has a variable origin, including the fibula, tibia, soleus, FHL, FDL and peroneus brevis. The muscle may have a single or double head. Within the tarsal tunnel, it is typically posterior to FHL, close to the neurovascular bundle (NVB). It may occasionally cross the NVB superficially. Once it exits tunnel, it inserts into the flexor digitorum longus prior to the latter's division into four tendinous slips. Alternatively, it may also insert onto the quadratus plantae, as in this case.

It's easy to miss this accessory muscle, as it is the same signal as the muscle tissue that is normally found in this area. Thus, it is prudent to specifically evaluate for the presence of accessory muscle tissue in this area in every patient.

This patient has a FDAL in the contralateral ankle as well:

The FDAL in this ankle is smaller, and the patient was less symptomatic in this foot.

Vic David MD