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Magnetic Resonance Imaging of the Foot and Ankle
I have always found topics on the foot and ankle to be exceedingly boring. This is mainly because I was never called on to use any of the information I learned; hence I quickly forgot all that I was told. I learned early in my residency that during lectures on the foot and ankle, I could find better use of my time—like running or playing golf. That has changed now that magnetic resonance (MR) imaging is being used in the foot and ankle. MR imaging is playing an increasingly important role in examining the foot and ankle. Orthopedic surgeons and podiatrists are learning that critical diagnostic information can be obtained in no other way and are relying on MR imaging for many therapeutic decisions.
When most of us first encounter an ankle MR image, we get out a cross-sectional atlas and start trying to determine where all the tendons, muscles, vascular structures, and so on lie. I can assure you this will be unnecessary after reading this chapter. Although the anatomy of the foot and ankle can be complex, the significant anatomy, that is, the anatomy that must be learned because it is affected by disease, is fairly straightforward and easily learned. There is no need to memorize tendons, ligaments, and muscles that are only rarely seen to be abnormal; therefore this chapter will dwell only on the pathologically significant areas.
I must admit, imaging the foot and ankle is getting more complex—it seems there is a new article every month on the utility of MRI in this area. Foot and ankle imaging is one of the fastest growing studies in musculoskeletal imaging.
Tendons
One of the more common reasons to perform an MR imaging exam on the foot and ankle is to examine the tendons. Although multiple tendons course through the ankle, only a few are routinely affected pathologically. These are primarily the flexor tendons, located posteriorly in the ankle. The extensor tendons, located anteriorly, are rarely abnormal.
Tendons can be directly traumatized or be injured from overuse. Either etiology can result in tenosynovitis , which is seen on MR imaging as fluid in the tendon sheath with the underlying tendon appearing normal; or tendinosis , which is seen as focal or fusiform swelling of the tendon with signal within the tendon that does not get bright on T2 images. This represents myxoid degeneration and can progress to a partial or complete tear. A partial tear has thinning or attenuation or has T2 high signal within it. A complete tear or rupture has a gap in the tendon. It is often diagnosed on axial images by noting absence of a tendon on one or more images.
It is important to distinguish between tendinosis and a partial tear or complete disruption because surgical repair is often warranted for the latter two and not for the former. It is often difficult to make the distinction clinically.
Complete tendon disruption can be difficult to see on sagittal or coronal images because of the tendency for tendons to course obliquely to the plane of imaging. An exception to this is the Achilles tendon, which is usually best seen on a sagittal image. The imaging protocol for the foot and ankle must include axial images with T1 (or proton density) and T2 (or T2∗) sequences. It is not recommended that both ankles be studied together. An extremity coil around one ankle with a small field of view will give the highest image quality. Both sagittal and coronal images with T1 and T2 weighting are also performed.
Achilles Tendon
The Achilles tendon does not have a sheath associated with it; therefore tenosynovitis does not occur. Tendinosis and partial tears are commonly seen in the Achilles. Complete disruption is commonly seen in athletes and in males around the age of 40; however, it is such an easy clinical diagnosis that MR imaging is usually not necessary. Complete tears are also commonly associated with other systemic disorders that cause tendon weakening, such as rheumatoid arthritis, collagen vascular diseases, crystal deposition diseases, and hyperparathyroidism.
Achilles tendon disruption can be treated surgically or by placing the patient in a cast with equinus positioning (marked plantar flexion) for several months. It is very controversial as to which treatment is superior, with both methods of treatment seemingly working well. I have known two surgeons who were dogmatic in their approach to always recommending surgery for torn Achilles tendons until they ruptured their own and opted for nonsurgical treatment.
MR imaging is being used by some surgeons to help decide if surgery should be performed. If a large gap is present ( Fig. 12.1 ), some surgeons feel surgery should be performed to reappose the torn ends of the tendon, whereas if the ends of the tendon are not retracted, nonsurgical treatment is preferred. No papers have been published to show that this is, in fact, scientifically valid.
Posterior Tibial Tendon
The flexor tendons are easily remembered and identified by using the mnemonic “Tom, Dick, and Harry,” with Tom representing the posterior Tibial tendon (PTT), Dick the flexor Digitorum longus, and Harry the flexor Hallucis longus ( Fig. 12.2 ). The posterior tibial tendon is the most medial and the largest, with the exception of the Achilles, of the flexor tendons. The PTT inserts onto the navicular, second to fourth cuneiforms, and the bases of the second to fourth metatarsals. As it sweeps under the foot, it provides some support for the longitudinal arch; hence, problems in the arch or plantar fascia can sometimes lead to stress on the PTT with resulting tendinosis or even rupture. Posterior tibial tendinosis and rupture are commonly encountered in rheumatoid arthritis.
Rupture of the PTT results clinically in a flat foot due to the loss of arch support given by this tendon. The spring ligament runs just deep to the PTT and then goes underneath the neck of the talus, which it supports in a sling-like fashion. When the PTT tears, the stress is then placed on the spring ligament to support the talus and the arch. The spring ligament has a high incidence of disruption when the PTT tears. Once the PTT and the spring ligament tear, the next structure to fail is the subtalar joint ligaments—the sinus tarsi. We looked at 20 patients with PTT tears and found that 92% of the cases had abnormal spring ligaments (thickened or torn), and 75% had an abnormal sinus tarsi. It’s clear these structures are linked, and injury or stress to one can affect the others.
Differentiation of partial tears from tendon rupture can be difficult clinically, and MR imaging has become very valuable for making this distinction. Most surgeons will operate on a disrupted PTT, whereas nonoperative therapy is often preferred for partial tears.
Posterior tibial tendinosis is seen on axial T1- or T2-weighted images as swelling and/or signal within the normally low-signal tendon on one or more images ( Fig. 12.3 ). The increased signal should not be fluid bright on the T2 images. Tendon disruption is diagnosed by noting the absence of low-signal tendon on one or more axial images ( Fig. 12.4 ). This typically occurs just at or above the level of the tibiotalar joint.
The spring ligament is identified on axial and coronal images just deep to the PTT. When it is stressed, it typically gets scarred and thickened ( Fig. 12.5 ). A tear can be diagnosed by noting a gap in the ligament.
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