Thursday, January 31, 2008

Posterior Interosseous Nerve Syndrome

In jurisprudence, entrapment is a legal defense by which a defendant may argue that he or she should not be held criminally liable for actions which broke the law, because he/she was induced by the police to commit those acts.

In the medical world, a peripheral nerve can cause severe symptoms, due to entrapment by local anatomic structures. The nerve is an innocent bystander in these cases.

Although this is supposed to be a MRI blog, I do run into interesting cases CT and ultrasound cases as well. So, at the risk of not being topical, here is a case of an 80 year-old man who presents with extensor weakness and a mass.

A CT scan reveals a lipoma (red arrows) that is inseparable from the supinator muscle, which can no longer be seen as a discrete structure. The posterior interossesous nerve (deep branch of the radial nerve) cannot be seen, as it is markedly compressed. The superficial radial nerve (green arrow) can be seen superficial to the lipoma, with mild compression by the lipoma.

The radial nerve divides into the posterior interosseous nerve and the superficial radial nerve at the level of the distal elbow. The posterior interosseous nerve pierces the supinator muscle, and provides motor innervation to the extensor muscles in the posterior compartment of the forearm. The superficial radial nerve runs superficial to the supinator, and supplies sensation to the skin of the thumb, index, and middle fingers.

In a more distal axial section through the forearm, there is marked fatty atrophy of the extensors of the posterior compartment (red arrows). Note the normal extensor carpi radialis brevis muscle (green arrow).

The nerve to the ECRB may be coming off the superficial radial nerve (which occurs in 25% of patients), which is less compressed in this case. Alternatively, the nerve may to the ECRB may have left the posterior interosseous nerve prior to the site of lipoma compression.

In most cases, the cause for radial nerve compression is not identified on MRI, with common sites of entrapment including the tendinous arch of the supinator muscle (arcade of Frohse), the ECRB, and a leash of recurrent radial vessels.

Posterior interosseous nerve syndrome can be mistaken for a C7 radiculopathy (distinguish by looking for weakness of triceps and wrist flexors in a C7 radiculopathy) and lateral epicondylitis. Radial tunnel syndrome can also cause pain along the proximal lateral forearm; it is also due to entrapment of the radial nerve, but there are no motor symptoms.

Vic David MD

Sunday, January 27, 2008

Magic Angle Effect

Ho, ho, ho it's magic....

Some of you may be old enough remember that lyric from the 1974 song "Magic", by the Scottish band Pilot.

A busy musculoskeletal radiologist could sing this lyric several times a day. Not talkin' about love, son, rather the magic angle effect.

The magic angle effect is a well-know phenomenon that results in artifactual increased signal in structures with ordered collagen, such as tendons, fibrocartilage, and hyaline cartilage.

Physicists tell us that this is because when collagen is oriented at 55 degrees to the main magnetic field of the magnet, dipole-dipole interactions go to zero, resulting in a prolongation of T2 relaxation time.

In English, that means that collagen-containing structures can exhibit increased signal when they are at certain angles with respect to the main magnetic field.

A picture is worth a thousand words. Here is a coronal gradient echo image showing magic angle artifact in the extensor pollicis longus (EPL) tendon (yellow arrows), which is oriented at 55 degrees to the main magnetic field:

Note that this tendon is perfectly normal; this increased signal is purely an artifact.

This phenomenon can be seen in multiple areas of the body. Common areas to see this artifact in high-field scanners include the posterior horn of the lateral meniscus, the patellar tendon, tendons in the ankle, and the glenoid labrum. This can confound accurate interpretation of these areas.

Finally, collagen fibers are oriented in various directions in tendons and cartilage. Thus, although magic angle effects tend to be greatest when the structure is at 55 degrees to the magnetic field, they can be seen at other angles as well.

Wednesday, January 23, 2008

Ligamentum Teres- Partial Tear

Wear and tear or pathologic?

That is a recurring question in orthopedic radiology.

For some reason, findings that are symptomatic in younger patients tend to clinically silent in older individuals. Part of senescence must be dead nerve endings....

Tears of the ligamentum teres of the hip can be symptomatic in younger individuals. Here is a proven partial tear of the ligamentum teres of the right hip:

The ligamentum teres is attenuated, and is imbibing contrast on this MR arthrogram.

For comparison, here is a normal ligamentum teres from the contralateral hip (I have flipped the images for clarity):

The ligamentum teres is often abnormal in the patient over the age of about 55 or 60, and this is often a bilateral finding. To my knowledge, there is no good literature on the significance of ligamentum teres pathology in the aged population.

In the younger patient though, it is certainly worth noting, as patients will respond to debridement.

Tuesday, January 22, 2008

Radiology Blogosphere

Blogging is fun, you just have to sleep a little less to do it.

Medical blogs are in their infancy, but there are a few out there. This blog just got mentioned in one of the earliest radiology blogs, Sumer's radiology site.

A partial list of radiology blogs and websites can be found at RadsWiki (and like all wikis, you can edit this list yourself).

Monday, January 21, 2008

Carpal Tunnel Syndrome- Pitfall

Carpal tunnel syndrome is a common affliction, but other entrapment neuropathies can be masquerade as carpal tunnel syndrome.

Pronator syndrome (also known as pronator teres syndrome) is an entrapment neuropathy of the median nerve that occurs at the level of the elbow. Other entrapment neuropathies of the median nerve are the supracondylar process syndrome, anterior interosseous nerve syndrome, and carpal tunnel syndrome.

Here is a patient with pronator syndrome. There is edema in the pronator teres and FCR (red arrows) and abnormal signal in part of flexor digitorum superficialis (green arrow). There is also edema in the muscles innervated by the anterior interosseous nerve branch of the median nerve, including the flexor pollicis longus and flexor digitorum profundus (white arrows).

In most cases of entrapment neuropathies, the characteristic MRI finding is edema and/or atrophy in those muscles innervated by the nerve in question. In most cases, there is no underlying mass, as most cases of entrapment neuropathies are due to anatomic variants such as anomalous muscles or fascial bands.

Many people expect to find a mass in these cases, but things are usually like the Wonder-Bra: you see the result, but the cause is hidden.

Sunday, January 20, 2008

Loose Bodies in the Knee

"Seek and ye shall find."

Although this phrase is abstracted from religious discourse, it applies to loose bodies as well.

Loose bodies gather in characteristic places in joints, including the knee. Common locations are the suprapatellar pouch, the posterior intercondylar notch, and the medial and lateral gutters. One should always look in the popliteus hiatus as well.

Here is an example of a 35 year-old patient with a history of a failed osteochondritis dissecans repair. Coronal proton-density weighted image demonstrates breakdown of OCD repair of the medial femoral condyle:

Here are the resulting loose bodies in the lateral gutter:

And the suprapatellar pouch:

And the popliteus hiatus; yellow arrows denote the popliteus tendon:

It's a good idea to look for loose bodies carefully, with special attention to areas where they characteristically gather, including the popliteus hiatus.

Monday, January 14, 2008

Selective Atrophy Abductor Digiti Minimi

The mysterious atrophied muscle, and no, I'm not talking about the generalized wasting of middle age.

The abductor digiti minimi (ADM) muscle of the foot is atrophied in many asymptomatic individuals. I used to worry about this being a sign of entrapment of the inferior calcaneal nerve, the first branch of the lateral plantar nerve. Here is an example of selective atrophy of the ADM:

My eyes were opened by an article by M. Recht et al. (AJR 2007; 189:W123-W127), who observed selective fatty atrophy of the abductor digiti minimi in 6.3% of examinations (38 patients out a total of 602 patients). Only one patient had their clinical management changed because of this finding.

Thus, I still mention this finding, but note that it can be seen in asymptomatic patients.

No need to cause a panic....

Friday, January 11, 2008

Orthopedic Problem or Rheumatology Problem?

When is a swollen knee a surgical problem, and when is it a medical problem?

Hey, if it’s my knee, I’d rather not get it scoped unnecessarily.

A large effusion without clear evidence of internal derangement (meniscal tear, bone bruise/fracture, hyaline cartilage damage, or tendon/ligament tear) is like a stop sign.

Stop and think about the possibility of an infectious or inflammatory cause. Possibilities include Lyme disease, crystal arthropathy, and other arthropathies.

A large effusion with a small meniscal tear should still make you do a double take, because small meniscal tears by themselves should not give rise to large effusions.

Here is a patient with a large knee effusion:

The only other finding was a small lateral meniscal tear (not shown). The patient was scoped, confirming the small lateral meniscal tear. There was also extensive crystal disease, which was the real cause of the effusion.

Thursday, January 10, 2008

Why Some MRI Scans Are Better than Others

A Ferrari and a Ford Pinto are both cars, but my guess is that you would much rather drive a Ferrari.

MRI scanners are a lot like cars- there are many kinds of scanners, made by different manufacturers, and some are newer and better than others. As a radiologist, I prefer scans from high-field (closed) scanners, over low-field (open MRI) scanners. I like driving Ferraris more than Pintos. Who doesn't?

You can’t change the laws of physics- there is more signal at high field, and this translates to better images and higher diagnostic confidence. Anybody that tells you anything different is trying to sell you something.

There is a wealth of experience at 1.5 Tesla and for most musculoskeletal imaging, this is a sweet spot, with well-developed coil technology and protocols. Three Tesla (3 T) scanners offer a boost in SNR, but coils are still being developed for this platform, and this is a relatively new technology. It will take some time before we can take full advantage of the inherent boost in SNR that 3 T offers. 3 T will have its biggest impact in the imaging of small joints and the imaging of hyaline cartilage.

Low-field (open) MRI scanners can give good quality images, but like high-field scanners, the hardware should be up-to-date, and the protocols massaged by a knowledgeable radiologist. In the end, however, one cannot compensate for the diminished signal at lower field strength. This becomes most noticeable when imaging small joints such as the wrist, and when imaging articular cartilage.

The biggest secret in MR imaging is that there is wide variation even among 1.5 T scanners. Important hardware parameters that have a large impact on image quality include gradient strength, the number of receiver channels, and coils that are available.

One should also not underestimate the importance of the “soft” side of this equation- the presence and active involvement of a dedicated radiologist that understands how to build and tailor imaging protocols. In the car analogy, you can think of the radiologist as a car driver. You want Mario Andretti driving your car.

An orthopedic surgeon recently told me that the two most important factors he considers when deciding where to send a patient for an MRI scan are: 1) image quality 2) accuracy of radiology report. This one is easy- the patient needs to go to a center with a MRI scanner of recent vintage, staffed by a dedicated, subspecialty-trained radiologist. Done deal.

Wednesday, January 9, 2008

Evaluation of Thumb UCL Tears

The ulnar collateral ligament of the first MCP joint can be evaluated by MRI, but like most things in life, some do it better than others. A high-field scanner (at least 1.0 Tesla) with strong gradients is critical. With good hardware and proper protocols, one can get excellent images.

The critical question in the setting of a tear of the UCL is whether there is a Stener lesion. This occurs when the torn UCL retracts and becomes superficial to the aponeurosis of the adductor pollicis. The ligament can no longer heal, and chronic instability results. A Stener lesion can be repaired surgically.

Example of a torn UCL, without a Stener lesion:

(click on image to enlarge)

Red arrows = torn UCL
White arrows = intact RCL (for comparison)
Yellow arrows = aponeurosis transverse head adductor pollicis
Pink arrows = aponeurosis oblique head adductor pollicis
Green arrow = intact accessory collateral ligament

Thus, in this case, the torn UCL is beneath the adductor aponeurosis, and there is clearly no Stener lesion.

MRI sometimes gets a bad rap for the evaluation of Stener lesions, because the test is often done poorly, and interpreted incorrectly. In the hands of a good MSK radiologist, however, it an excellent test.