Saturday, January 24, 2009

Darwin and an Anomalous Muscle

The theory of evolution posits that natural selection acts on the phenotypic expression of genetic variation, driving population changes over time. The classic example of genetic variation, observed by Charles Darwin, was the different appearance of the beaks of finches in the Galapagos islands:

The different beak shapes are adapted to different food sources, and affect the relative survival of various finch species. From these seminal observations, Darwin developed his theory of evolution.

Humans, like finches, also exhibit variations in their bodies. This variation is often plain to see, with different body shapes and sizes, skin color, and the like. Other variations are more subtle, and require cross-sectional imaging to detect.

In this case, a 34 year-old woman presented to a hand surgeon with a lump on the dorsum (back) of her wrist. The lump had been present for a few years, but had become more bothersome recently, exacerbated by certain activities. She was sent for an MRI:

Axial T1 and T2 fatsat images reveal a mass-like area (red arrows) corresponding to the clinical finding. The mass is close to muscle in signal intensity. A coronal image depicts the sharp contours of the mass-like area (red arrow), situated between the extensor tendons (green arrows):

Sagittal images help confirm the diagnosis of an accessory muscle, the extensor digitorum brevis manus (red arrow, figure A). Compare to an image from a normal individual (image B):

The extensor digitorum brevis manus (EDBM) muscle occurs in 1-3% of the population and may be mistaken for a mass or ganglion. The EDBM origin is variable, but the most common origin is the dorsal wrist capsule deep to the extensor retinaculum. The muscle may also arise from the distal radius or the deep carpal fascia. It typically inserts onto the extensor hood of the index finger or middle finger but may also insert into the extensors of the fourth and fifth fingers by way of either a tendon or a slip.

On MRI, extensor tendons should not have a muscular component at and distal to level of carpus. The presence of muscle tissue in this region indicates the presence of an accessory muscle.

The EDBM is usually asymptomatic, but increased use of the hand may lead to pain. Conservative treatment is pursued in most cases, but surgical removal may be necessary. Note that resection of EDBM should be avoided in cases where the EDBM is compensating for the absence of the extensor indicis proprius.

In this particular case, the patient elected to modify her activity level, rather than undergo surgery.

Nature gives finches different beaks, and humans different muscles.

Vic David MD

Friday, January 16, 2009

Acetabular Retroversion and the Library of Congress

Above is the Library of Congress— it is impressive looking, but it's very physical presence dooms it to obsolescence. Information wants to be free of physical presence and physical restraints. Wikipedia and Google "own" the Library of Congress, as my kids would say, when it comes to the accessibility of information.

Background information on the hip joint, straight from Wikipedia, one of those free sources of information:

There are three bones of the os coxae (hip bone) that come together to form the acetabulum. Contributing a little more than two-fifths of the structure is the ischium, which provides lower and side boundaries to the acetabulum. The ilium forms the upper boundary, providing a little less than two-fifths of the structure of the acetabulum. The rest is formed by the pubis, near the midline. The word acetabulum means "little vinegar cup", and was the Latin word for a small vessel for storing vinegar.

In the last few years, femoroacetabular impingement (FAI) has become recognized as a cause of hip pain in adults. Much has been written about this entity, and most physicians are now aware of FAI. The topic is complex, from a diagnostic as well as treatment standpoint.

The two main types of FAI are termed the "cam" and "pincer" forms. In this entry, we will not review FAI; rather, we will focus on how to recognize the condition of acetabular retroversion, which is associated with the pincer form of FAI.

In the normal hip, the acetabular opening is anteverted (opens anteriorly). In the retroverted condition, the superior aspect of the acetabulum is tilted posteriorly. In both normal and retroverted hips, the opening gradually tilts anteriorly as one proceeds inferiorly.

On X-ray, the anterior rim of acetabulum should always project medial to the posterior wall, in a normal anteverted acetabulum:

(A) Edge of anterior acetabular wall (green arrow) is medial to the edge of posterior wall, even in superior aspect of acetabulum. (B) Green and yellow lines denote the anterior and posterior edges of acetabular wall. Note that these lines never cross.
In cranial acetabular retroversion, the anterior rim will project lateral to the posterior wall in the superior aspect of the acetabulum. Views from a CT topogram, in a hip with cranial acetabular retroversion:

(A) Edge of anterior wall (green arrow) is medial to the edge of posterior wall (yellow arrow) in inferior segment of the hip, but in the superior segment, this relationship reverses. This leads to the "crossover sign". (B) Illustration of crossover sign, with lines drawn.
On CT and MRI, cranial acetabular retroversion is recognized by examining the first axial image that includes the femoral head. If the acetabulum is retroverted, the anterior rim of the acetabulum will be lateral to the posterior rim:

(A,C) Axial images that contain the top of the femoral head demonstrate cranial acetabular retroversion (B,C) Axial images from a different patient at a similar slice position demonstrate normal acetabular configuration at this level (anteversion).

Thus, one can recognize acetabular retroversion on both conventional radiography and cross-sectional imaging. Just know what to look for....

Vic David MD

Saturday, January 3, 2009

All that Meets the Eye

Photo by badboy69

"There is more than meets the eye"— a nice summary of the ability of MRI to peer deep inside tissues.

If we compare MRI and arthroscopy, it is clear that the arthroscopist has an unparalleled visualization of surface anatomic features. Once the arthroscope is introduced into a joint, the surgeon has a marvelous view of the surfaces of cartilage, ligaments, bone, and soft tissue. While diagnostic arthroscopy is a powerful tool, it is important to remember that it only sees what is on the surface.

MRI does not approach the resolution that arthroscopy provides, but enables us to look inside tissues. Exclusive reliance on what is seen at arthroscopy can lead to underestimation of pathology.

In this case, a 36 year-old female with shoulder pain was referred for an MR arthrogram. Coronal T1 weighted images with fat saturation demonstrate a SLAP lesion (yellow arrows). There is also globular hypointense signal within the distal supraspinatus tendon (red arrows), consistent with an area of calcification within the tendon. Calcification in this area was confirmed by a CT scan (not shown).

At arthroscopy, the SLAP tear was repaired, but the calcification within the substance of the tendon could not be observed. It is clearly present, but because it is within the tendon, it was not apparent at arthroscopy.

In this case, there was no edema within the tendon or in the peritendinous tissues on MRI, suggesting that this was a clinically silent area of calcification, rather than active calcific tendinitis. The patient responded well to her SLAP repair, and became pain-free postoperatively.

The radiologist should remember that the surface resolution of arthroscopy far exceeds that of MRI. The surgeon should remember that all that is important does not meet the eye of the arthroscopist.
Thus, MRI and diagnostic arthroscopy are complementary techniques. Melding the information gleaned from each discipline provides the orthopedic surgeon with maximal information.

Vic David MD