Friday, September 12, 2008

Diving and the Thumb CMC Joint

Platform diving is acrobatic and beautiful:

Photo by shutterhack

Inevitably, there are some serious g forces exerted on the human body, especially if the rapid deceleration is less like a knife-like entry into the water, and more like one of my dives, the classic belly-flop:

Photo by j. cliss

Recently, a hand surgeon called me and told me about a set of patients who are platform divers, with intractable pain at the base of the thumb, at the thumb carpometacarpal (CMC) joint. She remarked that it would be nice to get some imaging information about the ligamentous structures supporting this joint, particularly along the dorsal aspect of the joint, a region that is difficult to visualize at arthroscopy.

There is not a great deal written about MR imaging of the thumb CMC joint. One good article was written by Connell et al.; there is also a Wandering Radiologist blog post.

There is some variability in how these ligaments are named, but if we follow the convention of Connell et al., there are four main ligaments:

1. Anterior oblique (beak) ligament- extends from the trapezium to the first metacarpal base, on the volar side of the joint.

2. Posterior oblique ligament- extends from dorsoulnar trapezium to the first metacarpal base.

3. Dorsoradial ligament- extends from the dorsoradial aspect of trapezium to the first metacarpal base. This ligament is reinforced by the abductor pollicis longus tendon.

4. Intermetacarpal ligament- runs from the radial base of the index metacarpal to the ulnar aspect of the thumb metacarpal base.

The combination of direct coronal and
oblique sagittal images of the CMC joint visualize the first three of ligamentous structures quite well, at 1.5 Tesla. The intermetacarpal ligament can also be seen, but is more wispy in character.

Consecutive oblique sagittal intermediate-weighted images of a normal volunteer:

The anterior oblique ligament (red arrows) is well seen, as is the posterior oblique ligament (green arrows). The dorsoradial ligament is depicted on this next set of images:

(A) Oblique sagittal and (B) direct coronal images delineate the normal dorsoradial ligament (pink arrows).

Direct coronal images are usually the best way to visualize the anterior oblique (beak) ligament:

The MRI technologists I work were instrumental in determining the optimal imaging planes for us to visualize these structures. This post illustrates once again the vital role the MR technologist plays in producing high-quality MR images.

Vic David MD

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