Friday, February 29, 2008

Jersey Finger

Avulsion of the flexor digitorum profundus (FDP) tendon from its insertion on the base of the distal phalanx is known by the eponym "jersey finger". This injury of often a result from pulling on a jersey during an (American) football game. This injury was first described by VonZander in 1891, in a drummer.

Photograph of a flexor digitorum profundus tendon injury. Note that the injured finger is held in forced extension:

In the case of a FDP avulsion, MRI can be performed to see where the end of the torn, retracted tendon is.

Coronal images of the palm depict a flaccid FDP tendon in the palm, with surrounding edema:

Sagittal image of the ring finger reveals the
gap (red arrows) between the torn ends of the FDP and also depicts the intact flexor digitorum superficialis tendon (green arrows):

The first detailed classification of FDP avulsion was given by Leddy and Packer in 1977, and was elaborated upon by Buscemi and Page in 1987:

Type I- characterized by retraction of the tendon into the palm. There is often a painful nodule in the palm, where the retracted tendon lies. The vascular supply to tendon is disrupted, so it should be repaired as soon as possible, no later than 7-10 days after injury.

Type II- tendon retracts to the PIP joint. Further retraction is prevented by connections through the vincula longa; preserved vascularity through the vincula prevents necrosis and tendon contracture. Occasionally, a small bone fleck is avulsed, and this can be seen at the level of the PIP joint.

Type III- avulsion of a large bone fragment from the base of the distal phalanx. Tendon will not retract past the DIP joint, due to the A4 pulley. Must be treated with open reduction, because of possible associated avulsion of profundus tendon from the osseous fragment (Type IV lesion)

Type IV- type III lesions associated with a simultaneous avulsion of the FDP tendon from the fracture fragment.

Early diagnosis and treatment of these injuries is crucial. FDP avulsion is often incorrectly labeled a "sprain" or "jammed finger", treated with splinting, and the opportunity for timely treatment is lost. The cardinal sign of a FDP rupture is the complete loss of flexion at the DIP joint.

Vic David MD

Tuesday, February 26, 2008

Dinosaurs Bones and Foot Pain in a Runner

This is a metatarsal bone from the dinosaur Deltadromeus agilis:

(courtesy PaleoDirect)

The metatarsal bones of a human are a little smaller, but serve the same function. They help form the longitudinal arches along the inner and outer sides of the foot. Theodorou et al. (Radiology 2003;226:857-865) wrote a beautiful paper describing the anatomy of the base of the fifth metatarsal. Here is a photograph from that paper:

Surgical scissors are inserted under the peroneus brevis tendon (PB), close to the base of the fifth metatarsal (MT5). The lateral component of the plantar aponeurosis (PAL) that attaches to the fifth metatarsal base is well seen.

Thedorou et al. suggested that the tuberosity avulsion fractures are due to violent traction at the insertion of the conjoined fibers of the lateral component of the plantar aponeurosis and the peroneus brevis tendon.

When the lateral foot is stressed, biomechanical failure can occur at bone or in the soft tissue. When the bone fails, the result is a fracture. Soft tissue failure can occur at the insertion of the peroneus brevis and/or the lateral component of the plantar aponeurosis. Here is a 67 year-old runner with two months of lateral midfoot pain:

Sagittal (top set) and axial (bottow set) images demonstrate marked thickening and irregularity of the distal lateral component of the platar aponeurosis (red arrows). A comparison image from a different patient with normal anatomy is provided (green arrows).

Thus, this patient's symptoms are due to a partial tear of the lateral component of the plantar aponeurosis. Runners hate to stop running, but she is going to have to do that to get better.

I suspect that a dinosaur with the same problem would stop running, too.

Vic David MD

Friday, February 22, 2008

Neanderthals and Little League Shoulder

Americans are crazy about sports- witness that thirty million more people watched the most recent Super Bowl than the first Bush-Kerry presidential debate. I am ashamed to admit that I was one of the reprehensible Neanderthals that skipped the debate and ate pizza and drank beer through the Super Bowl.

This craze afflicts our youth as well, with surging injury rates in child athletes. We are seeing more and more acute and overuse joint injuries. Here is a 13 year-old male pitcher, who complained of pain while throwing. He was tender over the proximal humerus. (A) Coronal proton-density and (B) T2-weighted images show widening of the lateral humeral physis.

This injury is known by the eponym "Little Leaguer's shoulder", and is typically seen in the young teenage baseball player. It represents a stress-related injury to the growth plate. This injury is typically diagnosed using conventional radiographs, but one can occasionally run across it on MRI. Associated x-ray findings include sclerosis of the proximal humeral metaphysis and fragmentation of the lateral aspect of the proximal humeral metaphysis. It is treated conservatively, with several weeks of rest, and the majority of patients return to their athletic activity.

Pediatric musculoskeletal MRI can be challenging- one has to contend with the variable appearance of the skeleton through development (CRITOE, anyone?), and different diseases strike the pediatric population. I recently ran across an excellent book by J. Herman Kan and Paul Kleinman, devoted to pediatric musculoskeletal MRI. The pictures are of high quality, and the accompanying text is also excellent. Very much worth reading- nobody wants to miss an important diagnosis in a child.

Vic David MD

Saturday, February 16, 2008

Magnetic Susceptibility

Radiologists are fond of throwing around the term "susceptibility artifact", lexicon which can confuse rather than clarify. My teenage son is susceptible to the magnetic attraction of the opposite sex, but here we aim to demystify the radiologic term "magnetic susceptibility" .

"Magnetic susceptibility" refers to the internal magnetization of a substance when it is placed in a magnetic field. When two materials with different susceptibilites are placed next to one another, there will be a distortion in the local magnetic field. Since MRI scanners need a homogeneous magnetic field to create accurate images, any distortion of the magnetic field will lead to an artifact.

That, in a nutshell, is susceptibility artifact.

The main culprits in susceptibility artifact are metal and air.

This has real-world implications. Here is a patient who underwent prior surgery for osteomyelitis of the thumb. She had recurrent swelling of her thumb. Here is a sagittal gradient-echo image from her MRI:

The orthopedic surgeon called me and wanted to know what the black areas (yellow arrows) in the soft tissues represented. These could be areas of calcification (calcium can be dark on MRI), but the answer is revealed when we look at two additional images:

Note that these dark areas are prominent on the gradient echo image (green arrow), but are much less apparent on the T1 spin echo image (yellow arrow). The dark spots "bloom" on the gradient echo image. This is characteristic of susceptibility artifact- it is best seen on gradient echo (vs. spin echo).

The surgeon then told me that he did not use any instruments that would have left behind any metal, so how could this be metal (susceptibility) artifact? Well, there is a trail of metal debris left behind during surgery. One can almost always see some metal artifact, if gradient echo sequences are used, no matter how meticulous the surgical technique was.

The other major substance that can cause susceptibility artifact is air. Here is an example of a shoulder MRI arthrogram, with iatrogenic air (yellow arrows) in the subscapularis, introduced during the course of the joint injection. Note that the air cannot be seen on the axial T2 fatsat spin echo image, since spin echo images are much less prone to susceptibility artifact:

Whether you are a surgeon or radiologist, susceptibility artifact will reveal your tracks!

Vic David MD

Horse or Zebra?

Every clinician is eventually called on to evaluate a possible inguinal hernia. When confronted with a common clinical problem, it's easy to assume that it is yet another case of something you have seen a thousand times before. As we learned in medical school, "When you hear hoofbeats, think horses, not zebras".

Unfortunately, this is walking down the garden path. Every once in a while, the hoofbeats turn out to be a zebra!

Twenty-nine year-old male presented with an inguinal mass, suspected to be a hernia by the surgeon. The mass looked "odd" on an ultrasound and the patient had a follow-up MRI.

(A) Coronal T1-weighted image shows a predominantly fat signal mass (red arrows) occupying the right inguinal canal, and extending into the scrotum. (B) The mass (white arrow) is dark on a STIR image, confirming that it is primarily fatty in nature.

Axial T1-weighted image shows the mass (red arrows) in the scrotum, adjacent to a normal testicle (green arrow):

It's easy to call this an inguinal hernia, but there is absolutely no bowel in the mass. This is an example of a scrotal lipoma.

The earliest publication I can find is from 1925, when surgeon G. Paul LaRoque of Richmond, Virginia described this entity.

(click on image to enlarge)

I have seen two of these lesions in the last three years, and my practice is largely musculoskeletal radiology. These lesions, while rare, should be in the differential diagnosis when you are assessing an inguinal or scrotal mass, especially on ultrasound. 

Vic David MD

Saturday, February 9, 2008

Spring Ligament Tear

The ancient Greek physician Galen is said to have given the first written description of pes planus, or flatfoot. One common cause for flatfoot is tearing and dysfunction of the posterior tibial tendon, which helps maintain the medial longitudinal arch.

Another stabilizer of the medial longitudinal arch is the calcaneonavicular (spring) ligament. The spring ligament is composed of three components: superomedial, medioplantar oblique, and inferoplantar longitudinal. Of these, the superomedial component is the most important component. All three components of the spring ligament can be identified on MRI.

In this case, a 59 year-old woman presents with a flatfoot. (A) There is a complete tear of the superomedial portion of the spring ligament (red arrow). (B) Different patient, with a normal spring ligament (green arrow), for comparison.

Interestingly, this patient's posterior tibial tendon was thickened distally, but remained intact. The medioplantar oblique, and inferoplantar longitudinal ligaments were hypertrophied (not shown). In addition, there was extensive abnormal signal in the sinus tarsi, a common finding in patients with abnormalities of the posterior tibial tendon and spring ligament.

Spring ligament tears can be repaired (typically in conjunction with a repair of the posterior tibial tendon), enhancing the stability of the medial longitudinal arch. It's not enough to look for the integrity of the posterior tibial tendon on MRI- look carefully at the spring ligament as well.

Galen's fame is not restricted to the foot- his work dominated Western medicine for over a millennium. His vivisection experiments included tying off the recurrent laryngeal nerve to show that the brain controls the voice, and tying off the ureters to demonstrate kidney and bladder functions.

Vic David MD