Are there always 8 FDP tendons?
Hyatt B et al: Absent Ring Finger Flexor Digitorum Profundus Presenting as a Jersey Finger. J Hand Surg Am. 2016 May;41(5):e95-7
The authors present a case report of a 19 year-old man who caught his middle, ring, and small fingers in a football helmet forcing them into hyperextension. He had marked pain and tenderness at the PIP and MP joints of the ring finger and slightly limited active extension. He had no tenderness or active flexion at the DIP joint although he had a normal digital flexion crease. X-rays were normal.
The doctor logically assumed that the patient had a jersey finger—avulsion of the FDP tendon from the distal phalanx. The mechanism of injury, the finger involved, and the absence of active flexion at the DIP joint all certainly pointed to that diagnosis. At surgery, however, the FDP tendon and the A5 pulley were completely absent and there was no recent bleeding or evidence of tendon shredding. Exploration in the palm and forearm confirmed absence of the entire FDP muscle/tendon unit to the ring finger.
Post-surgical MRI and ultrasound evaluations confirmed the absence of the FDP muscle/tendon to the ring finger. The patient could not recall any old injury and had not previously noted whether or not he could flex the DIP joint in his ring finger. Careful examination of the patient’s other hand revealed no anomalies.
In the case report’s discussion, the authors cite 5 previous reports of congenitally absent FDP tendons and/or muscle bellies, but none involved the ring finger. Then they cited other case reports describing hypoplastic or unusual branching patterns of FDP and FDS tendons. Previous studies have determined that the FDS to the small finger is absent in 6-16% of individuals. Although the authors could not entirely rule out a long-forgotten avulsion of the FDP tendon in the patient reported here, the absence of the A5 pulley and lack of tendon findings by visualization and imaging support the idea of a congenital absence.
COMMENT: I am sure that the surgeon was shocked when what was going to be a routine FDP reattachment turned into an anatomical intrigue. In retrospect, the absence of tenderness and bruising at the DIP joint might have provided a clue. Conversely, the presence of a distal flexion crease implies that before injury the patient had flexion at the DIP joint via some means. The presence of the flexion crease remains unexplained.
This report highlights the truism that anatomical anomalies abound. Just as people vary in height, build, color, and facial characteristics, a great number of variations also occur internally.
A common one seen in the hand is the lack of independence of the FDS to the small finger. Usually FDS function is tested by holding the remaining fingers in extension and asking the person to flex their PIP joint. Try it yourself. Roughly 30% of individuals cannot flex their small finger PIP joint until their ring finger is freed, then they can flex the PIP joints in both fingers. This is because of a side-to-side tenodesis between the FDS to the ring and small fingers. Unaware of the existence of this rather common anomaly, one could conclude erroneously that the FDS to the small finger was absent, as it actually is in about 10% of hands. You can learn more about this tenodesis and the Linburg Comstock tenodesis and the Elson maneuver to test for an acute boutonniere injury HERE.
So it might be a few days before any of us spot a ring finger with a congenitally absent FDP tendon in the ring finger, but now that I know that it can happen, I will have my antenna up. Such anomalies keep anatomy interesting. Imagine how boring life would be if we were all exactly the same height, shape, and color? Same goes for inside: variety adds spice. It makes us professionals to be aware of such.