Case 1. A twenty-year-old woman was referred because of a three-month history of painful snapping of the left small finger. She had started to play the drums three weeks before the symptoms had developed, and she noted constant and painful snapping with flexion of the metacarpophalangeal joint.
On examination, the extensor tendon of the small finger dislocated radially at the level of the metacarpophalangeal joint with flexion of >30°. There was an obvious snapping sound at the time of dislocation (Fig. 1, A). This finding was not related to wrist or elbow motion. A diagnosis of extensor tendon dislocation of the small finger was made, and surgical reconstruction of the sagittal band was planned.
With the patient under local anesthesia, exploration through an oblique skin incision over the fourth intermetacarpal space revealed a thick cord-like structure (junctura tendinum) crossing obliquely from the extensor digitorum communis of the ring finger to the distal part of the metacarpophalangeal joint of the small finger. The radial sagittal band was intact. The junctura tendinum slipped off the metacarpal head radially as the joint flexed, leaving two slips of the extensor digiti minimi intact on the ulnar side (Fig. 1, B and C). The dislocating junctura tendinum was separated from its proximal attachment to the ring finger extensor digitorum communis and was sutured to the radial side of the extensor digiti minimi (Fig. 2, A and B). This effectively prevented any additional snapping. The small finger was immobilized in a plaster splint just short of full extension for two weeks. Passive range-of-motion exercise was advised for another four weeks, and unrestricted full active motion was allowed at the sixth postoperative week. The finger regained full range of movement without discomfort at twelve weeks postoperatively. There was no sign of recurrence or pain at the time of the latest follow-up, at twenty-eight months.
Case 2. A thirty-seven-year-old right-handed nurse, while working at the hospital, struck the dorsum of the metacarpal head of the right small finger lightly against a wall. Subsequently, she noticed the extensor tendon of the small finger slipping to the radial side of the metacarpal head each time she flexed the metacarpophalangeal joint. Its position returned to normal when the finger was extended. Over the next six weeks, the patient noted increasing pain with gripping and writing. On examination, dislocation of the tendon was produced fairly easily with flexion of the metacarpophalangeal joint. Radiographic examination revealed normal findings.
At the time of surgical exploration, a thick cord-like structure was found lying across the dorsum of the fourth intermetacarpal space, which dislocated radially over the small finger metacarpal head when the metacarpophalangeal joint was flexed actively by >30°. A junctura tendinum was identified, and it was separated from the extensor digitorum communis of the ring finger and was sutured to the radial slip of the extensor digiti minimi. Postoperative treatment was similar to that described for the previous patient (Case 1). At two years of follow-up, there was a full range of motion of the small finger without evidence of snapping.
Snapping of the extensor tendons of the middle and ring fingers at the metacarpophalangeal joint has been reported frequently4. However, we could find little discussion of snapping of the extensor tendon at the metacarpophalangeal joint of the small finger. In the present report, we describe the cases of two patients who were misdiagnosed as having snapping at the metacarpophalangeal joint of the small finger secondary to sagittal band injury when indeed the snapping was caused by a junctura tendinum.
There are numerous anatomic variations of the extensor tendons to the ulnar-sided fingers5-12. The ring finger is the least independent and typically has an oblique junctura tendinum originating from its extensor digitorum communis tendon7. The junctura tendinum can be either a fascial or a tendinous band on the dorsum of the hand, linking adjacent extensor tendons at a level proximal to the metacarpophalangeal joint. Although their anatomic arrangement has been well reported, their clinical importance has not been fully appreciated13.
Wehbé14 explored 240 hands and identified three types of juncturae: fascial, ligamentous, and tendinous. In the fourth intermetacarpal space, a tendinous type was predominant (79%) and originated from the extensor digitorum communis of the ring finger and ran to the small finger extensor digiti minimi.
Von Schroeder et al.15 explored forty hands and further classified three morphologic types of junctura tendinum. Type-3 juncturae were the narrowest and thickest, compared with the filamentous type-1 juncturae and the thicker connecting band type-2 juncturae. Type-3 juncturae were subclassified into "y" or "r" subtypes depending on the shape of bifurcation. When the extensor digitorum communis to the small finger was absent, type 3r was predominant, whereas when the extensor digitorum communis to the small finger was present, types 2 and 3y were common. Some of the variability noted in the literature concerning extensor digitorum communis-small finger tendon slips and juncturae may be in the identification of these structures. A tendon slip to the small finger may be similar to a junctura8.
Variations of common extensor tendon anatomy and the relationship with junctura tendinum have been well documented. Kaplan and Hunter16 reported that the extensor digitorum communis to the small finger is usually absent but that a junctura tendinum is usually present. Zilber and Oberlin11 noted the absence of the extensor digitorum communis to the small finger in 60% of specimens. When the extensor digitorum communis to the small finger was absent, it was replaced by a tendinous expansion arising from the distal part of the tendon of the ring finger, a junctura tendinum. Hirai et al.9 explored 546 hands and found that the small finger had a single extensor digitorum communis tendon or a single common extensor digitorum communis tendon distributed to both the ring and the small finger. El-Badawi et al.7 identified similar findings in the distribution of the extensor digitorum communis to the small finger.
The junctura tendinum that we observed in the fourth intermetacarpal space of our patients was a cord-like type-3r junctura tendinum that bifurcated from the extensor digitorum communis of the ring finger and attached distal to the metacarpophalangeal joint of the small finger. The slope and orientation of the junctura tendinum in the fourth intermetacarpal space demonstrated a rather distal attachment to the metacarpal head of the small finger, which may be related to a risk of radial slippage over the metacarpal head. There was no separate tendon that we could identify as the extensor digitorum communis to the small finger in either case. The cord-like type-3 junctura tendinum slipped radially over the metacarpal head with active flexion of >30°.
An anatomic and biomechanical study on the sagittal band by Young and Rayan3 showed that extensor tendon instability following sagittal band disruption was most common in the long finger and least common in the small finger. Those authors believed that the junctura tendinum between the ring and small fingers provided tendon stability after sectioning of the small finger radial sagittal band.
One possible cause of the snapping junctura tendinum in the fourth intermetacarpal space is trauma. One of our patients had repeated forced flexion to control a drumstick, and the other had a history of a minor blow to the dorsum of a partially flexed finger. Traumatic dislocation of the extensor tendon over the metacarpophalangeal joint occurs when the sagittal band is severed1,3,17,18. The mechanism of injury proposed by Bunnell19 (forced finger flexion while the wrist is flexed and in ulnar deviation) and that proposed by Wheeldon2 (forced finger flexion and ulnar deviation resisted by the extensor muscle) have been widely accepted. However, they do not completely explain the cause of snapping in our two patients. Despite the fact that both patients had relief of symptoms after simple surgical release, the exact cause remains unclear.
The surgical treatment for this type of injury remains controversial. In a case report, Benatar13 described the ring finger origin of the junctura tendinum being incised and split in a proximal direction parallel with the extensor digitorum tendon to the ring finger over a distance of 1 to 2 cm. Distally, the insertion of the junctura tendinum was reefed with the radialmost extensor digiti minimi tendon to the small finger. In so doing, the entire junctura tendinum is shifted in a more proximal position parallel to its original course; however, our approach differed. The procedure that we performed is simple and effective. Separating the junctura tendinum from the ring finger and suturing it to the extensor digiti minimi was successful for eliminating symptoms of snapping without recurrence.
When there is radial-sided snapping over the dorsum of the metacarpophalangeal joint of the small finger, the surgeon should suspect dislocation of the junctura tendinum in the fourth intermetacarpal space rather than a radial sagittal band injury. Knowledge of these anatomic variations in this area is important when treating these conditions. Simple release of the origin and suturing the junctura tendinum to the extensor digiti minimi provided satisfactory results without recurrence in our two patients. 
Note: The authors thank Youngsun Choi, MD, for her talented work on surgical illustrations in the paper.