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Corresponding author: Zachary L. Bernstein, BS, Leni & Peter May Department of Orthopaedics, Mount Sinai Hospital, 5 East 98th Street, Box 1188, New York, NY 10029.
The flexor digitorum superficialis tendon to the little finger (FDS-5) has been observed to have a higher degree of functional and structural variation than the FDS of other digits. FDS-5-deficient individuals necessarily rely on the flexor digitorum profundus tendon to the little finger (FDP-5) for flexion in their little fingers. FDS-5 deficient patients who experience a considerable injury to their FDP-5 are therefore at a risk of losing substantial little finger flexion. The purpose of this study was to evaluate the degree of flexion of the little finger at the metacarpophalangeal and proximal interphalangeal (PIP) joints in a cadaveric model of FDS-5 deficiency following amputation of the distal phalanx.
Methods
Ten fresh-frozen cadaveric upper extremities with no prior trauma were used. Loads were applied to the FDP-5. Flexion at the PIP and metacarpophalangeal joints was measured in degrees with a goniometer. Little finger flexion testing was conducted under 5 different conditions: “baseline,” “FDS-deficient,” “no repair,” “bone anchor” repair, and “A4 pulley” repair.
Results
The results were as follows: (1) no significant differences in the flexion between baseline and FDS-deficient conditions; (2) a significant decline in PIP flexion in the no repair condition after FDP-5 division compared with the FDS-deficient condition; (3) a significant restoration in PIP flexion in both surgical repair groups compared with the no repair group; and (4) no significant differences in PIP flexion between the A4 pulley and bone anchor groups.
Conclusions
The bone anchor repair and the A4 pulley repair demonstrate similar abilities to restore flexion of the little finger at the PIP joint to baseline levels in this cadaveric model.
Clinical relevance
A clinical protocol is yet to be established for the surgical treatment in FDS-5-deficient patients requiring amputation of the distal phalanx of the little finger. This study aims to address this area of uncertainty by comparing the little finger flexion after 2 different approaches to profundus tendon reattachment that may be applicable in this clinical scenario.
The flexor digitorum superficialis (FDS) is a multifunctional muscle that flexes the proximal interphalangeal (PIP) joint and, with further excursion, flexes the metacarpophalangeal (MCP) joint. The FDS tendon to the little finger (FDS-5) has been observed clinically to have a higher degree of functional and structural variation than the FDS tendons of other digits.
First reported by Shrewsbury and Kuczynski in 1974, the functional and structural variants of the FDS-5 have been described extensively in the literature.
provided a comprehensive report of all reported structural variants of the FDS-5; among the most common of these were aplasia of the FDS-5 and tendon insertion abnormalities.
FDS-5 structural abnormality resulting in functional deficiency is a reported phenomenon that may be more common than is currently appreciated, even though it can be easily identified with a simple physical examination maneuver.
FDS-5-deficient individuals necessarily rely upon the flexor digitorum profundus to the little finger (FDP-5) for all the flexion that they are able to achieve in this digit. Accordingly, FDS-5-deficient patients who experience a significant injury to their FDP-5 are at a risk of losing substantial little finger flexion.
to assert that when these FDS-5-deficient digits lose the profundus, they lose everything.
Injuries to the fingertips are among the most common hand injuries, but a protocol is yet to be established for treatment in FDS-5-deficient patients who require amputation or have undergone a traumatic amputation of the little finger distal phalanx.
The purpose of this study was to evaluate the degree of flexion of the little finger at the MCP and PIP joints in a cadaveric model of FDS-5 deficiency following the amputation of the distal phalanx. We postulated that the repair of the FDP-5 to either the middle phalanx or the A4 pulley would improve little finger flexion following a distal phalanx amputation compared with the no repair (NR) condition. We further hypothesized that no considerable differences in the little finger flexion would be detected between the 2 surgical repair conditions.
Materials and Methods
Specimen preparation
Ten fresh-frozen cadaveric upper extremities (distal to the elbow) with no prior upper-extremity trauma were used for this experiment. All soft tissues besides the interosseous membrane were dissected away from the proximal end of the specimens, leaving approximately 3 inches of free radius and ulna. The proximal radius and ulna, along with a hollowed cylindrical brass guide track, were potted in a cubic aluminum vessel (2.5 × 2.5 × 2.5 inch) using a 3:4 mixture of acrylic self-curing liquid and acrylic repair powder (Henry Schein Inc). A #2 FiberWire (Arthrex) suture was fastened to the FDP-5 at the level of the wrist, threaded proximally through the cylindrical brass guide track and was knotted on the other side of the acrylic potting apparatus. This knot allowed for later application of a load on the FDP-5. A 4-mm Steinmann pin was inserted down the third metacarpal, traversing the carpus, and inserted approximately 1 inch into the distal radius to immobilize the wrist during testing. All specimens were carefully inspected after pin placement to ensure that they were fixed in neutral flexion. During testing, the specimen was secured to the table with a standard C-clamp. This design was based on the report by Demirkan et al.
To account for tendon creep, a 1-kg weight was tethered to the suture loop for 5 minutes while a 5-kg weight was softly pressed down on the little finger to hold it in extension. To create a load, an exact mass was attached to the FDP-5 suture loop, which hung vertically and transmitted a horizontal tension force on the FDP-5 equal to the weight of the hanging mass. We based mass values on a prior work by Kursa et al,
who identified the amount of physiologic tension in the profundus to the index finger during active unresisted flexion and static held flexion to be 4.76 N and 7.01 N, respectively. The application of 475 g and 700 g mass transmitted a 4.65 N and 6.86 N horizontal tension force, respectively, on the FDP-5. Flexion at the PIP and MCP joints was measured in degrees with a goniometer such that 0° was the point of neutral flexion of the joint being measured. In each test condition, the load was applied for 1 minute before the angle of flexion was recorded.
Surgical conditions
Little finger flexion testing was conducted under 5 different conditions (Fig. 1). Three of these conditions served as controls: baseline, FDS-deficient, and NR, whereas the other 2 conditions were experimental surgical FDP-5 repairs: bone anchor (BA) and A4 pulley. In all specimens, baseline, FDS-deficient, and NR conditions were measured in that order before the 2 surgical conditions. Each specimen was randomly assigned into 1 of 2 groups (n = 5) to control for the order of application of the 2 surgical repairs. The first group received the A4 repair followed by the BA repair, whereas the second group received the BA repair followed by the A4 repair.
Figure 1Lateral view of condition groups. A FDS-deficient condition. Little finger with a red line labels the site where FDS-5 was cut. B A4 condition C BA condition. D NR condition. Baseline condition not pictured. Images used with permission from Mount Sinai Health System.
The baseline condition was defined as the normal and undisturbed specimen (all specimens had fully developed FDS-5). To establish the FDS-5-deficient condition, the FDS tendon to the little finger was carefully incised at the palmar level using a scalpel, eliminating the connection between the distal FDS-5 insertion at the middle phalanx and the rest of the FDS (Fig. 1A). The palmar incision was closed with a 4-0 Vicryl suture (Ethicon). Thereafter, a simple distal phalanx disarticulation was performed using a System 6 Sagittal saw (Stryker Corp), while preserving a finger flap for the closure of the amputation. The A4 condition was established by suturing the distal FDP-5 to the A4 pulley of the little finger (Fig. 1C). This was accomplished using 2 stitches running through the detached FDP-5 and continuing through the A4 pulley, similar to the Zancolli technique.
The BA condition was established using 2 micro-BAs (Dupuy Synthes MiniLok QuickAnchor Suture Anchor), which were inserted just proximal to the condyle of the middle phalanx and just distal to the distal edge of the A4 pulley (Fig. 1B). Once secured in place, the BA sutures were deployed and used to secure the FDP-5 to the BAs (Fig. 2). In both profundus repair conditions, the tendon was shortened approximately 1 cm to maintain the relative lengths of the tendon and bone.
Figure 2Anterior view of distal phalanx amputation, FDS-5 palmar incision, and surgical repair conditions. A Two horizontal red lines mark where the FDS-5 palmar incision was made (proximal red line) and where the distal phalanx amputation was made (distal red line). B A4 pulley suture repair following distal phalanx amputation. C BA repair following distal phalanx amputation. Images used with permission from Mount Sinai Health System.
The little finger flexion was recorded as degrees from the horizontal (full extension), defined as the neutral flexion of the joint. The mean ratio of flexion observed in a given specimen to the baseline flexion seen in that same specimen was also recorded in order to account for interspecimen variability in baseline flexion. Adequate adherence of the normalized data to a Gaussian distribution was confirmed using the Shapiro-Wilk test for all groups (Fig. 3). Statistical analysis was performed with a 1-way analysis of variance at each tendinous load level and with Sidak post hoc multiple comparisons test. Statistical significance was determined by P < .05. The sample size of 10 specimens per studied condition was selected to ensure that the power for all pairwise comparisons of PIP joint flexion would be ≥0.80. In determining the number of specimens required to supply the desired power, an effect size of 15° was used (ie, the mean difference in PIP flexion between the groups would have to be at least 15° to be considered clinically significant). Additionally, the expected variance of flexion data was approximated using the results of a pilot study conducted previously on 4 specimens.
Although there was no significant difference in the flexion achieved at the MCP joint among groups on 1-way analysis of variance, there was a significant difference in the flexion achieved at the PIP joint among groups at both levels of tendinous loads (P < .05, Fig. 4). A post hoc multiple comparisons analysis showed that at both tendinous loads, there was a significant reduction in PIP flexion in the NR condition compared with the FDS-deficient condition (4.65 N: P < .05, mean = −17.8°; 6.86 N: P < .05, mean= −18.5°, Fig. 4B). At both tendinous loads, we observed a significant increase in PIP flexion in the A4 condition compared with the NR condition (4.65 N: P < .05, mean = +23.4°; 6.86 N: P < .05, mean = +25.9°, Fig. 4B). Similarly, we observed a significant increase in PIP flexion in the BA condition compared with the NR condition (4.65 N: P < .05, mean = +29.3°; 6.86 N: P < .05, mean = +26.3°). There were no significant differences in PIP flexion between A4 and BA conditions (Fig. 5).
Figure 4Degrees of flexion between condition groups at both tendinous loads. Error bars represent standard error of the mean. A MCP joint. B PIP joint. Asterisk represents statistical significance (P < .05).
Figure 5Degrees of flexion in PIP joint, between condition groups at both tendinous loads, represented as deviation from the baseline condition (divided by the degrees of flexion at baseline). Above the bar represents an increase in flexion, and below the bar represents a decrease in flexion. Error bars represent standard error of the mean. Asterisk represents statistical significance (P < .05).
The motivation for this study stems from the literature concerning the functional and structural anomalies of the FDS-5. There remains a paucity of both biomechanical analyses of these anomalies as well as in-depth discussions of the implications they have on the surgery of the little finger. The following observations were made from this study: (1) there are no significant differences in flexion between baseline and FDS-deficient conditions; (2) there is a significant decline in PIP flexion in the NR condition after FDP-5 division compared with the FDS-deficient condition with the FDP-5 intact; (3) there is a significant restoration in PIP flexion in both surgical repair groups compared with the NR group; and (4) there are no significant differences in PIP flexion between the A4 and BA group. These findings suggest that the BA repair and the A4 pulley repair confer similar potential to restore the flexion of the little finger at the PIP joint to baseline levels. Of note, we did not observe a full loss (0°) of flexion in the NR condition, a finding that we attribute to tendinous forces transmitted to the middle phalanx by the vinculum that connects the FDP and FDS. Our results suggest that while there is still some flexion that may be achieved in the little finger following a distal phalanx amputation, it is significantly diminished, corroborating Baker et al’s
earlier contention. The repair methods used in this study, however, have the potential to significantly restore flexion of the little finger.
Our report suggests that in clinical practice the surgeon may consider using either of the FDP-5 repair techniques used in this study. Future clinical studies are needed to compare the efficacy of these 2 surgical repairs (or other similar repairs) in terms of recovery of flexion, as well as in terms of pain, overall function, complication rate, and cost. Future research may build upon this study’s experimental design to evaluate different metrics of tendon function, such as tendon pull-out strength and flexion during varying levels of wrist flexion.
Interestingly, contralateral asymmetry of the FDS-5 is a commonly reported phenomenon.
As such, contralateral examination cannot serve as a definitive method to determine if the afflicted finger is deficient of the FDS-5. Therefore, the results of our study are most appropriately applicable to clinical scenarios in which the patient is known to be FDS-5-deficient, especially if injury renders it impossible to test for the presence of FDS-5 deficiency. At a minimum, we emphasize that the appropriate physical examination for FDS-5 deficiency should be performed prior to any surgical procedure involving amputation of the distal phalanx of the little finger as part of a thorough preoperative assessment.
There are several limitations to this study. First, dividing the FDS-5 at the palmar level to model its deficiency is not perfectly analogous to what is seen clinically. As previously noted, FDS-5 deficiency has a variety of accompanying anatomic variations and a simple division of the FDS-5 at the palmar level does not capture the structural intricacies of all previously reported anatomic anomalies. In addition, this study was subject to limitations that affect all biomechanical studies that employ a cadaveric model, such as joint stiffness, tissue dehydration, and skin deterioration.
This experiment elucidates potential treatment options to restore flexion in an FDS-5-deficient patient requiring little finger distal phalangeal amputation and may help guide future biomechanical studies focused on the pathology, injuries, and surgical techniques that implicate the flexor tendons.
Acknowledgments
The authors acknowledge Jill Gregory for her kind support and illustrational contributions.
References
Yammine K.
Erić M.
Agenesis, functional deficiency and the common type of the flexor digitorum superficialis of the little finger: a meta-analysis.
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