Effect of a Flexor Digitorum Superficialis Hemitenodesis on Reducing Volar Plate Strains for Swan Neck Deformities

Abstract

Background: Flexor digitorum superficialis (FDS) hemitenodesis is a common procedure to treat swan neck deformity (SND). We hypothesize that this surgical technique is a biomechanically effective way to reduce strain in the volar plate at the proximal interphalangeal joint (PIPJ).

Methods: Fifteen digits from 5 cadaveric specimens were tested using a novel in vitro active finger motion simulator under 4 finger conditions: intact, SND, FDS hemitenodesis, and FDS hemitenodesis with distal interphalangeal (DIP) joint fusion. Tensile loads in FDS and flexor digitorum profundus (FDP) and joint ranges of motion were measured by electromagnetic tracking. In addition, strain gauges were inserted under the volar plate to measure strain during PIPJ hyperextension. Results were analyzed using 1-way repeated-measures analysis of variance tests.

Results: The SND condition increased volar plate strain by 176% ± 25% (P < .001) compared with the intact condition. The FDS hemitenodesis repair relieved more than 50% of the SND strain, restoring it to within no statistical difference from intact. The DIP fusion further reduced strain with no further statistical significance. At full flexion, FDS and FDP tendon loads diverged as a function of the test condition (P < .001). With the FDS hemitenodesis, the FDP load increased by 2.1 ± 1.5 N from the SND condition (P < .001), whereas the FDS load decreased by 1.3 ± 1.3 N (P = .012).

Conclusion: The FDS hemitenodesis repair restored strains to within 3.0 milli-strain of the intact condition with no significant difference. Application of DIP fusion did not further protect the PIPJ from increased hyperextension and further exacerbated the imbalance of flexor tendon loads.

Keywords: FDS hemitenodesis; hyperextension; strain; swan neck deformity; tendons; volar plate.

Figure 1.

Specimen setup in motion simulator: Linear servo actuators were used to generate active finger motion. Tendon loads were controlled and measured using uniaxial load cell feedback, and finger motions were recorded by electromagnetic tracking.

Figure 2.

Flexor digitorum superficialis hemitenodesis technique: One slip of the flexor digitorum superficialis was used as a static restraint secured to the A2 pulley. Note. (a) A transverse incision in the distal third of the A2 pulley was created, and a small tendon retriever was used to pull the ulnar slip through the incision. Subsequently, tenodesis was completed by (b) looping the slip over the A2 pulley and (c) suturing the slip onto itself with 2 figure-of-8 sutures.

Figure 3.

Insertion process of strain gauge: The volar plate was identified, and a 5-mm transverse incision was performed over its insertion to the middle phalanx. Note. A strain gauge was inserted underneath the volar plate (a, b) and secured with a suture (c).

Figure 4.

Finger conditions: The 3 finger conditions following intact are illustrated. Note. (a) Swan neck deformity, (b) flexor digitorum superficialis hemitenodesis, and (c) distal interphalangeal fusion. The hyperextension of the proximal interphalangeal joint and the flexion of the distal interphalangeal joint represent the characteristics of a true swan neck deformity case. A fulcrum in (a) and (b) was used underneath the digit to illustrate the significant deformation and subsequent repair in the finger.

Figure 5.

Change in PIPJ angle at terminal extension: At terminal extension, the change in PIPJ’s terminal angle is plotted as a function of finger condition, where whiskers denote the standard error of 15 finger specimens. Note. Simulation of an SND condition caused an overall increase of 0.2° ± 0.6° in PIPJ extension (hyperextension) compared with the intact condition; however, it was not statistically significant. Repair using FDS hemitenodesis and DIP fusion caused an overall decrease of 10° ± 4° and 11° ± 4°, respectively. PIPJ = proximal interphalangeal joint; SND = swan neck deformity; FDS = flexor digitorum superficialis; DIP = distal interphalangeal.

Figure 6.

Volar plate strain: At DIP fusion’s terminal extension, the volar plate strain is plotted as a function of finger condition (*P < .05), where whiskers denote the standard error of 15 finger specimens. Note. The SND condition caused a 176% ± 95% increase in strain (P < .001) compared with the intact condition. Subsequent repair with the hemitenodesis resulted in 181% ± 523% significant reduction in strain compared with the SND condition, restoring strains to within 3.0 milli-strain of the intact condition (P = .158). DIP fusion also produced an added apparent reduction of 87% ± 252% in strain compared with the hemitenodesis condition, though not statistically significant (P = .776). DIP = distal interphalangeal; SND = swan neck deformity; FDS = flexor digitorum superficialis.

Figure 7.

Flexor tendon loads: At full finger flexor, flexor loads, FDP and FDS, were plotted as a function of the finger condition (*P < .05), where whiskers denote the standard error of 15 finger specimens. Note. FDP and FDS load values diverged as a function of the finger condition (P = .001). With the hemitenodesis, FDP load caused a 99% ± 12% increase, whereas FDS load experiences an 77% ± 10% decrease from the SND condition. Simulation of the DIP fusion continued to cause a divergence in tendon load, further increasing FDP load by 86% ± 6% and decreasing FDS load by 81% ± 7% compared with the hemitenodesis condition. FDP = flexor digitorum profundus; FDS = flexor digitorum superficialis; SND = swan neck deformity; DIP = distal interphalangeal.