Feasibility of Homodigital Flexor Digitorum Superficialis transposition, a new technique for A2-C1 pulleys reconstruction: A kinematic cadaver study

Abstract

Introduction

Homodigital flexor digitorum superficialis transposition (HFT) is proposed as a new technique for A2-C1 pulley reconstruction. Flexor digitorum superficialis is transposed on the proximal phalanx and inserted on the pulley rims, crossing over flexor digitorum profundus and acting as a pulley.

Materials and methods

The kinematic feasibility was investigated in a cadaveric bowstring model (after A2 and C1 pulley removal) on 22 fingers (thumb excluded).

Results

HFT was effective in restoring the correct flexion of proximal and distal interphalangeal joints, compared to bowstring model. No adverse events were registered.

Conclusion

HFT is a feasible technique. Clinical application is encouraged.

1. Introduction

Flexor tendon system is frequently involved in complex hand trauma. Flexor tendons and pulleys lesions can simultaneously occur, and sometimes could be very difficult and laborious to reconstruct. A quick recovery of movement and function should be obtained, to avoid joint stiffness and motion impairment due to scar tissue adhesion.¹ Pulleys reconstruction is crucial to allow the normal tendons’ gliding, avoid bowstring effect of flexor tendons.^1,2

Pulleys reconstruction techniques usually require the use of tendon grafts, and sometimes they could be difficult to perform in surgical practice, because of the simultaneous presence of damage of skin, vessels, nerves, tendons (flexor and extensor), bone and joints.1, 2, 3, 4, 5, 6, 7, 8, 9, 10

We designed a new technique, that could be useful to restore the proximal fibrous digital sheaths, ensuring flexion of fingers without bowstring effect.

This technique consists in transposing the distal insertion of flexor digitorum superficialis (FDS) from the intermediate phalanx (P2) to the proximal one (P1) on the pulley rims, crossing ulnar insertion (UI) and radial insertion (RI) of the FDS over the flexor digitorum profundus (FDP). In this way FDS re-inserted on P1 can act as a pulley on FDP.

The simultaneous muscle activation of FDS and FDP was supposed to control the bowstring effect. We performed a cadaveric study on six hands, to investigate the kinematic feasibility of this new technique, that we called homodigital FDS transposition (HFT).

2. Materials and methods

Twenty-four fingers (6 index, 6 middle, 6 ring, 6 little finger), from 6 donors (4 men, 2 women). Thumb was excluded because of its anatomy. This study was performed to evaluate the PIPJ flexion after ICT. The mean age of the cadaver donors was 78.1 years (70–92 years). This study was performed from October 2019 to January 2020 in an authorized institutions for cadaver studies, in accordance with the approval of ethics committees.

All procedures were performed by two expert hand surgeons (M.P. and R.D.V.). The specimens were obtained within two days of death and stored at −5.2 °C.

Fingers which presented anatomical or pathological defects (e.g. finger overlapping for a possible previous disease, joint stiffness, tendons degeneration) were excluded from the study.

Specimens were thawed four hours before testing. Specimens included the forearm, to maintain the correct tracking of flexor tendons.

Brunner exposure was performed from distal interphalangeal joint (DIPJ) to metacarpophalangeal joint (MPJ). Surgical loupes were used to improve visualization of the pulleys. FDS and FDP were identified proximal to the wrist, where each tendon was sutured to itself, through a 2–0 vicryl one strand locking suture.

We know that a tension of 20 N or more provoke negative effects on a pulley which was reconstructed using a phalanx non-encircling technique.^3,4 The same thing applies to sutured tendons, where a suture site elongation can provoke healing impairment.¹¹ Hence a 20 N tension was enough to test the kinematic feasibility of HFT.

Goniometric values were obtained using a standard finger goniometer. Tension was measured using a dynamometer for each tendon (FDS and FDP). Both FDS and FDP were simultaneously and progressively pulled up to 20 N loading, that was maintained for 5 s. Goniometric assessments were performed during the maximum loading. Goniometric values were obtained as a mean of three consecutive measurements.

Once maximum flexion was obtained, range of motion (ROM) was estimated using a goniometer for each finger joint, MPJ, proximal interphalangeal joint (PIPJ) and DIPJ. Then, with the finger in full extension, A2 and C1 pulleys were identified and removed, making sure to maintain the insertions of the A2 pulley on the bone (pulley rims), that should be used as an anchor point for the FDS. Hence a Bowstring model was obtained (Fig. 1A). A simultaneous progressive and simultaneous tension was applied on FDS and FDP and goniometric measurements were performed, as described above. Then the HFT was performed.

Fig. 1.

A. A2 and C1 pulleys were removed, and bowstring effect occurred. B. Flexor digitorum superficialis (FDS) insertions were cut proximally to the A3 pulley. C. A tendon vinculum was removed, thus FDS was elevated, and ulnar and radial insertions of FDS were further separated through a cut, from distal to proximal. D. A Ulnar and radial insertions are inverted crossing over the flexor digitorum profundus (FDP), then they were pulled in divergent directions, to fix the FDS in the right position on the pulley rims. E. Ulnar and radial insertions crossed again over the flexor FDP, then they were fixed on the pulley rims on the opposite side. The end result was a four anchor-points transposition. In this photo, a blue background was positioned to highlight the virtual space between FDS and FDP. F. finger flexes without bowstring effect.

2.1. Surgical technique

A small scalpel (n. 15 blade) was introduced inside the A3 pulley to cut UI and the RI of the FDS on P2 (Fig. 1B). FDS was elevated and the P1 vinculum was removed.

At P1 FDS change its shape, from a unique tubular tendon it divides in a double-stranded flattened tendon. The two strands should be spread apart and divided for approximately 4 mm from distal to proximal (Fig. 1C). Thanks to the flattened shape of FDS, UI and RI could be inverted crossing over the FDP, without bulkiness. The order of inversion was considered accidental. Tendons should not be twisted.

At this point, FDS tendon was gently distally pulled, using forceps (Fig. 1D). UI and RI should be sutured using three simple stitches 5/0 prolene or nylon in the distal half of A2 pulley rims. The ends of the FDS should remain loose for almost 8–10 mm. The loose ends should cross again over the FDP distally, and should be sutured on the A2-C1 pulleys rims using simple stitches 5/0 prolene or nylon.

Further stitches could be added to get the FDS close to the pulley rims. Crossing points were reinforced with further stitches (Fig. 1E).

Finally a simultaneous progressive and simultaneous tension was applied on FDS and FDP and goniometric measurements were performed, as described above (Fig. 1F). A drawing of the final pulley reconstruction is presented in Fig. 2.

Fig. 2.

Drawing of the final pulley reconstruction. The flexor digitorum superficialis has four anchor points on the pulley rims, two on the ulnar side and two on the radial side.

2.2. Outcomes

The aim of the study was to compare flexion after HFT with pre operative values, with and without A2 and C1 pulleys. Adverse events were also registered (e.g. partial or complete tendon avulsion, other tendon lesions, tendon entrapment).

2.3. Statistics

The Kolmogorov-Smirnov test was used to check for normal distribution. Statistical analysis was performed using the Mann–Whitney U-test for two independent continuous variables. The significance was established for a value of p < 0.05. Data were reported as mean ± standard deviation (SD) for continuous variables. Statistical analysis was performed using the SPSS v.19.0 software (SPSS Inc.; Chicago, IL).

3. Results

Two fingers were excluded, one because of rigid PIPJ (ROM = 0–60°), and another one because of poor tendon quality (possible previous lesion of both FDS and FDP). The experimental study was executed on 22 fingers. Results are resumed in Table 1.

Table 1.

Comparison of flexion values. MPJ: metacarpophalangeal joint. PIPJ: proximal interphalangeal joint. DIPJ: distal interphalangeal joint. HFT: homologous Flexor digitorum superficialis Transposition.

	Normal finger	Bowstring	HFT	Comparison between normal finger and HFT (p value)	Comparison between bowstring and HFT (p value)
MPJ	88.2 ± 5.0	90.0 ± 4.9	88.0 ± 4.5	0.952	0.150
PIPJ	81.8 ± 5.9	70.5 ± 6.2	81.1 ± 6.2	0.387	<0.00001
DIPJ	65.2 ± 9.2	57.7 ± 7.0	65.2 ± 8.8	0.968	0.009

The mean flexion estimated on MPJ was 88.2.8 ± 5.0 in normal finger, 90.0 ± 4.9 in bowstring model, 88.0 ± 4.5 after HFT. No statistical differences were revealed between normal finger and HFT (p = 0.952), nor between HFT and bowstring model (p = 0.150).

The mean flexion estimated on PIPJ was 81.8 ± 5.9 in normal finger, 70.5 ± 6.2 in bowstring model, 81.1 ± 6.2 after HFT. No statistical differences were revealed between normal finger and HFT (p = 0.387). Difference between bowstring fingers and HFT was statistically significant (p < 0.00001).

The mean flexion estimated on DIPJ was 65.2 ± 9.2 in normal finger, 57.7 ± 7.0 in bowstring model, 65.2 ± 8.8 after HFT. No statistical differences were revealed between normal finger and HFT (p = 0.968). Difference between bowstring fingers and HFT was statistically significant (p = 0.009).

No adverse effects were observed (e.g. partial or complete tendon avulsion, tendon entrapment, other tendon lesions). A correct tendon gliding was observed in all cases.

4. Discussion

Three types of pulley reconstruction techniques have been described, phalanx encircling techniques,² through-phalanx techniques,⁵ and phalanx non-encircling techniques (all-palmar techniques).^12,13 HFT can be classified as an all-palmar technique.

Every pulley reconstruction procedure should respect the following principles. Firstly, the new pulley should be sufficiently long, to replicate the length of the original pulley.^6,14 Secondly, the new pulley should be sufficiently strong, to avoid a further rupture.^3,4,7 Hence synovial autologous structures should be preferred to non synovial one.^8,9,15 Lastly, local non-compromised tissues could be used, and autografts should be preferred to allograft and artificial tissues.² Phalanx encircling techniques might jeopardize bony vascularity.^2,16, 17, 18, 19 To avoid this, mini anchor techniques have been suggested, but foreign body reactions have been described.^15,20 Furthermore, phalanx encircling techniques can bother extensor apparatus.^2,5 Volar plate anchoring techniques have been described, but they can bother joint function.^2,7,21

Every hand surgeon should handle many different techniques for pulley reconstruction, to adapt to any case. In some cases many consolidated techniques are not possible. For example, phalanx encircling and through-phalanx techniques are not recommended in case of bone fracture and extensor tendon injuries. Every technique has some drawbacks.²

All-palmar techniques are considered weaker, if compared with phalanx encircling techniques, according to historical cadaveric studies.^3,4,7,22 Nevertheless cadaver models do not consider biological healing phenomena, thus quantitative estimates performed on cadaver models are incomplete. Indeed all-palmar techniques have been successfully used in high demanding patients too, with good results.23, 24, 25 In our experimental study, HFT demonstrated to be effective to maintain the FDP close to the bone, ensuring the restoration of the original ROM.

HFT is an all-palmar technique and does not bother dorsal structures, and, in our opinion, it is simple and easy to perform. It ensures a long four-point fixed pulley, FDS distally cross over the FDP and is fixed on P1 on two levels. A sacrifice of extra-digital structures is not required, and further surgical exposures can be avoided, reducing the duration of the surgical procedure. Then comes another advantage, HFT could be performed in a WALANT anesthesia.26, 27, 28 Hence the correct position of FDS can be intra-operatively checked, and eventually it can be fixed again. According to our findings, no suture detachments have been reported, and we reasonably presuppose that HFT could withstand a strain comparable with a flexor tendon suture in vivo. HFT allows to reconstruct proximal pulley system only, thus further reconstructions of more distal flexor tendon apparatus are still possible. A synovial tissue (FDS) is used, thus FDP can be safely close to the bone, with a minimal gliding stress.^29,30 Furthermore, after HFT is performed, the fibrous digital sheath is deprived of FDS, thus FDP gliding stress is reduced. We reasonably suppose that post-traumatic/surgical inter-tendon adherence should also be reduced.

FDS is maintained as a motor muscle. When FDS and FDP contracts, FDS allows P1 to follow the finger's movement, nevertheless it is moved from its original position. We do not know if such transposition can limit hand function. Dynamic in vivo study should be performed to clarify this point. Our experimental study focused on the feasibility of the technique we described in detail above. However, we personally consider that further technical improvement could be performed, based on further cadaver model studies and in vivo.

Being effective in ensuring a very satisfying ROM, we think that this technique is feasible in surgical practice, it can be assumed to be as strong as other similar techniques, that have been previously applied on climbers.^24,25

4.1. In vivo feasibility

We suggest this technique be performed in the following cases:

• •Both FDS and FDP acute lesion and contemporary A2 pulley severe damage
• •as a salvage procedure, when other techniques have failed.

According to our experience, this technique can be used when phalanx encircling techniques are not feasible, for example in some cases of complex hand and finger trauma. In those cases, the use of HFT could be helpful to quickly restore finger and hand function.

It may be argued that, after a complex trauma, local tendon tissue could be not sufficiently robust for performing HFT. Given our preliminary clinical results (not already published), we never observed the lack of local tissue for performing HFT. We obtained satisfying results in term of finger's motion, with no significant complication.

4.2. Limitations

The lack of dynamic tension esteems is a limitation of the present study. The tension applied on tendons was minimal to ensure tendons gliding, and was less than the minimal tension estimated for tendon rupture.¹¹ Further studies should be performed, also including minimal technical changes (e.g fixation system), and different tension application.^3,4,27

5. Conclusion

According to the abovementioned suggestions, we consider HFT as a feasible technique in clinical practice, with the limitations due to the early experimental data. Clinical application is encouraged.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Ethical approval

The participants had given informed consent for the use of their bodies for medical research. The study was performed during a training workshop from October 2019 to January 2020 in an authorized institutions for cadaver studies (ICLO, Verona, Italy).

Declaration of competing interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.