Flexor hallux tendon transfer: comparative study through double or single approach

OBJECTIVE: To quantify the FHL length difference obtained through a single approach and by a double combined approach. METHODS: 16 fresh cadavers, a total of 32 feet, were used to measure the FHL graft length. With the cadaver positioned in ventral decubitus, a posteromedial incision in the ankle and a second incision in the plantar cavus were performed. RESULTS: The average gain of tendon's length (GTL) was of 42.43 mm, the lowest value being 32 mm and the largest 48 mm. The comparative analysis of the GTL on the right and left sides through the paired "t" Test does not show statistical differences, with a p-value = 0.463 and a statistical power of 0.1443. The height analysis of the sample and the right and left GTL performed through linear regression do not show statistically significant differences, with a p-value of 0.38311 and 0.82640, respectively. CONCLUSION: Harvesting the FHL graft using a double combined approach yields a 42.43 mm length gain in comparison to harvesting using the single approach. Level of Evidence III, Case Control Study.


INTRODUÇÃO
Nonsurgical treatment of chronic tendinopathy of the Achilles tendon (TAT) is the initial form of approach and includes indications such as rest, anti-inflammatory medications, correction of extrinsic factors, stretching the sural triceps, and use of orthoses to alleviate the influence of intrinsic factors, among others. When conservative treatment is not well succeed, surgery is indicated, and injured Achilles tendon may be retained or replaced depending on its degree of degeneration. 1 In cases of resection of the Achilles tendon, using the flexor hallux longus tendon (FHL) as substitute has been used frequently in medical literature for presenting some advantages over other tendons available. They are: the long flexor hallux muscle goes into action at the same stage as the sural triceps during gait; 2 it has twice and 30% more power than the flexor digitorum longus and fibularis brevis flexor, respectively; 3 its axis contraction is the same as the sural triceps; 4 it can be dissected with the same approach as that used in the treatment of tendinopathy of the Achilles tendon; 5 and its muscular body sutured to the Achill tendon serves as a vascularization source for its regeneration. 6 Two dissection techniques for obtaining FHL are described, one in the posterior segment of the distal leg, the same used for the treatment of chronic tendinopathy of the Achilles ten-don, through which the tendon is cut along the osteofibrous tunnel located after the subsequent process the talus, after being pulled upwards with the foot and hallux in maximum plantar flexion to allow obtaining the greatest possible tendon length. 6 The other technique involves the preparation of a second access track to medial plantar cavus, in addition to the first way approach to the Achilles tendon, whereby the FHL is identified and transected immediately proximal to Henry's knot, and then pulled through the proximal track. 2 This procedure allows to obtain a longer tendon graft at the expense of a second incision with longer exposure and higher risk of injury to the neurovascular structures located near the plantar tendon. TAT is treated with graft FHL obtained by both single track and double track combined, with similar clinical outcomes. 1,[7][8][9][10][11][12][13][14] In an anatomical study on cadavers, Tashjian et al. 15 found that the average length of FHL obtained by the unique track is 5.16 cm, while on the dual track is 8.09 cm. It is not clear whether the increase in length of FHL achieved by combined dual track brings any benefits to the reconstruction of TAT. The objective of this study is to quantify the difference in length of FHL obtained by single track and double track combined in the Brazilian population.

MATERIALS AND METHODS
After approval by the Scientific Committee and the Ethics Committee of the University 16 fresh cadavers, totalizing 32 feet, were used to measure the length of the FHL graft. All cadavers were male, aged 25-45 years with no history or signs of previous injuries in the foot and ankle region. The height of the cadavers ranged 167-182 cm, with a mean of 173 cm and body weight ranged 62-78 kg, 70 kg on average. With the corpse placed in the prone position, one posteromedial longitudinal incision was made in the ankle and, after opening the flexor retinaculum, FHL was identified and isolated from the muscle-tendon transition to the entrance of the osteofibrous tunnel posterior to the talus. ( Figure 1A) FHL was pulled upwards while the foot and hallux were placed in maximum plantar flexion, a maneuver which is typically performed in the surgical field for obtaining the greatest possible length of FHL when using only one access track. In this condition, a mark was made on the tendon with a surgical pen on its lower visible limit, adjacent to osteofibrous tunnel, corresponding to the point where the tenotomy is held in single track surgeries. A second longitudinal incision was made in plantar cavus in the navicular topography and the middle compartment of the foot was accessed to identify Henry's node. FHL was isolated and transected immediately proximal to the node and retracted by the first access track. ( Figure 1B) With FHL stretched and supported on a flat surface, three measurements were made with a millimeter ruler of the distance between the most proximal portion of the pen mark to the most proximal portion of the edge of the sectioned tendon, and the average of the measurements was taken. ( Figure 1C) Table 1. Distribution of height and FHL gain of tendon's length (GTL) on the sample studied.

RESULTS
The correlation between the sample height and the left and right GLG performed by linear regression shows no statistical difference, p=0.38311 and p=0.82640, respectively.

DISCUSSION
The 42 mm GLG found when using the second access track to FHL section in the plantar region is greater than the 29.3 mm length found by Tashjian et al. 15 However, their methodology is totally different, since FHL was first sectioned in the transition to the post-talus osteofibrous tunnel and they considered the measure of the available tendon as a graft. In order to calculate this measure, the authors used as reference points the top edge of the tuberosity of the calcaneus, with the ankle in neutral position, and the end of the proximal stump of the transected tendon, keeping a light pull on the FHL. The average value found in 14 cadavers was 51.6 mm. In order to calculate the length of tendon graft available when using double track, to this value it was added the length of the tendon measured from the point of section to Henry's node, reaching 80.9mm. We believe that the length difference found between our study and the study of Tashjian et al. 15 is due to the elimination, in the current work, of subjectivity on the traction force applied to FHL during measurement of tendon available for graft, since the applied strength was not measured, and also to the different population studied. One issue that should be discussed is which is the advantage of having available 42 mm extra tendon in the reconstruction of Achilles tendon. Making a second track to isolation of FHL in the plantar region of the foot leads to increased surgical time, besides being a laborious dissection. FHL is located on the medial plantar compartment of the foot near the medial plantar neurovascular bundle, and to be accessed it is required to open the medial intermuscular septum. The retraction of the tendon to the proximal incision endangers both the medial plantar nerve as planting lateral. 5 Some authors consider as an advantage of the double track technique the possibility of performing a tenodesis between the distal stump of the FHL and the flexor digitorum longus tendon to decrease the weakening of hallux flexion strength. 16 In a study using pedobarograph by Coull et al. 17 found no differences in the feet where the FHL was isolated by single or double track. This conclusion, however, was not significant due to the small number of cases operated. All TAT patients treated using FHL, either by single track 1,4,9,18 or double track with FHL tenodesis of flexor digitorum longus [11][12][13][14]16 had loss of flexion strength of the hallux when examined, however this loss is rarely a functional deficit. The clinical results were similar with both techniques, but cannot be compared, because prospective studies have not been found comparing the complications of using single track with double track. On surgical treatment of insertional Achilles tendinopathy, transferred FHL should be reinserted into the calcaneus bone, as there is no remaining distal stump of the Achilles tendon to be used. If reinsertion is done by making a transverse bone tunnel along the axis of the calcaneal tuberosity at 10 mm from the upper limit, 14 and considering that the average width of the calcaneus in this region is 27 mm, 19 to which the tendon is sutured in itself, a length of at least 74mm would be needed. (Figure 2) Considering that the length of available graft taken by single track is 51mm, 15 this reintegration technique can only be accomplished with the removal of the graft by FHL by double track, as in this condition we have a 42 mm GTL.
Using an interference screw in the calcaneus FHL insertion, 3 the available tendon length required is much smaller. Using 23mm screws is suitable, 9 it only requires a bone tunnel filled with 30 mm tendon (Figure 3) a length seamlessly obtained by single track. Counting on a 42 mm GTL of in double track, 30 mm gain becomes embedded in the tunnel in retrograde direction ( Figure 4) with only 12 mm available by the upper limit of the calcaneal tuberosity. We consider that this short tendon segment is usually not useful in rebuilding the insertional TAT, not offsetting the risk of carrying out a second access track on the foot. In non-insertional Achilles tendinopathy, the FHL graft is sutured primarily in the distal stump of the tendon and then in the remaining proximal stump of the Achilles tendon,    defect after debridement of the Achilles tendon. Having a 42 mm GTL may be necessary or not, depending on the length of this defect. The weakness of the study lies in the fact that no bone tunnels were made in the calcaneus to assess the influence of the FHL length in the simulation with different types of its reintegration. The values used in the discussion are reported and there are not the ones found in our bodies. The importance of this work lies in the fact that it is the first paper to quantify the difference in length of the FHL graft obtained by single or double track in the Brazilian population.

CONCLUSION
Dissection of FHL for tendon transfer, by double access provides a 42 mm graft length gain, in comparison to the single track procedure. Thus, depending on the particular case chosen for each technique, the surgeon must choose using one or two access tracks, as he could rely on extra 4 cm of the transferred tendon.