Flexor hallucis longus tendon transfer through a single incision in the treatment of chronic Achilles tendon rupture with large defect

Background To evaluate the medium-to-long term clinical outcome of flexor hallucis longus tendon transfer through a single incision in the treatment of chronic Achilles tendon rupture with a defect greater than 5 cm. Methods Clinical data of patients treated with flexor hallucis longus tendon transfer due to chronic Achilles tendon rupture between January 2009 and December 2016 were reviewed retrospectively. All cases were presented with Achilles tendon rupture for more than 4 weeks after injury. The defect of the Achilles tendon was greater than 5 cm after debridement in all patients. Clinical outcomes were assessed with AOFAS Ankle-Hindfoot Scale (AOFAS-AH), Achilles tendon Total Rupture Score (ATRS) and AOFAS Hallux Metatarsophalangeal-lnterphalangeal Scale (AOFAS-HM). Results 18 patients were followed successfully for 24-83 months (mean follow-up time of 43.2 months) and 5 were lost. According to the complete datasets obtained from 18 patients, none of the tendons re-ruptured during the follow-up. The AOFAS-AH and ATRS at last follow-up visit was 94.4 ± 3.2 (87-100) and 89.6 ± 5.9 (72-98) respectively, which revealed statistically significant improvement from the preoperative score of 58.2 ± 6.3 (49-70) and 53.3 ± 7.3(42-68). The AOFAS-HM at last follow-up visit was 90.1 ± 5.2 (77-97). Conclusion The flexor hallucis longus tendon transfer through a single incision is a safe and simple method for chronic Achilles tendon rupture with minimal morbidity and complications.


Background
The Achilles tendon (AT) rupture is one of the most common tendon injuries following the rupture of patellar ligament and quadriceps femoris tendon [1,2]. It often occurs in people 30-39 years old especially the so called "weekend warrior". Patients can still perform 30° of plantar flexion after the rupture of AT because the tibialis posterior muscle, fibular muscle and flexor digitorum muscle have a synergistic effect on ankle plantar flexion. Therefore, it is easy for this injury to be ignored by both patients and doctors. Approximately 25-27% of acute AT ruptures became chronic ruptures because of wrong or missed diagnosis, leading to severely impaired function of the foot and ankle and even more complicated treatment [3,4].
There are multiple surgical options for the treatment of chronic AT rupture, for example, the Lindholm method, Bosworth method, V-Y extension, repair with autologous tendon transfers and so on. However, most surgical methods have the disadvantages of high rate of re-rupture, obvious adhesion, and various soft tissue complications [5][6][7][8]. To date, there is no gold standard method for chronic AT ruptures. Although there are some studies on the application of flexor hallucis longus tendon (FHLT) transfer in the reconstruction of AT for large defects [9][10][11][12]. Whether FHLT transfer is superior to other autologous tendons or allogeneic tendons remains controversial. This article aimed to describe our surgical experience and review its medium-to-long term clinical outcomes.

Patients
All patients with chronic AT rupture who were treated at Affiliated Hospital of Southwest Medical University with FHLT transfer between January 2009 and December 2016 were retrospectively analyzed. The diagnosis of chronic AT rupture was defined as delayed or neglected diagnosis for more than 4 weeks after injury, it was confirmed by clinical examination and MRI scan.

Inclusion Criteria And Exclusion Criteria
The inclusion criteria were patients diagnosed with chronic AT rupture, defect of the ruptured AT greater than 5 cm after debridement, performed with FHLT transfer. The exclusion criteria were patients with acute AT rupture (duration < 4 weeks), with systemic disorder and with neurologic problems. With the inclusion criteria and exclusion criteria, a total of 23 patients were invited for study inclusion. 18 patients responded to the invitation and 5 patients were lost to follow-up because their telephone number had changed and could not be contacted. The characteristics of the remaining 18 patients were summarized in Table 1.   Table 2.

Discussion
AT rupture tends to occur approximately 2-6 cm above the insertion of the tendon because the blood vessel distribution and blood supply are relatively poor [14]. The main reason for the AT rupture is the degeneration of AT fibers caused by poor blood flow and mechanical factors. Obesity, local repeated block therapy, application of hormones, and oral contraceptives also have potential impacts on the occurrence of AT rupture [15,16].
At 4 weeks after injury, highly vascularized collagen scar tissue fills the retraction gap of the ruptured AT, which is the earliest manifestation of chronic healing [17]. Therefore, it is generally accepted that the diagnosis of chronic AT rupture was defined as delayed or neglected diagnosis for more than 4 weeks after injury [18].
The nonoperative treatment of chronic AT rupture is only suitable for high-risk patients with surgical contraindications who do not have high needs for ankle function. The ankle plantar flexion can be partially strengthened with an ankle foot orthosis, however, the normal physiological gait cannot be restored. The prognosis of nonoperative treatment is poor compared to surgical options, and patient satisfaction is less than 55% [1].
Surgical repair of chronic AT rupture has been widely accepted by clinicians. Various surgical treatments have been reported in the literature, including direct suture, triceps tendon rotation flap, V-Y extension, tendon transfer, repair of scar tissue, artificial tendon transplantation, and allograft tendon transplantation [5][6][7][8]19]. However, neither a uniform standard nor evidence-based support has been established [20][21][22]. Generally, a surgical plan can be constructed based on the length of the defect according to Kuwada and Myerson classifications [14,23]. However, there are still controversies regarding the surgical methods for large defects of AT. The incidence of re-rupture after conventional surgical treatment is high. Moreover, conventional surgical treatment has disadvantages such as difficult procedure requiring skill, poor graft strength and poor blood supply particularly when the defect of AT rupture is greater than 5 cm. The reconstruction of AT with large defect using FHLT transfer was firstly reported by Hansen in 1991 [24]. This technique has been gradually recognized and continually improved over the years. Studies have shown that FHLT transfer is a safe and effective surgical approach for patients with chronic AT rupture [25][26][27][28]. However, the evidence of this technique is insufficient due to the limited number of reports and cases, and the lack of long term follow-up data.
It has been reported that the FHLT can be harvested through a single incision or double incisions. However, which incision is better and whether the distal end of the FHLT needs to be sutured to the FDLT remains controversial [29][30][31]. In this study, all the 18 tendons were harvested through a single posteromedial incision. According to our experience, harvesting FHLT through a single incision can be sufficiently long to prevent excessive muscle tension after transfer. The technique is simple and safe, no more incisions are need which may potentially increase the risk of infection and medial plantar nerve injury.
However, note that the FHL is located on the deep side of the tarsal tunnel, posterior tibial blood vessels and nerve in the tunnel should be carefully protected when a single incision is made.
Reconstruction of AT with FHLT transfer could reduce the plantar flexion strength of the hallux, resulting in reduced balance or strength of propulsion theoretically [26,32].

Richardson et al reported decreased distal phalangeal pressure and FHL weakness in 22
patients, but no difference was found in plantar pressure of the first or second metatarsal head [29]. We also noticed the slight weakness of hallux in all patients postoperativly, but the plantar flexion of the distal phalanx could be performed in some of the patients. The postoperative AOFAS-HM at last follow-up visit was relativly high. All patients recovered to their pre-injury daily activities and no patient complained of functional disorder or any noticeable weakness in this study. It is worth mentioning that one amateur football player returned to the pitch for football match 1 year after surgery. The reason for this phenomenon may be existence of flexor hallucis brevis and abnormal branch of other congenerous muscles. The tenodesis of FHLT to FDLT resulting in floating halluces with hyperextension and alignment problems were reported in some studies [26,27]. Some authors do not routinely perform a tenodesis of FHLT to FDLT, they also noticed the weaker hallux postoperatively, but it was not a clinical issue for most patients [32].
However, it is uncertain whether this technique is suitable for athletes and sports participants who have a high need for movement.