Comparative outcomes of different achilles stump management after flexor hallucis longus tendon transfer

Abstract

Various surgical techniques for reconstruction of chronic Achilles tendinosis have been developed, including free semitendinosus tendon autograft and allograft for larger Achilles tendon defects. Flexor hallucis longus (FHL) transfer has emerged as an effective treatment option for patients with insertional Achilles tendinopathy (IAT). During FHL transfer, FHL tendon is harvested at its distal part and rerouted to augment or replace the diseased portion of the Achilles tendon, providing improved structural integrity and function, but the necessity of reattachment of the Achilles stump in FHL transfer remains controversial. Patients who underwent FHL transfer surgery for IAT between 2013 and 2022 were enrolled in this retrospective study. Evaluating parameters included clinical evaluations, complications, a satisfaction questionnaire, preoperative and postoperative visual analog scale (VAS) as well as outcomes scores including American Orthopaedic Foot and Ankle Society (AOFAS), Achilles Tendon Total Rupture Score (ATRS), Foot Function Index (FFI). Furthermore, active range of motion (AROM) and torque of strength in both plantarflexion(PF) and dorsiflexion(DF) for both healthy and operated ankles of all patients were measured. The follow-up time was more than 12 months for all patients. A total of twenty-nine patients, with a median age of 63.0 years old and an average follow-up time of 28.1 months were enrolled. Thirteen patients were assigned to the Non-attached group and sixteen patients were assigned to the Reattached group. The functional outcomes such as AOFAS, ATRS, FFI, and VAS scores, as well as ankle strength in both groups were significantly improved after surgeries. However, no significant differences between groups in preoperative and postoperative VAS scores and functional outcomes were found. AROM and ankle torque were comparable, except for less plantarflexion torque loss at 30°/sec in the reattached group (p = 0.046). All evaluating parameters significantly improved without notable complications in patients after FHL transfer surgery whether with or without reattachment. There were no significant differences in the improvement of most parameters among subgroups. While reattachment of the Achilles tendon stump to the FHL transfer for IAT may not be crucial for superior outcomes, the torque improvement seen in the reattached group indicates possible advantages that require additional investigation.

Introduction

Insertional Achilles tendinopathy (IAT), characterized by pain and dysfunction, are common, particularly among the elderly, and elite athletes^1,2. The pathophysiology of IAT involves a combination of degenerative changes, inflammation, and calcification at the tendon-bone interface^3,4. Insertional Achilles tendinopathy is usually treated conservatively, including rest, elevated leg, cold packs, nonsteroidal anti-inflammatory drug, physical therapy, and exoskeletal rehabilitation assisstance^5–7. If conservative treatment fails, surgery is usually recommended.

Various surgical techniques for reconstruction of chronic Achilles tendinosis include free semitendinosus tendon autograft or allograft for larger Achilles tendon defects^8–10. Peroneus brevis tendon transfer^11,12, gastrocnemius turndown fascial flaps^13,14, and other synthetic materials have been reported^15,16. Flexor hallucis longus (FHL) tendon transfer has emerged as an effective treatment option for patients with refractory IAT or neglected Achilles rupture¹⁷. Clinical outcomes following FHL transfer for IAT or chronic Achilles tear have shown promise, with significant reductions in pain and improvements in functional scores postoperatively^17–19. The FHL tendon is anatomically suited for this role due to its strength, length, and close proximity to the Achilles tendon²⁰. Moreover, the highly vascularized muscle part of the FHL provides increased blood supply to the repair site of the defect in the Achilles, and aids in the success of the repair²¹.

During FHL transfer, FHL tendon is harvested at its distal part and rerouted to augment or replace the diseased portion of the Achilles tendon, providing improved structural integrity and function. ²² DeCarbo et al. first reported the technique of isolated short FHL transfer to the posterior calcaneus to substitute the resected Achilles insertion in chronic Achilles tendinopathy by using interference screw fixation with reattachment of the Achilles stump to FHL²⁵. However, there is still no consensus on the management of the residual Achilles stump in FHL transfer after resection of the distal part of the diseased Achilles tendon. Some surgeons reattached the Achilles stump to the transferred FHL in their cohort^17,20,26, while others performed FHL transfer without any reattachment^19,27. Given the limited comparative evidence and lack of consensus, it remains unclear whether reattachment confers additional clinical benefits.

This study aimed to elucidate whether performing reattachment of the Achilles during FHL transfer to the calcaneus has any effects on the postoperative outcomes of FHL transfer in patients with IAT. We speculated that reattachment of the Achilles stump to the FHL would enhance the clinical outcomes and improve heel plantarflexion strength. The results of this study provide clinicians with empirical evidence that may be useful in determining management strategies for the Achilles stump during FHL transfer.

Materials and methods

Patient recruitment and groups

This study was approved by the Institutional Review Board in our institution. (No. CE21051A) All patients in this study were diagnosed with insertional Achilles tendinopathy, which necessitated resection of the entire Achilles tendon insertion from the posterior calcaneus. They subsequently underwent FHL transfer surgery between January 2013 and December 2022. All participants enrolled in this cohort had a minimum follow-up period of 12 months. We retrospectively reviewed their medical records and images. The diagnosis of IAT was based on the patient’s history, clinical, radiographic, and Magnetic Resonance Imaging (MRI) assessments. The flowchart and demographics in this cohort are shown in Fig. 1 and Table 1, respectively. The inclusion criteria were: (1) age over 18 years, (2) patients underwent resection of Achilles tendon insertion site with fixation of the FHL tendon at the posterior calcaneus with a suture anchor. The exclusion criteria were: (1) inadequate radiographs or poor image quality, (2) follow-up less than 12 months, (3) without patient reported outcome.

Fig. 1.

Flowchart of the enrolled cases.

Table 1.

Patients’ demographics

	Total (n = 29)	Non-reattached (n = 13)	Reattached (n = 16)	p value
Age (years)	63.0 (57.5 ~ 69.0)	66.0 (57.5 ~ 67.5)	63.0 (54.5 ~ 69.0)	0.688
Gender, n,(%)				0.192
Male	7 (24.1%)	5 (38.5%)	2 (12.5%)
Female	22 (75.9%)	8 (61.5%)	14 (87.5%)
BMI (kg/m2)	27.2 (24.2 ~ 29.3)	26.2 (23.7 ~ 29.7)	27.6 (24.6 ~ 28.9)	0.779
Side, n,(%)
Right	17 (58.6%)	8 (61.5%)	9 (56.3%)	0.774
Left	12 (41.4%)	5 (38.5%)	7 (43.8%)
Pathology				0.844
Rupture, n,(%)	21 (72.4%)	9 (69.2%)	12 (75.0%)
Infection, n,(%)	5 (17.2%)	3 (23.1%)	2 (12.5%)
Tendinopathy, n,(%)	3 (10.3%)	1 (7.7%)	2 (12.5%)
Steroid injection history, n (%)	21 (72.4%)	10 (76.9%)	11 (68.8%)	0.697
DM	3 (10.3%)	2 (15.4%)	1 (6.3%)	0.573

Mann–Whitney U test, Chi-squared test, Fisher’s exact test. BMI, Body mass index; DM, diabetes mellitus; SD, standard deviation

The enrolled cases were classified into two groups, reattached and non-reattached groups, according to whether or not reattachment of the residue Achilles stump was performed during FHL transfer. (Fig. 2)

Fig. 2.

Reattached (A) and non-reattached (B) Achilles stump to transferred FHL.

Evaluations of functional outcomes and muscle strength

Follow-up data were collected and included clinical evaluations, complications, completion of a subjective questionnaire, preoperative and postoperative visual analog scale (VAS), as well as functional scores, and active range of motion (AROM) and strength in both plantarflexion (PF) and dorsiflexion (DF) of ankles. The assessments of clinical evaluation and complications included tendon integrity, incision healing, signs of infection, and the ability to perform bilateral-leg and single-leg heel raise maneuvers. We assessed clinical outcomes using American Orthopaedic Foot and Ankle Society (AOFAS), Achilles Tendon Total Rupture Score (ATRS)²⁸, and Foot Function Index (FFI)²⁹, while pain was evaluated using the VAS score calculated on a 10-grade scale. A questionnaire was administered to determine the patient’s satisfaction after surgery. Subjects rated their satisfaction with the surgical outcome as very satisfied (5 points), satisfied (4 points), fair (3 points), dissatisfied (2 points), or very dissatisfied (1 point). AROM, the strength of ankle plantarflexion, and dorsiflexion were measured using the Cybex (Biodex Medical Systems, Inc., Shirley, NY). Peak PF torque, peak DF torque, and total AROM for both healthy and operated ankles of all enrolled patients were measured three times at each of the three dynamometer speeds (30 degrees/sec, 60 degrees/sec, and 120 degrees/sec). The highest values of torques and AROM were recorded. Outcome assessments for all patients were conducted at the one-year postoperative follow-up visit by one author who was blinded to the surgical procedure and groups of the study.

Surgical technique

The FHL transfer procedure followed the technique note reported by DeCarbo et al. in 2008²⁵. All surgical procedures were done with the patient under general anesthesia and in the prone position. A thigh tourniquet was used, taking care to prevent squeezing of the gastrocnemius muscle, which would decrease the mobility of the muscle and prevent accurate tensioning of the tendon repair. A 10-cm longitudinal incision was placed medial to the Achilles tendon. Once the diseased portion of the Achilles tendon was identified, the diseased Achilles region, along with the nonviable ends of the tendon, was debrided and resected. The dissection was continued through the deep fascia of the posterior heel and the FHL muscle was identified. The FHL tendon was then isolated through the tendon sheath along the medial wall of the calcaneus. The FHL tendon was then harvested with the ankle and hallux fully flexed and maximal traction placed on the FHL tendon, ensuring that the tendon was transected as distally as possible. A tendon-passing stitch was then used at the distal margin of the transected FHL. A guided Steinmann pin was inserted from superior to inferior through the posterior tuberosity of the calcaneus. The drill and reamer through the Steinmann pin were sequentially used to make an intraosseous channel on the calcaneus such that it just penetrated the plantar cortex. The ankle was maintained at 15-degree plantarflexion with 90-degree flexion of the knee and the FHL tendon was passed through the calcaneal tunnel and exited the plantar heel. The FHL tendon was fixed at the attachment of the distal stump of the Achilles tendon with 5.5 mm tenodesis anchor screw (SwiveLock anchor, Arthrex Inc., Naples, FL, USA). The ankle was then gently stressed into the neutral position to evaluate the security of the transfer. The absorbable tendon-passing suture exiting the plantar heel was then cut. Thereafter, the posterior wound was closed and a splint was applied, maintaining the ankle in slight plantarflexion. After the FHL tendon transfer was completed, the residual Achilles stump was attached to the transferred FHL as distal as possible, for patients in the reattached group. Patients in the non-reattached group, who lacked sufficient residual Achilles stump after debridement, did not have their Achilles stump reattached to the transferred FHL.

Post-operative care

Postoperatively, the extremity was splinted for 4 weeks at 5 to 10 degrees of plantarflexion. Patients were restricted to partial weightbearing with the assistance of crutches for the first 4 postoperative weeks. At 2 weeks after surgery, incisions were checked, and the sutures were removed. After 4 weeks, patients were re-evaluated clinically. The tendon integrity was manually tested. The splint on the affected limbs were removed and patients were placed into walking boots for further protection. Patients were then allowed to start partial weightbearing on the affected extremity, as comfort allowed, in a protective boot. At that time, patients were instructed to actively move ankles up or down as tolerated and begin formal physical therapy. Therapy was performed two to three times a week for the subsequent 4 weeks and included passive Achilles stretching, an Achilles strengthening program, and gait training. Twelve weeks after surgery, the boot was discontinued for free ambulation, with the patient continuing the physical therapy program. Patients were advised to slowly resume their activity as comfort allowed, but to avoid sudden acceleration, cutting, or jumping activities until at least 6 months after surgery. The enrolled patients were then invited to return for follow-up evaluations.

Statistical analysis

All variables were assessed for normality of data distribution using the Shapiro-Wilk test. Continuous variables are reported as mean with standard deviation (SD), while categorical variables are expressed as frequencies (percentages). Group differences were evaluated using the Mann–Whitney U test for continuous data and are reported as median with IQR. The Chi-squared test and Fisher’s exact test were used for categorical data. Paired t-tests were used to compare preoperative and postoperative VAS scale and functional scores. Statistical analyses were carried out using SPSS version 25.0 (IBM, New York, NY, USA), with a p-value of < 0.05 considered statistically significant.

Ethics statement

All methods were carried out in accordance with relevant guidelines and regulations. The study was approved by the Institutional Review Board I & II of Taichung Veterans General Hospital (approval number CE21051A). Written informed consent was obtained from all participants and/or their legal guardians.

Results

A total of twenty-nine patients, with a median age of 63.0 (range 38–75) years old, meeting the criteria were included in our study. There were 7 males and 22 females with an average follow-up time of 28.1 (range 7–60) months. Patient demographics, coexisting diagnosis, and group comparison p values are shown in Table 1. There were thirteen patients who did not receive Achilles tendon reattachment after FHL transfer surgery and these individuals were assigned to the “Non-attached” group. The other sixteen patients received Achilles reattachment to the FHL during transfer surgery and thus were assigned to the “Reattached” group. The age, gender, Body Mass Index (BMI), involved side, pathology, and steroid injection history, as well as the proportions of patients with Diabetes mellitus (DM) and those who needed revision were comparable between the two groups. (Table 1)

The operative and postoperative status were shown at Table 2. The surgery duration was very similar between the two groups, with an average of 97 min. The length of the postoperative follow-up period was slightly higher in the reattached group compared to the non-reattached group, but the difference was not statistically significant. Regarding postoperative complications, only two patients experienced complications, both of whom were in the “reattachment” group. One patient, a 69-year-old female, developed a superficial wound infection, which was successfully treated with intravenous and oral antibiotics. The other, a 50-year-old male, had poor wound healing, but recovered well after intensive wound care. Both cases did not require further surgery.

Table 2.

Operative and postoperative status.

	Total (n = 29)	Non-reattached (n = 13)	Reattached (n = 16)	p value
Surgical time (min)	98 (81 ~ 108)	98.0 (84 ~ 108)	99.5 (80 ~ 110)	0.923
Post-op follow up (months)	26.0 (15 ~ 36.5)	24.0 (13.5 ~ 31)	30.5 (22.5 ~ 47.5)	0.093
Complications, n (%)	2 (6.9%)	0 (0%)	2 (12.5%)	0.488

Mann–Whitney U test, Fisher’s exact test

The clinical outcome measurements were shown at Table 3. Patients were requested to perform bilateral heel raise and then single heel raise including the affected leg and the contralateral (non-operative) side. All patients were able to complete bilateral-leg heel raise more than 3 times without assistance. Patients were then asked to attempt a single-leg heel raise on the operated (OP) side and the non-operated (non-OP) side. In this cohort, only six of the 29 cases (20.6%) were able to perform a single-leg heel raise on the operated side. Among them, five (31.3%) were in the reattached group and one (7.7%) was in the non-attached group. However, there was no statistically significant difference between the two groups in performing a single-leg heel raise. The whole AROM, AROM of plantarflexion, and dorsiflexion of ankles showed no significant differences among the subgroups.

Table 3.

Comparisons of clinical outcome measurements.

	Total (n = 29)	Non-reattached (n = 13)	Reattatched (n = 16)	p value
Single heel raise (n (%))	6 (20.6%)	1 (7.7%)	5 (31.3%)	0.110
ROM of ankles (degrees)
OP side	65.5 (± 11.5)	63.8 (± 10.2)	66.9 (± 12.6)	0.836
Non-OP side	67.4 (± 11.9)	64.2 (± 11.2)	70.0 (± 12.2)	0.125
Plantar flexion (degrees)
OP side	51.2 (± 8.5)	50.4 (± 8.5)	51.9 (± 8.7)	0.738
Non-OP side	52.9 (± 9.5)	51.9 (± 10.1)	53.8 (± 9.2)	0.406
Dorsiflexion (degrees)
OP side	14.3 (± 6.4)	13.5 (± 5.2)	15.0 (± 7.3)	0.449
Non-OP side	14.5 (± 5.4)	12.3 (± 5.3)	16.3 (± 5.0)	0.051

Mann–Whitney U test, chi-squared test, Fisher’s exact test. OP, Operative; ROM, range of motion;

The dynamometer evaluation of both the operation and contralateral side were performed in all participants. The strength of ankles in plantarflexion and dorsiflexion were measured. Table 4; Fig. 3 shows the differences in DF and PF torques between the operated and non-operated sides at 30 degrees/sec, 60 degrees/sec, and 120 degrees/sec. No statistically significant differences in the difference of dorsiflexion torque were found among subgroups. At an angular velocity of 30°/sec, the non-attached group exhibited a significant reduction in plantarflexion torque, with a loss of 21.1 N-m, compared to a loss of 7.8 N-m in the reattached group (p = 0.046).

Table 4.

Torques difference between contralateral and operation side.

	Non-reattached (n = 13)	Reattached (n = 16)	p value
∆ 30°/s PF (N-m)	21.1 (± 23.3)	7.8 (± 7.4)	0.046*
∆ 30°/s DF (N-m)	−0.9 (± 6.4)	2.8 (± 4.6)	0.170
∆ 60°/s PF (N-m)	8.9 (± 14.2)	5.5 (± 8.6)	0.209
∆ 60°/s DF (N-m)	3.0 (± 12.2)	1.2 (± 13.2)	0.390
∆120°/s PF (N-m)	5.1 (± 10.4)	4.4 (± 9.7)	0.655
∆120°/s DF (N-m)	7.9 (± 12.5)	0.1 (± 7.0)	0.177
∆ 30°/s PF TQ/BW	21.9 (± 28.5)	12.1 (± 11.4)	0.059
∆ 30°/s DF TQ/BW	−0.7 (± 8.8)	4.4 (± 7.5)	0.260
∆ 60°/s PF TQ/BW	13.0 (± 18.9)	8.2 (± 12.5)	0.201
∆ 60°/s DF TQ/BW	5.0 (± 18.2)	1.3 (± 19.4)	0.423
∆120°/s PF TQ/BW	7.5 (± 14.3)	5.4 (± 14.6)	0.723
∆120°/s DF TQ/BW	10.6 (± 16.9)	0.1 (± 10.7)	0.193

Mann–Whitney U test, * p < 0.05. ∆: the differences of torque between OP side and non-OP side. PF: plantarflexion; DF: Dorsiflexion; TQ: Torques; N-m: Newton-meter; BW: body weight

Fig. 3.

Box-and-whisker plot for torque difference between reattached and non-reattached Achilles, * p < 0.05.

As to clinical outcomes, VAS pain score and patient-reported outcome measures (PROMs) including AOFAS score, FFI score, and ATRS score, were applied to measure the clinical outcome at the last outpatient department follow-up. The results of the above clinical outcomes, shown in Table 5. The VAS scale, AOFAS score, FFI score, and ATRS score showed significant improvement, comparing before and after surgery. However, the clinical outcomes were comparable between the non-reattached and reattached group, based on a comparison of before and after surgery. Patients’ functional scores were significantly improved in both the reattached and non-reattached groups after FHL transfer, and all patients were satisfied with the surgery, including the two cases with minimal wound complications.

Table 5.

Comparisons of VAS scale and functional scores.

	Total (n = 29)	Non-reattached (n = 13)	Reattached (n = 16)	p value
VAS scale
Pre-OP	7.3 (± 1.2)	7.0 (± 1.0)	7.5 (± 1.4)	0.312
Post-OP	0.2 (± 0.5)	0.3 (± 0.6)	0.1 (± 0.3)	0.251
p valuew	< 0.001**	< 0.001**	< 0.001**
∆VAS (PreOP-PostOP)	7.1 (± 1.2)	6.7 (± 0.9)	7.4 (± 1.3)	0.110
AOFAS score
Pre-OP	57.3 (± 10.8)	57.5 (± 12.5)	57.1 (± 9.6)	0.655
Post-OP	97.4 (± 5.6)	97.9 (± 4.4)	97.1 (± 6.5)	0.857
p valuew	< 0.001**	< 0.001**	< 0.001**
∆AOFAS (PreOP-PostOP)	40.2 (± 9.4)	40.5 (± 11.6)	45.3 (± 12.6)	0.769
FFI score
Pre-OP	46.6 (± 12.0)	48.1 (± 11.6)	45.3 (± 12.6)	0.596
Post-OP	1.2 (± 2.3)	1.7 (± 3.1)	0.8 (± 1.3)	0.606
p valuew	< 0.001**	< 0.001**	< 0.001**
∆FFI (PreOP-PostOP)	45.4 (± 11.5)	46.4 (± 11.2)	44.6 (± 12.1)	0.787
ATRS score
Pre-OP	68.7 (± 9.9)	69.0 (± 8.7)	68.4 (± 11.0)	0.837
Post-OP	4.6 (± 5.3)	5.2 (± 7.1)	4.0 (± 3.4)	0.993
p valuew	< 0.001**	< 0.001**	< 0.001**
∆ATRS (PreOP-PostOP)	64.1 (± 9.4)	63.8 (± 9.0)	64.4 (± 9.9)	1.00
Satisfaction scale	4.9 (± 0.3)	4.9 (± 0.3)	4.9 (± 0.3)	1.00

Mann–Whitney U test, ^wWilcoxon Signed Ranks test, * p < 0.05, ** p < 0.01. VAS, Visual Analogue Scale; AOFAS, American Orthopaedic Foot and Ankle Society; FFI, Foot Function Index; ATRS, Achilles tendon Total Rupture Score; Pre-OP, preoperative; Post-OP, postoperative; SD, standard deviation

Discussions

In this study, we aimed to elucidate the differences in outcomes in terms of clinical performance, complications, ankle strength, and functional outcomes scores between reattached and non-reattached FHL transfer for IAT. Significant improvements in all evaluated parameters were observed following FHL transfer at the midterm follow-up. However, there were no significant differences in any of the analyzed parameters between the two groups. FHL transfer proved to be a safe and effective surgical procedure for the treatment of IAT. However, the findings of this study did not demonstrate that reattachment of the Achilles tendon stump to the transferred FHL yields superior outcomes compared to non-reattachment.

Chronic rupture or insertional tendinopathy of the Achilles tendon is associated with retraction and degeneration of Achilles tendon ends with short distal stumps, which makes its management a challenge with a relatively poorer expected outcome compared to management at the acute stage³⁰. While accommodative bracing may provide exterior ankle support, it leads to considerable ankle weakness and gait disturbances, and does not allow for a return to full function³¹. Surgical reconstruction offers the potential to restore patients to their full strength and activity level. In this cohort, patients’ symptoms were significantly improved and they regained near-normal walking ability with significant pain relief after the surgery.

In our cohort, the range of motion (ROM) between the operative foot and contralateral foot was similar (65.5 degrees vs. 67.4 degrees, p = 0.245) and there was no significant difference in AROM between the reattached and non-reattached group in total range of motion, dorsiflexion, and plantarflexion. Previous studies have also indicated that ankle ROM can be restored to levels comparable to the contralateral foot in patients with chronic Achilles tendon rupture undergoing FHL transfer³². However, some studies revealed that passive ankle ROM was reduced both in dorsiflexion and plantarflexion³³. The impact of FHL transfer surgery on ankle range of motion requires further research.

With regard to single heel raise after FHL transfer, Lee et al. reported all cases underwent acellular tissue graft augmentation for neglected Achilles ruptures were able to perform single heel raise on the reconstructed side after 6 months’ follow-up.³⁴ However, in our patient cohort, only 6 out of 29 patients were able to perform a single heel raise on the operated foot. Although the distribution of patients did not reach statistical difference, a higher proportion of patients in the reattachment group were able to perform single-leg heel raise than in the non-reattachment group. This discrepancy could potentially be attributed to the older age of our cohort and the specific characteristics of the Asian population, as only 15 out of the 29 patients in this study were able to perform a single heel raise on the non-operated side. This suggests that factors such as age, ethnicity, or baseline muscle function could have played a role in the clinical performance after FHL transfer surgery in our study. Further research is needed to explore these potential influences.

Isokinetic dynamometry is widely used to assess joint and muscle performance and is known for its high reliability across all angular velocities^35,36. The measure has been demonstrated to be a reliable procedure for assessing strength^31,37, for quantification of muscle function³⁸, and to test ankle function³⁹. Foot peak pressures, initially disrupted, were restored and became comparable to the normal foot following FHL transfer for chronic retracted Achilles tears after a two-year follow-up.¹⁸ Though the FHL muscle showed noticeable hypertrophy 2 years after the FHL transfer surgery, the muscle strength of the operated side was still weaker than that of the contralateral side^33,40,41. In our study, patients with reattachment of Achilles stump to FHL seemed to have less power loss than those without Achilles tendon reattachment, especially at an angular velocity of 30 degrees/sec plantarflexion (p = 0.046). However, the post-hoc power for this comparison was only 0.5, and the effect size was 0.77, indicating that this result should be interpreted with caution given the small sample size and lack of consistent differences in other outcomes.

All functional outcome measurements revealed significant improvement after FHL transfer in both the reattachment and non-reattachment of Achilles groups A systematic review reported significant improvements in functional scores in 141 patients undergoing FHL transfer for chronic Achilles tendon rupture. The mean preoperative AOFAS score was 54.7 ± 2.7 (), while the postoperative score increased to 90.8 ± 7.3 ().²² The findings in the present study were compatible with their results and all patients were satisfied with the FHL transfer surgery.

FHL tendon transfer is a widely used alternative surgical procedure for the treatment of chronic Achilles tendon rupture or insertional Achilles tendinopathy. Patients treated with FHL transfer reported improvement in subjective clinical outcome scores and had a low surgical wound complication rate. A previous study reported an overall complication rate of 11.3% following FHL transfer, with the most common complication being surgical wound infection, occurring in 7.8% of cases²². In our study, there were two surgical wound complications (6.9%), which healed well without further surgical intervention. However, FHL transfer might reduce interphalangeal joint (IPJ) plantarflexion strength of the hallux and could diminish push off during the stance phase^42,43. However, Ozer et al. indicated that there was no significant difference in vertical jump, forward jump, or balance performance⁴⁴. Thus, while IPJ plantarflexion may be reduced, overall functional performance might remain unaffected.

Limitations

There were some limitations in this study. First, due to the retrospective design, there will be unknown confounding factors and selection bias occurred since group allocation was determined by stump length and tissue quality, or other unknown confounders. Second, this study was limited by its relatively small sample size. Third, rehabilitation compliance beyond six months postoperatively was not systematically recorded, which may influence the variability in functional recovery. Finally, all patients were operated on and followed up in only one institute. A longer follow-up period with standard rehabilitation course and multi-center cooperation research may be necessary to fully assess the outcomes of the procedure.

Conclusions

FHL tendon transfer surgery has emerged as an effective and safe treatment option for patients with insertional Achilles tendinopathy. Reattachment of Achilles tendon after FHL transfer might improve ankle torque, especially at 30 degrees/sec plantarflexion. As to the clinical outcome, patients improved a lot after the surgery with or without reattachment of the Achilles tendon and there was no statistically significant difference between the two groups. While reattachment of the Achilles tendon stump to the FHL transfer for IAT may not be crucial for superior outcomes, the torque improvement seen in the reattached group indicates possible advantages that require additional investigation.

Author contributions

Lan-Li Hsueh has made substantial contributions to the conception, design, data acquisition, analysis, and drafting of the manuscript, and is designated as the first author. Shun-Ping Wang and Yuan-Yang Cheng are the corresponding authors and have contributed equally and overseen all aspects of the manuscript preparation and submission. Furthermore, Shih-Chieh Tang, Han-Ting Shih have contributed equally to the research work, data interpretation, and manuscript revision, and should be recognized as co-equal contributors.

Data availability

The data used in this study cannot be made publicly available due to privacy concerns. The data contain sensitive or identifiable information that must be protected in accordance with institutional policies and applicable laws and regulations. However, data are available from the authors upon reasonable request and with the appropriate legal and ethical approvals. Interested researchers may contact wsp0120@yahoo.com.tw to request access to the data.

Declarations

Competing interests

The authors declare no competing interests.

Conflict of interest

All the authors had no conflict of interest to disclose.