Clinical and Functional Outcomes of Gastrocnemius Recession for Chronic Achilles Tendinopathy

Gastrocnemius Release in the Treatment of Achilles Tendinopathy: A Systematic Review

Background

The aim of this study is to evaluate the use of isolated gastrocnemius release in the treatment of Achilles tendinopathy. The primary outcome is the change in patient-reported pain outcomes. Secondary outcomes include the change in patient-reported foot and ankle function, ankle range of motion and strength, patient satisfaction and rate of surgical complications.

Methods

A systematic review was undertaken of studies involving patients treated with an isolated gastrocnemius release for Achilles tendinopathy. Randomised controlled trials, cohort studies, case-control studies and case series were eligible and identified from the following databases: MEDLINE and EMBASE.

Results

Isolated gastrocnemius release results in improved patient-reported outcome scores for pain and foot and ankle function. There is an increase in ankle range of motion but a reduction in ankle strength. Patients report a high rate of overall satisfaction. The most common surgical complications are sural nerve injury and wound infection.

Conclusions

Isolated gastrocnemius release may offer maintained and clinically meaningful improvements in pain and foot and ankle function with an associated reduction in ankle strength.

Alteration of the calf strength by heel cord lengthening, gastrocnemius recession through tenotomy or fasciotomy. A retrospective clinical force analysis before and after surgery

BACKGROUND

Indications for surgical corrections about the Achilles tendon are increasing as additional measures in reconstructive interventions about the foot. These indications include shortened gastrocnemii, which cause a so-called 'functional pes equinus' and secondary forefoot imbalances, as well as corrections of pes planus and cavus. Surgery about the heel cord may also be indicated for achillodynia and diabetic pressure ulcers. However, there is a lack of evidence that quantifies the results of lengthening procedures about the heel cord. The aim of this study was to quantify the exact changes in calf strength one year after elongating the triceps surae, by measuring flexion forces in 90 degrees knee flexion and knee extension.

METHODS

This study involves 69 patients who were examined for calf strength preoperatively and 1 year after gastrocnemius release. A new device, the Leonardo Mechanograph® (Novotec Medical) was used to measure calf strength. Measurements were performed with the knee flexed and extended.

RESULTS

The operated leg had an overall statistically significant reduction in strength after surgery. Changes were similar on the contralateral leg. The difference in force reduction between the operated and non-operated leg was not statistically significant CONCLUSION: A correlation between measured plantar flexion forces of the foot after a reconstructive foot operation with or without a lengthening procedure about the calf musculature could not be established.

LEVEL OF EVIDENCE

Level III: retrospective cohort study.

Failed Surgery for Achilles Tendinopathy

Treatments of Achilles tendinopathy continue to evolve. The body of literature is inadequate to provide a comprehensive guide to evaluation and treat failed surgeries. Issues related to failed surgical treatment may be divided into infection/wound issue, mechanical failure, and persistent pain. Awareness of the potential problems described in this article will allow surgeons to have a foundation in clinical assessment and making accurate diagnoses. Various surgical treatment options are available and should be executed carefully to treat individualized patient conditions.

ICON 2020-International Scientific Tendinopathy Symposium Consensus: A Systematic Review of Outcome Measures Reported in Clinical Trials of Achilles Tendinopathy

BACKGROUND

Nine core domains for tendinopathy have been identified. For Achilles tendinopathy there is large variation in outcome measures used, and how these fit into the core domains has not been investigated.

OBJECTIVE

To identify all available outcome measures outcome measures used to assess the clinical phenotype of Achilles tendinopathy in prospective studies and to map the outcomes measures into predefined health-related core domains.

DESIGN

Systematic review.

DATA SOURCES

Embase, MEDLINE (Ovid), Web of Science, CINAHL, The Cochrane Library, SPORTDiscus and Google Scholar.

ELIGIBILITY CRITERIA FOR SELECTING STUDIES

Clinical diagnosis of Achilles tendinopathy, sample size ≥ ten participants, age ≥ 16 years, and the study design was a randomized or non-randomized clinical trial, observational cohort, single-arm intervention, or case series.

RESULTS

9376 studies were initially screened and 307 studies were finally included, totaling 13,248 participants. There were 233 (177 core domain) different outcome measures identified across all domains. For each core domain outcome measures were identified, with a range between 8 and 35 unique outcome measures utilized for each domain. The proportion of studies that included outcomes for predefined core domains ranged from 4% for the psychological factors domain to 72% for the disability domain.

CONCLUSION

233 unique outcome measures for Achilles tendinopathy were identified. Most frequently, outcome measures were used within the disability domain. Outcome measures assessing psychological factors were scarcely used. The next step in developing a core outcome set for Achilles tendinopathy is to engage patients, clinicians and researchers to reach consensus on key outcomes measures.

PROSPERO REGISTRATION

CRD42020156763.

Is Gastrocnemius Tightness a Normal Finding in Children?: A Cross-Sectional Study of 204 Norwegian Schoolchildren

BACKGROUND

Isolated gastrocnemius tightness has been associated with several foot conditions. We do not know whether gastrocnemius tightness is a normal finding in the developing child or whether tightness is associated with foot symptoms. Recurrent foot and leg pain is common in children, and more knowledge of gastrocnemius tightness is of interest.

METHODS

Passive ankle dorsiflexion was measured with the knee flexed and extended (Silfverskiöld test) in 204 Norwegian schoolchildren (408 feet). School classes from 4 different age groups were recruited (5 to 6 years, 8 to 9 years, 11 to 12 years, and 14 to 15 years). Footprints were also recorded in order to assess for any association between foot morphology and dorsiflexion results, specifically, gastrocnemius tightness.

RESULTS

Ankle dorsiflexion decreased with increasing age. The thresholds for equinus contracture were set at either ≤5° or ≤0° of ankle dorsiflexion when measured with the knee extended, and the Silfverskiöld test was considered to be positive when ankle dorsiflexion was reduced by ≥10° from the flexed knee position to the extended knee position. The rate of gastrocnemius tightness was 54.9% if the threshold was set at ≤5° and only 3.7% if the threshold was set at ≤0°. The data did not provide evidence of an association between dorsiflexion and footprints.

CONCLUSIONS

Ankle dorsiflexion decreased with increasing age in this population of schoolchildren, highlighting the importance of age-matched norms. A majority had a tight gastrocnemius when the ankle dorsiflexion threshold was set at ≤5°, indicating that isolated gastrocnemius tightness should not be interpreted as a pathological finding.

LEVEL OF EVIDENCE

Diagnostic Level II. See Instructions for Authors for a complete description of levels of evidence.

Patient reported outcomes and ankle plantarflexor muscle performance following gastrocnemius recession for Achilles tendinopathy: A prospective case-control study

BACKGROUND

Prospective studies to guide the application of a gastrocnemius recession for Achilles tendinopathy are limited. Our aim was to prospectively evaluate patient reported outcomes and muscle performance.

METHODS

Patients with unilateral recalcitrant Achilles tendinopathy who received an isolated gastrocnemius recession (n=8) and a healthy control group (n=8) were included. Patient reported outcomes, ankle power during walking and stair ascent, and the heel rise limb symmetry index (total work) were collected.

RESULTS

Improvements in pain and self-reported function were observed (six months and two years). Sport participation scores reached 92% by two years. Patients demonstrated lower ankle power during stair ascent and decreased limb symmetry during heel rise six months following treatment (p≤.02).

CONCLUSIONS

Study findings regarding long-term improvements in patient pain, self-reported function and sport participation, and early preservation of ankle function during walking, can help refine patient selection, anticipated outcomes, and rehabilitation strategies.

Macroscopic changes in sural nerve after gastrocnemius recession: a cadaver study

BACKGROUND

Gastrocnemius recession is a common foot and ankle procedure and various techniques that have been utilized are mainly delineated by the anatomic position of the gastrocnemius transection; the 2 common ones are the Baumann and Strayer procedure. Both can adversely affect the sural nerve. The objective of this study was to evaluate the macroscopic changes in the sural nerve following gastrocnemius recession, and to compare the efficacy of the two procedures, regarding the improvement of maximal ankle dorsiflexion.

METHODS

Ten fresh-frozen, above knee cadaveric legs were assigned to one of two gastrocnemius recession techniques: Baumann (n = 5) or Strayer (n = 5). A goniometer was used to measure degree of ankle dorsiflexion before and after the surgery. The sural nerve was meticulously dissected and marked with two suture knots, 2 cm apart. The ankle was passively dorsiflexed from 90° to maximal dorsiflexion in 5° degree increments, and the distance between two suture knots was measured at each increment. The distance between the two cut ends of gastrocnemius muscle was measured with the ankle at 90° and at maximal dorsiflexion.

RESULTS

Overall, a mean increase in length between the suture knots on the sural nerve was 0.2 cm, from 90° to maximum ankle dorsiflexion (130°); both the Baumann and Strayer techniques resulted in 0.2 cm increase. The mean improvement in maximal ankle dorsiflexion in the Baumann and Strayer group was 22.6° and 22°, respectively. The mean change in distance between the two cut ends of the gastrocnemius muscle in the Baumann and Strayer group was 1.0 cm and 0.9 cm, respectively.

CONCLUSION

Increased dorsiflexion of the ankle following Strayer or Baumann gastrocnemius recession resulted in similar macroscopic change in the sural nerve, which may contribute to the development of sural neuritis. Further clinical studies are warranted to assess clinical implications of these findings.

Changes in Muscle Morphology Following Gastrocnemius Recession for Achilles Tendinopathy: A Prospective Cohort Imaging Study

Background. The purpose of this study was to evaluate changes in posterior compartment muscle volume and intramuscular fat content following gastrocnemius recession in people with Achilles tendinopathy (AT). Methods. Eight patients diagnosed with unilateral recalcitrant AT and an isolated gastrocnemius contracture participated in this prospective cohort study. Magnetic resonance imaging was performed on both limbs of each participant before and 6 months following an isolated gastrocnemius recession. Involved limb muscle volumes and fat fractions (FFs) of the medial gastrocnemius, lateral gastrocnemius, and soleus muscle were normalized to the uninvolved limb. Preoperative to postoperative comparisons were made with Wilcoxon signed-rank tests. Results. Soleus or lateral gastrocnemius muscle volumes or FFs were not significantly different between study time points. A significant difference was found in medial gastrocnemius muscle volume (decrease; P = .012) and FF (increase; P = .017). Conclusion. A major goal of the Strayer gastrocnemius recession, selective lengthening of the posterior compartment while preserving soleus muscle morphology, was supported. The observed changes isolated to the medial gastrocnemius muscle may reduce ankle plantarflexion torque capacity. Study findings may help inform selection of surgical candidates, refine anticipated outcomes, and better direct postoperative rehabilitation following gastrocnemius recession for AT.Levels of Evidence: Level IV: Prospective cohort study.

Meta-analysis of Noninsertional Achilles Tendinopathy

BACKGROUND

Achilles tendinopathy is a common clinical problem that can be either insertional or noninsertional. A variety of treatment methods have been described, although little consensus exists on an optimal method or methods. We sought to investigate the current evidence on different treatment methods for noninsertional Achilles tendinopathy, with a focus on functional outcomes.

METHODS

We performed a review of the available literature in PubMed and the Cochrane Central Register of Controlled Trials. Data from included studies were categorized according to treatment method and analyzed with respect to functional outcome and complication rate.

RESULTS

In total, 1420 abstracts were reviewed, of which 72 articles containing 3523 patients met inclusion criteria. Within the 72 studies included, 6 operative techniques and 19 nonoperative treatments were evaluated.

CONCLUSION

A wide variety of treatments are available for noninsertional Achilles tendinopathy, although newer treatments and most operative methods lack high-level evidence. Eccentric exercise is the most thoroughly studied and supported nonoperative treatment, while tenotomy and debridement is the operative procedure with the most evidence of efficacy. Platelet-rich plasma injections and extracorporeal shockwave therapy have proven to be viable second-line nonoperative treatments. Gastrocnemius recession and flexor hallucis longus transfer have shown benefit in case series.

LEVEL OF EVIDENCE

Level II, systematic review.

Achilles tendoscopy for non insertional Achilles tendinopathy. A case series study

BACKGROUND

Achilles non-insertional tendinopathy is usually treated with conservative means. If resistant to a rehabilitation protocol surgical treatment could be proposed. The aim of this research is to report the mid-term clinical results of endoscopic assisted surgery for patients suffering from recalcitrant painful non-insertional Achilles tendinopathy.

METHODS

A consecutive series of 11 patients (6 men and 5 women), median age of 54 (range 40-67) years, with chronic recalcitrant painful non-insertional Achilles tendinopathy were included. All patients completed at least 20 physical therapy sessions and 6 months of no sports activities before surgery. All underwent Achilles tendoscopy, without tendon excision or transfer with a median follow-up of 87 (27-105) months. We report the preoperative symptoms duration, treatment before surgery, complications and satisfaction after surgery, return to previous sport level, and postoperative VISA-A score.

RESULTS

Mean preoperative symptoms duration was 1 year, having all performed at least 20 physical therapy sessions. No postoperative complications were reported, achieving a complete satisfaction in 10 of 11 patients. All patients returned to their preoperative sports level with a median postoperative VISA-A functional score of 100 (30-100) points.

CONCLUSIONS

The mid-term results of Achilles tendoscopy in patients with chronic painful non-insertional Achilles tendinopathy are satisfactory with a rapid rehabilitation. This procedure is safe and has a low complication rate.

LEVEL OF EVIDENCE

IV. Retrospective case series.

A Comparative Analysis of Clinical Outcomes in Noninsertional Versus Insertional Tendinopathy Using PROMIS

Background. Achilles tendinopathy is a common clinical disorder. Utilizing the Patient-Reported Outcomes Measurement Information System, we aim to determine clinical response to nonoperative achilles tendinopathy rehabilitative care of insertional achilles tendinopathy compared to non-insertional achilles tendinopathy. Methods. Prospective Patient-Reported Outcomes Measurement Information System Physical Function, Pain Interference, and Depression scores were collected for patients with achilles tendinopathy at presentation and following a standard course of nonoperative care. A distribution-based method was used to determine the minimal clinically important difference. Descriptive statistics were reported and bivariate analysis was used to compare insertional achilles tendinopathy and non-insertional achilles tendinopathy. Receiver operating characteristic curve analysis was used to predict clinical improvement. Results. A total of 102 patients with an average follow-up of 68 days were included. For the non-insertional achilles tendinopathy group: Fifteen (46%), 12 (36%) and 9 (27%) patients reached clinical improvement for Physical Function, Pain Interference and Depression, respectively. For the insertional achilles tendinopathy group: Seventeen (25%), 20 (29%) and 22 (32%) patients reached clinical improvement for Physical Function, Pain Interference and Depression, respectively. Physical Function scores improved more in non-insertional achilles tendinopathy patients (4.0 vs. -0.046; p = 0.035) and more patients clinically improved (45.5% vs. 24.6%; p = 0.034). Patients with non-insertional and insertional achilles tendinopathy clinically improved functionally when initial Physical Function scores were equal to or lower than 40.25 and 38.08, respectively. Conclusions. Nonoperative care in achilles tendinopathy is often successful. The Patient-Reported Outcomes Measurement Information System can be used to evaluate and help determine clinical success. Levels of Evidence: Level II: Prospective comparative study.

Achilles Tendinopathy and Associated Disorders

Degenerative disorders of the Achilles tendon are common, affecting up to 18% of the adult population. A thorough evaluation including a focused history, physical examination, and diagnostic studies helps in choosing the appropriate treatment. Initial treatment is usually nonoperative, consisting of activity modification, bracing, and physical therapy. Patents who fail nonoperative management may be treated operatively with a wide range of procedures from endoscopic surgery to open debridement and tendon transfer. Understanding a patient’s expectations and educating patients about potential treatments and their outcomes enables informed collaborative decision making. This article will review the evaluation and management Achilles tendinopathy and associated disorders.

[Insertional Achilles tendinopathy : Differentiated diagnostics and therapy]

Achilles tendinopathy at the calcaneal insertion is classified into insertional tendinopathy, retrocalcaneal and superficial bursitis. The aim of this study was to present the current evidence on conservative and surgical treatment of insertional tendinopathy of the Achilles tendon. Conservative first-line therapy includes reduction of activity levels, administration of non-steroidal anti-inflammatory drugs (NSAID), adaptation of footwear, heel wedges and orthoses or immobilization. In addition, further conservative therapy options are also available. Eccentric stretching exercises should be integral components of physiotherapy and can achieve a 40% reduction in pain. Extracorporeal shock wave therapy has been shown to reduce pain by 60% with a patient satisfaction of 80%. Due to the limited evidence, injections with platelet-rich plasma (PRP), dextrose (prolotherapy) or polidocanol (sclerotherapy) cannot currently be recommended. Operative therapy is indicated after 6 months of unsuccessful conservative therapy. Open debridement allows all pathologies to be addressed, including osseous abnormalities and intratendinous necrosis. The success rate of over 70% is contrasted by complication rates of up to 40%. The Achilles tendon should be reattached, if detached by >50%. No valid data are available for the transfer of the tendon of the flexor hallucis longus (FHL) muscle but it is frequently applied in cases of more than 50% debridement of the diameter of the Achilles tendon. Lengthening of the gastrocnemius muscle cannot be recommended because insufficient data are available. Tendoscopy is a promising treatment option for isolated retrocalcaneal bursitis and has shown similar success rates to open debridement with significantly lower complication rates.

Proximal Medial Gastrocnemius Recession and Stretching Versus Stretching as Treatment of Chronic Plantar Heel Pain

BACKGROUND:

Plantar heel pain is a common disorder that can lead to substantial pain and disability. Gastrocnemius recession has been described as an operative treatment option, but there is a lack of prospective clinical and biomechanical outcome data. The aim of this study was to evaluate the clinical and biomechanical outcomes of gastrocnemius recession and stretching compared with a stretching exercise protocol for patients with plantar heel pain lasting more than 12 months.

METHODS:

Forty patients with plantar heel pain lasting more than 1 year were randomized to a home stretching exercise program only or to surgery consisting of a proximal medial gastrocnemius recession in addition to stretching exercises. The main outcome was the American Orthopaedic Foot & Ankle Society (AOFAS) ankle-hindfoot score at 12 months. Secondary clinical outcomes were the Short Form-36 (SF-36) and visual analogue scale (VAS) pain scores. The biomechanical outcome parameters were ankle dorsiflexion, Achilles function evaluated by a test battery with 6 independent tests, and plantar pressure evaluated by pedobarography. All data were obtained at baseline and at 12-month follow-up.

RESULTS:

The AOFAS score increased from 59.5 (42-76) to 88.0 (50-100; P < .001) for the operative group and from 52.5 (37-73) to 65.5 (31-88; P = .138) for the nonoperative group. The AOFAS, VAS pain, and SF-36 scores were significantly better in the operative compared with the nonoperative group at 12-month follow-up ( P < .05). Ankle dorsiflexion increased from 6 degrees (-3 to 15) to 10.5 degrees (0 to 23; P < .001). No between-group difference was observed for Achilles function at follow-up. The average forefoot plantar pressure for the operative group increased from 536 KPa (306-708) to 642 KPa (384-885) at follow-up ( P < .001).

CONCLUSION:

Proximal medial gastrocnemius recession with a stretching program was a safe and efficient method of treating chronic plantar heel pain.

LEVEL OF EVIDENCE:

Level 1, randomized clinical trial.

Validation of a New Device for Measuring Isolated Gastrocnemius Contracture and Evaluation of the Reliability of the Silfverskiöld Test

BACKGROUND

Important aspects on the diagnostics of isolated gastrocnemius contractures (IGCs) have been poorly described. This study was designed to validate a new ankle range of motion (ROM) measuring device for diagnosing an IGC. In addition, we wanted to investigate the reliability of the clinical Silfverskiöld test.

METHODS

Twelve health care personnel (24 feet) were examined by 4 testers on 3 different occasions for the reliability testing of the new ankle ROM measuring device. The same participants were examined using the Silfverskiöld test to examine the reliability of the clinical test. Eleven patients (15 feet) with IGC were examined before gastrocnemius recession, immediately after surgery, and 3 months after surgery to examine the validity and responsiveness of the ankle ROM device.

RESULTS

An intraclass correlation coefficient (ICC) >0.85 was found for both inter- and intrarater reliability for the new ankle ROM device. The device confirmed an IGC in 13 of 15 feet before surgery and 3 of 13 feet at 3-month follow-up. At baseline, the measured ankle dorsiflexion was median 3 degrees with the knee in extension, which increased to 10 degrees ( P < .001) immediately after surgery and 12 degrees ( P = .003) at 3-month follow-up. ICC values of 0.230 to 0.791 were observed for the inter- and intrarater reliability testing of the clinical Silfverskiöld test.

CONCLUSION

The new ankle ROM measuring device was reliable and responsive for detecting IGC. The Silfverskiöld test had poor inter- and intrarater reliability. Level of evidence Level II, prospective cohort study.

Peritenolysis and Debridement for Main Body (Mid-Portion) Achilles Tendinopathy in Athletic Patients: Results of 107 Procedures

Achilles tendinopathy in the main body (mid-portion) of the tendon is a common pathologic finding among active and athletic populations and can be debilitating without proper and adequate treatment. Numerous surgical approaches for this common pathologic finding have been reported, with variable outcomes. We evaluated the surgical outcomes of peritenolysis and debridement of main body Achilles tendinopathy among athletic populations using the return to activity (RTA) and decreased desired activity (DDA) as our primary outcome measures. A total of 100 patients underwent 107 procedures by the senior author (A.S.) from January 2001 through December 2015 met the inclusion criteria, 65 (65%) of whom were runners. The mean follow-up duration was 106.6 ± 55.5 months from the index procedure, and the mean interval necessary to RTA for the entire group was 10.9 ± 5.3 weeks. The average RTA after debridement was 14.1 ± 5.2 weeks and after peritenolysis was 7.3 ± 2.0 weeks (p = .00001). Of the 100 patients, 3 (3%) had experienced a DDA at the last follow-up visit. With >97% of the patients able to return to their desired activities, we have concluded that peritenolysis and debridement are favorable surgical techniques for main body Achilles tendinopathy.

Gastrocnemius Recession for Metatarsalgia

Metatarsalgia is a common cause of plantar forefoot pain. Causes of metatarsalgia include foot anatomy, gait mechanics, and foot and ankle deformity. One specific cause, mechanical metatarsalgia, occurs because of gastrocnemius muscle contracture, which overloads the forefoot. Muscular imbalance of the gastrocnemius complex alters gait mechanics, which increases recruitment of the toe extensor musculature, thereby altering forefoot pressure. Patients with concomitant metatarsalgia and gastrocnemius contracture demonstrate ankle equinus and a positive Silfverskiold test. Nonoperative therapeutic modalities are mainstays of treatment. In patients in whom these treatments fail to provide metatarsalgia symptomatic relief, gastrocnemius muscle lengthening is a therapeutic option.

Isolated Gastrocnemius Recession for the Treatment of Achilles Tendinopathy

BACKGROUND

Various treatment modalities have been described for Achilles tendinopathy with varying degrees of success. The rationale for the gastrocnemius recession procedure is to decrease the mechanical overload of the Achilles tendon arising from an equinus contracture.

METHODS

We retrospectively reviewed 25 patients who underwent an isolated gastrocnemius recession procedure at our institution between May 2013 and April 2015 by a single surgeon. Clinical outcome was evaluated on the basis of pain, utilizing visual analog scale (VAS) scores and the Foot Function Index (FFI) by telephonic interview. Student's t test and one-way analysis of variance were used for statistical analysis.

RESULTS

The average age of patients was 53.2 years with an average body mass index of 35.8 kg/m². The average follow-up was 13.1 months. All 25 patients had a decrease in VAS scores from an average of 8.9 preoperatively to 2.0 at the 6-week visit. The average FFI went from 73.5 preoperatively to 27.4 at final follow-up. Two out of 25 patients had a postoperative sural neuritis with a total complication rate of 12%. Twenty-one of 25 patients (84%) reported total or significant pain relief.

CONCLUSION

The medium-term results of our study suggest that an isolated gastrocnemius recession is a simple, effective, and safe surgical procedure for the treatment of Achilles tendinopathy.

LEVELS OF EVIDENCE

Therapeutic, Level IV: Retrospective.

Noninsertional Tendinopathy of the Achilles

Noninsertional Achilles tendinosis is differentiated from insertional Achilles tendinosis based on anatomic location. Tendinosis, as opposed to tendonitis, is primarily a degenerative process and the role of inflammation is believed limited. The etiology of Achilles tendinopathy may include overuse leading to repetitive microtrauma, poor vascularity of the tissue, mechanical imbalances of the extremity, or combination of these elements. There is evidence to support eccentric exercise nonoperative management for patients with noninsertional Achilles tendinopathy. Operative treatment options include percutaneous longitudinal tenotomies, minimally invasive tendon scraping, open débridement and tubularization, and tendon augmentation with flexor hallucis longus.