Intratendinous pressure of the Achilles tendon during exercise is related to the degree of tendon torsion

Torsion of the Achilles tendon (AT) enhances tensile strength, but a high degree of torsion might also be a risk factor for Achilles tendinopathy, due to greater internal compression exerted during tensile loading. However, evidence supporting the grounds for this assumption is lacking. Hence, we aimed to investigate the impact of AT torsion type on intratendinous pressure. Eighteen human fresh frozen cadaveric legs were mounted in a testing rig and a miniature pressure catheter was placed through ultrasound‐guided insertion in the midportion region of the AT. Intratendinous pressure was measured during a simulated straight‐knee calf stretch and eccentric heel drop. The AT was then carefully dissected and classified into Type I (least), Type II (moderate), and Type III (extreme) torsion. Of the ATs examined, nine were found to have Type I torsion (50%), nine Type II (50%), and none Type III. It was found that the intratendinous pressure of the AT increased exponentially with ankle dorsiflexion during both exercises (p < 0.001) and that this increase was greater in ATs with Type II torsion than Type I torsion (p < 0.05). This study provides the first biomechanical data to support the hypothesis that in athletes with a high degree of torsion in the AT, the midportion area will experience more internal compression during exercise, for example, calf stretching and eccentric heel drops. Whether this phenomenon is also associated with an elevated risk for Achilles tendinopathy needs further prospective investigation.


| INTRODUCTION
Achilles tendinopathy is one of the most frequent overuse injuries among athletes, especially those involved in running sports.The lifetime risk is estimated at 52% for elite long-distance runners and 23.9% for athletes in general. 1 Achilles tendinopathy, which is characterized by tendon thickening, pain, and dysfunction, most often occurs in the midportion of the Achilles tendon (AT), located 2-7 cm above the insertion. 2Excessive overload is considered one of the main triggers of this pathology.Traditionally, it was assumed that the nature of this overload was purely tensile.7][8] Proximally, the fascicles of the subtendons initially run vertically, according to the position of their corresponding muscle bellies.][12][13][14][15] Indeed, this torsion-induced internal compression explains the fact that it is particularly in the midportion region of the AT that fluid exudation occurs during loading. 16The torsional structure occurs in all ATs and the direction of rotation is also consistent across individuals. 7,8However, the amount of torsion varies between individuals.8][9] It has been suggested that a higher degree of torsion might be correlated to a higher risk of developing Achilles tendinopathy, as it is theoretically associated with more internal compression. 9owever, there is currently a lack of supporting evidence for this theory.Hence, we aimed to evaluate the influence of torsion on intratendinous pressure, which correlates to the amount of internal compression, in the midportion region of the AT during stretching and eccentric loading.We hypothesized that with these exercises, the intratendinous pressure in this region would increase more in ATs with Type II or III torsion than those with Type I torsion.

| Specimen preparation and mounting
Ethical approval for this biomechanical study was provided by the Ethics Committee of the Ghent University Hospital (BC-09407).Eighteen fresh frozen human cadaveric legs from four male and five female donors (mean age 79.8 ± 7.9 years, mean BMI 24.1 ± 5.9 kg/m 2 ) were obtained from a tissue bank for experimental research.The legs were disarticulated at the hip, underwent a computerized tomography (CT) scan, and were then placed in a −20°C freezer for storage.Based on the CT scan, a 3D segmentation model of each bone was created using Mimics software (Mimics software®).Using OptiTrack cameras (OptiTrack®) and markers, ankle joint flexion angles were measured.The specimens were thawed at room temperature for 24 h before testing.To ensure a correct anatomical position of the bones in the test setup, guides were designed and printed based on the CT scan.The femur was sectioned 95 mm distal from the femoral hip center and potted in a container.A customized validated test setup was used as an activity simulator, which had the great advantage of preserving the anatomical connection of the gastrocnemius muscle to the knee (Figure 1). 4 This allows a more physiological tensile load of this biarticular muscle on the AT during plantarflexion or dorsiflexion of the ankle.The specimen's foot was inserted in the ankle-foot holder and fixed with three pins.One pin was drilled through the medial and lateral calcaneus and one through the medial talus so that movement in the subtalar joint was impeded and only plantar and dorsal flexion could occur in the tibiotalar joint.Finally, a connection was made between the cable pulley system (two gastrocnemius actuators) and the lateral and medial head of the gastrocnemius, so that additional tensile load could be applied to the AT.

| Intratendinous pressure determination
A Mikro-Tip® catheter transducer (Model SPR-524; Millar Instruments), a commercially available pressure sensor, was used to measure intratendinous pressure at the center of the midportion of the AT.The sensor functions by the piezoresistive principle and relies on chip technology to transform pressure into an electronic signal.The transducer is flexible, has high-fidelity pressure sensors mounted at the tip, a natural frequency of ≥10 kHz, and a diameter of 1.16 mm.Data from the implanted sensor were recorded using a digital acquisition system (PowerLab 8/30, AD Instruments) and analyzed with LabChart 8 software.The reliability of this transducer for measuring interstitial tissue pressure has been tested and is considered a gold standard. 179][20] Under ultrasound guidance, an experienced sonographer placed the pressure sensor in the midportion region of the AT, namely 25 mm proximal to the posterosuperior calcaneal border, where tendinopathy typically occurs. 21 GE Logic S8 ultrasound system (GE Healthcare) with a high-resolution linear ML6-15 transducer was used for ultrasonographic imaging.A 17-gauge catheter was inserted percutaneously and advanced until the tip of the needle was at the desired location, centered in the mediolateral and antero-posterior directions, which was assessed both longitudinally and transversally (Figure 1).Before each set of measurements, a dynamic ultrasonographic examination was performed using a probe fixation system (USONO) in order to verify that the pressure sensor remained in situ during each loading condition.

| Testing protocol
Before testing, 10 preconditioning cycles of ankle flexion (−15° plantarflexion to 15° dorsiflexion) were performed by tilting the ankle-foot holder up and down.Next, two different activities, a straight-knee calf stretch and an eccentric heel drop, were simulated for the actual experiments.To mimic a physiological straight-knee calf stretch, the knee was fixed in full extension and the ankle was moved from a plantar flexed position (−15°) to dorsiflexion (+15°).To simulate an eccentric heel drop, the previous movement was repeated, but an additional continuous tensile load was applied to the AT by activating the two gastrocnemius actuators.To preserve the integrity of the specimens, a downscaling by a factor five was applied to the average tensile force acting on the Achilles tendon during an eccentric heel drop, resulting in a total tensile load of 200 N (100 N for each head of the gastrocnemius). 22Each loading condition was repeated without removing the pressure sensor so that the mean intratendinous pressure could be determined.After completion of the biomechanical testing protocol, the specimens were again stored in a −20°C freezer so that they could later be dissected for torsion type determination.

| Torsion type determination
Before dissection, the specimens were thawed at room temperature for 18 h.To determine torsion type, the method developed by Edama et al. 7 was used and support was provided by two experienced anatomists.First, skin, subcutaneous tissue, and fascia cruris were removed from the posterior surface of the leg.Then, the triceps surae muscles along with the AT and calcaneus were extracted and placed in a dissection setup.In this setup, the muscle bellies of all of the medial gastrocnemius (MG), lateral gastrocnemius (LG), and soleus (SOL) could be suspended individually, recreating a "natural situation" as if the leg was standing upright, with the musculature displayed in its natural position.This setup facilitated the dissection and interpretation which followed (Figure 2).The tendon fascicles originating from the MG and LG were separated from those originating from the SOL, after which the tendon fascicles originating from the MG and LG were also dissociated.It should be noted that fusions exist between the fascicles of the three different subtendons, but, as also noted by Edama et al., 6 by following the course of their relatively thick tendon fibers, it was possible to establish the main boundaries of the fascicles of the subtendons.Following these boundaries allowed for careful separation of the individual fascicles during both steps.Finally, the course and arrangement of the different subtendons on the insertion site on the calcaneum were evaluated.Based on this evaluation, the degree of torsion of the AT was classified as Type I (least), in which the anterior layer of the AT insertion is formed only by tendon fascicles of the SOL, Type II (moderate), in which the layer is formed by tendon fascicles from both the LG and SOL, and Type III (extreme), in which it is formed only by tendon fascicles from the LG.

| Statistical analysis
A linear mixed model analysis (IBM® SPSS® 27.0 statistics) was used to investigate the effect of ankle flexion and torsion type on the intratendinous pressure of the AT.Analysis was conducted with the cadaver as a random factor and ankle flexion angle, loading condition, and torsion type as main fixed factors.A log transformation of the data was first done since the raw data showed an exponential relationship between intratendinous pressure and ankle flexion angle.The residuals were checked for normal distribution and homoscedasticity.Estimates of the fixed effects were determined for the significant fixed factors and pairwise comparisons of the resultant estimated geometric means of the intratendinous pressure were performed using Bonferroni correction.The relative differences in pressure were defined at 15° of ankle dorsiflexion after back-transforming the logarithmic values to the original scale.The level of significance was set at p value ≤ 0.05.The within-session reliability of intratendinous pressure was assessed using the intra-class correlation coefficient and was found to be 0.94 (CI, 0.85-0.98),which indicates a high level of reliability.

| Assessment of torsion type
Anatomical dissection revealed that of the ATs examined, nine were found to have Type I torsion (50%) and nine Type II (50%).None had Type III.Among the right legs, four (44.4%) were Type I and five (55.6%) were Type II.Among the left legs, five (55.6%) were Type I and four (44.4%) were Type II.Seven individuals (77.8%) had the same torsion type bilaterally.

| Assessment of intratendinous pressure
The results of the mixed model analysis are presented in Table 1 and show that intratendinous pressure of the AT is influenced by both ankle flexion angle (F = 2289.21,p < 0.001), torsion type (F = 5.06, p < 0.05) and loading condition (F = 83.86,p < 0.001).An interaction effect between ankle flexion angle and torsion type (F = 15.38,p < 0.001) was also identified.The estimates of the significant main and interaction fixed effects are presented in Table 2.

| Effect of ankle dorsiflexion
A positive correlation was identified between the ankle flexion angle and intratendinous AT pressure (p < 0.001; Table 2).Figure 3 shows that intratendinous pressure increases exponentially with ankle dorsiflexion for both AT torsion types and during both loading conditions.

| Effect of torsion type
As shown in Table 2, higher intratendinous pressures were found in ATs with torsion Type II than those with torsion Type I (p < 0.05).In addition, intratendinous pressure increased more during ankle dorsiflexion in ATs with torsion Type II (p < 0.05).Furthermore, pairwise comparisons in Table 3 show that intratendinous pressure at 15° ankle dorsiflexion in ATs with torsion Type II was 81% and 84% higher than in ATs with torsion Type I (p < 0.05) during a straight-knee calf stretch and an eccentric heel drop, respectively (Figure 3).

| Effect of loading condition
Higher intratendinous pressures were found during an eccentric heel drop than a straight-knee calf stretch (p < 0.001).Pairwise comparisons in Table 4 show that the intratendinous pressure at 15° ankle dorsiflexion during an eccentric heel drop was 46% and 48% higher than during a straight-knee calf stretch (p < 0.05), for ATs with torsion Types I and II, respectively.

| DISCUSSION
This study is the first to investigate the functional consequences of tendon torsion on the amount of intratendinous pressure in the AT during exercise.The results of our anatomical assessment confirmed torsion in the structure of all ATs, which has been reported to be present  without exception. 7Our observations on the prevalence of AT torsion types were also similar to previous studies where it was found that Type I and II are about equally common, but that Type III is unfrequent. 6,7,9In addition, we can confirm that the torsion type is not always identical on both sides. 7However, these previous studies did not couple the observed tendon torsion types to intratendinous pressure patterns of the AT during exercise.Our principal finding is that ATs with moderate torsion (Type II) experience significantly more intratendinous pressure during both stretching and eccentric heel drops than ATs with least torsion (Type I).These novel observations support the suggestion of Pękala et al. that ATs with more torsion might undergo more internal compression. 9Moreover, we found that for both AT torsion types, the amount of intratendinous pressure was greatest at terminal ankle dorsiflexion during eccentric heel drops.This finding can be explained by the fact that such condition leads to the highest tensile forces acting on the AT, which can translate into more torsioninduced internal compression. 22,23Indeed, more fluid exudation from the tendon core, which results from internal compression, is observed in vivo when more load is applied on the AT. 24he higher levels of internal compression during exercise for Type II AT may have detrimental consequences, especially in cases of overloading, and could therefore be an important risk factor for athletes.First, blood supply might be compromised as blood vessels can collapse during high intratendinous pressure, causing ischemia. 3,9Second, chondrogenic metaplasia might occur as the tenocytes try to adapt the tendon structure F I G U R E 3 Intratendinous pressure in the midportion region of the Achilles tendon (mm Hg) as a function of ankle flexion angle (°) during a straight-knee calf stretch (left) and an eccentric heel drop (right), comparing Achilles tendons with a Type I and Type II torsion.The data are presented as estimated geometric means with their corresponding limit of the upper 95% confidence interval.*p ≤ 0.05 Indicates a significant difference in intratendinous pressure.to this compressive load, a process known as mechanotransduction.Indeed, both ischemia and fibrocartilagelike tissue are common features of midportion Achilles tendinopathies. 13,25However, these possibilities remain speculative.Therefore, prospective studies are needed to verify whether athletes with Achilles tendinopathy do indeed have a higher degree of tendon torsion.In this regard, it should be considered that noninvasive assessment of AT torsion by MRI or ultrasound remains a challenge. 26ur fundamental findings are also of clinical interest, as load management and exercise are the cornerstones of treatment of patients with Achilles tendinopathy.As it is suggested that the accumulation of glycosaminoglycans and associated fluid in a tendinopathic AT induces higher intratendinous pressure for the same amount of tensile loading, it might be appropriate to limit the internal compression somewhat during rehabilitation. 3,28s we found that the amount of intratendinous pressure peaks at terminal ankle dorsiflexion, it might be better to restrict loading into excessive dorsiflexion as to limit torsion-induced internal compression, especially in patients with a high degree of AT torsion and during the initial phase of rehabilitation.It might partially explain why heel lifts are found to be more effective than eccentric training in reducing pain and improving tendon function in patients with midportion Achilles tendinopathy. 29inally, several limitations of this study should be considered.First, there are known limitations to cadaver studies such as the fact that the age of specimens and their quality, despite careful preservation, cannot exactly mimic the conditions of healthy athletes' tendons.Second, as only the center of the midportion was investigated, statements about the influence of torsion type on intratendinous pressure are limited to this specific region.Third, we found no AT with Type III torsion in our study.Although it seems plausible that intratendinous pressure is greatest in this torsion type, we cannot state this with certainty.Fourth, although it has been suggested that excessive ankle eversion can enhance torsion-induced internal compression, in our setup it was not possible to simulate eversion and therefore its effect could not be determined. 30Fifth, in order to keep the gastrocnemius muscles intact, it was decided not to perform a dissection.Therefore, the underlying soleus muscle could not be separately stitched and consequently could not be activated during the eccentric heel drop.Additional activation of the soleus muscle could have an impact on the intratendinous pressure, as it may accentuate torsion-induced internal compression.

| PERSPECTIVE
Our findings indicate a relationship between the degree of AT torsion and intratendinous pressure in the midportion region during exercise.More specifically, this study is the first to demonstrate that AT with a moderate degree of torsion (Type II) experiences more internal compression than AT with least torsion (Type I) during stretching and eccentric loading.In clinical practice, these findings provide biomechanical data to support the assumption that athletes with higher degrees of AT torsion are more prone to develop midportion Achilles tendinopathy. 9However, future research is needed to further investigate AT torsion type as potential risk factor and to determine whether adjusting rehabilitation programmes, based on the degree of AT torsion, may be beneficial.

F I G U R E 2
Posterior view of the dissection setup of a left Achilles tendon, showing the three subtendons that comprise it rotating distally clockwise along the tendon (A).Cross-sectional view of the proximal and insertional region of the Achilles tendon, showing that the tendon can be classified into three different types of torsion based on the course of the subtendons (B).

b
Straight-knee calf stretch is the reference category.c Type I × Ankle flexion angle is the reference category.*p ≤ 0.05.

3
Pairwise comparisons of estimated mean intratendinous pressure between the different torsion types for each loading condition.
Type III tests of fixed effects with log intratendinous pressure as dependent variable.Estimates of fixed effects with log intratendinous pressure as dependent variable.
T A B L E 1*p ≤ 0.05.T A B L E 2 aType I is the reference category.

Torsion type Mean fold difference (I-J) a
Pairwise comparisons of estimated mean intratendinous pressure between the different loading conditions for each torsion type.Estimated geometric mean fold differences are evaluated at an ankle flexion angle of 15°.
T A B L E 4 a *p ≤ 0.05.