Intratendinous pressure changes in the Achilles tendon during stretching and eccentric loading: Implications for Achilles tendinopathy

Pathophysiology and healing of insertional Achilles tendinopathy: Current concepts

Insertional Achilles tendinopathy (IAT) is a challenging condition that significantly impacts athletes and physically active individuals, often leading to chronic pain and impaired performance. IAT is characterized by a complex interplay of mechanical stress, vascular impairment, inflammatory responses, and extracellular matrix (ECM) dysregulation at the Achilles tendon insertion. This review integrates recent advancements in the understanding of IAT pathophysiology, with focus on the effects of tensile and compressive loads, intratendinous pressure changes, tissue hypoxia, and ECM water balance. Emerging evidence indicates that mechanical loading influences tendon homeostasis through mechanotransduction, leading to ECM remodeling and fibrocartilaginous adaptation. Although appropriate compressive loading is necessary to maintain ECM homeostasis and fibrocartilage regeneration, excessive or abnormal loading disrupts tendon repair mechanisms and contributes to degenerative changes. Furthermore, increased intra-tendinous pressure impairs capillary perfusion, thereby promoting a hypoxic microenvironment that exacerbates the inflammatory response. Dysregulated water retention due to glycosaminoglycans (GAGs) and hyaluronic acid affects intra-tendinous pressure, highlighting potential therapeutic strategies targeting ECM hydration. This review also explores the roles of macrophage polarization, cytokine regulation, and growth factors in tendon healing, emphasizing their potential therapeutic implications. By integrating the anatomical, biomechanical, and molecular insights, this review provides a comprehensive perspective of IAT pathophysiology and its healing mechanisms. Understanding these mechanisms is essential to optimizing conservative treatments, refining surgical approaches, and developing novel therapeutic strategies to enhance tendon repair and prevent disease progression.

Effectiveness of reducing tendon compression in the rehabilitation of insertional Achilles tendinopathy: a randomised clinical trial

OBJECTIVE

To assess the effectiveness of low tendon compression rehabilitation (LTCR) versus high tendon compression rehabilitation (HTCR) for treating patients with insertional Achilles tendinopathy.

METHODS

In an investigator-blinded, stratified randomised trial, 42 sport-active patients (30 males and 12 females; age 45.8±8.2 years) with chronic (> 3 months) insertional Achilles tendinopathy were allocated in a 1:1 ratio to receive LTCR or HTCR. Both rehabilitation protocols consisted of a progressive 4-stage tendon-loading programme, including isometric, isotonic, energy-storage and release and sport-specific exercises. The LTCR programme was designed to control Achilles tendon compression by limiting ankle dorsiflexion during exercise, eliminating calf stretching and incorporating heel lifts. The primary outcome was the Victorian Institute of Sports Assessment-Achilles (VISA-A) score at 12 and 24 weeks, which measures tendon pain and function and was analysed on an intention-to-treat basis using a linear mixed model. Significance was accepted when p<0.05.

RESULTS

20 patients were randomised to the LTCR group and 22 to the HTCR group. Improvement in VISA-A score was significantly greater for LTCR compared with HTCR after 12 weeks (LTCR=24.4; HTCR=12.2; mean between-group difference=12.9 (95% CI: 6.2 to 19.6); p<0.001) and after 24 weeks (LTCR=29.0; HTCR=19.3; mean between-group difference=10.4 (95% CI: 3.7 to 17.1); p<0.001). These differences exceeded the minimal clinically important difference of 10.

CONCLUSIONS

In sport-active patients with insertional Achilles tendinopathy, LTCR was more effective than HTCR in improving tendon pain and function at 12 and 24 weeks. Consequently, LTCR should be considered in the treatment of insertional Achilles tendinopathy.

TRIAL REGISTRATION NUMBER

ClinicalTrials.gov (ID: NCT05456620).

Novel Insights Into the Intratendinous Pressure Behavior of the Achilles Tendon in Athletes

BACKGROUND

In contrast to other musculoskeletal tissues, the normal pressure behavior of the Achilles tendon is poorly understood. This study aimed to explore the normal intratendinous and perfusion pressures of the Achilles tendon at rest and during exercise, and investigate potential correlations with tendon load and morphology.

HYPOTHESIS

Intratendinous and perfusion pressures of the Achilles tendon exhibit similarities to other musculoskeletal tissues and depend on tendon load and morphology.

STUDY DESIGN

Observational study.

LEVEL OF EVIDENCE

Level 3.

METHODS

A total of 22 recreational athletes were enrolled. Demographics, activity level, and blood pressures were recorded. Achilles tendon thickness and echogenicity were assessed 25 mm proximal to the posterosuperior calcaneal border. In this region, intratendinous and perfusion pressures of the Achilles tendon were measured at rest and during isometric plantarflexion up to 50 N, using the microcapillary infusion technique. Linear mixed models were used to investigate the effects of plantarflexion force, tendon thickness, and echogenicity on intratendinous and perfusion pressures.

RESULTS

At rest, intratendinous and perfusion pressures of the Achilles tendon were 43.8 ± 15.2 and 48.7 ± 18.4 mmHg, respectively. Intratendinous pressure increased linearly with plantarflexion force, reaching 101.3 ± 25.5 mmHg at 50 N (P < 0.01). Perfusion pressure showed an inverse relationship, dropping below 0 mmHg at 50 N (P < 0.01). Neither intratendinous nor perfusion pressures of the Achilles tendon correlated with tendon thickness or echogenicity.

CONCLUSION

The normal intratendinous resting pressure of the Achilles tendon is higher than other musculoskeletal tissues, making it more susceptible to ischemia. During exercise, intratendinous pressure increases significantly to a level that lowers perfusion pressure, thereby compromising blood supply at already low plantarflexion forces.

CLINICAL RELEVANCE

Given the potential role of ischemia in Achilles tendinopathy, our findings caution against intratendinous injections, as they may exacerbate high intratendinous resting pressure, and against prolonged postexercise tendon stretching, as the associated rise in intratendinous pressure may impair the required hyperemic response.

Subject-specific biomechanics influences tendon strains in patients with Achilles tendinopathy

The treatment of Achilles tendinopathy is challenging, as 40% of patients do not respond to existing rehabilitation protocols. These protocols neglect individual Achilles tendon (AT) characteristics, which are crucial for healing of the tendon tissue. Although prior studies suggest an optimal strain for AT regeneration (6% tendon strains), it is unclear if current protocols meet this condition. Our study aimed to analyse the impact of a selection of rehabilitation exercises on tendon strains in patients with Achilles tendinopathy, using subject-specific finite element (FE) models of the free AT. Second, this study aimed to explain the influence of muscle forces and material properties on AT strains. The 21 FE models of the AT included the following subject-specific features: geometry estimated from 3D freehand ultrasound images, Elastic modulus estimated from the experimental stress‒strain curve, and muscle forces estimated using a combination of 3D motion capture and musculoskeletal modelling. Exercises were ranked based on strain progression, starting from concentric and eccentric exercises, and going to more functional exercises, which impose a greater load on the AT. There was no significant difference between the unilateral heel drop and walking, and both exercises fell within the optimal strain range. However, when examining individual strains, it became evident that there was diversity in exercise rankings among participants, as well as exercises falling within the optimal strain range. Muscle forces notably affected strains more than material properties. Our findings indicate the importance of tailored rehabilitation protocols that account for individual morphological, material, and muscle characteristics.

Elevated fluid and glycosaminoglycan content in the Achilles tendon contribute to higher intratendinous pressures: Implications for Achilles tendinopathy

BACKGROUND

Tendinopathy alters the compositional properties of the Achilles tendon by increasing fluid and glycosaminoglycan content. It has been speculated that these changes may affect intratendinous pressure, but the extent of this relationship remains unclear. Therefore, we aimed to investigate the impact of elevated fluid and glycosaminoglycan content on Achilles tendon intratendinous pressure and to determine whether hyaluronidase (HYAL) therapy can intervene in this potential relationship.

METHODS

Twenty paired fresh-frozen cadaveric Achilles tendons were mounted in a tensile-testing machine and loaded up to 5% strain. Intratendinous resting (at 0% strain) and dynamic pressure (at 5% strain) were assessed using the microcapillary infusion technique. First, intratendinous pressure was measured under native conditions before and after infusion of 2 mL physiological saline. Next, 80 mg of glycosaminoglycans were administered bilaterally to the paired tendons. The right tendons were additionally treated with 1500 units of HYAL. Finally, both groups were retested, and the glycosaminoglycan content was analyzed.

RESULTS

It was found that both elevated fluid and glycosaminoglycan content resulted in higher intratendinous resting and dynamic pressures (p < 0.001). HYAL treatment induced a 2.3-fold reduction in glycosaminoglycan content (p = 0.002) and restored intratendinous pressures.

CONCLUSION

The results of this study demonstrated that elevated fluid and glycosaminoglycan content in Achilles tendinopathy contribute to increased intratendinous resting and dynamic pressures, which can be explained by the associated increased volume and reduced permeability of the tendon matrix, respectively. HYAL degrades glycosaminoglycans sufficiently to lower intratendinous pressures and may, therefore, serve as a promising treatment.

Longitudinal changes in Achilles tendon and triceps surae muscle architecture during a 156-km mountain ultramarathon

This study aimed to assess the longitudinal changes in triceps surae muscle-tendon architecture during a mountain ultramarathon. Experienced trail runners [n = 55, 78% men, age: 45.2 (13.5) yr] participated in a 156-km trail run (6,000 m climbing) consisting of six 26-km laps. The resting architectural properties of triceps surae muscle-tendon were measured using ultrasound imaging for Achilles tendon cross-sectional area (AT CSA), medial gastrocnemius muscle pennation angle, thickness, length, and fiber length. Measurements were performed the day before the race (baseline), at 52 km (T1), at 104 km (T2), at 156 km (T3), and 12 h after the race (H12). Among finishers (n = 41), there was a significant biphasic change in AT CSA during the race (P = 0.001). First, a significant decrease in AT CSA occurred between baseline and T1 (P = 0.006), with a greater decrease for participants averaging speed >8 km/h (P = 0.014). Second, there was a significant increase in AT CSA especially between T2 and T3 (P = 0.006) that was correlated with a decrease in average speed (P = 0.001) and alteration of spaciotemporal running parameters (P < 0.05). Changes in muscle-tendon architecture were not significantly different between finishers (n = 41) and nonfinishers (n = 14). In 47 participants (85.5%) who completed the follow-up, AT CSA at H12 was greater compared with baseline (P = 0.010). The main finding is the significant and biphasic modification of the AT CSA during a 156-km mountain ultramarathon with an initial decrease corresponding to mechanical stress followed by a secondary increase suggesting adaptive mechanotransduction persisting after 12 h.NEW & NOTEWORTHY Achilles tendon cross-sectional area (AT CSA) demonstrated significant adaptive modifications during a 156 km mountain ultramarathon in trained athletes. Initially, a decrease in AT CSA, especially at higher running speeds, is consecutive to the biomechanical stress on the plantar flexor muscle-tendon unit (MTU). Subsequently, there is a significant increase in AT CSA persisting up to 12 h after the race, which likely corresponds to an adaptive process to limit the compressive and tensile load on the tendon.

Effect of interstitial fluid pressure on shear wave elastography: an experimental and computational study

Objective. An elevated interstitial fluid pressure (IFP) can lead to strain-induced stiffening of poroelastic biological tissues. As shear wave elastography (SWE) measures functional tissue stiffness based on the propagation speed of acoustically induced shear waves, the shear wave velocity (SWV) can be used as an indirect measurement of the IFP. The underlying biomechanical principle for this stiffening behavior with pressurization is however not well understood, and we therefore studied how IFP affects SWV through SWE experiments and numerical modeling.Approach. For model set-up and verification, SWE experiments were performed while dynamically modulating IFP in a chicken breast. To identify the confounding factors of the SWV-IFP relationship, we manipulated the material model (linear poroelastic versus porohyperelastic), deformation assumptions (geometric linearity versus nonlinearity), and boundary conditions (constrained versus unconstrained) in a finite element model mimicking the SWE experiments.Main results. The experiments demonstrated a statistically significant positive correlation between the SWV and IFP. The model was able to reproduce a similar SWV-IFP relationship by considering an unconstrained porohyperelastic tissue. Material nonlinearity was identified as the primary factor contributing to this relationship, whereas geometric nonlinearity played a smaller role. The experiments also highlighted the importance of the dynamic nature of the pressurization procedure, as indicated by a different observed SWV-IFP for pressure buildup and relaxation, but its clinical relevance needs to be further investigated.Significance. The developed model provides an adaptable framework for SWE of poroelastic tissues and paves the way towards non-invasive measurements of IFP.

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.