Tendinopathy: the interplay between mechanical stress, inflammation, and vascularity

Key points

1. 01The old view that tendinopathy is purely degenerative with no inflammation has been overturned; inflammation is now recognized, especially in early disease.
2. 02Three core drivers, mechanical stress, inflammation, and new blood-vessel growth, interact in a dynamic, self-reinforcing loop rather than acting in isolation.
3. 03Predisposing factors such as metabolic disease, fluoroquinolone or statin use, and genetics can prime a tendon so that repeated overload tips it into disease.
4. 04Healthy tendons are kept hypovascular and have a blood-tendon barrier; disease is marked by new vessel and nerve ingrowth, matrix breakdown, and altered cell metabolism.
5. 05No single animal model or biomarker captures the whole disease, and the dominant human trigger remains unproven.

How it was conducted

Design

Narrative review synthesizing tendinopathy pathomechanisms into a unified concept

Scope

Mechanical stress, inflammation, and vascular changes as key drivers, plus emerging concepts (intratendinous pressure, metabolism)

Evidence base

Mechanistic and molecular studies, animal models, and human histology; epidemiology drawn from cited systematic reviews

Outputs

Two summary tables of differentially regulated ECM proteins and 14 schematic figures

What they found

• Healthy Achilles tendon resting intratendinous pressure is about 43.8 plus or minus 15.2 mmHg, rising above 100 mmHg at 50 N dorsiflexion, and perfusion pressure can fall below zero with dorsiflexion.
• Two recent systematic reviews (44 studies) and Hopkins et al. (2016) suggest sex is not a major risk factor for tendinopathy.
• VEGF is upregulated by cyclic strain at 1 Hz and 10% in tendon cells, and is high in fetal, injured, and early tendinopathic tendon but low in intact adult tendon.
• Single-cell RNA sequencing reveals at least 5 to 12 distinct cell populations in tendon, overturning the view of tendon as nearly acellular collagen.
• The blood-tendon barrier (ZO1, occludin, claudin 3/5) limits diffusion of molecules larger than 10 kDa, and healthy tendon vessels are non-fenestrated.
• Mechanical load triggers angiogenesis in a frequency-dependent way at 8 to 10% strain via HIF1 alpha.
• Hypoxia at 1 to 5% O2 enhances tendon cell proliferation and stemness, while severe hypoxia drives apoptosis.
• Across systematic reviews of RCTs, eccentric exercise was the most effective conservative treatment compared with shockwave, ultrasound, laser, and needling.

Limitations

• This is a narrative review, not a meta-analysis, so it pools no effect estimates and is subject to selective citation.
• The authors state the dominant human trigger of tendinopathy is unproven and that the disease likely has multiple trigger points.
• No single animal model reproduces pain, matrix remodeling, inflammation, and angiogenesis together, limiting how directly the mechanisms transfer to patients.
• Key mechanisms such as blood-vessel barrier disruption in vivo and a direct overuse effect on vessel tightness are hypothesized but not yet demonstrated in living tissue.

Why it matters

For patients

It explains why tendon pain is stubborn and why simple rest or anti-inflammatory pills alone often fall short, since several processes feed each other.

For clinicians

It supports a multifactorial view and combination strategies, with eccentric exercise as the best-supported conservative option and metabolic factors worth screening in predisposed patients.

For readers

It frames tendinopathy as an interacting loop of overload, inflammation, and vascular change rather than a single-cause disease, pointing toward future combination and metabolic therapies.