Physiology of Tendons

 

Introduction

Tendons are dense connective tissues that connect muscles to bones. They transmit forces generated by muscle contraction to produce movement. Tendons are composed of specialised cells called tenocytes and tenoblasts, surrounded by a complex extracellular matrix (ECM) rich in collagen fibres, proteoglycans, and glycoproteins.

There are 2 tendon types:

  • Paratenon-covered tendons:
    • covered in a membrane-like areolar structure
    • well vascularised
    • better healing
    • examples: Achilles, patellar tendon
  • Sheathed tendons:
    • covered by a thin layer of connective tissue
    • less vascularised (poorer healing)
    • receive nutrition from synovial fluid by diffusion
    • examples: flexor tendons

 

Structure

  • Tendons are composed of hierarchical structures of collagen fibres organised in fascicles, surrounded by endotenon, epitenon, and paratenon layers

*awaiting image upload*

  • The collagen fibres are highly aligned, with a characteristic crimp pattern; this allows the tendon to store elastic energy and resist tensile forces
  • The proteoglycans and glycoproteins in the ECM provide lubrication, hydration, and regulate collagen fibrillogenesis

 

Composition

Cells

Tendons consist of two types of specialised fibroblasts:

  • Tenocytes:
    • elongated, spindle-shaped cells
    • arranged in parallel rows in line of muscle pull
  • Tenoblasts
    • oval-shaped cells
    • involved in healing process of tendon injury, producing collagen and transform into tenocytes in final repair phase

Collagen and Extra Cellular Matrix

Collagen comprises the largest proportion (65-80%) of the extracellular matrix:

  • Type I collagen is the main constituent
  • Type III collagen is found in the epitenonium and endotenonium
  • Type II collagen is present in the fibrocartilaginous areas of the osteo-tendinous junction

The remainder of the ECM consists of elastin (4%), proteoglycans, (4%) and glycoproteins (2%)

 

Mechanical Properties

Shape and length

  • Tendons for delicate movements are long and thin
  • Tendons for power and endurance are shorter and more robust
  • Short tendons have greater tensile strength than long tendons of the same diameter

Deformation and rupture

  • Long tendons can undergo greater deformation before rupturing compared to short tendons
  • Tendon strength and resistance depend on the diameter and length of the tendon
  • Tendons subjected to high stress have larger diameter collagen fibrils that are less flexible

                                  

Stress-strain properties

This graph demonstrates the stress-strain curve of tendons (and ligaments). Note the toe region, in which the crimping of collagen fibres straighten during the early stages of loading.

Mechanical adaptation

Tendons adapt to the mechanical environment they experience. Mechanical tension leads to an increase in tendon diameter through collagen synthesis. Adaptation increases stiffness and Young's modulus, which relates mechanical stress to strain.

 

Tendon healing

Injuries can lead to a prolonged healing process due to the low metabolic activity and vascularisation of tendons. The healing process can be disrupted by several factors, including mechanical overload, infection, or systemic diseases.

Stage

Description

Time frame

Inflammatory Stage

Begins immediately after injury. Involves release of inflammatory mediators, such as cytokines and chemokines, which attract immune cells.

1 to 7 days

Repair Stage

Fibroblasts migrate to the site and begin to produce collagen fibers. These help bridge the gap between the torn tendon ends. New tissue is initially disorganised and weak.

7 days to 6 weeks

Remodelling Stage

Collagen fibers gradually realign along lines of stress to enhance the strength and functionality of the healing tendon. Involves ongoing collagen synthesis and degradation, leading to gradual maturation of the tissue.

Week 6 to several months

Maturation Stage

Tendon tissue further matures, and collagen fibers become more organised and stronger. The aim is to restore the normal structure and function to its pre-injury state. Complete maturation and remodeling of the tendon may take a considerable amount of time.

Several months to a year

 

 

References

  • Bordoni B, Varacallo M. Anatomy, Tendons. [Updated 2022 Jul 18]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK513237/
  • Lavagnino M, Wall ME, Little D, Banes AJ, Guilak F, Arnoczky SP. Tendon mechanobiology: Current knowledge and future research opportunities. J Orthop Res. 2015;33(6):813-822. doi:10.1002/jor.22871
  • Screen HR, Berk DE. Mechanobiology of tendon. Journal of biomechanics. 2015;48(7):1469-1480.
  • Sharma P, Maffulli N. Tendon injury and tendinopathy: healing and repair. The Journal of bone and joint surgery. American volume. 2005;87(1):187-202
  • Latash ML, Zatsiorsky VM. Stiffness and Stiffness-like Measures. In: Latash ML, Zatsiorsky VM, eds. Biomechanics and Motor Control. Academic Press; 2016. p. 25-47. ISBN 9780128003848. https://doi.org/10.1016/B978-0-12-800384-8.00002-8. Available from: https://www.sciencedirect.com/science/article/pii/B9780128003848000028