Soft Tissue

Tendon and ligament biomechanics

Tendon and ligament composition, toe crimp mechanism and non-linear stiffness

19 questions 3 source pages

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Q1Outline the hierarchical organisation of tendon, from procollagen to whole tendon.▸

Procollagen from cell (1.5nm) -> tropocollagen (280nm) processed by EC protein
Microfibril: many tropocollagen arranged in a quarter-staggered array, held by cross-links (collagen 11)
Fibril: multiple microfibrils
Fibre: multiple fibrils
5. fascicle(endotenon)=multiple fiber
Tendon (epitenon)

Q2What is the collagen building unit within a tendon?▸

Q3What connective tissue sheaths surround the subunits of tendon and muscle?▸

Q4Outline the hierarchical structure of muscle from sarcomere to whole muscle.▸

Sarcomere -> myofibril -> fibre (endomysium) -> fascicle (perimysium) -> bundles of fascicles -> muscle

Q5Compare the functions, microscopic make-up, macroscopic structure, strength and blood supply of tendon and ligament.▸

1. Function - ligament: bone to bone (provide stability to joint, proprioception, passive, designed to be stretched); tendon: muscle to bone (transmits tensile force, active, transmit contraction)
2. Microscopic - collagen proportion: ligament lower (type III); tendon higher (type I)
Ground substance (PG) proportion: ligament higher, therefore high H2O; tendon lower
Fibroblasts: ligament fewer (round); tendon more (spindle)
Elastin: ligament more elastic; tendon less elastin
3. Macroscopic - organisation: ligament more random; tendon organised
Orientation: ligament - weaving layered pattern with parallel fibres in each layer -> multidirectional stress, wavy fibres increase tension absorption capacity; tendon - long axis direction -> resists unidirectional tensile load
4. Strength: ligament weaker, more viscoelastic; tendon stronger, less viscoelastic
5. Blood supply: ligament via insertion site; tendon - (vascular, non-sheathed) surrounding superficial vessel via paratenon, (avascular, sheathed) via vincula, and via insertion site

Q6What is the general composition of tendon and ligament?▸

Q7What is the tendon hierarchy?▸

Tropocollagen > microfibril > fibril > fibre > fascicle (endotenon) > tendon (epitenon)
Described in the classic Kastelic et al paper

Q8How is the blood supply of tendon and ligament provided?▸

Ligament: via the insertion site
Tendon (non-sheathed, vascular): surrounding superficial vessel via the paratenon, travelling longitudinally in the endotenon
Tendon (sheathed, avascular): via the vincula (mesotenon)
Sources at the MTJ, OTJ and surrounding connective tissue (paratenon, vincula, mesotenon)

Q9Describe direct and indirect tendon/ligament insertion into bone.▸

Indirect: superficial layer continuous with periosteum; deep via Sharpey fibres
Direct: 4 transition zones - tendon (tenocytes), uncalcified fibrocartilage, calcified fibrocartilage, bone (osteocytes)
Gradual change in structure and mechanics avoids a stress riser

Q10Describe tendon and ligament healing.▸

Inflammatory (immediate): fibrin clot, macrophages remove necrotic tissue; weakest at D7-10
Proliferative (few days): fibroblasts lay down collagen III in random pattern; most strength by 1 month
Remodelling (6 weeks): collagen III to collagen I, fibres realign; max strength 6 months, ligament up to 18 months
May be enhanced by growth factors or mesenchymal cells

Q11Compare extrinsic and intrinsic tendon healing.▸

Extrinsic: ingrowth of cells from the paratenon into the gap -> more adhesions; occurs with immobilisation
Intrinsic: cellular growth directly from the epitenon across the tendon ends -> less adhesion; allows early mobilisation

Q12Why do midsubstance ruptures occur rather than bony avulsions at high strain rate?▸

Explained by strain rate sensitivity - a viscoelastic property of tendon and ligament
Bone is more sensitive to strain rate than tendon
At higher strain rate the bone becomes stiffer than tendon/ligament, so ligamentous/tendon injury occurs

Q13What are the histological changes in tendinopathy?▸

Collagen fibril disorganization
Increased proteoglycan and glycosaminoglycan content
Increased non-collagenous ECM, hypercellularity and neovascularisation
Described by the Cook and Purdam 3-phase pathology model

Q14What is the evidence for PRP in tendinopathy?▸

Lateral epicondylitis: Cochrane review 2021 - no benefit
Midsubstance Achilles tendinopathy: evidence does not support PRP
Patellar tendinopathy: AJSM 2016 (Laprade group) PRP vs saline - no difference
Not first-line; reserve PRP for cases that failed physiotherapy and NSAIDs

Q15Describe the stress-strain curve of tendon and ligament.▸

Toe region - crimp; Modulas of elasticity is not constant, ligament becomes stiffer as more collagen fibrils are recruited (ends about 2% strain)
Linear region - all fibres straightened; intermolecular sliding of collagen triple helices (<4% strain)
Yield point and ultimate tensile strength
Progressive failure (4-8% strain): sequential fibril failure, irreversible plastic deformation
Complete rupture (>8% strain)

Q16How do tendon and ligament stress-strain curves differ?▸

Q17Describe the stress-strain curve of ligamentum flavum.▸

High elastin content
Tolerates high strain up to 50% before stiffness increases
Stiffness then rises significantly with further loads before it fails abruptly

Q18What is the difference between load-elongation and stress-strain curves?▸

Q19What determines the overall behaviour of ligaments and tendons on the stress-strain curve?▸

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