Week 4 - Tendons and Ligaments (Biomechanics)

Ligament function (3)

Transmit forces between bones - strength provides stability and flexibility permits joint motion

ALL/PLL anatomy

Connect vertebral levels to limit F/E. Superficial fibres span several levels, deep layer crosses only adjacent vertebrae and attaches to AF. Thickest at thoracic spine

Ligamentum Flavum anatomy

Thick+elastic ligament connects laminae - under constant tension, providing constant compression force to spine + preventing buckling (spring back up!)

Plantar ligaments anatomy

Dampen and reduce impact forces

3 integrated arches (medial/lateral longitudinal + transverse) - shape determined by bony arrangement, plantar ligaments and plantar aponeurosis

MTP hyperextension and pulling of calcaneal tuberosity posteriorly by superficial posterior calf muscles in plantarflexion (triceps surae) tensions the PF and raises arches, holding the foot rigid for push-off. Known as windlass effect

ACL anatomy

resists anterior tibial translation and rotational loads

Stabilises knee in various positions and loading conditions - complex anatomy (2 bundles)

Tendon function (4)

Transmit force from muscle to bone (joint motion).

Absorb, store and release energy - conserves energy

Amplifies power - store slow and release fast

Protects muscle from damage - power attenuation (shock absorber) by lengthening before the muscle does to prevent eccentric damage

Predominant energy storage tendons

Achilles and patellar

note ITB has small role

Achilles tendon anatomy

Spiral of achilles tendon fascicles during descent - allows for elongation and elastic recoil. MG fascicles are parallel, LG and soleus insert with torsion.

Important for intratendinous strain distribution

How to calculate tendon stiffness

Slope (k) of tendon length (ultrasound) and ankle joint torque (strain gauge)

Electro-mechanical delay and tendon compliance?

delay between the activation of a muscle and its production of force. More compliant tendons have a greater electro-mechanical delay as more time taken to take up slack, and require more contraction before force is generated.

Aponeuroses

Fibrous or membranous sheet connecting a muscle and the part it moves - can be muscle to muscle, muscle to bone or muscle to fascia. BROAD SHEET of dense connective tissue. Distribute force and provide attachment

Enthesis - 3 types

Tendon/ligament insertion at bone - 100x stiffer than tendon/ligament. Can be fibrous (inserting directly to long bone) or fibrocartilagenous (4 zones, gradual transition) or muscular (No tendon)

Muscle-tendon junction

Abrupt - collagen fibres and muscle cell membrane interdigitate to increase surface area and reduce stress between tendon and muscle

PROTECTS AGAINST INJURY

Where are bursae located (general)

Sites of compression

Where are retinacula located (general)

Sites of joints - provides stability

Synovial sheaths function

Reduce friction

Collagen anatomy

Most abundant protein in body - Type 1 90% and Type 3 10% (IN TENDONS)

Elastin anatomy/function and what structure is it present more in?

3 dimensional branching protein

Highly elastic, resistant to fatigue

Stores and returns energy

Resists transverse and shear deformation in ligaments (little in tendons)

Yellowish

Proteoglycans anatomy - 2 types for tendons, distribution?

Most abundant non-fibrous protein - core protein with glycosaminoglycan (GAG) side chains

Attracts water with negative charge

Distribution varies across tendon length - mostly found in insertional region of tendons to resist compression at bone by attracting water

Lubricin provides lubrication for gliding at tendon surface

Decorin is most abundant, helps transfer load between collagen fibrils and regulate collagen fibrillogenisis

Ligament composition and structure

10-20% fibroblasts, 80-90% ECM

Mainly collagen I, some III-V. Also proteoglycans (decorin) and water

Varying elastin amounts - ratio of collagen and elastin determines mobility

Collagen arrangement varied for multidirectional force resistance

Tendon composition and structure

Same fibroblast/ECM ratio as ligament

More type I (85-95%) less type II/III collagen for strength

Small elastin amounts, proteoglycans,

Collagen is aligned long axis ways

More anisotropic

Tendon structures beyond fibrils/fascicles (2)

Paratenon (additional loose connective tissue layer) surrounds tendons in regions away from joints, to facilitate movement of tendons below the skin.

Synovial sheath (additional covering where a tendon passes around a joint) to ensure smooth gliding past surrounding structures

Mechanical behaviour of tendon/ligaments - structure?

DEPENDS on nature and direction of applied forces (ANISOTROPIC)

Behaviour is heterogenous - varies along the tendon (also recall zones of fibrocartilagenous enthesis)

Tendons and ligaments are viscoelastic

Note similar structures, except tendon collagen fibres are parallel (stiffer) whilst ligaments are almost - think direction of forces they experience. ALSO tendon collagen ruptures all at once, ligament ruptures at separate times - progressive failure

Also thicker CSA means stiffer

Maturation vs aging - examples

Maturation is dramatic increase in mechanical properties of tendon, whilst aging is gradual decrease.

Patellar tendon example - increased CSA + collagen fibril diameter (size and stiffness) in adults vs children

What group is more susceptible to avulsions

Children - less mature?

Synchronicity in maturation of bones, tendons and ligaments - implications?

Ligaments mature faster than bones - avulsion injuries more common when bones less mature than tendons/ligaments. LIgament failures more common after skeletal maturity

Severs disease

apophysitis at the attachment of the Achilles tendon on the calcaneus - inflammation of the growth plate. Overuse injury affecting children 8-11yrs

Osgood schlatters disease

An irritation of the patellar ligament at the tibial tuberosity - tibial tuberosity apophysitis. Boys 13-14 and girls 11-12

Tendon stress in late adolescents - adaptation?

Muscles and tendons experience non-uniform adaptation - muscles get stronger, but tendon doesnt get thicker.. Results in higher stress at max force

Can be resolved by strength training - tendon stiffness, youngs modulus and electromechanical delay can all be improved.

Effects of age on muscles - what if training involved? Rewatch lecture on MU remodellling

Sarcopenia, atrophy, decreased capacity to detect info and activate muscles (motor units remodel). Proportional changes with type I/II fibres is not clear

Resistance training can offset strength decreases and better maintain muscle mass.

Tissue turnover - tendons vs muscles - implications in aging?

Muscles have higher self-renewal potential compared to tendons

With age, therefore tendons will lose stiffness e.g. Achilles tendon - older tendons 15% more compliant, with decreased contractile force and decreased rate of force development. implications for falls

Hence why its said that muscles get stiffer when older - counteracts tendon compliance

Gender difference in tendon properties?

Achilles tendon stiffness linked to strength, not gender

Gender difference in ligament properties

Female athletes 4x more likely to sustain non-contact ACL injury - likely due to strength imbalance betweeen hammies and quads (hammies less likely to activate). NOTE increase of female participation in sports at the time of study - may be reason

Exercise effect on tendons

Exercise can induce tendon hypertrophy and increase tendon stiffness - variable results depending on types and intensities of exercise, and specific tendon

low strain exercise insufficient trigger for tendon adaptation (size, youngs modulus and stiffness in ACL experiment)

Training interventions effective, but diminishing returns long-term - 14weeks same results as 1.5yrs

Jump training protocol - Achilles tendon increased strain with added mass and patellar tendon decreased strain with added mass

Muscles vs tendons - synchronicity in adaptations to training and implications?

Tendon stiffness adapts slower than muscle strength when trained - also detrains quicker. Imbalanced period of adaptation could cause tendon injury

For adaptation of tendon mechanical properties, we need: (4)

High loads

High strain

Performed at long lengths (IF ISOMETRIC)

Performed consistently for 8-14weeks

Mechanics of ligament injury

Ligaments fail when tensile load exceeds capacity

Often awkward landing position, associated with joint dislocation and articular damage

3 overlapping phases of tissue/ligament healing

Inflammation, proliferation and remodelling

MCL (and ligaments in general?) rupture process (in 5 parts)

10 days - defect filled with vascular inflammatory tissue

3 wks - inflammatory cells subsided, active fibroblasts dominated

6 wks - decrease size/number of fibroblasts, some longitudinal alignment of fibroblast nuclei

14 wks - increased re-alignment and decreased cell numbers (remodelling).

>14 wks - few changes, cells remained larger and more numerous

CSA is still larger than uninjured - but from 6 weeks onward decreases from largest size.

Mechanical changes of a ligament after healing

Decreased stiffness, decreased load at rupture, changed failure site (more at midsubstance)

Increased CSA despite increased laxity

Healing capacity for extra- vs intra-articular ligaments - implication

Extra-articular ligaments heal faster than intra-articular - intra-articular often requires surgery

Implications of ligament injury on joint function

Can be associated with instability, bony bruising and OA

3 things to consider for ligament rehab

What structure is damaged and to what extent?

What is the timeframe of the healing response?

What are the patient priorities?

Mechanics for tendon injury (5 main conditions)

Stress shielding - underloading of some fibres (Stress-shielded) and overloading of others in normal overall loading conditions - e.g. walking in excess supination leads to overloading of lateral region and underloading of medial

Excessive force

Repeated overload

Normal forces applied to a weakened tendon - degenerative changes weaken mechanical properties, predisposing to injury (97% ACL ruptures had preexisting asymptomatic degenerative changes)

Forces applied in alternative direction - e.g. tendon compression contributing to insertional tendinopathy at achilles and glut med tendon especially.

Non-mechanical factors that can affect tendon injury?

Ageing

Changes in physical activity levels

Diseases e.g. diabetes, RA

Medications

Alcohol (inhibits fibroblast proliferation)

Where does overuse tendinopathy occur - examples?

Mid-substance (achilles)

insertional (Achilles, lateral elbow (wrist extensor tendons), patellar tendon)

Musculotendinous junction e.g. hamstrings, quadriceps tendon

paratendinitis

When a tendon rubs over a bony protuberance, causing inflammation and acute swelling e.g. FHL, De Quervains (APL. EPB)

Changes in tendon appearance in overuse tendinopathy vs normal tendon

grey and amorphous compared to glistening white

Collagen disorganised compared to highly organised

Lack of immune cells and vascularity and increase of cellularity (??) and PG+water, compared to relatively few cells

Changes in tendon mechanical properties in tendinopathy

Reduced stiffness

Ultrasounds for tendinopathy - good and bad?

Good for finding shape changes e.g. thickening and tears, but more useful for ruling out than in (low specificity, high sensitivity). Also limited correlation with pain severity

Achilles vs Patellar tendon elasticity as a result of tendinopathy?

Lower achilles tendon youngs modulus, higher patellar tendon youngs modulus (compared to normal)

Considerations for rehab of tendinopathy

Avoid compressive loading, especially if insertional

Exercise - reduces pain, improves function - progressive loading to remodel tendon

What is loading history?

It will be slow - slow turnover - insertion sites even slower

Effects of immobilisation on tendons/ligaments

few weeks can reduce structural properties of tissue:

Immature/weaker/disorganised collagen

50% tissue stiffness decrease after 8 weeks

LEss deteria

Function of fibrocartilagenous enthesis in injury prevention, and enthesis organs?

4 zones from collagen to unmineralized to mineralized fibrocartilage to bone - increasing stiffness progressively plays role in force dissipation and shock absorption

Graduated transition zone for stiffness levels

ENTHESIS ORGANS - BURSA fluid filled sacs that promote free movement of tendons, and FAT PADS help absorb shock and prevent friction

Tension vs compression of tendons - which injury site on tendon?

Mid-substance injury in tension, compression causes insertional site injury (e.g. calcaneus pressing into tendon in dorsiflexion).

How to measure tendon stiffness

Force-deformation curve - gradient of elastic region. Isnt this the same as stress-strain bruh (stiffness comparison between individual tendons - youngs modulus is material bassed)

YOUNGS MODULUS VALUE TELLS STIFFNESS OF MATERIAL (NORMALISED) - stiff is quality between individual tendons

Fasicle/tendon length ratio implication

High tendon/muscle length ratio - greater elastic storage for improved efficiency in cyclic movements, and lower energy cost for movement

Low tendon/muscle ratio - Faster contractions for more explosive and fine motor control, higher energy cost, less elastic storage potential

a.       Where will most injuries occur within the musculotendinous unit?

b.       Using your knowledge on the mechanical properties of the structures as discussed in previous lectures, describe why this location is most prone to injury. 

Is this because the load is not absorbed and dissipated like they are in entheses?