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Composition and Mechanical Properties of Connective Tissue

Hamill & Knutzen (Ch 2) Nordin & Frankel (Ch 1) or Hall (Ch. 4)

The Musculoskeletal System

Ø  bone Ø  tendons Ø  ligaments Ø  cartilage Ø muscle

Ø  Over the next few weeks we will look at the biomechanical properties of these tissues.

Ø  I will follow this list rather than the order in the text chapter

Connective Tissues Obviously muscle is not a connective tissue despite being discussed in this text chapter

Connective Tissue Functions

Ø  Storage of energy (both chemical food energy and mechanical energy)

Ø  Protection of organs Ø  Providing structural framework for the

body (e.g. bone) Ø  Connection of body tissues (e.g. tendons,

ligaments)

Connective Tissue Composition Ø  Cells

Ø  Fixed (fibroblasts, chondroblasts, osteoblasts) and migratory (e.g. macrophages, neutrophils, mast & plasma cells)

Ø  Extracellular Matrix 1.  Fibres {collagen (collagenous &

recticular), elastic} 2.  Ground Substance (calcium, lipids,

glycoproteins, proteoglycans) Ø  Tissue Fluid

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Blood

Ø  Not all types of connective tissues are fibrous.

Ø  Blood is one example where the matrix is plasma.

Ø  As blood does not have a movement related mechanical function, we do not study blood in Kin 201.

Skin Ø  Skin is not a connective tissue. Ø  Skin is not that important to our study in

Kin201. Ø  Skin is isotropic (the strength and

elasticity is the same in all directions). Ø  However, as discussed in the text

damage to the skin can affect joint mobility.

Adipose Tissue Ø  Adipose tissue gives "mechanical cushioning"

to our body (in addition to energy storage!). Ø  There is no dense collagen network in adipose

tissue compared to a tissue like tendon. Ø  However, groups of

adipose cells are kept together by collagen fibers and collagen sheets in order to keep fat tissue under compression in place.

Skeletal System Functions

Ø Movement Related Functions Ø  Levers Ø Support

Ø Non-Movement Related Functions Ø Protection Ø Storage of fat and minerals Ø Blood cell formation

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Composition of Human Bone WATER 25-30% MINERAL 60-70% (Resists compression)

Ø Calcium phosphate 85% Ø Calcium carbonate 10% Ø Calcium fluoride 2-3% Ø Magnesium fluoride 2-3%

PROTEIN (Collagen) 5-15% (Resists tension) Bone is termed a two-phase material

Bone Structure (density)

Ø  Compact (Cortical) Bone Ø porosity < 15%

Ø  Cancellous (Spongy)

Bone Ø porosity > 70%

Flexible, Strong

Flexible, Weak Stiff, Weak

Stiff, Strong

lead

oak BONE

spider web

silk

fiberglass steel

iron

gold

copper

glass

Anisotropic Characteristics

Stress to Fracture

Compression

Tension

Shear

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Force vs. Stress

=

=

Cross-Sectional Area

of Vertebrae

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Compression apparatus in which the specimens were subjected to pressure (maximum 300 kp) and displacement (compression) was recorded.

A = Specimen B = Mechanically Driven Screw C = Strain gauge D = Measuring Bridge

A

C

B

D

Axial compression of the spinal unit results in a loss of height measured between the vertebrae. As the disc material itself is essentially incompressible, height decrease must result in a radial bulge of the disc and a corresponding axial disc bulge (an inward deformation of the vertebral end plates).

A centrally situated, postmortem fracture of the end-plate

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Vertebrae

% of Body

Weight Carried

Mass kg Carried by

72.7 kg Man

Breaking Strength

(N)

Breaking Stress in

Compression+

% of L4 Breaking

Strength

T1 9 6.5 1,605 25.0 16.6 T2 12 8.7 2,140 25.0 22.1 T3 15 10.9 2,675 25.0 27.7 T4 18 13.1 3,211 25.0 33.2 T5 21* 15.2 3,746 25.0 38.7 T6 25* 18.1 4,459 25.0 46.1 T7 29* 21.0 5,173 25.0 53.5 T8 33* 23.9 5,864* 24.9 60.7

Calculation of Vertebral Strengths T9 37* 26.9 6,657* 25.2 68.9 T10 40* 29.1 7,277* 25.5 75.3 T11 44* 32.0 7,580* 24.2 78.4 T12 47* 34.2 7,835* 23.4 81.0 L1 50* 36.4 7,982* 22.4 82.6 L2 53* 38.5 8,584* 22.7 88.8 L3 56* 40.7 9,636* 24.1 99.6 L4 58* 42.2 9,667* 23.4 100.0 L5 60* 43.6 10,550* 24.6 109.1

*Single asterisk represents data collected experimentally by Ruff (1950). Unmarked values are calculated or assumed.

<40 40-50 50-60 >60

Mean and Range of Disc Compression Failures by Age (Adapted from Evans, 1959, and Sonoda, 1962)

10000

8000

6000

4000

2000

0

AGE

Compressive Forces Resulting in Disc-Vertebrae Failures at L5/S1 Level (Newtons)

Should job design factor in age?

Compressive Strength (N) Estimated for L4/L5 Spinal Unit from Mechanical Testing of Lumbar

Spinal Units (males 20-40 years, n = 17). Porter, Hutton and Adams, 1989: Hutton and Adams, 1982

Age Compressive Strength (N)

Mean 28 10,093

Std. Dev. 9 1,924

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Bone Injury and Low Back Pain Ø  Bone injury (e.g. endplate fracture) is far from the

most common cause of back pain. Ø  However, extensive research has been conducted

into disc compression as it is thought to be largely responsible for vertebral end-plate fracture, disc herniation, and resulting nerve root irritation.

Ø  Back compression has been argued to be a good predictor of low-back and other overexertion injuries [Herrin+, 1986]

Ø  Due to the clinical interest in this area data exists on the compressive strength of the lumbar vertebral bodies and intervertebral disks

Like most things in the human body – it is a little more complex that simply looking at the compressive stress on the disks.

Tibial Boot-Top Fracture Model opposite shows the lever arms (A-D) from L3-L4 for the head, trunk, arms and lifted weight.

Data in table overleaf was from calculated for world championship level power lifters.

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Fatigue Failure Ø  Compression fracture is the common failure

mode of the vertebra-disc complex in severe axial loading.

Ø  This mechanism does not apply to repetitive loading within the linear portion of the stress-strain curve.

Ø  Low back pain and back disorders associated with frequent lifting, whole-body vibration and repeated shocks point to a chronic degeneration of tissues, rather than acute failure.

Repetition

Load

Tolerance

Injury Threshold

Acute trauma

Chronic

Repetitive

Tissue Tolerance Stress analysis of the proximal end of the femur (Koch)

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Avoiding Tension and Shear Brachioradialis Origin: Humerous -

Lateral Condyle Insertion: Radius (Lateral

Distal) - Styloid Process

Balanced Loads

There are many examples where carrying is designed to carry two balanced loads in each hand rather than one heavier load in one hand.

Stress in the Human Heel. The model (left) with forces applied indicated by arrows. Stress pattern indicated by polarized light (right).

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Continuous lines = compressive stress.

Dotted lines = tensile stress.

Red line shows epiphysial plate

Resolution of Vectors

Compression across an epipheseal plate is less damaging than tension.

Where there is tensile stress across an epiphysial plate (such as the proximal end of the tibia) a lot of collagen fibres are present to protect the plate from excess tension.

Quadriceps muscle force pulls on insertion point (via patella tendon)

Stress Fractures

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Repetition

Load

Tolerance

Injury Threshold

Acute trauma

Chronic

Repetitive

Tissue Tolerance

Paper Example

Bone Remodeling

Load

Deformation

Normal

Immobilized

(Wolff’s Law)

Issues of degeneration and regeneration

Loading, Muscular Activity, and Injury

Ø  Injury vs. Loading Ø complex problem depending on loading

level, direction, speed, skeletal maturity and conditioning.

Ø  Muscular Activity vs. Loading Ø muscular activity influences loading (often

reducing tensile loading). If muscles fatigue their ability to do this is compromised.

Sample Problem

Ø  What is the compressive force on the L5/S1 vertebral disk of the 50% male?

Ø  What is the compressive stress on this disk if it is aligned horizontally and its cross-sectional surface area is 24 cm2?

Ø  What is the compressive force on one tibia if the 50% male stands in the anatomical position (symmetrical weight bearing between both feet)?

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Biomechanics of Joints, Ligaments and Tendons.

Nordin & Frankel (Ch 2 & 3) or Hall (Ch. 5) Hamill & Knutzen (some in chapter 2 and 3, but ligament

and tendon mechanics is not well covered in this text)

Hippocrates (460-377 B.C.) “All parts of the body which have a function, if used in moderation and exercises in labours to which each are accustomed, thereby become healthy and well-developed: but if unused and left idle, they become liable to disease, defective in growth, and age quickly. This is especially the case with joints and ligaments, if one doe not use them.”

LeVay 1990. p30.

Joints Ø  Review architecture of cartilaginous joints

(specifically the vertebrae). We will look at these again.

Ø  Review architecture of synovial joints. Ø  This will help with understanding the

structures we are discussing. The anatomy of the synovial joint will not be specifically examined.

Articular Cartilage"

Ø  The joints of a mechanical device must be properly lubricated. Articluar cartilage, a dense white connective tissue coats (1-7 mm thick) the ends of bones articulating at synovial joints. It serves two purposes:

1: Spreads the load. Cartilage can reduce the maximum contact stress by 50% or more.

2: Reduces friction during movement. "

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Athletic Warm-Up

Ø  Warming up increases the production of synovial fluid.

Ø  In addition increasing the temperature of the synovial fluid will reduce its viscosity.

Ø  This ultimately reduces the friction within the joint allowing for more efficient movement.

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Articular cartilage has a combination of elastic and viscoeleastic properties. As load is applied, deformation increase with time, first in an elastic fashion, then with a slow creep. With the removal of the load there is an elastic recoil and the a slow recovery to the base line.

Articular Fibrocartilage Ø  Found in pubic symphysis, annulus fibrosus of

intervertebral discs, meniscus and the TMJ. Ø  The function of fibrocartilage includes: Ø  Absorption and distribution of loads Ø  Improvement of the fit of articulating surfaces. Ø  Increase in joint stability. Ø  Protection of the periphery of the articulation. Ø  Lubrication.

Knee Menisci Stress distribution in a normal knee and in a knee with the menisci removed. With the menisci removed the contact area is limited to the centre of the tibial plateau hence increasing the stress.

In the average male the knees support 88% of the body weight.

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Articular Connective Tissue"Ø  Tendons and ligaments are much less extensible

than muscle and do not have the ability to contract. Ø  Made primarily of collagen fibres (with some elastin

fibres) they will return to their normal lengths when unloaded.

Ø  However, there is an elastic limit (bone lecture) after which the tendon or ligament will not return to resting length (region of plasticity - 2nd degree strain). This will take time for the body to repair.

Ø  If the ligament completely fails (3rd degree strain) this can often only be restored by surgery.

Ligament Composition

Fibre Arrangement

Tendon

Ligament

Skin

Ligament Crimp

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Unloaded Ligament Loaded Ligament

Collagen Fibres

Stress

Strain (percent) 5 0 10

Collagen Fibres

Ø Deformation Range Ø  small 6 - 8%

Ø Strength

Ø  50% of that of cortical bone tested in tension

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Young’s Modulus Young’s Modulus is the ratio of:

tensile stress / tensile strain

Young’s Tensile Modulus Strength

Tendon 2 x 109 1 x 108 Bone 1.7 x 1010 1.8 x 108 Carbon Steel 2 x 1011 3 x 109

Soft rubber c.106

Fibre Arrangement

Tendon

Ligament

Skin

Elastic Fibres (elastin and microfibrils)

Stress

Strain (percent) 100 0 200

Elastin Fibres Ø Deformation Range

Ø  large >100% (150% Fawcett 1986)

Ø Strength Ø  weak

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Joint Stability"Ø  Stability to resist dislocation and damage to

the ligaments, tendons and muscles surrounding a joint.

Ø  The shape of the articulating surfaces is important.

Ø  Some joints are obviously not designed to be as stable as others as range of motion can be compromised in a very stable joint.

Ø  Hip versus shoulder? "

The knee is a good example of a joint where the articulating surfaces are not shaped like each other (although the menisci increases the contact area)."

Tendons & Ligaments

Ø  Tendons and ligaments are predominantly made up of collagen.

Ø  Hence their stress / strain relationship will mirror that of collagen.

Ø  The less-structured orientation of the collagen in the ligaments will provide additional elastic properties (directional) compared to tendons.

Load-elongation curve for rabbit tendon tested to failure in tension

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Load-Elongation Graph for Primate Ligament (Noyes 1977)

Load

Progressive failure of the anterior cruciate ligaments (cadaver knee tested in tension to failure at a physiologic strain rate, Noyes 1977)

Curve from previous slide divided into three regions correlating with clinical findings

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Tendonitis Ø  Tendons have a very similar structure to

ligaments. Ø  Ligaments work in the toe region of their stress

strain curves. Ø  Tendons work at 30-40% of ultimate strength

(up to 80%) Ø  You do not get liagament-itis!! Repetitive stress causes failure at lower load than

that required to cause failure in a single application. As a ligament undergoes cyclic loading it relaxation behaviour results in continuously decreasing stress (protecting ligament from fatigue failure)……..

Hysteresis during cyclic loading of a knee ligament.

……therefore, there is a time dependant increase in elongation when a viscoelastic material is subjected to a repetitive constant stress (cyclic creep).

Schematic representation of cyclic creep in the MCL of the knee Max. load to

failure for primate anterior cruciate ligaments Noyes 1977 Maximum load to failure

30 years on and ligament is still not 100%

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Ligament Scar

0 weeks post injury

Ligament Scar

6 weeks post injury

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Ligament Scar

14 weeks post injury

Quantity not Quality? Ø  Eighteen weeks of remobilization were

necessary to reverse the detrimental effects of a six-week immobilization on the structural properties of ligaments (Laros et al., 1971).

Ø  Structural properties nearly normal but mechanical properties of healed ligaments almost always remain inferior when compared to normal tissue.

Ø  This is possible as the tissue accumulates mass to compensate for inferior tissue quality.

Ø  Some areas of healed MCL were up to 2.5 times larger than controls (Ohland et al., 1991)

Healed MCL exhibits inferior mechanical properties after injury (intact state). Note the graph opposite is a load elongation curve for a MCL specimen

This graph is a stress-strain curve for the femur-MCL-tibia complex. Therefore this graph shows fundamental tissue properties compared to the previous specimen load-deformation curve.

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Joint Flexibility - Range of Motion"Ø  Properties of soft connective tissues (collagen

and elastin) are crucial. Ø  Extensibility of muscles Ø  Elasticity of the articular capsules & fluidity of

discs Ø  Extensibility of the longitudinal ligaments Ø  Anatomical architecture of the articulations Ø  Resistance of the surrounding tissues. Ø  Collagen shortens in the absence of tension

but shows plasticity. Everyday movement keeps ROM acceptable, but specialized stretching routines also help. "

Temperature Ø  Cold connective tissue and muscles are stiffer –

due to increased viscosity of the fluids in them. Ø  When connective tissue is warmed up its

viscosity is reduced. Ø  Kerr calls this property thixotropy, but that is

related to agitating a substance. Ø  However repetitive loading does heat a tissue up

so they are similar processes. Ø  So an increase in temperature via repetitive

movement or a direct heat source (hot tub?) will make stretching easier.

Deform (strain) tissue by fixed amount

Load tissue with constant stress (F/A).

Stress relaxation

Creep relaxation

As with bone (mentioned in Chapter 6) all connective tissues have viscoelastic response to loads applied at different speeds.

Stre

ss

Strain

Rapidly applied stress

Slowly applied stress

Viscoelastic Characteristics