1

CHAIRPERSON: DR.RUPA KUMAR CS MODERATOR: DR. SHESHAGIRI V

BIOMATERIALS AND APPLIED BIOMECHANICS

2

CONTENTS

BASIC CONCEPTS

BIOMATERIALS

BIOMECHANICS OF FRACTURES

BIOMECHANICS OF FRACTURE HEALING

3

BIOMECHANICS OF INTRAMEDULLARY NAILS

BIOMECHANICS OF BONE SCREWS

BIOMECHANICS OF BONE PLATES

4

BASIC CONCEPTS

STRESS Intensity of internal force Stress = Force/Area (pascals)

Depends on mode of application of force

TWO TYPES: Normal (compressive and tensile) and Shearing(bending and torsional)

5

STRAIN

Relative measure of deformation resulting from loading

Strain = Change in length/original length

Can also be normal or shearing

6

CONCEPT OF COLUMN BENDING

Occurs only when a beam is loaded eccentrically

Generates compressive forces on the concave side and tensile forces or the convex side

Analogous to most weight bearing bones

7

CANTILEVER BENDING

Horizontally disposed beam with one end fixed to a wall when loaded at its free end leads to bending of the beam

8

ELASTICITY Behavior of elongation when loaded and

recovery to its original state when unloaded

PLASTICITY Permanent deformation of material under load

Hooke’ s law Deformation is proportional to the applied

load upto a limiting value

9

Young’s modulus of elasticity (E)

a measure of the material’s ability to resist deformation in tension

E = stress/strain

E is the slope in the elastic range of the stress-strain

curve

10

11

BIOMATERIALS

Brittle materials (e.g., PMMA)

Stress-strain curve is linear up to failure.

These materials undergo only recoverable (elastic)

deformation before failure.

They have little or no capacity for plastic deformation

12

Ductile materials (e.g., metal) These materials undergo large plastic deformation before failure. Ductility is a measure of post yield deformation.

Viscoelastic materials (e.g., bone and ligaments) Stress-strain behavior is time-rate dependent.Properties depend on load magnitude and rate at which the load is applied. A function of internal frictionThese materials exhibit both fluid (viscosity) and solid (elasticity) properties.

13

These materials exhibit both fluid (viscosity) and solid (elasticity) properties.

Modulus increases as strain rate increases.

These materials exhibit hysteresis.

Loading and unloading curves differ. Energy is dissipated during loading.

14

INERTIA

Force of resistance tending to prevent any change in the existing state of its motion.

Proportional to the mass of the body

MOMENT OF INERTIA : resistance of a body at rest capable of rotatory motion

3 types MASS MOMENT OF INERTIA AREA MOMENT OF INERTIA POLAR MOMENT OF INERTIA

15

MASS MOMENT OF INERTIA

Depends on the distribution of material around the axis of rotation rather than the total mass of the body

I=mr2

m = mass of the body, r = radius of gyration (perpendicular distance to center of the mass)

16

AREA AND POLAR MOMENT OF INERTIA

AREA MOMENT OF INERTIA:Resistance offered by a structure when placed under a bending load

Depends on the shape of its cross section

Formulae differ depending on the different geometric cross sections used

POLAR MOMENT OF INERTIA: rigidity or strength of a rod or tube against torsional stress.

17

ORTHOPEDIC IMPLANTS

METALS Steel based, cobalt based, titanium based

NON METALS Polyethylene, PMMA, Silicones, Ceramics

18

STAINLESS STEEL (316 L)

Iron-carbon, chromium, nickel, molybdenum,manganese Nickel: increases corrosion resistance and

stabilizesmolecular structureChromium: forms a passive surface oxide,improving corrosion resistanceMolybdenum: prevents pitting and crevicecorrosion Manganese: improves crystalline stability “L” = low carbon: greater corrosion resistance

19

COBALT BASED

Cobalt-chromium-molybdenum (Co-Cr-Mo)65% cobalt, 35% chromium, 5% molybdenum Special forging processNickel may be added to improve ease of

forgingCo-Cr: macrophage proliferation and synovial degenerationIons excreted through the kidneys

20

TITANIUM BASED

Titanium is extremely biocompatible:Rapidly forms an adherent oxide coating

(selfpassivation); decreases corrosion A nonreactive ceramic coatingRelatively low EMost closely emulates axial and torsional

stiffness of boneHigh yield strength

21

PROBLEMS WITH METALS

Fatigue failure Occurs with cyclic loading at stress below ultimate

tensile strength Depends on magnitude of stress and number of cycles Endurance limit: Maximum stress under which the

material will not fail regardless of number of loading cycles

If the stress is below this limit, the material may be loaded cyclically an infinite number of times (more than 106 cycles) without breaking. Above this limit, fatigue life is expressed by the S-n curve: Stress (S) versus the number of cycles (n)

22

Creep (cold flow) Progressive deformation response to constant force over an extended period of time Sudden stress followed by constant loading causes continued deformation Can produce permanent deformity May affect mechanical function (e.g., in TJA)

23

24

Corrosion can be decreased in the following ways:

1.Using similar metals 2.Proper implant design 3.Passivation by an adherent oxide layer

effectively separates metal from solution For example, stainless steel coated with

chromiumoxide

25

NON METALS

POLYETHYLENE Ultra–high-molecular-weight polyethylene (UHMWPE) Polymer of long carbon chains Used in weight-bearing components of TJA Acetabular

cups, tibial trays

Wear characteristics superior to those of high-density polyethylene Tough, ductile, resilient, resistant to wear, low friction

26

DISADVANTAGES Major disadvantage is : WEAR DAMAGE Can be decreased by: 1.GAMMA IRRADIATION increases polymer chain cross-links. Greatly improves wear characteristics However, reduces resistance to fatigue and

fracture Decreases elastic modulus, tensile

strength, ductility, and yield stress

27

Annealing Heating to below melting point Decreases free radicals

Good mechanical properties; does not disrupt crystalline areas

28

PMMA(POLYMETHYL METHACRYLATE)

Used for fixation and load distribution for implants

Act as a grout, not an adhesiveMechanically interlocks with boneReaches ultimate strength within 24 hours Can be used as an internal splint for patients

with poor bone stock PMMA can be used as a temporary internal splint until the bone heals. If the bone fails to heal, the PMMA will ultimatelyfail.

29

Poor tensile and shear strengthIs strongest in compression and has a low E Not as strong as bone in compression Reducing voids (porosity) increases cement

strength and decreases cracking. Vacuum mixing, centrifugation, good technique Cement failure often caused by microfracture andfragmentation Insertion can lead to a precipitous drop in bloodpressure.

30

SILICONESPolymers for replacement in non–weight-bearingJoints. Poor strength and wear capabilities. Frequent synovitis with extended use.CERAMICSMetallic and non metallic elements bonded ionically in a

highly oxidized state Good insulators (poor conductors) 1. Biostable (inert) crystalline materials such as Al2O3(alumina) and ZrO2 (zirconium dioxide) 2. Bioactive (degradable), noncrystalline substancessuch as bioglass

31

Typically brittle (no plastic deformation) High modulus (E) High compressive strengthLow tensile strength Low yield strain Poor crack resistance characteristics Low resistance to fracture Best wear characteristics, with polyethylene

and a low oxidation rate

32

High surface wettability and high surface tension Highly conducive to tissue bonding Less friction and diminished wear (“smoothsurface”) Small grain size allows an ultrasmooth finish Less friction Calcium phosphates (e.g., hydroxyapatite) may be useful as a coating (plasma sprayed) to increase

attachment strength and promote bone healing.

33

34

35

BIOMECHANICS OF FRACTURES

PERKINS classification (1958)(A)DIRECT TRAUMA Tapping fractures Crush Fractures Penetrating or Gunshot fractures(B) INDIRECT TRAUMA Compression fractures Tension Fractures Angular/bending fracture Torsional/rotational/spiral fracture

36

TAPPING FRACTURES

Linear, complete, transverse fracture caused by a small force of “dying momentum” acting over a small area. Surrounding tissue often normal.

37

CRUSHING FRACTURES

Direct application of large force over large area. Usually commmunited

38

PENETRATING FRACTURES

Due to large force over a small area.Usually due to gunshot injuries

39

TENSION FRACTURES

Occur in Cancellous or corcticocancellous bones and are usually transverse

Fracture of patella is classical example

40

COMPRESSION FRACTURES

Compression is better tolerated than tensionVertebral and calcaneal fractures are

classical examples

41

ANGULATION/BENDING FRACTURES

Green stick fractures are a classical exampleAnalogous to a ripe bamboo stick

42

TORSIONAL/SPIRAL FRACTURES

Fracture surface is circular/ oval with vertical spicules at both ends

Combination of compressive and tensile forces

Torsion is directly proportional to the RadiusTorsion is inversely proportional to the polar

moment of inertiaExplains why rotational fractures are more

common in lower one third of tibia than upper one third

43

44

FRACTURE HEALING

FRACTURE HEALING:PHASE I: CALLUS FORMATION

CELLULAR RESPONSEVASCULAR RESPONSE

PHASE II: REMODELLING

45

CELLULAR RESPONSE

46

VASCULAR RESPONSE

47

BIOMECHANICS OF FRACTURE HEALING

PRIMARY HEALING Without callus formation Seen in rigid internally fixed fractures CONTACT HEALING GAP HEALING

SECONDARY FRACTURE HEALING With callus formation

48

HEALING UNDER BONE PLATING

Based on the relation between the magnitude of gap between fracture fragments remaining after fixation and maximum movement permitted by the stability achieved after fixation.

Depends on strain at the fracture site (change in length/original length). Different tissues tolerate different strain values.

Perren et al described it.

49

If interfragmentary gap = 3mm , interfragmentary movement = 3mm, Strain = 100%, promotes Collagen formation

If interfragmentary gap = 6mm, interfragmentary movement = 3mm , Strain = 50%

If interfragmentary gap = 6mm, interfragmentary movement = 1mm , Strain = 10%, promotes collagen formation

In a similar fashion, if strain levels of <2% are achieved it promotes osteoblast formation directly

50

51

BIOMECHANICS OF INTRAMEDUALLRY NAILS

IM nails can be broadly categorised as 1.Sliding or gliding nails Eg: - K (kuntscher) nails, schneider nails,

hansen street nails, sampsons fluted nails, rush nail.

2.Interlocking nails Screws bind the nail. Prevent any kind of

motion.

52

STRUCTURAL ANALYSIS OF NAILS

EFFICIENY OF A NAIL DEPENDS ON 1 . The material used for its construction

(imparts strength) 2.The design geometry (imparts

rigidity/stiffness)

Choice is guided by above two considerations

53

SOLID OR A HOLLOW NAIL?

Area moment of a solid nail = 3.14 x r4 / 4 If dia = 10mm, r = 5mm , Area moment = 490.6 If nail is hollow with 10 mm dia and 2mm thickness

Area moment = 3.14(r1 – r2)/4 = 427 (slightly stronger)

If a 2mm thickness hollow nail is made using the same material used to make the solid nail, diameter will be 16mm

Area moment now will be 2198 Rigidity is thus 4.5 times greater than that of solid

nail

54

Polar moment of inertia(solid nail) = 3.14 x r4 / 2 = 981.25

Hollow nail constructed the same way = 3.14 x 84

=6430.72 (6.6 times greater)

Therefore, a hollow nail is much stronger than a solid nail i.e. further the material is spread away from the neutral axis, greater is its resistance to bending and torsional forces

Similar calculations can be done for nails with different cross sections showing similar results

55

CLOSED OR OPEN SECTION?

Closed section: Russell Taylor nailOpen section : k nail Area moment is almost similar for both nails

as circumference is almost same Polar moment significantly changes due to

discontinuity which leads to interruption in transmission of stress forces leading to reduced resistance to torsional forces

This effect can be avoided to some extent by making the cross section clover leaf shaped

56

57

STRENGTH OF NAIL BONE CONSTRUCT

Depends on working length and gripping strength.

Working length is that part of the nail which is not covered by the bone after completion of the surgery.

Part which underlies residual fracture gap Stiffness of the nail against bending force is

inversely proportional to the square of working length

Rigidity of nail against torsional forces is also inversely related to working length

Shorter the working length, greater is the bending and torsional rigidity of the nail bone construct.

58

GRIPPING STRENGTH

magnitude of force by which slipping of nail axially at the bone nail interface at the time of transmission of forces between the fracture fragments is prevented.

Bone tissue exerts an equal and opposite force on the nail which is designated as hoop stress

Slotted hollow nails have a distinct advantage in this regard

Compression during entry exerts elastic force on the canal wall which mantains acceptable magnitude of gripping strength in the post operative period.

59

60

DISADVANTAGES OF INTERLOCKING NAILS

1. Residual inter fragmentary gap is always there since the nail is locked. Increases due to necrosis at the bone metal interface. Fall in hoop stress.

Not seen in gliding nails. Fragments glide along surface of nail , reduction in fracture gap leading to healing.

Can be avoided in interlocking nails with the help of dynamisation

61

2. Working length of the locked nail spans between the locking screws leading to less resistance to bending and torsional forces

This can only be averted by using nails of increased thickness which can be achieved by keeping the inner diameter constant

Proximal part of the nail should be hollow and round and close sectioned because the subtrochanteric region is the area of maximum stress and such design helps in increased resistance against bending and torsional forces

62

3. Holes or gaps within the nail act as stress raisers.

Since lower limbs undergo cantilever bending the stress is maximum at the distal end of the nail

The lower the fracture is, lesser is the supporting effect of the bone

Therefore fractures of the distal end of the femur are not amenable to fixation by nails introduced proximally.

Nails introduced from the intercondylar notch of femur with the locking sequence reversed are used

63

BIOMECHANICS OF BONE SCREWS

64

65

CORE

It is the solid shaft on which the thread is spiralled

There is a core diameter/root diameter and a major/ outer diameter

The cross sectional diemeter of the root or the core determines the tensile and torsional strength of the screw

66

67

SCREW THREADS

Most important constituent of the screw.

May be visualized as a narrow width inclined plane spiralled around the core like a helix to conserve space

Based on the mechanics of an inclined plane.

Any load can be lifted to the same height with the use of a lesser force than needed to lift it vertically up, when it is pulled along a sloping ramp

Similarly, driving a peg in to a block of wood requires more force than to drive a screw

68

69

W =weight of object = 100 kgW2 = normal component of w = wcosθW1 = shearing component of w = wsinθ which

tends to pull the object towards O.F = pulling forceIf θ = 20 degrees, w1 = 100sin 20 = 0.34 x 100

= 34kgTherefore, magnitude of pulling force is much

lesser than 100 kg. Same priniciple helps screws function more

efficiently

70

ANGLE OF REPOSE

Upto a point on the inclination of the inclined plane, no sliding will actually take place

Until an angle is reached following which the tendency of the object to slide down progressively increases leading to increased work against the pulling force and decreased efficiency. Known as angle of repose.

This angle is the factor guiding the magnitude of inclination of the plane to make it a most efficient simple machine. Inclination of screw threads is so adjusted that maximum mechanical advantage is matched with the maximum number of threads accommodated in unit length of screw

71

SCREW TIPS

Blunt rounded off tips without flutes

Blunt rounded off tips with flutes

Trocar point tips

Cork screw tips

72

73

MECHANICS OF DRIVNG A SCREW

Torque is directly transmitted to the core and the threads of the screw. Ultimate movement is translatory

Example of coupling of motionIf applied force = 2NHead dia = 18mm, outer dia = 4.5mm, core dia=3mmTorque at head = force x dia = 36N-mForce at the thread = torque / dia = 16 N Force at Core = 24 NTherefore the force is magnified 8 times and 12 times

respectively

74

PITCH AND LEAD

75

TYPES OF SCREWS

1. CORTICAL SCREWS may either be self tapping or non self

tapping Self tapping may have a prismatic trocar tip

with three sharp edges or rounded tip with flutes covering the last 3 threads

Non self tapping have blunt rounded tips and cannot be inserted without pretapping with a special instrument.

However they offer better precision and lesser force.

76

CANCELLOUS SCREWS

Narrower core diameter, wider threads, no tapering towards the tip

May be fully or partly threaded. Unthreaded part is called shank. Core diameter always lesser than the shank

Tapping usually not required unless it is the epiphyseal ends of long bones where the bone is corticocancellous or in younger subjects having tougher metaphyseal bones

77

78

SLIDING COMPRESSION SCREWS

Specially designed partly threaded large cancellous screws

Cannulated and headlessThe headless shaft of the screw can be telescoped into

the barrel of the angled blade plate. The barrel of the plate is so designed that the head of

the locking nut cannot pass through the barrel and as the nut is tightened it forces the unthreaded shaft of the screw to slide backwards

The threadless shaft of the screw slides along the barrel with each turn of the nut and at one point starts compressing the fracture surfaces. This is why it is called a sliding hip screw.

79

LAG EFFECT

The word lag means unable to keep pace with fellows and to fall behind during movement.

In mechanics, it means lack of movement of one of the two fracture fragments under the process of fixation by a screw

Can be achieved in two ways 1. Using a oversized hole in the proximal fragment so that

screw threads do not take purchase in proximal fragment 2. Using a partly threaded screw ONLY THE FRAGMENT THROUGH WHICH THE SCREW

THREADS ARE MOVING WILL MOVE IN THE DIRECTION OPPOSITE TO THAT OF THE SCREW ALONG ITS AXIS.

LAG EFFECT IS A TOOL TO APPLY INTERFRAGMENTARY COMPRESSION

80

81

BIOMECHANICS OF BONE PLATES

Design: Semitubular plate, one third tubular plate

Material: SS plates, titanium platesShape of hole: round slots, oval slotsShape: Angled blade plates, clover leaf

plates, cobra head platesBiological factors: limited contact plates,

semitubular platesFunctional classification: Neutralization

plates, compression plates, Butress plates

82

NEUTRALIZATION PLATES

Act as bridge between fragmentsEvery bone plate is a neutralization plateEg: sherman and lane plates were

neutralization platesOffer poor resistance against bending,

shearing and torsional loadingNeeds to be supplemented with compression

(introduced by danis in 1949) either by altering its design or applying screws

83

COMPRESSION PLATES

May be applied with the screw or with the plate itselfMay be used in two different modes 1. Inter fragmentary compression Between the fracture fragments, can be applied

only with screw itself 2.Axial compression Line of force passes through the plate itself as bone

plate is a splint which is applied to the surface of the bone.

So the line of action of compressive force applied through a bone plate not in line with the neutral axis i.e. it is a contra axial application mode.

84

85

METHODS OF OVERCOMING EFFECT OF ECCENTRIC LOADING

1. To apply the plate on the TENSION side of the bone. However, since the bone is an anisotropic material, the compression or tension on the bones changes according to instantaneous loading pattern and is difficult to identifu

2. PREBENDING of plates: Bending effect caused by eccentric loading imposed by the compression plate will be balanced by the counter bending effect of prebending leading to uniform compression

86

87

INTERFRAGMENTARY COMPRESSION

SISK (1989) recommended insertion of screw at right angles to long axis of bone for fixing long oblique fractures and spiral fractures without communition.

P = force acting perpendicular to boneF-f = fracture lineR = force pulling the far fragment towards near

fragment impressed on F-f at an angle <90Hence force will resolve into normal and shearing. Shearing component will cause sliding of fracture

surfaces with consequent loss of alignment

88

89

Compression screw placed perpendicular to the fracture surface does not generate any shearing force, but stability under axial loading is less which allows interfragmentary sliding

To overcome this difficulty, Muller et al in 1990 recommended placement of one central screw at right angles to the long axis of bone and one screw at right angles to the fracture plane

90

91

COMPRESSION IN LONG OBLIQUE FRACTURES WITH BUTTERFLY FRAGMENT

Use of multiples screws is recommendedMultiple screws also do not offer enough stability and

need to be supplemented with neutralization platesPrimary lag screw fixation to stabilize fragments

followed by supplementation with neutralization plate without compression is the rule.

If the plane of the fracture changes from place to place, (spiral) placement of lag screws should be such that each screw is perpendicular to the fracture surface underlying that area.

In fractures with butterfly fragment, two screws must be placed following the principle of “bisecting angle”

92

93

APPLICATION OF COMPRESSION

PRINCIPLE: tensioning the plate and fixing it on the bone across the fracture line

May be achieved using

1. MULLERS apparatus2. PLATES WITH OVAL SLOTS (semitubular

and one third tubular plates.3. DYNAMIC COMPRESSION PLATING

94

MULLERS APPARATUS

Plate is fixed to the smaller fragment. Hook of the tension apparatus with the jaws fully

open is engaged to the notch in the last hole of the plate overlying the unfixed fragment.

The small plate with a single hole hinged to mullers apparatus is now fixed to the larger unfixed fragment with screw.

Gadget applies tensile force to the plate which in turn applies compression to the bone

It is an excellent method with the drawback of a wider exposure

95

96

PLATES WITH OVAL SLOTS

Maximum transverse diameter of the elliptical or oval slot of the plate is a little less than the maximum head diameter and lesser towards the ends,

Only if the plate moves in the direction of the arrow to bring major axis of the elliptical hole to match the maximum diameter of the screw head, the head can enter into the slot

Once the screw is introduced the plate along with the fragment anchored to it, undergoes linear acceleration to its counterpart.

Strength and rigidity of these plates is not high, since only the margins and not entire surface is contact with bone surface. Compression of periosteum is mimimal

These plates are biologically superior but mechanically inferior

97

98

DYNAMIC COMPRESSION PLATES

Also self compressing plates but the geometry of plate holes makes it more versatile

Much thicker and strongerOne or both margins of the oval hole are

slanting inwards to make it an inclined plane.Downward movement of the screw thus gets

an adjunct forward movement imparting axial compression .

99

100

101