IMPLANT FAILURE

Presenter : Dr. Saumya Agarwal

Junior resident Dept of Orthopaedics J.N.Medical College and Dr. Prabhakar

Kore Hospital and MRC, Belgaum

1. Introduction

2. Methods Of Metal Working And Their Effects On Implants

3. Terminologies in Biomechanics

4. Characteristics and behaviour of implant materials

5. Materials Used In Orthopaedic Implants

6. Causes of implant failure

7. Corrosion

8. Screw failure

9. Implant failure in plating

10. Implant failure in IMIL nailing

INTRODUCTION

Implants:

Implant is an iatrogenic foreign body

deliberately induced by surgeon into human

body where it is intended to remain for a

significant period of time in order to perform a

specific function

• In orthopaedics, implants are used to reconstruct a fractured bone

• screws• plates • nails• wires • components- external fixator help orthopaedic surgeon

Implant Failure An implant is said to have failed if it ceases to

perform the function for which it is inserted

may be due to :• Deformation• Fracture of implant• Loosening of fixator• If implant causes undesirable consequences like

pain, infection or toxicity leading to rejection

Saying”

the true cause of implant failure is not the failure

of device but infact the failure of surgeon to

understand the principles of fixation and limitations of

implant

It is essential for an orthopaedician to know the

biomechanical aspects of the tools of his trade

Defect in manufacturing is also a major concern

Methods Of Metal Working And Their Effects On Implants

• Forging• Casting• Rolling and drawing• Milling• Coldworking• Case hardening• Maching• Broaching• Polishing and passivation

Forging

Casting

Rolling and drawing

Milling

Coldworking

Case hardening

Broaching

Polishing and Passivation

Terminologies in Biomechanics:

Force:

Is an action or influence, such as pull or push

which when applied to a free body tends to

accelerate or deform it

Load: refer to an application of a force to an object

5 types-

1. Axial load - tension – traction or pulling - compression – pressing together2. Bending load - simple three point - cantilever3. Torsion twisting 4. Direct shear - II forces in opposite direction5. Contact load

A body under load reacts in two ways ;

• It deforms – changes it shape strain

• It generates internal force stress

Deformation:

change of shape

represents a change in dimension

Strain : a technical term used to express deformation

defined as : change in linear dimensions of a body

resulting from application of a force or a load

strain = change in length/original length

3 types :1. Compressive strain : represented by ↓ in length of straight

edge or a line drawn on a body

2. Tensile strain : represented by ↑ in length of straight

edge or a line drawn on a body

3. Shear strain : represented by change in angular

relationship of two lines drawn on the surface

Stress: the internal forces resisting deformation

are called stress

defined as : internal force generated within a

substance as a result of application of external load

stress = load / area on which load acts

3 types :

1. Compressive stress acts perpendicular

. to a given plane

2. Tensile stress

3. Shear stress acts parallel to given plane

Stress risers (stress concentrators)

a point at which stress is appreciably higher than

elsewhere due to geometry of the stressed object is

called a stress riser

Stress riser produces ↑ local stresses , which may be

several times higher than those in the bulk of the

material and may lead to local failure.

Stresses also concentrate around discontinuities such as

• holes• sharp angles • notches• grooves • threads in a structure all stress risers greatly

weaken a structure

Stress Protection or Shielding

used to describe the reaction of bone to unloading

when a fractured bone is fixed with plate , both bone and plate share the limb load

bone is relieved of some of its original load by plate

results in reduced density of bone under plate because of reduced functional stimulation

Characteristics and behaviour of implant materials

Three properties:1. Mechanical controls functional .

characteristics of implants 2. Physical

3. Chemical determine biocompatibility between implant

and environment of body

Stress - strain curve

Mechanical Properties

Four properties :

1. Elasticity

2. Plasticity

3. Viscosity

4. Strength

Elasticity :

is the ability of a material to recover its

original shape after deformation on removal

of the force or load

Plasticity :

is the ability of a material to be formed

to a new shape without fracture and retain

that shape after load removal

Viscosity :

exhibited by viscoelastic materials;

shows progressive deformation with time under

constant stresses

Strength :

ability of a material to resist an applied

force without rupture

Stress strain curve :

defines certain universal qualities of the

behaviour of materials under load

Elastic Limit :

is that point on the stress strain curve

beyond which removal of applied load does not

result in full recovery of deformation

Yield Point :

denotes end of the elastic region of curve

Ultimate Tensile Strength :

with application of load , a maximum stress

will be achieved; this maximum stress attained

during a single loading is called the ultimate tensile

strength.

Beyond this; the metal will break or rupture

Physical Properties

1. Radio Transparency : opaque to x-rays – located and examined

2. Heat and Irradiation : sterilization of implants

Chemical Properties

When a material is exposed to a water containing solutions, one of the three conditions exist when the system reaches the equilibrium

1 corrosion 2 immunity

3 passivation

Corrosion :

is destruction of the metallic structures

by action of surrounding medium

No. of metal atoms > 106 gm atoms/lit – a state

of corrosion exists

Immunity :

if no. of metal atoms < 106 gm atoms/lit

– metal is said to be immune

does not possess enough energy to initiate a

significant reaction

Passivation :

brief period of corrosion

that results in an intimate

layer of oxide or hydroxide

being formed on the surface

that mechanically separates

the metal from solution

Materials Used In Orthopaedic Implants

Three types

1. Metals and Alloys

2. Polymers

3. Ceramics

Metals and Alloys :

1. Iron based alloys

2. Cobalt based alloys

3. Titanium based alloys

Iron based alloys (stainless steel)

Contains:- Iron (62.97%)- Chromium (18%)- Nickel (16%)- Molybdenum (3%)- Carbon (0.03%)

The form used commonly is 316L (3% molybd, 16% nickel & L = Low carbon content)

Cobalt based alloys (stelites)

a) Cast Co-Cr

- Chromium (27-30%)- cobalt (60%)- Molybdenum (5-7%)- nickel (2.5%)

- Carbon (0.35%)

Highly abrasion resistant and gives reasonable bearing properties

Used in two piece joint replacement

b) Wrought Co-Cr

- Chromium (19-21%)- Cobalt (46-53%)- Nickel (9-11%)

- Tungston (14-16%)

quite ductile and strong

Titanium based alloys :

recently introduced Ti 6Al 4V ELI

lower modulus of elasticity, good corrosion resistance, lower tensile strength

Different Components- Different Properties

- Chromium : corrosion resistance and hardenability

- Nickel : easy fabricability and corrosion resistance

- Molybdenum : brittleness and corrosion resistance

- Carbon : generates oxide film – corrosion resistant

- Manganese and silica : controls problem of manufacture

Was the design of implant adequate or faulty?

Was the choice of materials satisfactory with regard to strength, hardness, corrosion resistance, and ductility?

Were defects due to errors during fabrication??

Was the clinical condition adverse?

Did surgeon apply proper mechanical and surgical principles in implantation?

During after-care, any mechanical lapses?

Deformation

Fracture of implant

Loosening of fixator

or

Undesirable consequences like

Pain

Infections

Toxicity

leading to rejection of implant

Four principle modes of failure:

1. Excessive Deformation : most common due to introduction of large, irrecoverable strains

following static or dynamic loading

2. Fracture

3. Abrasion or Erosion : repeated surface contact

4. Chemical attack : sudden failure by corrosion fatigue

Willenegger 1. Instability :

a) Inadequate implant

b) Incorrect positioning of implant

c) Insufficient bone support - inadequate interfragmentary compression - inadequate reduction - remaining defect - absence of cancellous bone graft - weak bone

d) Bone necrosis

e) Inadequate post operative treatment

2. Complications :

1. Locala) Skin necrosisb) Wound infection

2. General thromboembolism

Black classified into

1. Functional failures

2. Material failures

3. Mechanical failures

Functional failure : those in which desired effect is

not achieved, but no frank defect is observed

Causes :a) Wrong device used b) Incorrect applicationc) Post-op infectiond) Inadequate post-op management

Material failure : due to problems associated with

device; characterised by failure of materials in device

May occur :a) Secondary to corrosionb) Tissue reaction to corrosionc) hypersensitivity

Mechanical failure : due to errors in implant design,

intra-op deformation of device

Three categories :a) Ductile failureb) Brittle failurec) Fatigue failure

Ductile failure : mechanical failure under static load

with excessive plastic deformation, long before physical separation has occurred is . ductile failure

Can be avoided by extension of design against excessive plastic deformation

Brittle failure : mechanical failure under static load without

plastic deformation

due to either defect in implant design or metallurgy can be prevented by avoiding use of stress risers in

implant designing or by taking care not to damage implant during insertion

Fatigue failure : primary concern results from cyclical loading on a device;

rhythmic nature of human locomotion imposes such cyclic stress on the bones and soft tissues in the limbs and thus on fracture fixation devices

When internal fixation device is implanted

fracture healing fatigue failure of device

Mechanism of fatigue failure involves formation of stress concentration points at superficial irregularities

irregularities may consist of holes in the plate, abrupt change in cross sectional area etc.

Small and round screw holes acts as stress concentration points

can be avoided by using large screw holes

CORROSION

is the gradual degradation of metals by electrochemical attack and is concern when a metallic implant is placed in the electrolytic environment of the body

initiation of corrosion depends on pH and O2 tension at the implantation site

Corrosion :

weakens implanted metal; changes surface of metal; releases metal ions into body fluids

Types of corrosion :

1.Galvanic 2.Crevice3.Pitting4.Fretting5.Stress6.Intergranular7.Ion release

Galvanic corrosion : mode of metallic deterioration in

which two dissimilar metals in content with one another are immersed in solution

A battery is formed

Anode undergoes more rapid dissolution

Cathode undergoes less rapid dissolution

CAUSES :

1. use of stainless steel screw in a cobalt chromium plate

2. when an impurity is accidentally included during manufacturing

3. rubbing of implants

Crevice corrosion : in a narrow gap (crevice) between

implants e.g. screw head and plate;

high concentration of cl- or h+ ions destroy this passive layers and local corrosion commences

Pitting corrosion : localized reaction

Starts as a defect in the surface layer

Chromium, nickel and molybdenum are added to stainless steal to the resistance to pitting corrosion

Fretting corrosion : results from very small oscillating

movements or vibrations

Causes abrasive damage to the passivating layer

a multicomponent weight bearing implant may be affected

Stress corrosion : high mechanical stress may alter the

activity of metal and rupture a protective passive surface layer, thereby increasing its susceptibility to corrosion

Intergranular corrosion : if impurities aggregate b/w grains of

relatively pure alloy a localized galvanic cell may exist between crystals and alloy in the grain boundaries

Ion release : implanted metal releases ions into

tissues

occasionally patients may be sensitive to chromium or nickel found in stainless steel implants requiring removal

Implant failure is an interplay of multiple factors and can be broadly classified into

1) implant related

2) patient related

3) technique related (surgeon related)

Implant related:

An ideal implant should be : • Chemically inert• Non-toxic to the body• Great strength• High fatigue resistance• Low Elastic Modulus• Absolutely corrosion-proof• Good wear resistance• Imaging compatible• Inexpensive

So metallurgical problems contribute to implant failure

Patient related :

Osteoporosis Comminuted fractures Bone loss Unstable fracture Premature weight bearing in lower limb fractures High velocity trauma with extensive injury to soft

tissues Degenerative disease, alcohol intake, drug addiction,

over weight

Surgeon related :

wrong selection of patient wrong selection of implant wrong selection of operation and technique

Technique related : 1) Excessive stripping of soft tissues resulting in

wide spread devascularization

2) Inadequate interfragmentary compression

3) Inadequate purchase on fracture fragments

4) Early mobilization without adequate stability

5) Application of plate on compression side

6) Inadequate bone support failure to use bone graft

7) Inadequate prebending of plate

8) Scratches on the implant

9) Improper placement of IM nail Improper

10) Dynamic locking of unstable fractures leads to failure of intramedullary fixation device

Idiosyncratic failure: originates from corrosion products

induced hypersensitisation phenomenon resulting in implant rejection or loosening

~ 6% of population has existing hypersensitivities to one or more constituents of stainless steel or cobalt-chromium alloys, suggesting a need for routine hypersensitivity screening prior to surgery.

SCREW FAILURE

Conical1. Countersink Hemispherical

Conical undersurface should be inserted centered & perpendicular to hole in plate

If set to any other angle

Undersurface does not adapt well to plate hole

Due to which Its wedge sharp create undesirable high forces and uneven contact which predisposes to corrosion

Both factors weakens screw Screw failure

2.Run out : screw may break at run out during

insertion if it is incorrectly centered over the hole or is not perpendicular to the plate.

3. screw may break : during insertion; if applied torsional

load exceeds its torsional strength

Not tapped in hard bone

Due to lack of lubrication

High stress

Implant Failure In Plating

Plate failure occurs because of interference with periosteal blood supply

Brittle and Plastic failure occur due to - minor loads in small plates - secondary major trauma in large plates

The most common failure of plate is fatigue failure

• The ends of plate act as stress riser leading to a fresh fracture proximal or distal to the original one

• Improper application of plates and poor technique

• Fatigue failure of plate is inevitable if healing fails to occur

Left. When a gap is left on the cortex opposite that to which the plate is attached, bending of the plate at the fracture site can cause the plate to fail rapidly in bending.

Right. Compressing the fracture surfaces not only allows the bone cortices to resist bending loads, but the frictional contact and interdigitation helps to resist torsion.

Breakage of Fracture Fixation Plates

The application of a plate on the compressive as opposed to the tensile side of a bone subjected to bending causes a gap to open on the opposite side of the plate during functional loading.

Plate Failure Through a Screw Hole

Placing the plate so that an empty screw hole is located over the fracture will significantly increase the potential for fatigue fracture of the plate.

A second consideration---

The greater the span or distance of a beam is between its supports, the lower its stiffness will be, and the more it will deform under load in bending and torsion. For this reason, screws should be placed as close together across the fracture site as possible.

IMPLANT FAILURE IN INTERLOCKING NAILING

associated with either insertion of a small dmt nail or use of an interlocking nail for a very proximal or distal shaft #

Plastic deformation (bending) of IM rod mainly occurs with nails < 10 mm in dmt;

minimal nail diameters range 12-14 mm for women 13-15 mm for men

Bending of nail at # site usually occurs as an early complication caused by premature wt bearing, lack of adequate support, or deficient material (nail) strength

Bent distal screws may result from early wt bearing if screws are too close to # site

Weak part of nail is proximal of the 2 distal holes

Fractures located within 5 cm of this hole will be stressed above endurance limit with ambulation

These fractures must have delayed wt bearing until callus is present

Femoral Splitting Due to IM Rod Insertion

Mismatch of the curvature between the IM rod and the medullary canal results in bending stresses that could cause splitting of the femur during insertion

If the same force acts on IM rods placed in femur with more proximal (left) or more distal (right) fractures, the moment arm of the force will be longer in the case of the more distal fracture, and therefore the moment, acting at the fracture site, on the implant, will be larger.

IM Rod and Locking Screw Breakage

Because the distal end of the femur flares rapidly, the length of the locking screw required to cross lock the rod can be quite variable.

If the screw is not well supported by trabecular bone but mainly by cortex, then its stiffness and strength decrease with the third power of its length between cortices.

If the screw length doubles, the deformation of the screw under the same load increases by a factor of eight.

A proposed mechanism for loosening external fixation pins involves under- or oversizing the diameter of the pin relative to the bone hole.

A. If the pin and bone hole are the same diameter, micromotion can occur with bone resorption.

B. If the pin is more than 0.3 mm smaller in diameter than the hole in bone, microfracture may occur during insertion.

C. If the bone hole diameter is about 0.1 mm smaller than the pin diameter, the bone is prestressed but does not fracture, micromotion is eliminated, and pin stability is maintained

Loosening of External Fixator Pins

To produce more rigidity in construction of an external fixator, the basic principles that should be considered are that for pin-and-rod-type sidebars; stiffness increases with the fourth power of the cross-sectional area (the moment of inertia, and decreases with the third power of their span or unsupported length . This explains why it is beneficial to decrease sidebar to bone distance, increase pin diameter, place pins as close together across the fracture site as possible, and use larger-diameter or multiple sidebars in frame construction

IMPLANT FAILURE IN ARTHROPLASTY

ASEPTIC LOOSENING :The most important cause of aseptic loosening is an inflammatory reaction to particles of wear debris.

Abrasive, adhesive, and fatigue wear of polyethylene, metal and bone cement produces debris particles that induce bone resorption and implant loosening.

Particles can cause linear, geographic, or erosive patterns of bone resorption (osteolysis), the distributions of which are influenced by the implant design.

Micromotion of implants that did not achieve adequate initial fixation is another important mechanism of loosening.

What should we do ? ? ?

Surgeon encounters evidence of failure of an appliance by

•Breakage•Tissue reaction •Or suspect failure

What he will do ?

He will plan to remove the implant and plan for another operative procedure BUT

Most important now is surgeon has to investigate and analyze what caused the failure

3. During removal of implant; surgeon should record his operative findings carefully, and, in particular, the orientation of the device or of its fragments with respect to grossly visible tissue reaction-discoloration, granulation tissue, hemorrhage, or pus formation.

1.Details of condition for which device was originally inserted, including dates, place of operation, operative procedure

2. Details of postop treatment and any episode of premature weight-bearing or undue loading, which directly preceded the failure.

4. Should then obtain enough material for biopsy and label it.

5.If there is a suspicion of infection bacteriological cultures of suspicious material are mandatory.

Beautifulppt.com

Adequate knowledge of implant materials is an essential platform to making best choices for the patient

Most of the existing implant material falls short of one or the other criteria to be an IDEAL IMPLANT.

Advances in biomedical engineering will go a long way in helping the orthopedic surgeon

The search is on…

02/05/2023 110

Questions in exams ??

Long question : What is implant failure ? Enumerate the causes for the

same and management.

Short questions :

1) Corrosion 2) Materials Used In Orthopaedic Implants