IMPLANT FAILURE

PRESENTER : Dr ANKUR MITTAL

INTRODUCTION

METALS IN ORTHOPAEDICS USE AND THERE COMPARISON

An ideal implant material should be: inert non-toxic to the body absolutely corrosion proof inexpensive great strength high resistance to fatigue easily worked

But all properties in a single implant cant be found

Different metals use are 1. Stainless steel 2. Cobalt – chromium alloys 3. Titanium alloys

STAINLESS STEEL ALLOY:

Effects of composition of this alloy on implant:

•Chromium produce a protective self regenerating chromium oxide layer that protect against corrosion.

•Molybdenum decreases the rate of slow passive dissolution of chromium oxide layer by upto 1000 times

•Nickle imparts further corrosion resistance

Short come of stainless steel alloy:

Though it is strong , stiff , and biocompatible material it has slow and finite corrosion rate

So long term effects of nickle ions however prevails

So best suited for short term implantation.

Titanium alloys

Highly reactive rapidly metals get coated with oxide layer making it phisiologically inert and resistant to most chemicals

Titanium has elastic modulus of approx. half that of stainless steel and cobalt chromium alloy

Lower stiffness of bone plate made of titanium reduce severity of stress shielding and cortical osteoporosis

It is less prone to fatigue failure than stainless steel

Elastic modulus of stainless steel is 12 times EM of cortical bone

Em of titanium is 6 times of cortical bone

Ductility of titanium is lower than stainless steelSo due to this difference surgeon require some adaptation of his feel while determinig the optimal amount of torque to be applied to the screw.

METHODS OF METAL WORKING AND THEIR EFFECTS ON IMPLANTS

VARIOUS METHODS ARE:

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

Forging: metal is heated and hammered or squeezed into shape. It produces an orientation of the grain flow making the metal strong.

Case hardening: metals are treated to cause the outer surface of rod to be harder than inner core

Advantage is harder outer surface will resist indentation while core is able to absorb more energy

Most important is Polishing removes scratches which could act as local stress risers Passivation produce protective oxide layer

Passivation can be damaged by – cold working - scratching - other mechanical traumaTherefore care is needed in handling implants.

WHAT IS IMPLANT FAILURE?

The term implant failure implies that failed implant was inadequate for the function expected of it.

OR

Clinically , implant failure may be defined as a failure of implantation procedure to produce satisfactory results.

Was the design of the implant adequate or faulty?

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

Were defects in the implant are due to errors during fabrication??

Was the clinical condition adverse Or

The surgical judgment in selection of the implant and the conditions under which the implant was used such that there was a high probability of failure because of difficulties in attaining satisfactory mechanical relationships between the implant and bone fragments?

Did the surgeon apply the proper mechanical and surgical principles in implantation of the device?

During after-care of the patient, were there any mechanical lapses which might have caused the implant to fail and which might have been avoided with proper precautions?

CAUSESIt is divided into:

1. Surgical

2. Material

3. Idiosyncratic

•Surgical technique•Surgical judgement•Surgically introduced infection

•Chemistry•Structural metallurgy•Engineering design

Selective rejection of implant by certain patients often associated with:•Pain•Hypersensitivity reaction•Implant loosening•Sinus tract infection

4. Patients compliance

Patients post operative management programme

Significant Re trauma during the consolidation phase of healing

Inadequate postoperative immobilization

5. Other causes : fresh trauma overweight

early weight bearing before significant union may lead to loosening or fatigue failure of implants..this more commonly occur in obese patients then underweight patients.

Surgical failure can be due to -

1. Mechanics of fracture fixation

2. Material limitation of devices and implants

3. Mixing of implants

Mixing of implants means mix and match implants from different manufactures in fracture fixation.

The mixing of implants from different producers can lead to high risk of corrosion, jamming , broken drills and taps, gaps, loose fits, and loosening.

So it is therefore good clinical practice to use instruments and implants from one manufacturer.

Material failure 1. Deficiency in engineering design 2. Manufacturing processing 3. Handling in operating room

Clinically , material failure fall into 1. Pure mechanically 2. Pure environmental 3. Conjoint

1. Pure mechanically is due to direct overload including impact or design.

2. Environmental failure is due to reaction of physiological environment with the metal , resulting in corrosion which either weakens the device mechanically elicits a adverse tissue response neccessiating device removal.

3. Conjoint i.e. mechanical and environmental failures produced by applied stress in corrosive environment. Conjoint faiure modes include fretting corrosion and corrosion fatigue.

Mechanical failure of implant falls into 3 categories:

1. Plastic failure is one in which implant failed to maintain its original shape resulting in clinical failure.

2. Brittle failure is effect in the design or metallurgy

3. Fatigue failure is due to repetitive loading on device therefore when surgeon inserts a implant he must realize that he

is entering a race between fatigue of implant and healing of fracture.

Environmental failure is due to corrosion

Corrosion is the gradual degradation of metals by electrochemical attack

Usually orthopaedic implants have inert protective layer to prevent corrosion

Whenever there is change in pH or oxygen tension in tissue

Damage the oxide layer

Produce corrosion

Types:

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

Effects of corrosion :

•Weakens the implanted metal•Changes the surface of the metal•Metal ions into the body

Infection in orthopedic surgery is a disaster both for the patient and surgeon.

HOW???

•Increase antibiotic use •Prolonged hospital stay•Repeated debridement•Prolonged rehablitation•Morbidity and mortality

Although its incidence has been reduced due to modern theatre facilities and aseptic measures but in developing countries its prevalence is still high. It is better to prevent infection rather than to treat it.

Follow up x-rays of implant failure due to infection

Not united after 4months of surgery

After 6 months of surgery

9 months after surgery 18 months after surgery

Probable risk factors

•Advanced age ( > 60 yrs )•Prolonged surgery time•Smoking•Co morbidity in patients like DM•Skin abrasion at fracture site•Skin at risk

Commonest organisms: Staphylococcus aureus E coli and proteus Klebsiella

Pathogenesis:Infection is related to microorganism which grow in biofilm

Therefore its eradication is difficult

Diagnosed by: Clinical examination Lab investigation Histopathology Microbiology Imaging like USG, MRI, Bone scan, CT

Idiosyncratic failure:

It originates from corrosion products induced hypersensitisation phenomenon resulting in implant rejection or loosening.

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

Localized attack in the form of fretting (mechanical) or fretting corrosion (mechanical-environmental) was commonplace at points of metal-on-metal contact in multi component implants. Only rarely was corrosion of the bone/plate interface observed.

LOCAL TISSUE RESPONSE:

The biologic environment walls off the implanted alloy by interposing a relatively acellular tissue capsule.

With accelerated implant degradation, however, inflammatory cells, macrophages, and occasionally foreign body giant cells may be found adjacent to the device.

Adverse tissue response to the presence of an implant stems from

- toxic nature of the corrosion process - an individualized sensitivity to certain corrosion products - biologically accelerated corrosion rate in certain patients.

Removal of the device usually effects prompt relief (24-48 hours), and very commonly there is obvious tissue discoloration from corrosion.

Typically, from device retrieval and analysis studies in humans, a small number will present with evidence of local infection (pain, inflammation, edema, fluid accumulation, or draining sinuses) 6months or more after the original surgery.

Typically, culture and sensitivity testing reveals no growth of microorganisms, thus the designation of "sterile abscess

Another well-recognized local effect of fracture-fixation devices is the osteoporotic remodeling of bone immediately beneath the plate.

Such a response is thought to stem from the load-sharing capacity of the plate, which acts to bypass forces around the underlying bone.

Accordingly, Wolff's law of dynamic remodeling can in theory not operate to maintain the appropriate balance of osteoblastic versus osteoclastic activity, and osteoporosis ensues.

In this regard much emphasis has been directed to reducing the rigidity (modulus) of the fracture fixation device and its attachment to bone. However, to date, no suitable low-modulus (or low-rigidity) system is available commercially.

SCREW FAILURE

Conical1. Countersink Hemispherical

Conical undersurface should be inserted centered and perpendicular to the hole in plate

If set to any other angle

Undersurface does not adapt well to the plate hole

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

Both factors weakens the screw Screw failure

2.Run out:

The screw may break at the run out during insertion if it is incorrectly centered over the hole or is not perpendicular to the plate. Typically it breaks with spiral configuration indicating failure under torsional load

3. IT may break

•During insertion if applied torsional load exceeds its torsional strength•When pilot whole is too small •Not tapped in hard bone•Due to lack of lubrication •High stress develop in screw when there is significant resistance to insertion causing screw to shear at a cross section and leave a part lodged in bone

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 the plate act as stress riser leading to a fresh fracture proximal or distal to the original one.

Improper application of plates and poor technique are other causes of plate failure.

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 HolePlacing 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.

• Associated with either the insertion of a small diameter nail or use of an interlocking nail for a very proximal or distal shaft fractures

• Plastic deformation (bending) of the IM rod mainly occurs with nails that are less than 10 mm in diameter; minimal nail diameters range 12-14 mm for women & 13-15 mm for men

•Early dynamization, especially of subisthmal fractures, is associated with increased risk of developing a valgus deformity at the fracture site

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

IMPLANT FAILURE IN INTERLOCKING NAILING

• Bent distal screws may result from early wt bearing if the screws are too close to the fracture site;

• Weak part of the nail is proximal of the 2 distal holes;

          - Fractures located with in 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 femora 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. For the more distal fracture, the high stress region, close to the fracture site, is also significantly closer to the distal locking screw holes, which are significant stress risers.

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.

Infection causes failure of about 1–5% of cases of primary arthroplasty.

Clues to the presence of infection include clinical signs, a periosteal reaction, a positive culture of aspirated joint fluid, and acute inflammation identified in tissue around the implant.

Fatigue failure at the bone/cement and bone/implant interface may cause aseptic loosening, and may be especially important for implants with relatively smooth surfaces.

Stress shielding can influence local bone density, but is rarely an isolated cause of implant loosening.

Now, 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

BUTMost important now is surgeon has to investigate and analyze what causes the failure

1 . The details of the condition for which the device was originally inserted, including dates, place of operation, operative procedure, and so forth.2. The details of postoperative treatment and, in particular, any episode, suchas premature weight-bearing or undue loading, which directly preceded the failure.

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.

He should then obtain enough material for biopsy amid label it as to origin amid orientation with respect to the device. Only with this information can the pathologist interpret the findings in a pertinent fashion.

If there is a suspicion of infection superimposed on a tissue reaction to time device, bacteriological cultures of suspicious material are mandatory.

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