INTRODUCTIONMETALS IN ORTHOPAEDICS USE AND THERE COMPARISONAn ideal implant material should be: inert non-toxic to the body absolutely corrosion proof inexpensive great strength high resistance to fatigue easily workedBut all properties in a single implant cant be foundDifferent 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 oxidelayer that protect against corrosion.•Molybdenum decreases the rate of slow passive dissolution ofchromium oxide layer by upto 1000 times•Nickle imparts further corrosion resistanceShort come of stainless steel alloy:Though it is strong , stiff , and biocompatible material it has slowand finite corrosion rateSo long term effects of nickle ions however prevails So best suited for short term implantation.
Titanium alloysHighly reactive rapidly metals get coated with oxide layer making itphisiologically inert and resistant to most chemicalsTitanium has elastic modulus of approx. half that of stainless steeland cobalt chromium alloyLower stiffness of bone plate made of titanium reduce severity ofstress shielding and cortical osteoporosisIt is less prone to fatigue failure than stainless steelElastic modulus of stainless steel is 12 times EM of cortical boneEm of titanium is 6 times of cortical boneDuctility 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 IMPLANTSVARIOUS METHODS ARE:•Forging•Casting•Rolling and drawing•Milling•Coldworking•Case hardening•Maching•Broaching•Polishing and passivationForging: 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 rodto be harder than inner coreAdvantage is harder outer surface will resist indentation while core isable to absorb more energyMost important isPolishing removes scratches which could act as local stress risersPassivation produce protective oxide layerPassivation 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 wasinadequate for the function expected of it. ORClinically , implant failure may be defined as a failure ofimplantation 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, corrosionDuring after-care of the patient, resistance, and ductility?were there any mechanicallapses which might have causedthe implant to fail and whichmight have been avoided with Were defects in the implant areproper precautions? due to errors during fabrication?? Was the clinical condition adverseDid the surgeon apply the proper Ormechanical and surgicalprinciples in implantation of the The surgical judgment in selection of thedevice? 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?
It is divided into:1. Surgical •Surgical technique •Surgical judgement •Surgically introduced infection2. Material •Chemistry •Structural metallurgy •Engineering design3. Idiosyncratic Selective rejection of implant by certain patients often associated with: •Pain •Hypersensitivity reaction •Implant loosening •Sinus tract infection
4. Patients compliancePatients post operative management programmeSignificant Re trauma during the consolidation phase of healingInadequate postoperative immobilization5. Other causes : fresh trauma overweight early weight bearing before significant union maylead to loosening or fatigue failure of implants..this more commonlyoccur in obese patients then underweight patients.
Surgical failure can be due to -1. Mechanics of fracture fixation2. Material limitation of devices and implants3. Mixing of implants
Mixing of implants means mix and match implants from differentmanufactures in fracture fixation.The mixing of implants from different producers can lead to high riskof corrosion, jamming , broken drills and taps, gaps, loose fits, andloosening.So it is therefore good clinical practice to use instruments and implantsfrom one manufacturer.
Material failure 1. Deficiency in engineering design 2. Manufacturing processing 3. Handling in operating roomClinically , material failure fall into 1. Pure mechanically 2. Pure environmental 3. Conjoint1. 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 metallurgy3. 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 corrosionCorrosion is the gradual degradation of metals by electrochemical attackUsually orthopaedic implants have inert protective layer to preventcorrosionWhenever there is change in pH or oxygen tension in tissue Damage the oxide layer Produce corrosion
Effects of corrosion :•Weakens the implanted metal•Changes the surface of the metal•Metal ions into the bodyTypes:1. Galvanic2. Crevice3. Pitting4. Fretting5. Stress6. Intergranular7. Ion release
Infection in orthopedic surgery is a disaster both for the patient andsurgeon. 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.
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 riskCommonest organisms: Staphylococcus aureus E coli and proteus KlebsiellaPathogenesis:Infection is related to microorganism which grow in biofilmTherefore its eradication is difficultDiagnosed by: Clinical examination Lab investigation Histopathology Microbiology Imaging like USG, MRI, Bone scan, CT
Idiosyncratic failure:It originates from corrosion products inducedhypersensitisation phenomenon resulting in implantrejection 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 ofthe bone/plate interface observed.
LOCAL TISSUE RESPONSE:The biologic environment walls off the implanted alloy by interposing arelatively acellular tissue capsule. With accelerated implant degradation, however, inflammatory cells,macrophages, and occasionally foreign body giant cells may be foundadjacent 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.
Typically, from device retrieval and analysis studies in humans, a smallnumber will present with evidence of local infection (pain, inflammation,edema, fluid accumulation, or draining sinuses) 6months or more afterthe original surgery.Typically, culture and sensitivity testing reveals no growth ofmicroorganisms, thus the designation of "sterile abscess Removal of the device usually effects prompt relief (24-48 hours), andvery commonly there is obvious tissue discoloration from corrosion.
Another well-recognized local effect of fracture-fixation devices is theosteoporotic remodeling of bone immediately beneath the plate.Such a response is thought to stem from the load-sharing capacity of theplate, which acts to bypass forces around the underlying bone.Accordingly, Wolffs law of dynamic remodeling can in theory not operate tomaintain 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 availablecommercially.
SCREW FAILURE Conical1. Countersink HemisphericalConical undersurface should be inserted centered and perpendicular tothe hole in plate If set to any other angleUndersurface does not adapt well to the plate holeDue to which Its wedge sharp create undesirable high forces anduneven 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 incorrectlycentered over the hole or is not perpendicular to the plate. Typically itbreaks with spiral configuration indicating failure under torsional load3. IT may break•During insertion if applied torsionalload 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 whenthere is significant resistance toinsertion causing screw to shear at across section and leave a part lodgedin bone
IMPLANT FAILURE IN PLATINGPlate failure occurs because of interference with periosteal bloodsupply.Brittle and Plastic failure occur due to -minor loads in small plates -secondary major trauma in large platesThe most common failure of plate is fatigue failure.The ends of the plate act as stress riser leading to a fresh fractureproximal or distal to the original one.Improper application of plates and poor technique are other causes ofplate failure.Fatigue failure of plate is inevitable if healing fails to occur
Breakage of FractureFixation Plates 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.
The application of a plate on thecompressive as opposed to the tensileside of a bone subjected to bendingcauses a gap to open on the oppositeside of the plate during functionalloading.
Plate Failure Through a Screw HolePlacing the plate so that an empty screw hole is located over the fracture willsignificantly 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 ININTERLOCKING NAILING• Associated with either the insertion of a small diameter nail or use of aninterlocking nail for a very proximal or distal shaft fractures• Plastic deformation (bending) of the IM rod mainly occurs with nails thatare 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 withincreased risk of developing a valgus deformity at the fracture site•Bending of the nail at the fracture site usually occurs as an earlycomplication caused by premature wt bearing, lack of adequate support, ordeficient material (nail) strength;
• Bent distal screws may result from early wt bearing if the screws are tooclose 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 aboveendurance limit with ambulation - These fractures must have delayed wt bearing until callus ispresent
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
IM Rod and Locking Screw Breakage 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.
Because the distal end of the femur flaresrapidly, the length of the locking screwrequired to cross lock the rod can be quitevariable. If the screw is not well supported bytrabecular bone but mainly by cortex, then itsstiffness and strength decrease with the thirdpower of its length between cortices. If thescrew length doubles, the deformation of thescrew under the same load increases by afactor of eight.
Loosening of External Fixator PinsA proposed mechanism for loosening external fixationpins involves under- or oversizing the diameter of thepin 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 diameterthan the hole in bone, microfracture may occur duringinsertion.C. If the bone hole diameter is about 0.1 mm smallerthan the pin diameter, the bone is prestressed but doesnot fracture, micromotion is eliminated, and pin stabilityis maintained
To produce more rigidity in construction of an external fixator, the basicprinciples that should be considered are that for pin-and-rod-type sidebars;stiffness increases with the fourth power of the cross-sectional area (the momentof 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, increasepin diameter, place pins as close together across the fracture site as possible, anduse larger-diameter or multiple sidebars in frame construction
ASEPTIC LOOSENING :The most important cause of aseptic looseningis an inflammatory reaction to particles of weardebris.Abrasive, adhesive, and fatigue wear ofpolyethylene, metal and bone cement producesdebris particles that induce bone resorption andimplant loosening. Particles can cause linear, geographic, orerosive patterns of bone resorption (osteolysis),the distributions of which are influenced by theimplant design. Micromotion of implants that did not achieve adequate initial fixation is another important mechanism of loosening.
Fatigue failure at the bone/cement and bone/implant interface may causeaseptic loosening, and may be especially important for implants withrelatively smooth surfaces. Stress shielding can influence local bone density, but is rarely an isolatedcause of implant 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.
Now, surgeon encounters evidence of failure of an appliance by•Breakage•Tissue reaction•Or suspect failureWhat he will doHe will plan to remove the implant and plan for another operativeprocedure BUTMost important now is surgeon has to investigate and analyze whatcauses 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, such as premature weight-bearing or undue loading, which directly preceded the failure.
During removal of implant surgeon should record his operative findingscarefully, and, in particular, the orientation of the device or of its fragmentswith respect to grossly visible tissue reaction-discoloration, granulationtissue, hemorrhage, or pus formation.He should then obtain enough material for biopsy amid label it as to originamid orientation with respect to the device. Only with this information canthe 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.