Metals in orthopaedics

METALS METALS IN IN

ORTHOPAEDICSORTHOPAEDICS

PROPERTIES

HIGH ELASTIC MODULUSHIGH DUCTILITYCAN BE FABRICATED INTO VARIOUS SIZES

AND SHAPESPROPERTIES CAN BE ALTERED BY PHYSICAL

PROCESSESCAN FORM ALLOYSGOOD RESISTANCE TO EXTERNAL AND

INTERNAL ENVIRONMENTS

MAJOR METALS USED1.STAINLESS STEEL2.COBALT BASED ALLOYS3.TITANIUM BASED ALLOYS

NEWER METALS1.OXINIUM2.TRABECULAR METAL3.NITINOL-NICKEL TITANIUM ALLOYS

STAINLESS STEEL

PLATES,SCREWS,PINS AND RODSCONTAINS CARBON,MOLYBDENUM,CHROMIUM

AND NICKEL APART FROM IRON CARBON-INCRESES STRENGTH,DECREASES CORROSION

RESISTANCE CHROMIUM-INCREASES PASSIVITY MOLYBDENUM-INCREASES PASSIVITY NICKEL-HELPS TO MAINTAIN AUSTANTIC STRUCTURE OF

STEEL.

COMMONLY USED TYPES OF STAINLESS STEEL ARE AISI 316 L,AISI 440 B.

ADVANTAGES AND DISADVANTAGESGOOD MECH STRENGTHHIGH DUCTILITYSHOWS WORK HARDENING EFFECTSMAY SOMETIMES SHOW LOCAL COROSSION

AND PITTING CORROSION-LOCAL IRRITATION AND WEAKENING OF IMPLANT

COBALT-BASED ALLOYS

MAINLY HIP AND KNEE PROSTHESESCORROSION RESISTANCE AND

BIOCOMPATIBILITY-EXCELLENTFATIGUE RESISTANCE AND INCRESED TENSILE STRENGTHQUITE DIFFICULT TO MACHINEEXPENSIVEVITALLIUM IS THE MOST COMMONLY USED ALLOY

TITANIUM-BASED ALLOYS

CONTAINS ALUMINIUM,VANADIUM ALONG WITH TITANIUM

MOST FLEXIBLE AMONG THE METALS USED IN ORTHOPAEDICS

LIGHT WEIGHT THAN OTHER METAL ALLOYSBIOCOMPATIBLEEXCELLENT CORROSION RESISTANCE[Ca-P

LAYER AND SURFACE OXIDE LAYER MAKES IT INERT]

PURE TITANIUM IS ALSO USED WHERE HIGH STRENGTH IS NOT REQUIRED

NEWER METALS

OXINIUM• OXIDIZED ZIRCONIUM IS A METALLIC ALLOY WITH A CERAMIC SURFACE.• ZIRCONIUM: A BIOCOMPATIBLE METALLIC

ELEMENT IN THE SAME FAMILY AS TITANIUM• COMBINES THE BEST OF BOTH METAL AND

CERAMICS.• EXCELLENT FRACTURE TOUGHNESS LIKE

COBALT CHROME.• CERAMIC SURFACE THAT OFFERS

OUTSTANDING WEAR RESISTANCE

TRABECULAR METAL• ELEMENTAL TANTALUM METAL• VAPOR DEPOSITION TECHNIQUES THAT

CREATE A METALLIC STRUT CONFIGURATION SIMILAR TO TRABECULAR BONE.

• CRYSTALLINE MICROTEXTURE IS CONDUCTIVE TO DIRECT BONE APPOSITION.

• INTERCONNECTING PORES – 80% POROUS– 2-3 TIMES GREATER BONE INGROWTH COMPARED TO

CONVENTIONAL POROUS COATINGS – DOUBLE THE INTERFACE SHEAR STRENGTH

TRABECULAR METAL MATERIAL IS MADE FROM TANTALUM OVER CARBON.

IT IS STRONG, FLEXIBLE, AND BIOCOMPATIBLE. THE STRUCTURE OF TRABECULAR METAL MATERIAL IS SIMILAR TO THE STRUCTURE OF BONE.

IT IS VERY POROUS, WHICH MEANS IT HAS SMALL SPACES OR PORES. NEW TISSUE CAN GROW INTO THESE PORES AND HELP HOLD THE IMPLANT IN PLACE.

THE LATEST TECHNIQUE FOR PRODUCTION OF LESS STIFF TOTAL JOINT PROSTHESES IS THE TRABECULAR METAL TECHNOLOGY. A METALLIC SPONGE MADE FROM TUNGSTEN HAS ABOUT THE SAME STIFFNESS AS BONE. WHEN A LAYER OF THE METALLIC SPONGE IS PLACED ON THE SURFACE OF THE TOTAL HIP PROSTHESIS, IT WILL MAKE A SMOOTH TRANSITION FROM THE STIFF METAL TO THE WEAK BONE. THE SCIENTISTS HOPE THAT THIS TECHNOLOGY WILL DIMINISH THE STRESS SHIELDING EFFECT OF THE TOO STIFF TOTAL HIP AND KNEE PROSTHESES

IMPLANTS

SUCCESS OF AN IPLANT IS DETERMINED BY

1.CONDITION OF THE PATIENT2.SURGEON’S SKILL3.BIOCOMPATIBILITY OF IMPLANT4.MECHANICAL PROPERTIES5.WEAR/CORROSION RESISTANCE

IDEAL METAL FOR IMPLANT

• BIOCOMPATIBLE – NON-TOXIC, NON-CARCINOGENIC, NON-IMMUNOGENIC

• STRENGTH – COMPRESSIVE, TENSILE, TORSIONAL

• FATIGUE RESISTANCE, CONTOUR ABILITY• CORROSION AND DEGRADATION RESISTANCE• OSSEO INTEGRATION (WHEN APPLICABLE)• IMAGING COMPATIBLE – MRI, CT SCAN

• INEXPENSIVE

PROBLEMS ENCOUNTERED

IN METAL IMPLANTS

INFECTION IN PRESENCE OF AN IMPLANT

EARLY INFECTIONS THROUGH SKIN,AIR OR SURGICAL

INSTRUMENTATION INFECTION DOESN’T SUBSIDE BCOZ

REVASCULARISATION BLOCKED BY IMPLANTLATE INFECTIONS HEMATOGENOUS IN ORIGIN BACTERIA PROTECTED BY GLYCOCALYX

PRESENT ON THE COATING FORMED ON THE SURFACE OF THE FOREIGN MATERIAL

BIOCOMPATIBILITY

BIOCOMPATIBILITY - ( MEANS WELL TOLERATED BY BODY'S TISSUES )

ALL MODERN ALLOYS ARE WELL TOLERATED BY BONE TISSUE - IN BULK FORM. THE BEST TOLERATED IS TITANIUM IN PURE FORM. FOR THIS EXTREME BIOCOMPATIBILITY, PURE TITANIUM IS OFTEN USED AS POROUS COATING FOR THE SURFACES OF TOTAL HIP PROSTHESES. 

IN DUST FORM, AS WEAR PARTICLES, ALL THESE ALLOYS, EVEN A PURE TITANIUM, MAY, HOWEVER, TRIGGER OSTEOLYSIS IF THEY LAND IN THE TISSUES AROUND THE TOTAL HIP PROSTHESIS. METALLIC WEAR PARTICLES IN THE SOFT TISSUES PAINT THE TISSUES BLACK, THIS IS CALLED METALLOSIS.

UNTOWARD HOST RESPONSE –CLINICAL MANIFESTATIONS

C/C INFLAMMATION-METALLOSISOSTEOLYSIS AND LOOSENINGSTERILE ABSCESSNEOPLASIA

METAL FAILURE

BRITTLE FAILURE – SCREW HEAD WITH POOR DUCTILITYPLASTIC FAILURE –LOAD > ENDURANCE LIMIT. IMPLANT BENDS PERMANENTLY.FATIGUE FAILURE – CYCLICAL (REPETITIVE)

LOADING.

FATIGUE FAILURE

FATIGUE FRACTURES

THE EVERYDAY LIFE PUTS ASTOUNDING DEMANDS ON THE MATERIALS OF THE TOTAL HIP JOINT. THE SHAFT OF THE MODERN TOTAL HIP PROSTHESIS WILL SUSTAIN SUCH LARGE LOADS, IF THEY OCCUR OCCASIONALLY; THE SHAFT MAY FAIL, HOWEVER, EVEN FOR LOWER LOADS, IF THEY OCCUR VERY OFTEN. THE METAL ALLOY WILL SUCCUMB TO THE SO- CALLED FATIGUE FAILURE AND BREAK.

THERE IS A LIMIT, HOW MUCH REPETITIVE LOADS 

THE PROSTHESIS WILL EVENTUALLY SUSTAIN. THIS LIMIT IS SPECIFIC FOR EVERY FORM OF THE TOTAL HIP PROSTHESIS AND FOR THE METAL ALLOY USED FOR MANUFACTURE.  ABOVE THIS LIMIT, THE PROSTHETIC SHAFT WILL SUSTAIN THE FATIGUE FRACTURE

STRESS SHEILDING

STRESS SHIELDING - A TOO STIFF SHAFT. THE PROSTHETIC SHAFT TAKES OFF A PART OF THE

STRESS THAT WALKING AND OTHER EVERYDAY ACTIVITIES PUT ON THE UPPER PART OF THE THIGH BONE HOLDING THE PROSTHESIS. A TOO STIFF SHAFT OF A TOTAL HIP PROSTHESIS "STRESS SHIELDS" THE UPPER PART OF THE THIGH BONE TO MUCH. THIS IS SO BECAUSE THE ALLOYS USED FOR FABRICATION OF THE SHAFT ARE MUCH STIFFER THAN THE SKELETON OF THE THIGH BONE. THE SHIELDED BONE DOES NOT THRIVE, LOSES ITS SUBSTANCE, AND BECOMES WEAK. THE TOTAL HIP JOINT HAS WEAK ANCHORAGE IN A WEAK SKELETON AND MAY FAIL

CORROSION

DAMAGE OF MATERIAL DUE TO ACTION OF THE ENVIRONMENT

EFFECTS- TISSUE INFLAMMATION AND NECROSIS,WEAKENING OF IMPLANT

CLINICALY RELEVANT FORMS OF CORROSION

1.UNIFORM ATTACK 2.GALVANIC OR BIMETALLIC 3.PITTING CORROSION 4.FRETTING

PRECAUTIONS1.USE OF CORROSION RESISTANT

MATERIAL2.USE OF SAME MATERIAL FOR DIFF PARTS

OF THE SAME IMPLANT3.AVOID DAMAGES DURING

TRANSPORTATION4.AVOID INSTABILITY OF FIXATION

METAL TOXICITY

• NICKEL – CYTOTOXIC AGENT & ALLERGEN• TITANIUM– INHIBIT OSTEOCLASTIC ACTIVITY AND REDUCE

OSTEOBLASTIC PROTEIN SYNTHESIS (THOMPSON & PULEO 1996).

– CONTACT DERMATITIS (LAYOR ET AL. 1991).

• COBALT-CHROMIUM– METALLOSIS, OSTEOLYSIS, – FORMATION OF SOFT TISSUE MASSES, – INFLUENCE PROLIFERATION AND FUNCTION OF HUMAN OSTEOBLASTS

POLYETHYLENE

POLYETHYLENE IS A TYPE OF PLASTIC COMMONLY USED ON THE SURFACE OF ONE IMPLANT THAT IS DESIGNED TO CONTACT ANOTHER IMPLANT, AS IN A JOINT REPLACEMENT 

POLYETHYLENE IS VERY DURABLE WHEN IT COMES INTO CONTACT WITH OTHER MATERIALS.

WHEN A METAL IMPLANT MOVES ON A POLYETHYLENE SURFACE, AS IT DOES IN MOST JOINT REPLACEMENTS, THE CONTACT IS VERY SMOOTH AND THE AMOUNT OF WEAR IS MINIMAL.   

PATIENTS WHO ARE YOUNGER OR MORE ACTIVE MAY BENEFIT FROM POLYETHYLENE WITH EVEN MORE RESISTANCE TO WEAR. THIS CAN BE ACCOMPLISHED THROUGH A PROCESS CALLED CROSSLINKING, WHICH CREATES STRONGER BONDS BETWEEN THE ELEMENTS THAT MAKE UP THE POLYETHYLENE. THE APPROPRIATE AMOUNT OF CROSSLINKING DEPENDS ON THE TYPE OF IMPLANT. FOR EXAMPLE, THE SURFACE OF A HIP IMPLANT MAY REQUIRE A DIFFERENT DEGREE OF CROSSLINKING THAN THE SURFACE OF A KNEE IMPLANT.

CERAMICS

• CERAMIC MATERIALS ARE USUALLY MADE BY PRESSING AND HEATING METAL OXIDES (TYPICALLY ALUMINUM OXIDE AND ZIRCONIUM OXIDE) UNTIL THEY BECOME VERY HARD.

• THESE CERAMIC MATERIALS ARE STRONG, RESISTANT TO WEAR, AND BIOCOMPATIBLE.

• THEY ARE USED MOSTLY TO MAKE IMPLANT SURFACES THAT RUB TOGETHER BUT DO NOT REQUIRE FLEXIBILITY, AS IN THE SURFACES OF A HIP JOINT.

COMPOSITE MATERIAL

COMPOSITE MATERIALS ARE MADE BY MIXING TWO OR MORE SEPARATE MATERIALS WITHOUT CREATING A CHEMICAL BOND BETWEEN THE MATERIALS.

METAL ALLOYS AND CERAMICS ARE NOT CONSIDERED TO BE COMPOSITE MATERIALS BECAUSE THEIR INGREDIENTS ARE CHEMICALLY BONDED TO CREATE A NEW MATERIAL. 

ON A LARGER SCALE, TWO LAYERS OF DIFFERENT MATERIALS CAN BE COMBINED TO CREATE A COMPOSITE MATERIAL WITH THE DESIRED CHARACTERISTICS.

THE STEM OF A HIP IMPLANT, FOR EXAMPLE, MAY CONSIST OF LAYERS OF TWO DIFFERENT MATERIALS THAT TOGETHER PROVIDE THE DESIRED COMBINATION OF STRENGTH AND FLEXIBILITY.

BIOABSORBABLE MATERIALS

• BIOABSORBABLE MATERIALS ARE DESIGNED TO BE ABSORBED BY THE BODY WHEN THEIR JOB IS COMPLETE.

• THEY ARE MADE FROM A BIOCOMPATIBLE PLASTIC THAT CAN BE DISSOLVED BY NORMAL BODY FLUIDS.

• THESE BIOABSORBABLE MATERIALS MAY ALSO BE USED IN IMPLANTS THAT REATTACH SOFT TISSUE TO BONE.

TANTALUMTANTALUM IS A PURE METAL WITH EXCELLENT

PHYSICAL AND BIOLOGICAL CHARACTERISTICS. IT IS FLEXIBLE, CORROSION RESISTANT, AND

BIOCOMPATIBLE.

SILICONE SILICONE IS A RUBBERY MATERIAL THAT IS VERY

FLEXIBLE. IN ORTHOPAEDICS, IT IS MOST COMMONLY USED

IN IMPLANTS THAT REPLACE THE JOINTS OF THE TOES

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