Mechanical Prosthesis

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Mechanical Prosthesis

• By Michael Shackcloth

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History

• Hufnagel began experiments on ball valves in
  1946
• 1952 valve inserted into descending aorta

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Hufgnals Results

• Operated on 80 patients
• Hospital mortality 20
• 10 of survivors suffered thrombosis or embolism
• Many survivors dramatically improved
• Regurgitation reduced by 70
• Above all demonstrated foreign material could be
  placed in the bloodstream
• Metal ball replaced by a silicone covered ball to
  reduce loud click

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History

• 1952 Bailey experimented with plastic flaps and
  balls
• 1950s Murray experimented by bypassing the valve
  with a homograft
• 1955 Murray inserted a homograft successfully
  into the descending aorta of a 22 yr old man
• 1959 Hufgnal developed a single cusp valve
• Cusp made of dacron cloth impregnated with
  silicone rubber
• Reported use in 150 patients in 1961
• Experience showed him patients did better when 3
  cusps sewn together

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Valves Developed at the National Heart Institute
of Japan in the 1950s
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Mitral Valve Replacement

• Braunwald Morrow developed a prosthesis made of
  polyurethane reinforced with dacron fabric.
  Besides to leaflets it had tails which passed
  through the ventricular wall and were secured to
  imitate chordae
• 1st clinical placement 10th March 1960

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Aortic Valve Replacement

• March 1960 Harken inserted ball and cage valve in
  aortic annulus. Prosthesis consisted of a
  stainless steel cage containing a lucite ball and
  a sewing ring backed with teflon

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Albert Starr and M Lowell Edwards
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Aortic Valve Prosthesis

• August 1960 inserted aortic valve
• One piece stainless steel cage, silastic ball and
  teflon sewing ring
• Edwards conceived idea from an 1858 wine bottle
  stopper

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Aortic Model 1260

• Various changes in design and manufacturing
  technique led to this model becoming commercially
  available in 1966

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Starr Edwards Valve

• Since mid 1970s sewing ring made of teflon and
  polypropylene
• Mid 1980s silicone ball was used
• Time has showed that this is an excellent valve
  withvery good long-term results

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Developments in Mechanical Valve Technology

• 1962 Barnard and Goosens developed the lenticular
  mitral valve and biconical aortic valve
• Made of stainless steel and silicone rubber
  (replaced later with polypropylene
• Successfully implanted but soon replaced by valve
  with better flow characteristics

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Gott and Daggart 1963

• Polyvinyl fluoride with lexan ring coated with
  colloid graphite
• Withdrawn early. Stasis distal to and between
  leaflets led to embolic problems
• Structural failure

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Other Mechanical Valves Of This Era

• 1964 Harry Cromie impregnated Barium sulphate
  to make the valve radiopaque
• 1964 Smeloff-Cutter ball valve allowed some
  regurgitation theoretically to wash the ball and
  prevent thrombosis

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Origin of Disc Valves

• 1965 Cross and Jones developed a valve with a
  lens shaped disc of silicone rubber in a low
  profile cage with a woven teflon fixation flange.
  The disc was reinforced with a titanium ring
• First human implant January 1965

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Lillehei Valves

• 1966 Lillehei-Nakib - free floating titanium
  disc
• 1967 Lillehei-Kaster - pivoting disc valve
• Opened to 80 degrees
• Machined from a single block of titanium
• Greatly improved central flow
• Rotatable sewing cuff
• 6500 inserted between 1971 and 1990
• Event free survival at 13 years was70 for aortic
  
• 1968 Kalke-Lillehei - bileaflet valve
• Good haemodynamics
• Leaflets opened to 60 degrees
• Forerunner to St Jude prosthesis

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Further Developments

• 1967 DeBakey used pyrolytic carbon making the
  valve relatively thrombogenic
• Due to advances in manufacturing techniques
  valves were fairly durable by the mid 1970s.
  however there was room for haemodynamic
  improvement

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Second Generation Mechanical Valves

• 1960s Jura Wada developed concept of the
  pivoting disc
• Manufactured by Cutter laboratories, USA
• Titanium housing
• Teflon cloth sewing ring
• Hard teflon disc

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• Bjork found that fibrin deposition occurred in
  the hinge mechanism leading to failure
• Developed a new valve withShiley
• Inlet strut was part of the orifice ring but the
  outlet strut was welded in place
• Problems with outlet strut fractures led to the
  development of the monostrut valve

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This episode led to much tighter regulatory rules
with regards valve development
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The St Jude Valve

• Nicoloff and the engineer Possis had developed a
  crude bileaflet valve
• Approached Villafana, a founder of Cardiac
  Pacemakers Incorporated
• Rejected by board of directors so he set up St
  Jude Medical Incorporated

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Bileaflet Design

• Proposed independantly by Gott, Lillehei and Wada
• Pyrolytic hydrocarbon introduced by Jack Bokros
• This resolved the question of thrombogeicity and
  durability of the pivots
• Valve first implanted in 1977

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Ideal valve

• Function free of mechanical failure for the life
  span of the patient
• Should not increase the work of the heart
• Should not cause cellular damage to the
  constituents of the blood
• Should not activate the clotting cascade

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Three Types Commercially Available

• Caged Ball

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Three Types Commercially Available

• Caged Ball
• Bileaflet

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Three Types Commercially Available

• Caged Ball
• Bileaflet
• Monoleaflet

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Valve Constituents

• Sewing ring
• Occluder mechanism
• Occluder

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Haemodynamics
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Ball Valve

• Minimal leakage
• Annular area for flow creates turbulence
• High profile may occlude VOT
• Cage may contact ventricular
• wall during contraction
• Partial obstruction by the
• ball in aortic position

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Tilting Disc Prosthesis

• Less obstruction to blood flow
• Gradients of 6-7 mmHg
• Opening angle
• High low gradients, increased regurgitation
• Small High gradients less regurgitation

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Bileaflet Prosthesis

• Lowest gradient due to wide opening angle
• Minimal turbulence due to
• Wide opening angle
• Thin leaflets
• Large cross-sectional area

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Advances in materials

• Pyrolite carbon leaflets
• Titanium or pyrolite housings
• Tungsten to radiopacify leaflets

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Advances in Design

• Retrograde washing reduces blood stasis and
  prevents thrombus formation
• Monoleaflet prosthesis have crossing bars or
  central guides to prevent leaflet travel
• Bileaflet prosthesis have pivot recesses in the
  orifice to prevent leaflet travel

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Problems With Mechanical Valves

• Thromboembolism
• Haemorrhage
• Endocarditis
• Periprosthetic leak
• Structural valve degeneration
• Nonstructural valve dysfunction

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Thromboembolism

• Defined as valve thrombosis or embolus
• Valve thrombus may be occlusive or non-occlusive
• Occlusive usually catastrophic
• High risk in first 14 months
• Then 0.5 per patient year
• Majority occurs when anti-co-aggulation is
  disrupted
• Thrombogenicity of a valve depends on
• Valve design
• Surface area and texture
• Turbulence
• Stagnent areas

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Haemorrhage

• Defined as any episode of bleeding that causes
  death, stroke, requires operation or blood
  transfusion
• Related to levels of anti-coaggulation
• Incidence of anti-coaggulation related death is
  0.2 per patient year

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Endocarditis

• Any infection involving a prosthetic valve
• Mortality ranges from 23-69
• Most frequently occurs in first few months
• Late incidence 0.17 per patient year
• Most common site at annular tissue interface
• Early aggressive surgery is usually required

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Periprosthetic Leak

• Leakage of blood between sewing ring and host
  tissue
• Incidence 1
• Aetiological factors
• Annular calcification
• Infection
• Annuloprosthetic mismatch
• Excessive tension on sutures, annulus or both
• Suture misplacement
• Inadequate fibrous ingrowth
• Abnormal annular tissue

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Structural Valve Degeneration

• Mainly due to leaflet fracture
• Cracks in pivot guard reported
• If occurs in the intraoperative or early
  postoperative period then it is probably due do
  mishandling of the valve

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Nonstructural Valve Dysfunction

• Defined as any abnormality that lead to stenosis
  or regurgitation at the valve that is not
  intrinsic to the valve
• Incidence 0.3 per patient year
• Causes
• Sterile or infected thrombus
• Pannus formation
• Retained strands of chordal tissue
• Excessively long sutures
• Annular calcification nodules

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Mechanical Prostheses in the Aortic Position
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St Jude Medical

• Pyrolytic carbon over graphite substrate for
  housing and leaflets
• Leaflets impregnated with tungsten for
  radiopacity
• Leaflets open to 85o resulting in near central
  laminar flow
• 10 regurgitant flow
• Leaflet motion by rotation

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Carbomedics Prosthesis

• Solid pyrolite housing
• Pyrolite coated leaflets over tungsten
  impregnated carbon
• Radiopaque titanium stiffened ring
• Opening angle of 78o
• Leaflets can be rotated in the orifice

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Edwards MIRA Prosthesis

• Bileaflet prosthesis
• Pyrolite coated leaflets over tungsten
  impregnated carbon
• Curved leaflets enhance central flow and leaflet
  closure
• Retrograde washing by relative high frequency
  jets
• Closes by rotation and translation

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Biocarbon Prosthesis

• Sorin Biomedica
• Titanium housing covered by pyrolite carbon
  strengthens prosthesis
• Curved leaflets
• Two effluent passages provide continuous washing
  even in the closed position
• Orifice divided into three sections
• Sewing ring dacron and carbon coated teflon
• Hinge mechanism supports a rolling motion

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ATS Medical

• Pyrolyte housing and pyrolite carbon leaflets
  with graphite substrate
• Convex housing with protrusions on the inner
  aspect that support the leaflets therefore no
  protruding struts
• Convex hinge mechanism facilitate retrograde
  washing
• Opening angle of 85o

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Bjork-Shiley Monostrut

• Monoleaflet prosthesis
• Orifice ring and integral struts are constructed
  from a single piece of cobalt-chromium alloy
• Opening angle of 70o
• Relatively low velocity retrograde washing
  between leaflet and orifice
• Leaflet motion is by rotation and translation

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Sorin Allcarbon Monoleaflet

• Chromium alloy housing coated with a thin film of
  carbon
• Pyrolytic carbon monoleaflet
• Strut mechanism integral with housing
• Sewing ring carbon coated

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Medtronic Hall Prosthesis

• Monoleaflet
• Central guide for leaflet travel
• Housing and central guide made of titanium
• Opening angle of 70-75o
• Leaflet motion is by translation and rotation

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Omnicarbon Prosthesis

• Monoleaflet prosthesis
• Titanium orifice ring
• Pyrolite carbon disc
• Disc motion controlled by short struts
• Opening angle 80o