On-X® Carbon is a pure isotropic pyrolytic carbon free from silicon carbide and associated imperfections therefore reducing possible shear damage to blood cells and therefore reducing clot formation.
- Less blood damage from a smoother, pure carbon surface resulting in lower hemolysis levels and complication rates.
- More flexible, stronger carbon allows the incorporation of a flared inlet that combined with natural length-to-diametre ratio organises flow, reduces turbulence and increases effective orifice area.
Supra-annular Sewing Rings
The sewing ring of the valve is attached on the outer portion of the carbon ring of the valve with titanium bands. For On-X valve sizes 19-25mm, the sewing ring is positioned on the aortic valve above the flared inlet that sits intra-annular so that it will be stitched in position above the annulus of the natural valve. Allowing removal of sewing material from the annular area and allows more area for flow through the annulus. In size 27/29 it is both intra-annular and supra-annular – in large sizes flow is optimized and sewing ring presence in the annulus is not as crucial.
- Greater geometric and effective orifice areas are achieved with On-X size-per-size compared to other valves, resulting in all sizes providing optimal hemodynamic – patients recover faster when there is optimal flow.
Optimum Geometric Orifice Area****
Geometric orifice area (GOA) is determined by taking measurements of the valve with a calliper and by calculating the area with equations. Optimal GOA refers to the most efficient orifice size available, most important in small aortic valves because these valves restrict flow unless there is a large area available for flow.
- Greater geometric orifice area contributes to all On-X valve sizes providing optimal haemodynamics – a gradient less than 10mmHg
- Optimal haemodynamics in every On-X valve size means that patients with small aortic roots will be relieved of stenosis and high gradients – improving quality of life postoperatively
- The supra-annular sewing ring placement, thinner orifice walls and leaflets that open to 90o contribute to On-X Valves having the largest geometric orifice area available in bileaflet valves
The length-to-diametre ratio of the On-X valve is optimal at a little more than 0.6, which closely resembles the natural valve at about 0.7. The combination of the supra-annular sewing ring, near natural length and intra-annular inlet flare work in combination to obtain unsurpassed flow.
- Less turbulence that causes less blood damage
- Hemolysis level is less than competitor valves
- Lowest complication rates due to reduced turbulence
- Reduced turbulence in the orifice leads to greater flow area, resulting in optimal haemodynamics and faster patient recovery
The valve orifice on the inflow end of the valve is flared about one millimetre larger than the stated valve size and is tapered inward to organise blood flow like that of air into a jet engine. The flare is on the bottom of an aortic valve and inside the sewing ring at the top of the mitral valve.
- Organisation of flow to a more natural laminar flow that creates less turbulence inside the orifice that short orifice valves
- Less turbulence reducing blood damage
- Greater effective orifice area (flow area) is achieved with On-X size-per-size compared to other valves
- Protection from pannus overgrowth is the aortic position. The flare abuts and supports the annulus providing a barrier to the bodies scarring tendency.
Full Annular Support
When the natural valve leaflets are removed from the annulus, the area of removal will tend to scar and overgrow (pannus formation). The inlet flare of the On-X aortic valve will abut the area of removal (annulus). It will provide a barrier to tissue proliferation. The guard of the mitral valve and thin rim of carbon on the inflow side protect from pannus in that position.
- In 150,000+ implants over 17 years as of 2014, there has been no reports of pannus ingrowth of the On-X valve
- The protection given to the annulus area by natural length of the On-X valve provides a barrier to pannus overgrowth that can lessen effective orifice area and create morbid events
- The protection given to the annulus area by the natural length of the On-X valve may prevent overgrowth that can hinder or impinge leaflet motion
Aortic and Mitral Leaflet Guards
Leaflet guards are orifice extensions on the outflow side of the valve orifice for both valve positions, aortic and mitral.
Mitral – The leaflet guards are on the outflow side of the valve (bottom) to provide a generous shelf to pull the retained leaflets or “reef” or “gather” them behind the valve. This helps prevent contact and potential lock-up with paravalvular and/or retained mitral valvular tissue and prevents pannus ingrowth. Leaflet contact with tissue is termed ‘impingement’ and can cause a hindrance or lock-up of leaflet motion. Lock-up is caused by the entrapment of tissue or suture ends between the leaflet edge and orifice
Aortic- The guards in the aortic position are on the outflow side of the valve and prevent pannus ingrowth and interference with the leaflets from the retained suture ends.
- Preventing impingement and lock-up of the leaflet as well as pannus ingrowth leads to decreased complications especially thrombosis formation
- Fully guarded leaflet swings decrease morbid event rates by prevention of entrapment or impingement of leaflets that can lead to thrombus formation or lock-up
- Mitral leaflet guards provide an area to move retained natural leaflets out of the way of the implanted leaflets.
The leaflet pivot is a recessed area in the carbon wall of the ring of the valve where the leaflet ‘tabs’ are inserted to hold them in place. The actuated pivot is only available with On-X valves; it ensures that the leaflets will always close rapidly because of the closing ramp that is built into the design.
- 90o opening angle made possible by the presence of closing ramp in the actuated pivot allows greater geometric and effective orifice areas with On-X size-per-size compared to other valves
- 50o degree travel angle of the On-X leaflet insures lower closure volume that allows more thorough pivot washing
- Greater washing by a backflow purge designed into the On-X pivot minimizes the opportunity for thromboembolic and thrombotic complications
- Closing ramp in the On-X pivot insures that leaflets will always function and not increase morbid event development even in low flow situations
90o Opening Angle of Valve Leaflets
Valve leaflets are the semi-circular moving parts of the valve. There are two in each valve and both have two leaflet tabs on either end of the curved edge for insertion into the pivot area of the wall or the ring of the valve. When the leaflets are perpendicular to the horizontal axis of the orifice, they are open to 90o to allow maximum flow. This means that leaflets in the open position are vertically aligned with the blood flow. On-X valve leaflets have about 10% weight tungsten in them for visibility on X-ray.
- Alignment of flow for less turbulence
- Increased effective orifice area from leaflets not deflecting flow
- More cavitation resistance because of less turbulence. Cavitation is damage that can result in the valve structure due to the bursting of bubbles created by the motion of valve parts. On-x valves have the highest cavitation resistance.
- Less morbid events because of less turbulence
40o Closing Angle
On-X valve leaflets contact the walls of the orifice at two points on the sides of the leaflets at an angle that is 40o to the horizontal alignment of the orifice; this is different from 25o angle of the other mechanical valves on the market.
- Closing regurgitant volume is less as the leaflets close sooner than they would if they had to travel a great distance. Closing volume is the same amount of blood trapped behind the leaflets as they swing shut. Due to a shorter leaflet swing and greater closing angle, the On-X valve design minimizes closing volume. This allows a design for greater pivot washing where it is needed.
- Reduced contact velocity of the leaflet with the orifice wall leads to less blood damage. Due to the shorter leaflet travel angle, there is a resultant lower contact velocity meaning less blood destruction and less likelihood for cavitation and morbid events.
- Noise potential is minimized as a result of shorter travel angle and less contact velocity. Noise related to mechanical valves is one of the implanted patients major complaints, many patients report ultra-quiet On-X valves
Optimized pivot washing
The pivot is a recessed area in the carbon wall of the ring of the valve where the leaflet ‘tabs’ are inserted to hold the leaflets in place. The On-X valve is designed with flow channels in the pivot area that allow complete purging (washing) of the pivot when the valve is closed, the contoured surfaces of the pivot provide room to continue purge and prevent stasis.
- Pivots are continuously flushed during the cardiac cycle leaving less possibility for stasis and clot formation
- Pivots are built with contoured surfaces which allow blood to go around closed leaflets leaving less possibility of stasis and clot formation
- Precent of pivot regurgitation is slightly higher in the On-X valve that other valves insuring better pivot wash
Soft Landing Closure of Valve Leaflets
On-X valve leaflets contact the orifice of the valve at two points instead of one like the other mechanical valves. The contact velocity as a result is distributed equally between two points instead of one. The leaflets will close sooner at two peripheral points instead of on the tip.
- Two point leaflet contact at closure enables the leaflet travel angle to be shorter than other valves and contact velocity to be less
- Noise potential is minimised
- Contact velocity with the orifice wall is less which leads to less turbulence, less turbulence means less blood destruction and less likelihood for cavitation and morbid events
- Lower closing volume because the leaflet close sooner than it would if contacting on one point at its tip. Closing volume is the amount of blood trapped behind the leaflets as they swing shut. Due to a shorter leaflet swing and a greater closing angle, the On-X valve design minimizes closing volume. This allows a design for greater pivot washing where it is needed
Cavitation resistance is the ability of a mechanical valve design to suppress cavitation. Cavitation is microbubble formation from the movement of parts through fluid. These bubbles on a surface can release high local energy focused on a small area. This can damage mechanical valve components. Cavitation can also damage blood and therefore cause haemolysis. On-X valve features which can lead to cavitation resistance include the opening and closing angles, optimized purging paths, and soft landing closure.
- Reduced blood damage, evidenced by near normal lactate dehydrogenase (LDH) levels
- Decreased possibility of structural failure caused by cavitation
- Decreased possibility of high intensity transient signals (HITS) which is linked to higher morbid event rates in some studies