Porous cardiovascular implants

~.~G925Z This invention relates to novel prosthetic devices and implants for cardiovascular use.

It is well known that the introduction of foreign bodies into the blood stream, for example, the polished metal surfaces of artificial heart valves, tends to cause the formation of blood clots which may break loose and embolize to various parts of the body. Such throm~oembolic problems have led to the administration of ant:icoagulants to patients with artificial heart valves. The effects of these anticoagulants on the blood clotting mechani~m cause difficulties in stopping the flow of blood through even a minor flesh wound.

In addition, flexible plastic conduits are used for vascular graft purposes and such surfaces also are thrombogenic.

To date, the art has been unable to avoid the dual problem of avoiding thrombus formation while also avoiding the bleeding problem associated with the use of antLcoagulants.

It has now been surprisingly found that these prior art difficulties can be overcome and foreign bodies may be introduced into thecardiovascular system without blood clotting problems.

The present invention provides a cardiovascular ~ prosthetic device or a cardiovascular implant having a blood .

engaging porous surface region in fluid flow communication with~ a subsurface net~ork of interconnected pores. Such a device may beused bo prevent t~e formation of clots in the blood stream.

It was particularly surprising to find that nucleated cells circulating in the blood stream colonize onto the porou9 blood-engagin~ suxface and subsequently differentiate into other cell ty~es to form a thin, smooth, generally uniformlythick, firmly-attached tissue covering on the surface. The tissue covering is formed rapidly over about a one month period and does not appear to increase significantly in thickness~ thereafter. The tissue formation is not accom~ ~ ied by throm Go~ bosis or embolism owing to its blood- ~re.

- 2 ~ ~06925Z In U.S.

Patent No. 3,855,638, there is described a surgical prosthetic device consisting of a metal substrate with a porous metal coating into which bone tissue may grow - for incorporation of the prothesis into the body. ~he porous coating used in this prior art device has several essential require- ments, including restrictions on coating thickness, interstitial pore size and coating porosity. These parameters are dictated by the strength requirementsof the surgical pros~hetic device, namely, that the coating and the coating-substrate interface have strengths at least that of bone, so that there is no danger of failure of the prosthesis after ingrowth of bone tissue.

In cardlovascular uses, however, strength is less of an important consideration, and the ranges of parameters chosen are dictated to some degree by the intended use of the prosthetlc aevice-or ~mplant.

Further, the mechanism of incorporation of the sur- ` gical prosthetic device of this prior art into the body is by ingrowth of tissue into the coating while the present invention involves quite a different mechanism which arises from the different environment of the devices of the invention as ~ ~ .

compared with that of the prior art. ~-The clK~ovascular prosthetic devices~Dr imDla~ use~ in the~ ~ present invention may, in some-cases, be wholly porous with the ;

network of interconnected pores extending throughout the body `~ ,:

`~ ~ -of the device.

Alternatively, the prosthetic device or implant may be in the form of a porous coating on a coherent substrate,~ with the network of interconnected pores extending throughou~ ~ ~ the-coating only. ~ :, . . . . . .

~ An example of the use of the former type of device ;~ 30 is as the metal electrode tiP of a heart pacemaker ~ while an example of the use of the latter type of device is as a ,:

! heart valve prosthesis, typically having a statianary member - ', ' .. -.

- 3 - ~ , 069252 and at least one moving member, the latter being freely moving or connected to the stationary me~ber.

Both of these applications use metal as the material of construction. The present invention is not limited to metal as the material of construction and many other constructional materials inert to blood may be used, either alone or in combinations of two or more such materials, provided that they can be provided in a porous form. Typical materials include flexible or rigid plastics, cera~ics and carbon.

The present invention may be used for a variety of cardiovascular applications in addition to those mentioned, ` including partially or totally implantable blood pumps, such as artificial hearts and ventricular assist devicas, heart valve - - components, such as flexible flap-type valve members, other heart pacemaker electrode parts, rigid or flexible blood vessel grafts and patches, blood stream fil~ rs, intracardiac patches or baffles and in vascular access tubes.

In the latter case, typically for use in haemodialysis, the inner surface of the tube is porous coated to promote .

; 20 colonization and tissue growth, while the outer surface also may bè porous coated for soft vessel tissue, soft tissue and skln tissue ing,rowth.

In many applications of the present invention, the .

promotiQn of colonization and tissue growth is accompanied by true soft tissue ingrowth into the porous surface at the . . . : :- margins or on the outer surface from adjacent body tissue, to provide bonding between the host and the member. ~ ;

The body tissue ingrowth combined with promotion ~f tissue growth c?n the porous surface from the nucleated blood ~3Q stream cells is important in many applications of the present invention.

For example, in an artificial heart, a ~orous ~69Z52 coating on all the elements provides a means of fixation to host tissues by soft tissue ingrowth and provides surfaces which are blood compatible arising from colonization and tissue formation on the blood-contacting surfaces.

The parameters of the porous surface may vary widely.

The surface must, however, have an interconnected network of pores underneath the surface in fluid flow communication with the surface pores to promDte the colonization by nucleated cells and subsequent differentiation into other cell types so that the tissue which is formed is interlocked in the subsurface network.

- The interstitial surface pore size may vary widely, typically from about 1 micron such as up to about 1000 microns, although it may be preferred to use pore sizes below 20 microns.

- As the pore size decreases, the surface becomes smoother, decreasing blood turbulence and abrasion on movin~ parts of the device.

. . .

The porosity also may vary widely, from about 8% upwards.

Where a coating is provided on a coherent substrate, the thickness may vary from a single layer of particles upwards, ~ ;

- typically from about 1 to about 1000 microns.

Thin layers are prefèrred in devices having close tolerances.

- The devices may be made porous in any desired manner, the manner chosen usually depending on the material of construc- ~ tion.

Where the material is metal, for example, powder metal- lurgy techniques may be used to form a coating consisting of a plurality of small, discrete generally ball-shaped particles of the metal bonded together at their points of contact with each other and the substrate to define the network of connected interstitial pores uniformly distributed throughout the coating.

After formation of theporous ooating, it may be machined ~ ~ and refined, ii.- desired, to improve its surface characteristics. ~;

The formation of the adherent tissue coating from ` ~ 5 - ~ ~ : .: .

~s~zsz nucleated blood cells also alla~s thecardiovascular prosthetic device or implant of the present invention to be incorporated into the cardiovascular system, thereby achieving a more secure attachment than has previously been the case.

The invention is illustrated by the following Example:

Example Conventionally-cast heart ball-valve cages of cobalt base alloy composition were coated with a porous suxface layer.

'0 The surface to be coated was initially roughened by sand blasting and then cleaned and degreased using ultrasonic cleaning methods. A uniform Co-Cr-Mo alloy powder coating of about 500 micron thickness was applied to the surface using an aqueous methylcellulose binder.

After air drying, the coated samples were sintered in a dry hydrogen atmosphere at about 1220C for 2~ hour ~periods to form an integral structure well-bondea to the sub strate surface.

Both coarse and fine powder coated samples were formed, 2Q the coarse powder having a particle size between 50 and 200 ;

microns and the fine powder having a particle size less than 50 microns.

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; Ten of the coated cages, with either fine or coarse powder-made metallic surfaces, were implanted in the right atria ~ in five dogs for periods-of time ranging from two weeks to `~ three months.

Upon removai, each valve was examined grossly .: . : : and by light and scanning electron microscopy.

Ten uncoated ~ . .

control valves were similarly implanted in five additional dogs.

No prophylactic anticoagulants were used in either the experimental or the control group.

, ,:

All porous-coated valve cages developed a thin, smooth firmly-attached tlssue coverlng with no evidence of thrombosis ~:

6925;Z or embolism in the coated region. The process of coating appeared to be well established at two weeks and complete at one month.

On the other hand, the uncoated polished metal valve cages sho~ed no evidence of tissue ingrowth and massive thrombus formation was a consistent findillg.

The present invention/ therefore, provides a novel cardiovascular prosthetic device or implant which avoids prior art thro~bogenic problems. Modifications are possible within the scope of the invention. ~ " ' '~' ~'''',.

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