Biological aortic heart valve calcification

Calcification is the key factor role in the failure of bioprosthetic 

and other tissue heart valve substitutes

David Reineke MD

Thierry Carrel MD

Department of Cardiovascular Surgery, University Hospital Berne (Inselspital), Berne, Switzerland

Heart valve substitutes are of two principle types: mechanical valves of non-biologic material or tissue valves, which are constructed of either human or animal tissue.1,2 Since the early 1960s, tissue valves have been used in the form of aortic valves harvested from fresh human cadavers and transplanted to other individuals (homografts).

Chemically preserved stent-mounted tissue valves  (bioprostheses) were introduced and implanted a decade later. Today, glutaraldehyde preserved bovine pericardial bioprosthetic heart valves and porcine aortic valves are used widely and also stentless valves were introduced. Of approximately 300,000 biologic heart valves implanted worldwide each year half are mechanical and half are tissue with a clear shift towards biological prostheses in developed countries.

The frequency and nature of specific valve-related complications depends on the prosthesis type, model, site of implantation and characteristics of the patient.

Mechanical valves can cause systemic thromboemboli and thrombotic occlusion. In addition, the chronic anticoagulation therapy required in all patients with mechanical valves potentiates haemorrhagic complications. Contemporary mechanical valves are, on the other hand, extremely durable. 

In contrast, biological valves have a very low rate of thromboembolism without the need for anticoagulation owing to a so-called central pattern of flow similar to that of human heart valves and cusps. The attractiveness of biological valves is undermined by a high rate of valve failure with structural dysfunction due to progressive tissue deterioration.2,3

Structural valve degeneration is the major cause of failure in bioprosthetic heart valves. The principle underlying process is cuspal degeneration. This either leads to secondary tears followed by regurgitation, or stenosis of the valve owing to the stiffening of the cusps. Calcification is accelerated in younger patients, with children and adolescents showing signs of degeneration only after a few years. Older patients have a markedly lower rate of degeneration. Apart from the calcification progressive collagen deterioration is also an important contributor to limited durability of biological prostheses.4,5

Todays pericardial heart valves have little tendency of thrombosis and no anticoagulation is required due to their physiological characteristics for patients from the age of 65 years onwards. Their increasing longevity in the range of 15–20 years makes reoperations due to structural valve degeneration unlikely. Nonetheless unexpected early structural valve degeneration is an issue of concern, which then necessitates early re-intervention.6

Interventional or minimally invasive valve implantation techniques can at this time only be performed with pericardial heart valves. Owing to their flexibility they can be introduced through transfemoral and transapical access points. As these techniques are mainly applied in patients of old age and certain morbidity with a reduced life expectancy, early degeneration of biological prostheses does not play such an important role. With increasing technical advances in this field, the age of patients receiving these devices will surely go down, making the structural valve degeneration in these patients an issue.7

The fixation and anticalcification processes

In general, valves undergo two major processes to increase durability in terms of structural degeneration due to calcification and collagen deterioration.

Tissue fixation process 

The first generation of bioprostheses in the 1960s were fixed in formalin. It was soon noticed that this led to unstable collagen crosslinks and collagen degeneration. In the late 1960s formalin fixation was replaced by a far more stable glutaraldehyde cross-linking technique. While in the 1970s the use of glutaraldehyde in a concentration of 0.2–0.5% proved to be inadequate in eliminating certain mycobacteria, the use of sodium metaperiodate glutaraldehyde in the higher concentration of 0.652% eliminated all bacteria but disrupted the collagen structure.

Today, the fixation of choice is the use of glutaraldehyde at a concentration of 0.625% in low-pressure setting. This uniformly inhibits all mycobacteria and preserves collagen structure. In general after being decelluralised and fixated no immunologic reactions due to remnant animal cells should occur. A common problem with glutaraldehyde treatment of tissue is the residual aldehydes, free acids and so-called Schiff bases, which serve as possible binding sites for immunogenic agents, phosphates and calcium. The attempt to reduce this bioburden with agents such as proteins, amino acids or amines is under constant research. But these agents are never permanent and tend to leave the tissue over time.8

Anticalcification process

The fixation process leaves several binding sites for calcium in the form of unstable glutaraldehyde formations and phospholipids, which will lead to early and severe calcification of pericardial heart valves if left untreated. Anticalcification treatment therefore aims to extract phospholipids and residual glutaraldehyde components.

Each company treating pericardial heart valves adheres to the above named principles. We would like to outline their strategies and present different approaches.

Edwards Lifescience

The pioneer in the invention and commercialisation of biological heart valves has relied on an anticalcification technology called the Therma Fix Process (TFX), which uses buffered glutaraldehyde and formaldehyde-Tween 80 solution (FET) for tissue fixation, sterilisation and bioburden reduction. This technique is able to effectively remove unstable glutaraldehyde residuals through a combination of both thermal and chemicals agents. TFX, mainly applied to bovine pericardial valves has demonstrated excellent clinical safety and durability. 

In order to enhance the TFX technology for increased valve durability, an innovative anticalcification technology was created to irreversibly cap the aldehyde groups through reactive amination. The technology platform GLX introduces the additional step of capping, reducing and glycerolisation and thereby eliminating sites that would be exposed as possible binding sites for calcium, phosphates and other immunogenic agents. The GLX process also allows for non-liquid storage, inhibiting oxidation and thus prolonging the lifetime of the tissue. In animal studies, GLX-processed tissue demonstrated a 93% reduction in calcium content compared to TFX-processed pericardial tissue at 35 days.

St Jude Medical

Several competitive valves utilise an ethanol-based anticalcification treatment. St Jude Medical utilises Linx AC technology. Clinical studies have shown that ethanol treatments are most effective at reducing calcium formation in animals at concentrations above 50%.9 The patented Linx AC technology utilises such concentrations. The treatment also works on the tissue collagen structure and is effective on both porcine and bovine tissue. Linx AC technology is a valve treatment that has demonstrated to resist calcification in four ways in animal studies: 

1. Reduction of free aldehydes.

– Free aldehydes have been observed to promote calcification in glutaraldehyde fixed tissue.

2. Extraction of lipids.

– Lipids are calcium-binding sites.

3. Minimising uptake of cholesterol.

– Cholesterol provides calcium- binding sites.

4. Stabilising leaflet collagen.

– Stabilisation may prevent calcification of the collagen.

Medtronic

Medtronic utilises a unique tissue preservation process called Physiologic Fixation. Zero-pressure fixation of the cusps with glutaraldehyde preserves the leaflet structure and provides leaflets that function in a similar manner to fresh aortic valves. The AOA treatment process to mitigate tissue calcification follows this. 2-α-amino-oleic acid (AOA, Biomedical Design, Inc, Atlanta, GA) bonds covalently to bioprosthetic tissue through an amino linkage to residual aldehyde functions and inhibits calcium flux through bioprosthetic cusps. The AOA is effective in mitigating cusp calcification in rat subdermal and cardiovascular implants. This compound is used in FDA-approved bioprosthetic valves.10,11

Sorin

Sorin valves are, as the majority of tissue valves on the market, treated with glutaraldehyde (GA), as GA has been proven to provide the tissue with stability, mechanical durability and reduction of immunogenicity. After being assembled, their valves are fixed by means of a fluidic system with GA. The essential features of the Sorin fixation system are the following: (1) in-depth GA diffusion: This slow process allows GA to enter the collagen bundles of the tissue, in order to treat them homogeneously and gently. A shorter and more abrupt process could damage the collagen fibres. 

(2) The fluidic system works at a low pressure, in order to preserve the structural characteristics of the tissue and preserve collagen bundles.

To reduce the risk of tissue calcification, Sorin Group has two different proprietary tissue treatment processes, separately applied to two different families of products after GA fixation. 

Sorin’s patented homocysteic acid treatment (HAT) neutralises unsaturated aldehyde groups, which may enhance collagen affinity to calcium binding and reduce the endothelisation, using homocysteic acid, a natural aminoacid, which is able to bind free aldehyde groups. Aldehyde neutralisation on fixed pericardium is widely recognised to decrease propensity to mineralisation and thus calcification.12–14 

Sorin’s patented phospholipid reduction treatment (PRT) addresses phospholipids that are present in the pericardial tissue and that may facilitate calcification, acting as potential binding sites for circulating calcium ions. The PRT process proved to decrease phospholipid content in pericardial tissue using octanediol and ethanol solution, leading to a 97% reduction of calcium uptake compared to control tissue in animal studies at 60 days.15–17

Outlook

As described above, calcification of stented bioprosthetic heart valves is probably the key process limiting the expected durability and use of tissue valves. The pathophysiology has been largely understood through investigation using animal models. Several strategies based on altering biomaterials or local drug administrations appear to be promising but no clinically useful approach is available yet. 

Antimineralisation strategies in tissue heart valve substitutes

Systemic drug administration

Localised drug delivery

Substrate modification

• Inhibitors of calcium phosphate mineral formation

– Biphosphonates 

– Trivalent metal ions 

– Amino-oleic acid

• Removal/modification of calcifiable material

– Surfactans 

– Ethanol 

– Decellularisation

• Improvement/modification of GA fixation

– Fixation in high concentrations of GA 

– Reduction reactivity of residual chemical groups 

– Modification of tissue charge 

– Incorporation of polymers

• Use of tissue fixatives other than GA

– Epoxy compounds 

– Carbodiimides 

– Acyl azides

As some of the anticalcification techniques have been used in clinical valves for a decade, studies on 15–20 year outcomes will surely require another decade for representative results. But already to today it can be stated with quite some confidence that the efforts undertaken in developing sufficient anticalcification treatments have led to an impressive increase of durability in stented biological valves.

References

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