Allograft Valves and Other Replacement Heart Valves

The first clinical implantation of a mechanical replacement heart valve was performed in 1958 using a ball-in-cage valve inserted in the descending thoracic aorta.6 The first orthotopic valve replacements were accomplished in 1960 (aortic position)7 and 1961 (mitral position).8 Mechanical heart valve designs continued to evolve, with the current prosthetic designs consisting of ball-in-cage, tilting disk and bileaflet valves. The occurrence of thromboembolic events and of sudden, life-threatening modes of structural failure9,10 have continued to stimulate interest in the use of biologic (allografts and autologous tissues) and bioprosthetic (glu-taraldehyde-crosslinked porcine aortic valve and bovine pericardial tissues) heart valves.

Bioprosthetic and biologic heart valves became the prosthetic valves of choice during the 1970s, since they did not require long-term anticoagulation and demonstrated less restriction of central flow through the valve orifice. However, as long-term clinical experience increased, the durability of biologic and bio-prosthetic valves became a limiting factor, principally because of primary tissue failure (e.g., tissue abrasion, cuspal dehiscence and calcification) and cuspal thickening (e.g., fibrous sheath formation) resulting in significant alterations in hemodynamic performance (i.e., stenosis; regurgitation). Leaflet tears and dehiscence have also occurred as a consequence of the technique used to attach pericardial and porcine valve tissue and allograft valves to polymeric and metallic stents (Figures 21.1 and

21.2).11-16 The incidence of tissue failure secondary to cuspal calcification (Figures 21.1 and

21.3) was shown to be particularly high in porcine aortic valve bioprostheses implanted in children and adolescents.1718 Currently, allo-graft valves are used extensively for the reconstruction of the right ventricular outflow tract in children.19,20 The clinical modes of failure of biologic and bioprosthetic valves are typically more gradual and less catastrophic than those

Autograft Valve
Figure 21.1. Gross photographs depicting biopros-thetic valve primary tissue failure. (A) Inflow surface of a porcine aortic valve showing cuspal dehiscence

(arrow) and calcification (arrowhead). (B) Ionescu-Shiley bovine pericardial valve demonstrating leaflet tears located adjacent to the commissural stent posts.

of mechanical valves. However, the rate of reoperation after implantation of bioprosthetic valves has been reported to be approximately 40% following 8 to 10 years of use.21 The current usage of cardiac replacement valves world wide is estimated to be: approximately 70% mechanical valves (predominantly the St. Jude bileaflet valve); 28% bioprostheses (predominantly porcine aortic valves); and 2% cryopre-served allograft heart valves.22

Jude Mechanical Aortic Valve
Figure 21.2. Scanning electron micrograph of the commissural region of an explanted Ionescu-Shiley bovine pericardial valve. A leaflet tear resulting from the use of an alignment suture to ensure leaflet coaptation is shown. X 15.
Cheddar Micrograph
Figure 21.3. Transmission electron micrographs entire collagen fibrils (B).Uranyl acetate/lead citrate depicting intrinsic leaflet calcification localized to the stain. A, X 14,000; B, X 10,000. surface of the collagen fibrils (A) and involving

The development of replacement heart valves continues to progress with the use of a variety of biomaterials and tissues (e.g., pyrolytic carbon, polyurethane, parietal pericardium, dura mater, and aortic, mitral and pulmonary valvular tissues)23-26 as well as with modifications in tissue processing (e.g., zero-pressure tissue fixation, anti-calcification treatments, non-aldehyde cross-linking agents),27-37 the development of stentless bioprostheses,3637 and the application of tissue engineering concepts.38,39 However, with the exception of the use of cryopreserved allograft valves and pulmonary autografts (i.e., Ross procedure), which have demonstrated a modest increase in long-term durability,40-43 the clinical efficacy and actuarial freedom from primary tissue failure of the next generation of stentless bioprosthetic and autologous tissue valves remains to be demonstrated.

There has been a renewed interest in the use of allograft and heterograft mitral valves for the replacement of atrioventricular valves.24,44-46 However, despite optimistic initial reports, the demonstration of long-term mitral valve allograft function remains to be estab-lished.46 Preclinical studies of mitral valve allo-grafts used as mitral valve replacements in juvenile sheep (surviving 12 to 24 weeks) have demonstrated two distinct mechanisms of allo-graft failure.24 Characteristic morphologic changes occurred in the allografts depending on whether the mitral valves where glutaralde-hyde-treated or stored in a cold antibiotic solution before implantation. Marked calcification and chordal rupture (secondary to calcific deposits) were observed in the glutaraldehyde-treated allografts, while leaflet perforations and ruptured chordae due to connective tissue deterioration were noted in the antibiotic stored valves (Figure 21.3). Cryopreserved mitral valve allografts have been used as either partial or total mitral valve replacements in conjunction with the use of an annuloplasty ring.46 Clinical findings following fourteen months of implantation are encouraging with continued retention of valve function; however, long-term studies will be required to establish the effectiveness of this reconstructive mitral valve technique.

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