Soft Tissue Replacements - Biomaterials - page 192

blood-interfacing implants and significantly improve the quality of life for patients with implants. This
chapter discusses the development of biomaterials for blood-interfacing implants as well as problems
associated with and future directions in the development of such implants.

9.1.2 Heart Valve Prostheses
Attempts at replacing diseased natural human valves with prostheses began about four decades ago. The
details of the development of heart valve prostheses, design considerations, in vitro functional testing,
and durability testing of valve prototypes can be found in several monographs (Shim and Lenker, 1988;
Chandran, 1992). The heart valve prostheses can be broadly classified into mechanical prostheses (made
of nonbiological material) and bioprostheses (made of biological tissue). Currently available mechanical
and tissue heart valve prostheses in the United States are listed in Table 9.1.

9.1.2.1 Mechanical Heart Valves
Lefrak and Starr (1970) describe the early history of mechanical valve development. The initial designs
of mechanical valves were of centrally occluding caged ball or caged disk type. The Starr-Edwards caged
ball prostheses, commercially available at present, were successfully implanted in the mitral position in
1961. The caged ball prosthesis is made of a polished Co-Cr alloy (Stellite 21 ® ) cage and a silicone rub-
ber ball (Silastic ® ), which contains 2 wt% barium sulfate for radiopacity (Figure 9.1). The valve sewing
rings use a silicone rubber insert under a knitted composite polytetrafluoroethylene (PTFE- Te l o n ® ) and
polypropylene cloth. Even though these valves have proven to be durable, the centrally occluding design
of the valve results in a larger pressure drop in flow across the valve and higher turbulent stresses distal
to the valve compared to other designs of mechanical valve prostheses (Yoganathan et al., 1979a,b, 1986;
Chandran et al., 1983). The relatively large profile design of caged ball or disk construction also increases
the possibility of interference with anatomical structures after implantation. The tilting disk valves , with
improved hemodynamic characteristics, were introduced in the late 1960s. The initial design consisted
of a polyacetal ( Delrin ® ) disk with a Teflon sewing ring. Delrin acetal resins are thermoplastic polymers
manufactured by the polymerization of formaldehyde (Shim and Lenker, 1988). Even though Delrin exhib-
ited excellent wear resistance and mechanical strength with satisfactory performance after more than 20

TABLE 9.1 Heart Valve Prostheses Developed and Currently Available in the United States

Type Name Manufacturer

Caged ball Starr-Edwards Baxter Health Care, Irvine, CA

Tilting disk Medtronic-Hall Medtronic Blood Systems, Minneapolis, MN

Lillehei-Kaster Medical, Inc., Inner Grove Heights, MN

Omni-Science

Bileaflet St. Jude Medical St. Jude Medical, Inc., St. Paul, MN

Carbomedics Carbomedics, Austin, TX

ATS Valve ATS Medical, St. Paul, MN

On-X Valve Medical Carbon Research Inst., Austin, TX

Porcine bioprostheses Carpentier-Edwards Edwards Lifesciences, Irvine, CA

(CE) porcine
CE SAV
Hancock II

Medtronic, Santa Ana, CA

Hancock-modified orifice
Freestyle
Biocor
Mitroflow

St. Jude Medical, Inc., St. Paul, MN
CarboMedics, Inc., Austin, TX

Pericardial bioprostheses Carpentier-Edwards

CE PERIMOUNT

Edwards Lifesciences, Irvine, CA