Developments in cardiovascular valve technology - Biomaterials and Devices for the Circulatory System - page 108

deposit a biofi lm. 92 The use of valves containing the new pure pyrolytic
carbon, e.g. the On-X valve, may ameliorate biofi lm deposition.
A novel approach to dampen bacterial growth in biomaterials is by appli-
cation of silver, which inhibits bacterial growth and biofi lm deposition. 93
Silzone, a silver-impregnated coating manufactured by St. Jude Medical and
applied to the sewing ring, represents an application of this technology to
reduce PVE. The Silzone coating is applied by an ion-beam-assisted deposi-
tion process, which results in a layer of metallic silver bonded to each strand
of the fabric. The Silzone coating polyester was investigated in vivo in a
guinea pig direct-contamination model. The investigators found that the
Silzone coating reduced infl ammation and impeded biofi lm deposition by
S. epidermidis . 94-96 Shortly after use of Silzone-coated valves became wide-
spread, reports of valve-related complications began to surface. There
appeared to be an increase in thrombogenicity, as well as paravalvular leaks.
One report of 40 consecutive implants revealed the following incidences:
hospital mortality 7/40 (17.5%), early PVE 2/40 (5%), relevant paravalvular
leak 2/40 (5%), and minor paravalvular leak rate 5/40 (12.5%). 97 A plausible
explanation for the increased incidence of paravalvular leak may be sup-
pression by silver of normal tissue ingrowth and epithelialization. 98 A larger
multi-centered study comparing Silzone against non-Silzone-coated valves
was undertaken to determine the prevalence and severity of paravalvular
leak. The Artifi cial Valve Endocarditis Reduction Trial (AVERT) echocar-
diography study of 678 patients revealed that paravalvular leaks occurred
in 10% (29 of 285) of the Silzone-coated valves, versus 7.5% (21 of 290,
p NS) of the non-coated valves. 99 The differences between the Silzone-

coated prosthetic valves vs. the conventional devices were not statistically
signifi cant.

=

4.6 Future trends

￿ ￿ ￿ ￿ ￿

The ultimate goal in new valve designs is to replicate as closely as possible
the normal anatomical and physiological properties of a natural heart
valve. Although its long-term durability, non-thrombogenicity, and hemo-
dynamic effi ciency make pyrolytic carbon one of the best available bioma-
terials, there is still a clinical dependence on anticoagulation. 100 The need
for newer valve designs and materials persists. Owing to regulatory con-
straints, the potential for litigation, and skepticism of new ideas among
many surgeons, new valve designs are costly to develop and slow to bring
to clinical practice. 101 Newer biomaterials are being investigated and
developed, including advanced pyrolytic carbon, ceramics and biocompat-
ible plastics. 102 The possibility of a ceramic-based heart valve, which could
be durable and thromboresistant, is under active investigation. 103 Identify-
ing interfacial reactions between clotting plasma proteins and the solid