Developments in cardiovascular valve technology - Biomaterials and Devices for the Circulatory System - page 104

of biological valve substitutes, the major problem of durability persists. With
the advances in technology and material science, artifi cial mechanical valves
are the most durable at present. Among the prosthetic devices the pyrolytic
composition valves have the best hemodynamics and durability. The fl ow
across the leafl ets of these valves is nearly laminar, without stagnation
points.
Design changes over the years along with material improvements have
reduced hemolysis, thromboembolic complications, and catastrophic valve
failures. The transition from rubber-based poppets to pyrolytic carbon and
the change from caged-ball to tilting discs have improved hemodynamics
and provide central or near-central laminar fl ow with much less trans-
valvular pressure drop and shear stress. Catastrophic occluder fracture and
weld fractures were eliminated with the development of machined valve
housings. Recently the St. Jude medical valve was redesigned to increase
the effective valve orifi ce area at a given tissue diameter. This improvement
was achieved by modifying the thickness of the sewing ring and its attach-
ment to the valve housing. This modifi cation has resulted in the avoidance
of a prosthesis mismatch in patients with a narrow aortic root. Despite these
improvements in design and materials, two other challenges are not yet
completely resolved. One is the development of prosthetic leaks requiring
re-operation, which carries inherent risks. The second is bleeding or throm-
boembolic complications from anticoagulants.
The original mechanical caged-ball prostheses are no longer in use. The
Starr-Edwards ball valve (Fig. 4.3), the earliest valve model with the 'cured'
silastic ball, was available for more than 45 years. The Starr-Edwards valve
was characterized by its durability and the low incidence of valve-related
events. However, the ball produced unacceptably high pressure gradients,
particularly during exercise. It was not unusual to see a systolic gradient of
25-30 mmHg across the aortic prosthesis and 18 mmHg across the mitral
prosthesis. Initially there was a high incidence of thromboembolic events
which prompted development of the cloth-cover models that eventually
failed because of cloth wear and tear. The inadequacies of the cloth-cover
models forced a return to the non-cloth-cover ball-valves.
The Björk-Shiley prosthesis was a low profi le valve, with a Stellite
housing and a Tefl on sewing ring. Two struts welded to the housing ring
retained a free-fl oating Delrin disc that opened at a 60° angle. The Delrin
disc had a tendency to expand, which interfered with its free motion. 76 In
1971 a pyrolytic carbon disc was introduced.
The Björk-Shiley valve had several reports of strut fractures (Fig. 4.6).
Careful examination of the specimens revealed the cause to be a defect in
the weld points of the outlet struts in the larger 60° convexo-concave mitral
prosthesis. 77 The manufacturing technique of the entire housing was rede-
signed, the welding points of the struts were eliminated, and the entire

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