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The Björk-Shiley aortic prosthesis: flow characteristics, thrombus formation and tissue overgrowth
Insights
Thrombus formation and tissue overgrowth in Björk-Shiley aortic prostheses are linked to abnormal blood flow patterns. These issues, including stagnation and low shear stress, occur in the minor outflow region, potentially causing device complications.
Area of Science:
- Biomedical Engineering
- Cardiovascular Research
- Prosthetic Devices
Background:
- Björk-Shiley mechanical heart valves are used to replace damaged aortic valves.
- Thrombus formation and tissue overgrowth are potential complications following prosthetic valve implantation.
- Understanding the hemodynamic factors contributing to these complications is crucial for improving patient outcomes.
Purpose of the Study:
- To investigate the relationship between the flow characteristics of Björk-Shiley aortic prostheses and observed thrombus formation and tissue overgrowth.
- To identify specific regions within the prosthesis where adverse flow patterns occur.
Main Methods:
- Analysis of nine explanted Björk-Shiley aortic prostheses implanted for six months or longer.
- In vitro velocity measurements using a laser-Doppler anemometer to assess blood flow patterns.
- Calculation of average velocities and peak-shear stresses in different outflow regions.
Main Results:
- Thrombus formation and tissue overgrowth were observed in the explanted prostheses.
- A stagnation zone was identified near the aortic face of the disc.
- The minor outflow region exhibited significantly lower average velocities (25 cm/sec) and peak-shear stresses (150 dynes/cm2) compared to the major outflow region (100 cm/sec and 700 dynes/cm2).
- Tissue overgrowth exacerbated flow restriction in the minor outflow region.
Conclusions:
- The study attributes thrombus formation and tissue overgrowth to the identified stagnation zone and low shear stress in the minor outflow region of the Björk-Shiley aortic prosthesis.
- These hemodynamic factors are likely contributors to the pathological findings observed in explanted devices.
- Findings suggest that prosthesis design influencing flow dynamics may play a role in complication development.
Abstract:
Thrombus formation and tissue overgrowth were observed in nine Björk-Shiley aortic prostheses recovered six months or longer after implantation. These pathologic findings may be attributed to the flow characteristics of the prosthesis. The open disc of the valve separates the flow into two unequal regions. Varying degrees of thrombus formation were observed in the minor outflow region, including the depression in the aortic face of the disc and the metal strut bridging this area. Tissue overgrowth was noted along the perimeter of the prosthesis adjacent to the minor outflow region. That overgrowth further reduced the available cross section for flow in this already constrained area. In vitro velocity measurements with a laser-Doppler anemometer identified a zone of stagnation about 20 mm wide near the aortic face of the disc. The average velocities in the major and minor outflow regions were around 100 and 25 cm/sec, respectively, and the corresponding peak-shear stresses were approximately 700 and 150 dynes/cm2. There is reason, then, to attribute the thrombus formation and tissue overgrowth to the stagnation zone and the low shear in the minor outflow region.