Biomedical Engineering Reference
In-Depth Information
Chapter 12
Heart Valve Substitutes
12.1
Introduction: Valve Explants
On 19/5/2005 a male patient, aged 44, underwent surgery for explanting his mitral
bioprosthetic heart valve, which was malfunctioning, having become stenotic. The
valve had remained in the patient's heart for 63 months. It was made out of bovine
pericardium sheet, i.e., the membranous structure that envelopes the bovine heart,
in the form of three cusps sewn on a low profile stent, produced from a polyacetal
resin, while the sewing ring was coated with a carbofilm. Examination of the explant
revealed ingrowth of vascularized tissue on the pericardial leaflets, as well as calci-
fied areas in the leaflets. Furthermore, fibrotic tissue was present on the stent and the
sewing ring. These alterations resulted in compromising the initial ideal matching of
the cusps and the failure of the prosthesis. To state it in an other way: the failure of
this prosthetic device was not due to material's failure but, rather, to harmful changes
in the structure/architecture of the device, probably invoked by the interaction of the
materials that the device was made of with the biological millieu of the patient, i.e.,
blood and surrounding cardiovascular tissues. Figure
12.1
shows a similar valve
before implantation and after explantation. The failed explanted valve showed pres-
ence of vascularized tissue and calcified areas on pericardium leaflets. Abundant
growth of fibrotic tissue on stent and sewing ring. Not perfect cusps matching.
On 20th December 2005, a female patient, aged 74, had her mitral monostrut
tilting disc prosthesis explanted, after it being in place for 18 years. The clinical evi-
dence necessitating the removal of this mechanical heart valve was severe stenosis.
The materials used for this valve were graphite coated with pyrolytic carbon (for the
disc), Haynes 25 (for the housing) and Teflon (for the sewing ring).
1
All the materials of this explant were very well preserved, no macroscopic traces
of wear were present. However, the device could not function properly because pan-
nus had grown over the sewing ring on the outflow side of the valve, had extended
onto the valve housing, therefore causing stenosis and hindrance of the proper
movement of the tilting disc (see Fig.
12.2
).
2
1
Haynes 25 is a Cobalt-based alloy, which includes Chromium, Tungsten and Nickel.
2
Pannus, a term used to describe a flap of fibrous tissue, sometimes calcified, and vascularized.
