Beall Disk Heart Valve Prosthesis and the Durability of Pyrolytic Carbon

The Beall heart valve prosthesis, introduced in 1967, was a low-profile disk valve composed of a disk fabricated from extruded Teflon and metal struts coated with Teflon. This design and material were intended to minimize thromboresistance. Following realization of poor wear properties of the disk, with sequelae of disk abrasion including hemolysis and abnormal disk motion67 (Fig. 6), its composition was changed to a denser compression-molded Teflon. Nevertheless, this prosthetic design continued to exhibit wear-related problems concentrated at the struts,62 and a new model of the valve with disk and struts fabricated from pyrolytic carbon was introduced in the early 1970s.

Presently, as a result of favorable mechanical and biological properties of pyrolytic carbon, virtually all mechanical heart valve prostheses in use have pyrolytic carbon occluders and some have both carbon occluders and carbon cage components.63 Pyrolytic carbon is strong, resistant to fatigue and abrasive wear, thromboresistant, and can be fabricated into a wide variety of shapes.

Fig. 6. Severe abrasive wear of disk poppet of Beall Teflon caged-disk mitral valve prosthesis.

In the early 1980s, clinical experience suggested that pyrolytic carbon had contributed to a major advancement in the durability of prosthetic heart valves; however, this had not been verified by direct valve observation. To confirm the anticipated favorable wear resistance of pyrolytic carbon in the clinical environment, we recovered at necropsy or surgery eight carbon-containing mechanical valve prostheses after long-term implantation and analyzed them by surface scanning electron microscopy and surface profilometry.64 None of the prostheses had clinical or gross pathological malfunction or abrasive wear, but minimal strut wear was demonstrated by scanning electron microscopy. No appreciable wear on carbon valve occluders was demonstrated by analytical surface profilometry. Our study suggested that the use of pyrolytic carbon as an occluder and as a strut material for mechanical heart valve prostheses has minimized progressive abrasive wear as a long-term complication of cardiac valvular replacement. Favorable durability of pyrolytic carbon has subsequently been well documented.65

Analysis of implanted medical devices has traditionally concentrated primarily on those devices that failed in service and paid little attention to those

Fig. 7. Cryopreserved/thawed allograft heart valve. Removed from a patient who died, this aortic valve is ready for implantation in another as an aortic valve replacement (courtesy of CryoLife, Inc., Marietta, GA).

serving the patient until death or removal owing to unrelated causes. The study described above emphasizes that detailed examination of functional (not failed) prostheses recovered from patients following extended duration after implantation may yield worthwhile data to answer a focused question.

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