Mechanical mitral prostheses are frequently employed due to their long-term durability. Despite advancements in design, a dysfunction rate of 10–30% is still observed after ten years. One primary complication is the growth of fibrous tissue known as pannus, which is a consequence of chronic inflammation and, in some cases, can cause prosthesis dysfunction by restricting disc mobility. The exact pathophysiology of pannus is not fully understood, although it is more common in younger patients, regions with turbulent or high-shear flow, patients with double valve replacements, and smaller prosthetic rings. Diagnosing pannus-related prosthetic dysfunction in mechanical mitral valves is challenging. Clinical presentation is insidious, and it can take up to two decades post-surgery for overt dysfunction to emerge.

The aim of this study was to investigate pannus formation in bileaflet mechanical mitral prostheses. It is a single-center study where all transesophageal echocardiograms from patients with mechanical mitral prostheses performed between May 2017 and April 2021 were retrospectively reviewed. Patients with less than one year since prosthesis implantation and those with inadequate anticoagulation were excluded. The 141 patients with bileaflet mechanical mitral prostheses were divided into two groups: those with anatomical disc orientation (parallel to the native leaflet axis) and those with non-anatomical orientation (perpendicular or oblique to the native leaflet axis). Forty-six patients had anatomical orientation, and 95 had non-anatomical orientation. Since there are no guidelines for diagnosing pannus via echocardiography, Barbetseas et al.’s recommendations, which assess a combination of echogenicity and the echotexture of fibrous tissue on ultrasound, were utilized.

Out of 141 patients, pannus was identified in 26 patients with anatomically oriented prostheses (56.5%) and in 71 patients with non-anatomical orientation (74.7%) (p = .03). Patients with pannus generally had a longer postoperative duration (13.4 years vs. 6.8 years; p < .01). Of the 97 patients diagnosed with pannus, 11 required reoperation due to symptomatic obstruction. Among these, 3 had anatomical orientation, and 8 had non-anatomical orientation. The most common cause of dysfunction in these patients was pannus growth in the disc hinges. Multivariable analysis showed that anatomical orientation acted as a protective factor against pannus development (OR = 0.39; p = .04), while mitral-aortic prosthesis increased the risk (OR = 2.73; p = .04).

The authors concluded that anatomical orientation of mechanical mitral prosthesis discs reduces the incidence of pannus overgrowth.

COMMENTARY:

Traditionally, selecting a non-anatomical orientation for mechanical mitral prostheses has been preferred to avoid interference with the preserved subvalvular apparatus, either partially or completely. However, this study conveys a significant message: non-anatomical orientation of discs may increase the long-term incidence of pannus. Two mechanisms could explain why patients in the non-anatomical orientation group might be more prone to pannus formation. First, turbulent flow and shear stress across the discs may activate platelet function and promote pannus formation. In fact, several studies show that flow across anatomically oriented bileaflet mitral prostheses is less turbulent than in non-anatomical settings, as anatomical orientation generally allows for greater and more stable disc opening angles. Second, all patients in this study were diagnosed with rheumatic valve disease, which is itself a risk factor for pannus development. Preserving the posterior leaflet to prevent atrioventricular groove rupture places the prosthesis, especially the disc pivots, in contact with rheumatic tissue when oriented non-anatomically.

Therefore, does this justify implanting mechanical mitral prostheses in anatomical orientation? The data must be considered with perspective: out of 141 patients, two-thirds were diagnosed with pannus growth, but only 11 required surgery. It should also be noted that all patients in this study had rheumatic valve disease, which may not be representative of our clinical environment. Moreover, the study has significant limitations that prevent extrapolation to our clinical practice. Information on the brand and size of the prosthesis was not available for over half of the patients. Additionally, there were no other imaging studies to corroborate pannus diagnosis beyond echography, which is operator-dependent. Lastly, surgical reports were unavailable for most patients, leaving uncertainties regarding implantation techniques (intra- or supra-annular) and the degree of subvalvular apparatus preservation.

In conclusion, today’s article does not provide enough evidence to change our approach to implanting mechanical mitral prostheses. However, it prompts reflection while we await larger studies with more diverse patient diagnoses and more specific imaging studies. Until then, the best orientation is the one that ensures optimal disc opening and closure.

REFERENCE:

Duman ZM, Apaydın Z, Can İ, Kaplan MC, Buğra AK, et al. Impact of Bileaflet Mechanical Mitral Valve Orientation on Pannus Overgrowth. Innovations (Phila). 2023 Sep-Oct;18(5):466-471. doi: 10.1177/15569845231199100.