Uneven MitraClip Application Does Not Increase Leaflet Stress in a Finite Element Model

To the Editor

The MitraClip (Abbott Laboratories) is a percutaneous device for mitral valve (MV) repair, which connects the anterior and posterior leaflets of the mitral valve to eliminate mitral regurgitation (MR). Durability of MitraClip repair is an issue, with a 44% recurrence rate of 3–4+ MR within two years of clip application (1). Uneven MitraClip application is a common procedural occurrence, resulting in more posterior than anterior leaflet tissue grasped by the clip (2). To determine whether uneven clipping plays a role in MR recurrence, we investigated the effect of MitraClip application on leaflet stress, which is implicated in repair failure: Leaflets excised after failed MitraClip show significant tissue damage, including large perforations; (3) Furthermore, in-vitro studies have shown that increased leaflet stress alters collagen and proteoglycan synthesis, resulting in leaflet thickening and increased compliance. (4, 5) Leaflet stress resulting from uneven clip application has not been previously studied. To determine whether the common occurrence of asymmetric MitraClip application contributes to increased stress and therefore to repair failure, we simulated a variety of MitraClip grasps, testing the hypothesis that asymmetric grasp does not increase leaflet stress.

Methods

A finite element (FE) model was created and described previously, based on imaging of a single human patient with isolated posterior MV prolapse (6). Using this model and the FE modeling software LS-Dyna (Livermore Software Technology Corporation, Livermore CA) we applied a simulated MitraClip consisting of virtual sutures, connecting the anterior and posterior mitral valve leaflets. Three leaflet grasps were tested: No clip; 4.5 mm posterior leaflet, 4.5 mm anterior leaflet; 4.5 mm posterior leaflet, 1.3 mm anterior leaflet; 4.5 mm posterior leaflet, edge of anterior leaflet. Stress was recorded for the anterior and posterior leaflets as the average over the entire leaflet at end systole, over a range of systolic blood pressures from 100–200 mmHg. Leaflet coaptation length at end-systole was measured for each clip type to ensure that MR was completely eliminated.

Results

Clip application resulted in stable or decreased leaflet stress, relative to the baseline model with no clip. End-systolic leaflet stress increased linearly with increasing systolic blood pressure for all clip configurations. [Figure 1] Anterior leaflet stress was higher than posterior leaflet stress for all configurations. Grasp of less anterior leaflet tissue resulted in lower overall leaflet stress. Stress for all clip configurations was concentrated at the area of the clip. [Figure 2]

Figure 1.

Average anterior and posterior leaflet stress at end systole, vs systolic blood pressure.

Figure 2.

End-systolic leaflet stress in hectoPascals by type of clip application.

Discussion

In our single patient model, with a prolapsing posterior leaflet and a normal anterior leaflet, uneven MitraClip application resulted in lower anterior and posterior leaflet stress over all blood pressures. All clip configurations resulted in stable or decreased stress relative to the model with leaflet prolapse and no clip. Because all clip applications restored leaflet coaptation, or the normal close contact of the anterior and posterior leaflets during systole, stress in the clip models is shared across a wider surface and is therefore lower than stress in the baseline regurgitant model. Although this experiment is only representative of one particular pattern of MV disease, this pattern is the most common cause of degenerative MR in the developed world. Based on this single patient model, the common procedural occurrence of uneven MitraClip application does not result in increased leaflet stress and therefore should not increase the incidence of recurrent MR. Further studies are indicated to examine this phenomenon over a range of MV pathologies, including functional MR. As novel percutaneous MV therapies emerge, finite element modeling as used here can be a powerful tool to examine their effect on the tissues of the heart.