How Local Annular Force and Collagen Density Govern Mitral Annuloplasty Ring Dehiscence Risk

Abstract

Background Annuloplasty ring dehiscence is a well described mechanism of mitral valve repair failure. Defining the mechanisms underlying dehiscence may facilitate its prevention. Methods Factors governing suture dehiscence were examined using an ovine model. Following undersized ring annuloplasty in live animals (N=5), Cyclic Force (FC) acting on sutures during cardiac contraction were measured using custom transducers. FC was measured at 10 suture positions, throughout cardiac cycles with peak left ventricular pressure (LVPmax) of 100, 125, and 150mmHg. Suture pullout testing was conducted on explanted mitral annuli (N=12) to determine suture holding strength at each position. Finally, relative collagen density differences at suture sites around the annulus were assessed by two-photon excitation fluoroscopy. Results Anterior FC exceeded posterior at each LVPmax (e.g. 2.8±1.3 vs. 1.8±1.2N at LVPmax=125mmHg, p<0.01). Anterior holding strength exceeded posterior (6.4±3.6 vs. 3.9±1.6N, p<0.0001). Based on FC at LVPmax=150mmHg, margin of safety before suture pullout was vastly higher between the trigones (exclusive) versus elsewhere (4.8±0.9 versus 1.9±0.5N, p<0.001). Margin of safety exhibited strong correlation to collagen density (R2=0.947). Conclusions Despite lower cyclic loading on posterior sutures, the weaker posterior mitral annular tissue creates higher risk of dehiscence, apparently due to reduced collagen content. Sutures placed atop the trigones are less secure than predicted, due to a combination of reduced collagen and higher overall rigidity in this region. These findings highlight the inter-trigonal tissue as the superior anchor, and have implications on the design and implantation techniques for next-generation mitral prostheses.

Mitral annuloplasty ring suture forces: Impact of surgeon, ring, and use conditions

“…By using a live ovine model coupled to in vitro imaging, we recently found that the clinical tendency for dehiscence from the posterior annulus [12][13][14]22 likely derives from this region's reduced collagen density. 18 We further observed a stark redistribution of force after a single suture's tear-out to its 2 adjacent sutures, highlighting a potential dehiscence cascade mechanism. 17 These initial findings now motivate our pursuit to understand the impact of ring type, ring-annulus sizing, suture position, surgeon, and left heart hemodynamic conditions on suture loading.…”

“…Although clinical reports may be naturally attributed to surgical error, multiple groups have questioned the extent to which dehiscence likelihood is determined systematically. [17][18][19][20] Quantification of such systematic factors would offer 2 principal benefits. First, in a review of recent literature (with mean patient follow-up >1 year), Khamooshian and colleagues 21 found insufficient evidence to establish any superior ring type, in terms of New York Heart Association class and ventricular function.…”

“…For additional details on force transducer technology and experimental procedures, see Supplemental Materials and Methods or our previous publications. 18,19 Tie-Down and Cyclic Contractile Force Analyses For each suture, tie-down force (F TD ) was computed as the difference in force immediately before and after its tie-down ( Figure 2, A). Cyclic contractile force (F C ) was computed as the difference between the minimum and the maximum force recorded within a cardiac cycle (Figure 2, B), averaged across 10 consecutive cardiac cycles.…”

The tip of the iceberg: Evaluating the mechanism behind dehiscence of mitral annuloplasty rings

“…In the current issue, Pierce and colleagues 3 explore the variable forces to which annuloplasty rings and their anchoring sutures are subject over the cardiac cycle using an ovine model. This study builds on previous publications 4,5 by authors who have pioneered the study of mitral valve biomechanics. In previous work, 4,5 the development of suture force transducers applied to an implantable annuloplasty ring demonstrated that force distribution along the mitral annulus is not uniform and is variable throughout the cardiac cycle.…”

“…Additionally, it appears that collagen content (there is reduced collagen density in this region) is directly related to the force necessary to tear out an anchoring annuloplasty stitch, shedding light on why dehiscence along the posterior annulus is more common. 4,5 In the study by Pierce and colleagues, 3 23 sheep were implanted with either Physio (Edwards, Irvine, Calif) or Profile 3D (Medtronic, Sunnyvale, Calif) annuloplasty rings all custom engineered with suture force transducers. Groups were further divided into true-sized annuloplasty rings or undersized annuloplasty rings (by 2 sizes).…”