Gross necropsy observations
Representative images for each of the test samples at each time point are provided in Fig. 1.
There were no signs of herniations in any of the animals. However, the two Gentrix Surgical Matrix Plus implants at 4 weeks had torn away from approximately 75% and 50% of the defect perimeter, respectively, and those at 12 weeks could not be identified in the defect area (Supplementary Fig. 1). Based on this, Gentrix Surgical Matrix Plus was not included in any further analyses.
All animals showed surface adhesions of the omentum, which were freed with blunt dissection except for Phasix and Physiomesh, which required sharp dissection to remove at 24 weeks (Table 3).
There was a wide variety of implant geometry at the various time points as the implants contracted or stretched to different degrees in length and width (Fig. 1). The geometry was analyzed by measuring the percent reduction in defect area, and the aspect ratio (measure of length over width) (Table 3). All implants at all time points showed a reduction in area (Table 3), except for Strattice Firm, which at 24 weeks had stretched to 110% of the original defect area. The most contracted implants were the synthetics, which had reduced to less than 50% of the defect area by 12 weeks and further reduced by 24 weeks, forming rolls in the mesh (Fig. 1i, l, o). At 4 weeks, test articles essentially retained their aspect ratios (approx. 2.33). However, by 24-week differences between the test articles were observed. For example, at 24 weeks, the reinforced biologics best preserved the original aspect ratio of the defect area (2.45 average), whereas Physiomesh was substantially distorted (0.45 average).
Representative low and high magnification histology images for each of the test samples at each time point are provided in Figs. 2, 3.
Inflammation at 4 weeks was mild-to-moderate (score 2–3) across all groups (Fig. 4a). Among implants at 24 weeks, inflammation was minimal (less than 1) with OviTex 1S Resorbable and Strattice Firm, mild with OviTex 1S Permanent, and mild–moderate for the synthetic meshes Phasix, Physiomesh, and Ventralight ST.
Biologics and reinforced biologics were initially infiltrated primarily by lymphocytes and macrophages, and in the case of reinforced biologics, neutrophils (Fig. 4b–d). At 12 and 24 weeks, macrophage infiltrate decreased to near minimal and near absent, respectively, as the implants were integrated into host tissue. Synthetic devices were primarily infiltrated by histiocytic cells, with giant cells and macrophages in the areas immediately surrounding the synthetic materials. The inflammatory response to these devices persisted at elevated levels throughout the study (Fig. 4a).
Host cellular infiltration and fibroproliferative remodeling: biologics and reinforced biologics
Implants containing biologic materials were evaluated for the degree and timing by which they were infiltrated by host tissue such as fibroblasts and collagen, and for the formation of vasculature. At 4 weeks, infiltration by spindle cells was mild-to-moderate (score 2–3) with both OviTex implants, Strattice Firm and Zenapro, while extensive with SurgiMend 1.0 (Figs. 3a, d, p, s, 5a). Collagen deposition and blood vessel infiltration were minimal-to-mild with all groups (score 1–2). OviTex 1S Resorbable at 12 weeks and all other biologic containing implants had diffuse tissue infiltration throughout the interstitium between individual collagen bundles at 12 or 24 weeks (Fig. 5a, c, e). This evaluation was not able to be performed for knit meshes (Phasix, Ventralight ST, and Physiomesh), because they lacked an implant interstitium. This evaluation was also not possible for the biologics after they had remodeled into host tissue.
Implant-to-tissue ratios at 4 weeks were highest with Strattice Firm, SurgiMend 1.0, and OviTex 1S Permanent (Figs. 2p, s, d, 6b). By 12 weeks, the biologic (Strattice Firm and SurgiMend 1.0) and reinforced biologic implants (both OviTex implants) had remodeled into host tissue which occupied nearly the entire defect area (Fig. 2b, e, q, t). This finding continued at 24 weeks (note SurgiMend 1.0 was not evaluated at this time) (Fig. 6b, d). At both 12 and 24 weeks, the synthetic component of OviTex 1S Permanent was detectable, while the reinforcing component of OviTex 1S Resorbable had resorbed (Fig. 6c). This explains the slightly elevated implant-to-tissue ratio of both OviTex implants in comparison to pure biologic devices. The 24-week remodeled tissue of both OviTex implants and Strattice Firm was comparable to the thickness of the native abdominal wall tissue, with increasing organization of the collagen, consistent with maturation over time (Fig. 6a, e, g).
The synthetic implant components of Zenapro, Phasix, Physiomesh, and Ventralight ST were persistently detectable throughout the study, reflecting the abundant content and durable nature of the synthetic polymers in these implants (Figs. 2i, l, o, 6c). Although synthetic implants could not be evaluated for the degree of infiltration, there were host cells and loose connective tissue surrounding the implant fibers; and the organization of an adjacent layer of tissue, which developed asymmetrically on the subcutaneous side of the implant, was analyzed.
Implant-to-tissue ratios among synthetic implants were quantified but confounded by the persistent presence of synthetic meshes occupying a greater portion of the defect thickness over time (Fig. 6b, c). This was further confounded in events where the thickness in relation to the abdominal wall was considerably thin. Furthermore, there were other cases where the mesh bunched up and minimal amounts of deposited collagen were present, resulting in the mesh occupying significant portions of the cross section, and, therefore, a bias in the implant-to-tissue ratio.
Delamination, longitudinal splitting, or clefting along the defect wall repair tissue was also used to qualify the defect tissue area. Delamination was primarily observed in synthetics, in which the mesh was loosely adhered or entirely split from the asymmetrically deposited layer of tissue (Figs. 2g, h, n, o, 6f and Supplementary Fig. 2). Delamination was persistently mild with Phasix, variable but typically mild with Physiomesh, and absent to minimal with Ventralight. Of note, the biologics and reinforced biologics did not develop an asymmetric layer of tissue.
Implant tissue organization and architecture
Organization (i.e., maturity) of collagenous/fibrous connective tissue was determined by the presence of histologic features including dense lamellar collagen, low cellularity, and predominance of quiescent (fibrocytic) compared to active (fibroblastic) spindle cell morphology (Fig. 6e). At 12 weeks, the organization of tissue spanning the abdominal wall defects was moderately amorphous for Strattice Firm and OviTex 1S Permanent, and markedly lamellar for OviTex 1S Resorbable and SurgiMend 1.0 (Fig. 3q, e, b, t). At 24 weeks, biologics and reinforced biologics had mature lamellar collagen (Fig. 3c, f, r). The tissue adjacent the synthetics was predominantly amorphous, while the area immediately surrounding the mesh fibers was predominantly loose connective tissue with some thin strands of lamellar tissue (Fig. 3i, Supplementary Fig. 2). This finding was one of the more distinct findings between the classes of materials.
Adverse implant findings were limited to mineralization and osseous metaplasia at 24 weeks which was marked/severe in one Phasix implant (also observed at 4 and 12 weeks), moderate in one Strattice Firm implant (also observed at 12 weeks), and moderate in one Physiomesh implant (Supplementary Fig. 3). There were no implant associated cavities/pockets at 24 weeks, regardless of group.