Prototyping and Finite Element Analysis of Tissue Specific Barbed Sutures.

Abstract

[Ingle, Nilesh P. Prototyping and Finite Element Analysis of Tissue Speciﬠc Barbed Sutures (Under direction of Dr. Martin W. King & Dr. Elizabeth G. Loboa)]

The project titled ’Prototyping and Finite Element Analysis of Tissue Speciﬠc Barbed Sutures’ is focussed on understanding the relationship between barb geometry and mechanical behavior of barbed sutures. In this study size ’0’ polypropylene monoﬠlament sutures of diameter 0.4mm were used for creating barbs at 150o , 160o and 170o cut angles and 0.07, 0.12 and 0.18mm cut depths. A new prototyping method was developed to create barbed sutures with precisely controlled ge- ometries. This method used the upper crosshead of an MTS tensile testing machine for controlling the displacement of a vertical blade which corresponds to the actual cut depth of the prototype barbed suture. The cut samples were then characterized by image analysis so as to measure the reproducibility and variability of the barbs geometric dimensions, before they could be tested exper- imentally for their mechanical performance. Special specimen mounting ﬠxtures were developed to mount the samples under a light microscope which was attached to a camera and then a computer. The computer recorded the images of the barbs which were later converted to measurements of cut angle and cut depth using Image J software. Tensile testing and stress and bulk relaxation experi- ments were performed to obtain viscoelastic constants for ﬠnite element modeling. An experiment was run to quantify the peeling properties of a barb under point-pressure load by attaching a metal wire to the end of the barb. Suture/tissue pullout experiments were also performed using bovine tendon and porcine skin tissues. The ﬠnite element simulation of the point-pressure loading of a barb tip in ANSYS was validated by experimental results of the same materials by a margin of only 4%. Three sets of FEA simulations were then performed for each of the nine blocks of combination of barb geometries. The same three levels of cut angle and three levels of cut depth were selected. In addition point-pressure loading simulations were run and experimental suture/tissue pullout tests were performed on tendon and skin tissues. The experimental results revealed that since tendon tissue has a higher modulus than skin it needs a more rigid barb to penetrate and anchor into the surrounding tissue. A cut angle of 150o and 0.18mm cut depth are recommended. On the other hand for the softer skin tissue a cut angle of 170 degrees and 0.18 mm cut depth provided a more flexible barb that gives superior skin tissue anchoring. The simulations helped identify the areas of stress concentration in the barb as well as in the surrounding tissue using a suture/tissue pullout test model. The cut line at the base of the cut appears to be the weakest part of the barb. So the geometry or design should be modiﬠed so that the stresses generated along the base of the barb should be lower than the peel-initiation stress regardless of the particular type of tissue. A new design with a circular cut line has been virtually prototyped and tested in ANSYS. This new and improved design helps to redistribute the stresses along the barb and its cut line so that peeling is initiated at higher stresses and improvements are achieved in the barb’s anchoring performance.