The flexor system of the fingers has a particular anatomy and a specific gliding process which contribute to the flexion and mobility of the different joints of the fingers. This flexor system is composed by two tendons, the flexor digitorum superficialis (FDS) and the flexor difgitorum profondus (FDP). They originate from their corresponding flexor muscles, cross the carpal tunnel at the wrist level and the hand palm. At the base of each finger, they enter the flexor sheath and end at the base of the second phalanx for the FDS and at the base of the third phalanx for the FDP. The flexor sheath is composed by five annular pulleys and three cruciform pulleys. The integrity of the whole flexor system “muscle, tendon, flexor sheath and pulleys” is crucial for a normal hand and fingers function.

The anatomical position of the FDS and FDP tendons is relatively superficial in the hand and fingers and any sharp skin laceration at this level may lead to the rupture of any of them or both[

Techniques of flexor tendon repair are well established and they have been subject to a great number of research and comparison studies. Usually a core suture with two strands is performed using non absorbable material to hold the two stumps together followed by a circumferential running suture to redesign the external shape of the tendon[

To avoid this phenomenon of adhesion, a particular protocol of passive tendon mobilization can be applied in order to prevent the extrinsic healing to be constrictive[

In order to avoid the previously stated problems, we have imagined the use of a barbed suture device inserted into the two flexor tendon stumps. The barbs should have the ideal shape to anchor into the tendon structure and enough mechanical properties in order to maintain the stumps connected while performing the physical therapy courses.

This article is an original research study with different circular and elliptical cross sectional sutures provided with 9 different types of barbs “3 different depth of cut and 3 different angle of cut” aiming to define the ideal suture shape, the best elliptical cross sectional ratio and the most strong barb configuration for flexor tendon repair.

The suture used for this experimental study is modeled as a solid cylinder. A circular cross sectional suture was first selected a 0.4-mm diameter and a 10-mm length

Design of the suture sample used for the experimental study

The barb is created by performing an extruded cut in this cylinder using slot geometry. The dimensions of the slot were chosen in accordance to the finite analysis software in order to obtain feasible meshing and appropriate results.

As for the barb, a slither of the suture material was removed with a cut radius at the base of the barb of 0.005 mm. This was done in order to avoid high stress concentrations that arise due to the presence of sharp cuts in the geometry. Two variables were changed in this study: the depth of the barb and the cut angle

The two variables used for barbs design

Variables | Value 1 | Value 2 | Value 3 |
---|---|---|---|

Depth of cut (mm) | 0.07 | 0.12 | 0.18 |

Angle of cut (°) | 150 | 160 | 170 |

Given that the cross-sectional area of the suture remains constant, the geometry of the ellipse was defined such that the total area of the ellipse is equal to that of the circular cross-sectional area.

Area of circle = π*r^{2}

Area of ellipse =π*a*b

Area of ellipse = π*r^{2} =π*(0.4/2)^{2} = 0.12566 mm^{2}

In addition to the circular cross-section suture selected (ratio = 1), we have considered 6 different ellipse geometries with the following ratios of minor to major axes (ρ = b/a): 0.25, 0.5, 0.75, 2, 3 and 4 such that the area of the obtained ellipse remains equal to that of the circle. In

Major "a" and minor "b" axes of the ellipses considered for the study

a | b | ρ | 1/ρ |
---|---|---|---|

0.4 | 0.1 | 4 | 0.25 |

0.34641 | 0.11547 | 3 | 1/3 |

0.2828 | 0.1414 | 2 | 0.5 |

0.2 | 0.2 | 1 | 1 |

0.1732 | 0.231 | 0.75 | 4/3 |

0.1414 | 0.2828 | 0.5 | 2 |

0.1 | 0.4 | 0.25 | 4 |

Given that the uncut area of all suture geometries remains constant, thus the uncut cross-sectional area of the ellipse is equal to that of the circular part with the same depth of cut. Thus, the depth of cut for all elliptical geometries is calculated from the following equation of the area of an ellipse with a cut:

A = a*b*[π/2 + sin^{-1} (d/a)] + [b*d*(a^{2}-d^{2} )^{0.5}]/a

Where “d” is the length from the origin to the cut and “a” is the length of the major axis while b is the length of the minor axis

Different parameters on the ellipse used for the calculation of the depth cut

For d = 0.07 mm, A_{circle remaining} = 0.11089 mm^{2} and the depths of cut are given in _{circle remaining} = 0.093957 mm^{2} and depth of cut 3 for d = 0.18 mm, A_{circle remaining} = 0.0708185 mm^{2}.

Different depth of cut for the different ratios "ρ" in the elliptical sutures

ρ | Depth of cut 1 (d = 0.07) | Depth of cut 2 (d = 0.12) | Depth of cut 3 (d = 0.18) |
---|---|---|---|

4 | 0.035 | 0.065 | 0.0901 |

3 | 0.04047 | 0.075 | 0.10397 |

2 | 0.04972 | 0.0917 | 0.12742 |

1 | 0.07 | 0.12 | 0.18 |

0.75 | 0.081 | 0.15 | 0.208 |

0.5 | 0.0993 | 0.1835 | 0.2548 |

0.25 | 0.1405 | 0.2595 | 0.3605 |

The uncut area is chosen in blue

Different elliptical geometries with different ratios "ρ" selected

Summary of all geometry sets considered (7*3*3 = 63 sets)

A static structural project is created in ANSYS and all the geometries drawn in SolidWorks are imported into DesignModeler. Meshing is performed under Mechanical in Workbench. Every geometry model used for the experimentation is meshed by using nodes and elements.

In finite element analysis, it is known that for geometries with corners, convergence of the stresses can hardly be reached. Resolving to a finer mesh for the analysis might not be the proper solution to reach convergence. Thus, during meshing, a set of different mesh configurations was considered and the one chosen was found to ensure proper convergence of the stress in the model with an acceptable duration for the simulation

Converged stress versus mesh refinement

Different element sizing cases were considered without having either a fine relevance center or a refinement at the cut location. The values of the stress of the different cases of element sizing are depicted in

Mesh refinement analyzed in order to have the best stress convergence

Mesh number | Refinement characteristics |
---|---|

1 | Growth rate 1.4 |

2 | Growth rate 1.2 |

3 | Fine smoothing (growth rate 1.2) |

4 | Smooth transition (growth rate 1.2) |

5 | Virtual topology (growth rate 1.2) |

6 | Element sizing |

Since the material considered is neoprene rubber which has elastic and plastic properties, a nonlinear analysis is performed with large deformations. Therefore, aggressive shape checking is specified in ANSYS Mechanical rather than standard shape checking. This ensures more conservative element shape checking criteria to account for possible distortion of the elements during the nonlinear analysis. In addition, the default volume mesher in ANSYS includes some defeaturing automatically. This will consequently lead to ignoring the small, narrow cuts. In our case, this is undesirable and thus the automatic mesh based defeaturing option was switched off.

On the other hand, a virtual topology is inserted. Including virtual cells properly merges our tiny edges into one edge for better meshing to preserve the rounded shape of the cut. Tetrahedral meshing in ANSYS is associated with virtual cells, allowing for a greater flexibility. Several meshing criteria are primarily considered in order to ensure proper stress convergence and a mesh independent problem. This is performed on one of the cases considered, 0.07 mm depth of cut and 150° angle of cut. In addition to the above defined meshing criteria, a growth rate of 1.2 is considered for better meshing results. This is defined with advanced size function on proximity and curvature. A sample of the actual meshing of the barbed suture is shown in

Sample of the final meshed geometry

Based on the actual scenario

Pulling force is applied on the upper surface while the lower surface is fixed

Upper and lower boundaries are fixed while the force is applied on the barb

Thus, the simulations for the above defined geometries were performed for each of the cases separately. Finally, concerning the material chosen to perform these different experimentations, neoprene rubber was selected since it showed similar elastic and plastic behavior to the actual material used for flexor tendons repair, the polypropylene.

In conclusion, seven different cross-section areas were considered with three different cut angles and three different depths of cut. Each of these cases was studied for the two boundary conditions of fixed barb and fixed edges. Thus, a total of 126 simulations were performed (7*3*3*2 = 126 simulations).

After the proper mesh was assigned for the geometry, simulations for the circular cross-section area were analyzed. The equivalent Von Mises stress

Circular cross sectional suture area Von Mises Stress

Circular cross sectional suture area Maximum Shear Stress

The suture with 150° cut angle and 0.18 mm depth of cut showed minimal Von Mises and maximum shear stresses when compared to the other configurations considered for a circular cross-section area.

Based on these preliminary results obtained from the circular suture different configurations, we moved to the analysis of the different elliptical sutures with the different preselected depths and cut angles barbs.

For the cut angle 170°, when the loading of the second situation was applied: depth of cut = 0.07 mm force was decreased by a factor of 10

Equivalent Von Mises stress for d = 0.07 mm

Equivalent Von Mises stress for d = 0.12 mm

It was noted that for the first situation, fixed barb with a vertical load applied on the upper edge of the cylinder, in all types of elliptical configurations, resulted in lower stresses. Furthermore, further investigation of the data showed that, the 0.18-mm depth of cut results in minimal Von Mises and maximum shear stresses. When compared with Ingle’s results for the circular cross sectional area, the ellipse with a ratio of major to minor axis equal to 3 (r = 3) is seen to have the lowest Von Mises and shear stresses as can be seen by the blue bars

Equivalent Von Mises stress showing comparison between best ratios 3 and 4 for the first experimental situation of boundary conditions

Equivalent Von Mises stress showing comparisons between best ratios 3 and 4 for the second experimental situation of boundary conditions

The results showed that as “r” of the elliptical cross sectional area increases for 0.18 mm and 150° case, the equivalent Von Mises stress will reach a minimum at r = 3 and then start to increase slightly after that at r = 4

Equivalent Von Mises Stress r = 3, 0.18 mm, 150° Fixed Barb

Equivalent Von Mises Stress r = 4, 0.18 mm, 160° Fixed Barb

The work on barb sutures and anchoring devices dates back to 1945 with McKee[

Another technique was launched by with Lengemann, an Austrian physician, in 1950 and was later introduced in the United States by Jennings and Yeager[

The first report for a nylon barbed suture was used by McKenzie[

McKenzie barb sutures configurations (form A and B)

In London, one year later in 1968, Shaw[

Shaw technique with the use of a bidirectional barb suture

More recently, in the International Conference and Exhibition on Healthcare and Medical Textiles in the United Kingdom in 2003, Leung[

Magnified barbed suture with the geometry measurements of a single barb

The revival of barbed sutures and anchoring devices was launched by Su ^{TM}. This device has been used for zone II flexor tendon repair. It is composed of two intra-tendinous stainless steel anchors joined by a single monofilament 2-0 stainless steel suture. As it began to be used by surgeons, it was shown to withstand greater forces and to result in lower rupture rates than the traditional modified Kessler repairs

The Teno-Fix system

Another anchoring device was developed in 2010 by Hirpara

Nitinol tube with anchors

Finally, a device, similar to McKenzie’s and Shaw’s sutures described earlier, was introduced by Quill^{TM} as a monofilament suture made from polypropylene, nylon and glycolic acid derivatives. It had two needles at each end with each side of the barbs being oriented away from the needle. It was primarily introduced into face lifting surgeries but is more commonly used now in sub-dermal skin closure. Silhouette suture has a resembling design and was developed by FeatherLIFT® being different than the Quill^{TM} in having flexible, absorbable hollow cones instead of the barbs that allow the growth of tissue inside and around and thus improve the anchoring mechanism to lift the tissues

Face lifting procedure using the Silhouette lift threads

Considering the flexor tendons repair, in a study performed in 2005 by Lawrence and Davis[

A four strand flexor configuration was performed on all 150 fresh porcine flexor tendons. All repairs failed by suture rupture at the locking loop. It was observed that nylon sutures consistently produced the poorest mechanical performance in all outcome measures. Polypropylene and braided polyester showed similar results with respect to the gap formation and ultimate forces. Stainless steel and braided polyethylene sutures were significantly stronger than other suture groups. Upon exceeding the strength of the suture material, all repairs failed rather than when the holding capacity of the repair loops was exceeded. The study concluded that braided polyester is favored for core repair although some researchers prefer monofilament sutures such as nylon and polypropylene.

Another study was done by Viinikainen

The yield force and stiffness in the linear region began to increase with the number of core suture strands. It was observed that the ultimate force increased significantly when the number of strands increased or the suture caliber increased. However, the suture configuration did not influence the ultimate force. In addition, the peripheral suture contributed to the strength of the tendon repair. Thus, the tendon repair was defined as a composite of core and peripheral sutures.

Furthermore, it was observed that increasing the number of strands crossing the repair site increases the strength of the repair technique and thus averts breakage of the suture. Nevertheless, multiplying strands are also found to jeopardize the integrity of the tendon and its ability to glide. This results in adhesion formation in undesirable location and further a loss in the gliding movement of the tendon as depicted by Zhao

Therefore, barbed sutures were introduced into tendon repair surgeries in an attempt to improve the strength of the repair site and without impairing the healing process. The barbs distributed along the surface of the suture are oriented in order to allow passage of the suture in one direction through the tendon tissue but exhibit resistance due to the barbs in the opposite direction. Thus, barb sutures exhibit non-slip attributes. In addition, they do not require knotting like conventional sutures; thus, failure due to knot breakage is avoided. Barb sutures are also found to reduce the straight pull tensile strength since the effective suture diameter is decreased.

In October 2009, Parikh

The most relevant study was the one performed by Ingle

Based on all these previous experimental studies, we have performed experimentation on both circular and elliptical cross sectional sutures. Elliptical sutures were selected in order to approach the most the elliptical shape of the flexor tendon. Our experimentation and results were promising.

The two loading situations, one applied on the suture and the other on the barb itself, were considered because they represent the load to which the sutures are exposed to

Our experimentations showed that for a depth of cut of 0.18 mm with 150° cut angle, the best elliptical cross sectional area was a/b = r = 3, while for a cut angle of 160°, the best elliptical cross sectional area was with r = 4.

These findings are crucial to invoke further research on better and improved barbed sutures with non circular cross sectional areas. The design of the barbed suture will be studied using extended finite element analysis in order to define the one which fit the best the shape of the flexor system. Also, future work can be done to account for the biological interaction of the suture with the surrounding tendon tissues. The interaction between the barb and the tendon tissues ensures a more realistic model and could provide more insights on the mode of failure. In addition more research can be performed on the best barb configuration, their arrangement and distribution over the length of the suturing wire, based on the findings done in the use of this concept in the field of aesthetic surgery[

Medical and surgical part of this work: Bakhach J, Oneissi A, Karameh R

Literature review, photos design and tables: Bakhach E

Biomechanical investigations and studies: Hantouche M, Shammas E

Data in this study were derived from searches of the PubMed database.

None.

There are no conflicts of interest.

Not applicable.

Not applicable.

© The Author(s) 2018.