Despite considerable improvements in surgery, the incidence of failure of surgical sutures, remained widely constant throughout the last decades [12
]. The negative influence of high suture tension on the structural and mechanical quality of the healing incision has been clearly demonstrated by several authors, focussing on parameters like suture material, suture technique and the suture-length to wound-length ratio [7
]. Nevertheless, as several meta-analysis outlined we seem to be unable to substantially reduce our rate of anastomotic leakage or of incisional hernia by changing surgical technique [14
]. However, the disappointing lack of improved results by optimising suture material or suture technique may find its explanation in the negligence of suture tension in most of the experimental settings. Until now surgeons have no other criteria than their purely subjective 'feeling' of what the tissue needs in terms of suture tension and the local tissue damage. In regard to the limited visual control in the process of suturing, the surgeon has to rely on his firm belief that his suture technique and suture tension are 'appropriate' for the tissue. Repeated measurements of the suture tension, 5 sutures subsequently done by one surgeon, demonstrates a considerable variation between these sutures [9
]. Furthermore, the range of the tension considered as "appropriate" showed wide overlap to too high or too loose tension. Because of missing data, which define the optimum of a suture tension, surgical suture repair is mainly based on an individual feeling for suture tension.
The tension within a suture loop will be affected by the volume (bite) and type of tissue included, the size and diameter of the suture, and the force applied during knotting. A rabbit model was the most suitable model for our setting since the sensors are too big for a rat model. In our study we used only three rabbits but not the amount of animals is important but rather the amount of measurements per tissue in order to obtain reliable data. We used 3/0 monofilament polypropylene single sutures which is an established standard suture material. Although we tried to place similar sutures with similar bite and tension there still was a considerable variation of 0.7 to 5.9 N for the peak tension. Obviously, we were unable to apply constant peak tension to a knot, which is a clear limitation of our study but in accordance with findings of Butz et al. [9
]. Obviously not only between surgeons it seems to be impossible to standardize suture tension in hand-knotted sutures [10
] but also the same surgeon underlies a great variability of suture tension since all sutures were performed by the same person. Therefore, it might be favourable to develop a suture device which provides standardized suture tension. However, we usually saw a rapid decrease within the first minute, which was interpreted as initial cutting through. This loss was higher in case of high peak tension, but was affected as well by the resistance of the tissue. A high amount of collagen seems to withstand better to the forces and thus reduce the cutting through. It may be speculated that the extent of cutting through damage of the tissues impairs wound healing and favours failures e.g. incisional hernia or anastomotic leakage. Adoption of the peak tension within the suture to the tissue should reduce the amount of damage, and is that what the experienced surgeon is able to consider already today reducing this surplus to a minimum for achieving less necrosis and improved wound healing.
The constant decline of the tension during what we call Phase II is characterized by a constant loss of suture tension of 8 - 15% per minute. This can be interpreted as plastic deformation and was slightly different between the tissues. The differences may be caused by distinct composition of the tissues and their ECM, however it could not been related to the collagens or any other biometric variable. Further measurements will show whether this phase and its area under the curve indicating time with increased tension may be reducible with stretchable sutures.
With all sutures we could see a rapid loss of tension, though sometimes it takes more than 1 hour to reduce the initial peak tension to half. The strength of the remaining plateau tension mainly depended on the tissue, and furthermore, was closely related to the amount of collagen. The collagen per protein content was found to have a negative correlation to the decrease of tension in the rapid cutting phase RCP. This fact indicates that especially in tissue with low collagen content (liver) high tension can lead to overwhelming tissue damage whereas in tissue of high overall collagen content (stomach, skin) this cutting reaction is not as severe since collagen is one of the structural proteins that is responsible for the tissues' stability [17
]. There are many other factors supposedly effecting the stability of tissue, like elastin [19
], chondroitin sulphate [20
], the composition of the various collagens and its cross-linkings, [21
], the junction between the cells [24
], or adhesion molecules [25
]. All this may influence the resistance of tissue, its plastic deformation or its resident plateau, and should be considered in further experiments.
The design of our device allowed only the evaluation of single sutures so far. Although with running sutures a more even distribution of suture tension along the incision is attained and tension peaks are avoided, the question of adequate suture tension remains unanswered. It is technically not easy to maintain identical tension levels from stitch to stitch in a running suture. This might lead to a generally lower suture tension in running sutures compared to single sutures, which can be a possible explanation for the superior quality of fascial healing after running sutures [26
]. We are working on an experimental layout of our sensor in order to investigate tension of running sutures. Furthermore, the investigation of elastic fibres might highlight important findings of tension deviation.
With knowledge of the influence of inadequate suture tension on the healing of laparotomy wounds, further research work needs to focus on the definition of a tissue specific optimum for suture tension, and the development of sutures and measurement devices which help the surgeon to suture according to this tension optimum.