Abstract. The specific aim of this study was to measure the TS of rat skin wounds during the first week follow- ing surgical injury. Biomechanical and histological data were collected daily (days 1 to 7 following surgery) from separate groups of Sprague-Dawley rats (N = 12) each with two 3 cm long parallel skin incisions on the back. The wounds were immediately closed by four simple sutures. A control group (N = 15) was used to obtain TS measurements of unwounded skin. TS was measured by applying a ramp load until wound separation and estimated by dividing the yield strength by the wound area. The time course of biomechanical recovery followed a step-plateau pattern with the largest increase in TS observed one day after surgery (0 – 1.60 g/cm 2 ). The plateau stage extended from day 1 to 5 (1.60 – 3.88 g/cm 2 ). The final step (day 5–7) indi- cated a period of rapid rise in wound TS (3.88 – 11.57 g/cm 2 ). Since even on day 7 the mean TS was only 4% of unwounded skin, the wound had to be protected from tensile loads. Histological analysis confirmed that the early changes in TS (day 1) correlated with the fib- rin accumulation of the wound edges followed by a plateau stage caused by the tissue proliferation. The rapid increase in wound TS was characterized by cross-linking the incisions with collagen fibres with escalating organization. We conclude that from a bio- mechanical perspective, sutures can be removed dur- ing the “plateau phase”, but the wound must be protected from tensile loads. Introduction It is estimated that the morbidity associated with delayed wound healing increases, which costs the health services over $9 billion per year (Ashcroft at al., 2003). Many factors such as diabetes mellitus, AIDS, bad nutrition as well as chronic application of corti- coids may negatively affect the process of wound heal- ing in patients. Therefore, a number of experimental studies deal with new approaches improving the repar- ative and regenerative processes of tissues (Paul et al., 1997; Menetrey el al., 2000; Medrado et al., 2003; Milgram et al., 2004; Gál et al., 2005). Skin sutures can be removed as soon as the wound is fixed enough to no longer need mechanical support (sutures). Early sutures resection assures a uniquely better cosmetic result (Burkitt et al., 1990). Therefore, tensile strength (TS) of wounds is objective and the preferred method for wound healing evaluation and is often used in numer- ous experimental studies (Davidson, 1998; Dorsett- Martin, 2004). Nevertheless, no detailed biomechanical study was performed during the fist week of healing. Published studies evaluated biomechanically only cer- tain time intervals or they observed, compared to con- trols, the influence of various factors on the healing process on selected days (7 th and 14 th day by Allendorf et al. (1997), 7 th , 10 th and 20 th day by Andreassen and Oxlund (1987), 7 th , 10 th 14 th , 19 th 22 nd , 28 th 29 th 84 th by Paul et al. (1997) in his biomechanical and biochemical study of a standardized wound healing model, etc.). However, the most significant changes occur during the first week of wound healing and this contributes to the importance of our study (Medrado et al., 2003). As an experimental animal model we decided to use the rat, since rat skin represents one of the most com- mon models used in experimental studies concerning the skin wound healing. It is a useful model because we can study the healing of three different tissue types (epi- dermis, dermis and striated muscle) (Menetrey et al., 2000; Vidinský et al., 2006). Only the epidermis has the Received July 27, 2006. Accepted August 28, 2006. This study was supported in part by the P. J. Šafárik University institutional grants for young scientists and students Nos. VVGS-56/2006 and VVGS-58/2006 and by the Ministry of Edu- cation, Youth and Sport of the Czech Republic No. MSM0021620806. Corresponding authors: Peter Gál or Ján Sabo, Department of Medical Biophysics, Faculty of Medicine, Pavol Jozef Šafárik University in Košice, Trieda SNP 1, 040 66 Košice, Slovak Republic, e-mail: galovci@yahoo.com; jan.sabo@upjs.sk. Abbreviations: ECM – extracellular matrix, PMNL – polymor- phonuclear leukocytes, TS – tensile strength. Early Changes in the Tensile Strength and Morphology of Primary Sutured Skin Wounds in Rats (wound healing / tensile strength measurement / tissue repair and regeneration) P. GÁL 1 , T. TOPORCER 1 , B. VIDINSKÝ 1 , M. MOKRÝ 1 , M. NOVOTNÝ 1 , R. KILÍK 2 , K. SMETANA JR. 3,4 , T. GÁL 3 , J. SABO 1 1 Department of Medical Biophysics, 2 1 st Clinic of Surgery, Faculty of Medicine, Pavol Jozef Šafárik University in Košice, Slovak Republic 3 Institute of Anatomy, 1 st Faculty of Medicine, Charles University in Prague, Czech Republic 4 Centre of Cell Therapy and Tissue Repair, 2 nd Faculty of Medicine, Charles University in Prague, Czech Republic Folia Biologica (Praha) 52, 109-115 (2006)