Determination of Wound Breaking Strength
A critical outcome of the wound repair process is restoration of the mechanical properties of tissue strength. Measurement of wound strength provides highly quantifiable estimates of the efficacy of the aggregate healing process. Determination of various individual components of the phases of healing can provide important insights about events operative during repair. However, if sufficient wound strength is not attained, the net effect may be wound failure. Factors that modulate wound repair can be evaluated according to their influence on the development of wound strength.
Various methods have been used to estimate the strength of healing wounds. Harvey (23) developed a technique of estimating the tensile strength of wounds in hollow viscera by removing them from the body, tying one end, and pumping air into the other until the wound burst. The pressure at which bursting occurred was recorded on a revolving drum attached to a sphygmomanometer (23). The method for testing hollow viscera was modified by Lanman and Ingalls (24), who used a lumbar-puncture needle attached by rubber tubing to a sphygmomanometer. The needle could be inserted into the peritoneal cavity or any hollow viscus to measure the wound bursting pressure (24). Howes and Harvey (25) also tested the breaking strength of excised wounds by attaching them to a standard thread-testing machine. Tension was sequentially increased and wound strength determined at the load value of wound disruption (25). Testing equipment for incisional wounds was also developed by Hartzell and Stone (26) and Jones et al. (27). In 1944, the apparatus used for testing tensile strengths of incisional wounds by Bourne (5) was a simple gallows device. Excised pieces of skin bearing the wound were hung onto the top of a rack by one end and weights were attached to the free end until the wound disrupted (5).
During the past several decades, manufactured materials testing instrumentation have improved in both sophistication and availability. Instron brought one of the first commercially available material tensile testers to the market in 1946. The Instron model series 5540 single-column tester combines a broad range of testing capabilities with a computerized operating system (28). The materials tester used in our own laboratory was designed and built locally by the Department of Surgery and Instrument Models Facility at the University of Vermont (29). The degree of elongation and load applied to a tissue specimen is determined via Wheatstone bridges, which are coupled to a differential transformer and load transducer (Entran Devices, Fairfield, NJ). The load deformation curves are obtained by a continuous recording on an X-Y plotter, and the maximum load or wound breaking strength (in grams) is displayed via a digital readout. This particular design has proven to be accurate, stable, and simple to use; to provide reliable results; and to be economically feasible to fabricate (Fig. 1).