The impression technique for assessment of tissue oedema : instrumentation, evaluation and applications

An instrument for clinical noninvasive assessment of tissue oedema based on an impression method was developed. The method measures and evaluates the decaying force, due to translocation of tissue fluid, during mechanical compression of any site of tissue. We applied the impression method on physical models, animal models, and patients. Significant parameters for the assessment of tissue oedema that estimated tissue fluid translocation and tissue pressure could be derived from the registered impression force curves.Accuracy was determined theoretically and reproducibility was estimated on plastic foam. We described the clinical procedure for the instrument, and preliminary results from patients with chronic pitting oedema showed that the instrument detected larger fluid translocation on oedematous sites than on non-oedematous sites. We concluded that the instrument was acceptable for accurate measurements on biological tissue.Evaluation was performed in a rat testis model in which testicular interstitial fluid volume could be changed both artificially by 30-min infusions of different fluids with different fluid resistance properties, and pharmacologically by administration of hormones. We found that tissue pressure increased with infused fluid volume, and changes as small as 16 μl (7 % of total testis interstitial fluid volume) could be detected. Fluid translocation changed depending upon the infused fluid's resistance properties. Hormone-induced changes in rat-testis oedema altered both fluid translocation and tissue pressure. Discrete changes in vascular permeability were monitored.Investigation of generalised oedema in patients suffering from burn injury showed that tissue fluid translocation increased up to a maximum value after 6 days postburn and declined thereafter. We found tissue pressure to be relatively high during the first 7 days postburn as compared with 3-week postburn values. Force curve analysis suggested a flux of water-like fluid from the vasculature to the interstitial space during the first 6 days postburn. The course of postburn tissue swelling could be followed and estimated with the impression technique.Comparison with a new tactile sensor that measured physical properties of soft tissue showed that both methods detected changes in silicone hardness/softness and in hormone-induced changes of rat-testis interstitial fluid. We concluded that impression force estimated hardness of soft tissue, which can be helpful when investigating hardness of oedematous tissue.