[Experimental studies on the mechanical properties of tympanic membranes after repeated myringotomy or tube...

Although several complications have been reported at present, myringotomy and ventilation tube insertion are frequently used in the treatment of secretory and recurrent otitis media. There have been many studies on histological changes of tympanic membranes (TM) after surgery, however, experimental studies on the mechanical properties of the TM are very few. In animal models, repeated myringotomies were performed on the TM of 39 guinea pigs (Group A), and polyethylene tubes were inserted into the upper anterior quadrants of the TM in 19 guinea pigs (Group B). In Group A two types of incisions, circumferential and radial, were made in the TM. In group B the ventilation tubes were removed after either one week or five weeks. After the final incision or removal of the tubes, structural changes of the TM were examined by otomicroscope once a week for a duration of four weeks. The animals were sacrificed after one week or four weeks and the TM were dissected out, and observed under both light and scanning electron microscopes. A microtesting system was developed to measure the mechanical strength of the TM. This system consists of three components: a microscope, a specimen holder and a force sensor. The specimen was stretched over the hole (500 microm in diameter) of the holder, and was penetrated by a force sensor needle, 130 microns in diameter. The load-displacement curve began ascending when the sensor touched the specimen, and disappeared after breakage. The results were as follows: 1) Repeated myringotomies caused a dramatically thickened TM in not only the operated area, but also the posterior quadrants. The TM in group A were more thickened than those in group B. Tympanosclerosis was observed more frequently in the TM with circumferential incisions as compared to those with radial incisions. Retraction of the TM was observed only in group B. 2) In normal TM, the load-displacement curve showed a linearly ascending curve with some small peaks. The load at breakage was 72.9 mN on average. The load-displacement curve at the thickened area showed a concave ascent without small peaks. The load at breakage ranged from 10 to 660 mN after circumferential incisions, and from 40 to 240 mN after radial incisions. At the tube insertion region, the load at breakage ranged from 53 to 237 mN.