The measurement of pure-tone bone-conduction thresholds figures prominently at present in the selection of cases for stapes surgery for otosclerotic deafness and in the prediction of postoperative results.1
In the period preceding the development of modern stapes surgery, the importance of the mechanical shift in bone-conduction thresholds following successful fenestration surgery of the horizontal canal was accepted generally along the lines suggested by Carhart, Shambaugh, and McConnell.2,3 The average improvement in these thresholds as postulated by Carhart was as follows: no shift at 250 cps, 5 db. at 500 cps, 10 db. at 1,000 cps, 15 db. at 2,000 cps, and 5 db. at 4,000 cps. A carefully controlled study by McConnell and Carhart yielded somewhat different values of shifts following successful fenestration nov-ovalis, namely: 1.2 db. at 250 cps, 1.4 db. at 500 cps, 6.5 db. at 1,000 cps, 7.4 db. at 1,500 cps, 8.5 db. at