HYPOTHESIS: Tight fixation of stapes prostheses yields better functional results because sound transmission from the incus to the prosthesis is improved. BACKGROUND: The optimal prosthesis to use for otosclerosis surgery is still a matter of debate. It has been proposed that using prostheses made of Nitinol, a shape-memory metal, produces better functional results with less variability and reduced risk for middle and inner ear damage. This is thought to be because heat activation rather than manual crimping of the prosthesis loop forms a tighter fixation. METHODS: Functional results of two groups were compared 1 year after surgery. In one group were 75 cases of stapedotomy performed using Nitinol prostheses. Results were analyzed prospectively and compared with 75 retrospectively analyzed matched controls with conventional stapes prostheses. Crimping quality was measured in 23 patients by intraoperative laser Doppler interferometry (LDI). Causality was assessed by correlating results of intraoperative LDI and postoperative pure-tone thresholds. RESULTS: Nitinol and conventional prostheses yielded postoperative air-bone gaps (ABGs) of 8.0 and 11.6 dB with 71 and 43% ABG closure within 10 dB, respectively. Intraoperatively, sound transmission was improved by 2.5 dB with the Nitinol prostheses as compared with conventional prostheses. These differences were statistically significant. Intraoperative fixation quality was positively correlated to functional outcome, but results were not statistically significant. CONCLUSION: Tight fixation, as provided by Nitinol prostheses leads to improved functional results because of better sound transmission properties at the incus-prosthesis interface. The improvement in ABG closure is in the range of 3 dB pure-tone average and more pronounced at higher frequencies. Nitinol prostheses provide an effective treatment option in otosclerosis surgery.