Conclusions. Use of the recently introduced intraoperative loading instrumentation during surgery improves the coupling efficiency and consistency of a fully implantable hearing device to the ossicles and ultimately leads to better patient performance with the device. Background. Patient performance with an implantable hearing device is dependent on effective coupling of the implant to the middle ear ossicles. New intraoperative instrumentation aids the surgeon in coupling the implantable hearing device and consistently optimizes patient performance. Materials and methods. The middle ear transfer function for the Otologics MET Ossicular Stimulator (METF_MET) is a measure of transducer coupling efficiency, defined as acoustic threshold (in dB HL) minus implant threshold (in dB MET). This transfer function was measured in the Otologics US Phase I Semi-Implantable Clinical Trial without intraoperative loading instrumentation and in the Otologics US Phase I Fully Implantable Clinical Trial with intraoperative loading instrumentation. The same operative techniques and transducer design were used in both studies. The theory of operation and surgical technique for using this instrumentation are described. Results. In the semi-implantable clinical trials, METF_MET was -10 to -15 dB for frequencies above 1 kHz, and dropped to -15 to -20 dB for lower frequencies. Fully implantable clinical trial results were 10-20 dB better than the semi-implantable trial results, with less variability at all frequencies. These findings indicate more consistent and better coupling efficiency of the transducer to the ossicles.