The method you will need to use to get this information from the database is very much dependent on how and what you have entered.  The most accurate method involves the Clinical Audit, but this is only an option if you have set up your Clinical Audit Codes and then select the relevant codes for the Site, Diagnosis, Treatment etc. for each patient you see..

Clinical Audit:
If you have been entering Clinical Audit Codes, you would be able to run the Audit Query (Reports - Clinical Audit).  You would need to know the codes that were used for this.

Appointments:
If you have booked a patient for surgery using an Appointment type specific to say, Cochlear Implant, you could run the Appointments in Period report (Reports - Appointment).

You would run this for a single Appointment Type for your Cochlear Implants.

Billing:
The Sales Analysis Report (Reports- Financial - Analysis) will provide a list of all the Items that you have invoiced and group them by the Item code (eg Cochlear Implant). 

The 'Summary' option will display the Invoice code, the number of times a code was used and also the combined value, on a single line.  Otherwise the patient names will also be displayed.

If you want to look at one Invoice code you can use the option 'Filtered by F8 Code' and enter in the code that you use when invoicing.  If you bulk bill any of these, ensure that you also tick the 'Include Schedules' option.

You may need to run the report multiple times if you have more than one invoice code.  Alternatively if all of your Cochlear Implant invoice codes are in a particular category, you could use the option 'Filter By Category' and select the relevant category.
 
Text Search:
The final option is to run a text search through the Notes and Letters tables in the database.  This can be done via Utilities - Data Repair - Find Note Text.  You will need to enter the appropriate text to search for, together with a date range.  Please note that you will need to look at the patients that this report brings up as the text might be 'I recommend that ABC has a cochlear implant' but this does not mean that they have had this procedure performed.