It sounds like you have tried the non-surgical methods to control the ear infection. Infections are very hard to resolve with topical medication once the lining of the ear canal is thickened and there is exudate (pus). Generally, bacterial cultures and multiple antibiotics both systemic and ear drops have been tried at this point. Neither systemic nor topical antibiotics are able to penetrate pus to kill the bacteria. In addition, the bacteria are often antibiotic resistance.
There are two major types of surgery. The lateral canal resection (Zepp) and total ear canal removal. The Zepp removes the cartilage over the part of the ear canal that on the outside under the skin. This opens the canal so that it can be treated more effectively, drain and air can reach it.
Total ear canal ablation removes the vertical and horizontal parts of the ear canal, the ear drum and the bones of the middle ear. The tympanic bulla is opened and infected material removed.
There are good descriptions of both of these surgeries at the following website. http://www.marvistavet.com/html/body_lateral_ear_resection.html
Let me know if you have follow up questions.
Have bacterial cultures been done to determine what antibiotics would be most appropriate? Have you used both topical and systemic antibiotics at the same time?
Cleaning the ears by filling the canal with an ear wash and repeating 10 times a session usually gets most of the pus and debris out. This can be repeated several times a week. Antibiotic (not gentomycin) ear drops 2-3 times a day and systemic antibiotics for 2-3 weeks as determined by the sensitivity of the culture would give the best chance of control.
If a lateral ear canal resection is not sufficient, the above non surgical treatment is very unlikely to help. Lateral resection often resolves chronic infection and inflammation resolves over time unless the ear drum is ruptured and the infection is into the middle ear. Sometimes intermittent treatment is still required particularly if there is an underlying allergic component, which is very common.
Severe ear infections that require total ablation are generally very painful and are constant rather than having only intermittent flare ups. The pain is not always obvious to the owner. The smell is generally horrible. They also have perforated the ear drum and are in the middle ear. The dog is already deaf at that point.